Manual 21425907

Manual 21425907

Introduction

The past few decades have witnessed intense research work worldwide in the field of conductive polymers, a class of conjugated organic materials which combines the mechanical qualities of polymers with the electrical conductivity of metals

1

.

In this context, polythiophenes and their derivatives have occupied an important position, firstly because of their environmental stability and secondly because of the versatility of the thiophene moiety in lending itself to synthetic modification. Polythiophenes have been identified as molecules with potential electro-optical properties and the most stable molecular switching devices discovered to date are thiophene derivatives

2

.

Coordination of transition metals to thiophene or its derivatives leads to the formation of complexes, in this case carbene complexes, with unique properties and diverse application possibilities.

1. Complexes with unsaturated metal-to-carbon double bonds

1.1 Carbene complexes

Complexes containing metal-carbon double bonds are generally referred to as metal-carbene complexes. Without a heteroatom directly bonded to the carbene carbon atom, the compounds are called metal-alkylidene complexes. The first stable transition metal carbene complex was synthesized and characterized by Fischer and Maasb61 in

1964

3

.

Since then different synthetic routes have been developed for the synthesis of these and similar complexes.

1

N.J. Long,

Angew. Chern. Int. Ed. Engl.,

34,

1995,21.

2

G.M. Tsivgoulis, J.-M. Lehn,

Angew. Chern. Int., Ed. Engl.,

34,

1995, 1119.

3

E.O. Fischer and A. MaasbOl,

Angew. Chern.,

76,

1964,

645.

Two different complexes are carbon atom carbonyls of transition complexes can be distinguished. The first Fischer-type by an electrophilic

I"'<:II"I"\QI'",Q carbon atom, the metal-coordinated

1.1). These complexes are readily synthesized by the reaction of metal like chromium, tungsten, molybdenum, iron, rhenium, manganese with various organolithium reagents.

co oc I

CO 1. LiR

"'-w/ - - -....

2. [E!;30]BF4

oc/I"'-co

CO

Figure 1.1 An of a Fischer carbene complex

The second type is the carbene complexes in which the carbene carbon atom is nucleophilic and displays an ylide-like reactivity (figure 1 metals in high oxidation donor ligands, complexes are afforded instance alkyl or cyclopentadienyl, and weak acceptor the carbene ligand is simply a methylene group.

+

-

CH~H

Figure 1.2 An example a Schrock-carbene complex

In years there much interest in the activity of carbenes with respect to their ability to act as reagents for synthesis of organic compounds. Several review articles and

books have addressed this topic

4

, which has been explored extensively and applied in various organic syntheses. Fischer carbene complexes can undergo reactions at several sites and the chemical properties of these complexes are outlined in figure 1.3.

CO CO

8

(a)

6R

\ / / 2

OC--M=c .........

E - - - - -

N (b)

01

\CO "O-R

1

(d)

E

(c)

Figure 1.3 Reactivity of Fischer carbene complexes

Owing to the acidity of the a-CH groups, alkylcarbene complexes are deprotonated by bases (8) to form metal carbene anions (route a), while nucleophilic attack (N) occurs at the electrophilic carbene carbon atom (route b) e.g. aminolysis. Electrophiles (E), for instance Lewis acids, are coordinated to the alkoxy substituent (route c), leading to the formation of metal-coordinated carbyne complexes, while carbonyl substitution by other ligands can occur

via

route d.

Applications of carbene complexes in organic syntheses have been widely employed and recognized for their usefulness. The strategy is to use metal complexes to establish a metal-to­ carbon bond, modify it by further reactions and subsequently cleave the new ligand from the metal moiety. Applying this concept to various carbene complexes, a large range of compounds were formed including

~-Iactams from imines (figure 1.4), cyclobutanones from alkenes and esters from

4

(a) L.S. Hegedus, Transition Metals in the Synthesis of Complex Organic Molecules,

University Science Books, Mill Valley, California, 1994. (b) K.H. Obtz, H. Fischer, P.

Hofmann, F.R. Kreissl, U. Schubert, K. Weiss, Transition Metal Carbene Complexes, VCH

Verlag Chemie, Weinheim, 1983. (c) M. Schuster, S. Blechert, Angew. Chem. Int. Ed. Engl.,

36, 1997, 2036. (d) W.O. Wulff, Organometallics, 17, 1998, 3116. (d) R. Aumann, R.

Frohlich,

J.

Prigge,

O.

Meyer, Organometallics, 18,1999,1369. (e) M.M. Abd-Elzaher, H.

Fischer, J. Organomet. Chem., 588, 1999,235.

6, 7

+

+

Me

~CO)6 )~

0

OMe

Ph M complex to synthesize a

~-Iactam

Figure 1 Employing a complexes been employed in carbon-carbon bond formation in organic

.""rotn.:• ." react under mild conditions with a number of non-heteroatom substituted alkynes to yield annelated reaction products. These are referred to in literature as reactions.

Complexes containing pentacarbonylchromium moieties with phenyl-g, naphtyl-10, furyl- and thienyl11 substituted ligands been with various to give substituted naphthol, phenanthrene, benzofuran and benzothiophene ligands n-coordinated to a tricarbonylchromium fragment.

-

Figure 1.5 Reaction of carbene with alkyne

6

L.S. Hegedus, Acc. Chern. Res., 28,1995,299.

MA L.S.

J.

Arn.

Chem. Soc.,

111,1989,2335.

7CA

D. Xu, S.l.

Organometallics,

11,1992,412.

8

KH. DOtz,

Chern., Int. Ed.

23, 1984, 587.

9

Chern. Ber.,

111, 1978, 2059.

10

K.H.

Angew. Chern. Int. Ed. Engl.,

14, 1975,644.

11

K.H. D6tz, W. Kuhn,

J. Organomet. Chern.,

1983, C78.

On t"IQf1,AlQ&.~n pentacarbonyl[methoxy(phenyl)carbene]chromium an alkyne (figure 1 we find the reaction product to be 4-methoxy-1

"' .... i"tn.f'\' which is n-bonded to a Cr(COh moiety. second ring unsubstituted ring of the naphthol ligand from the carbene ligand, while the C(OH) group is ligand. Alkoxy ,...",rhQr'Qcarbonyl complexes are employed in the c\lr,.n",,,, as peptides, vitamins K and E and antibiotics, by forming cycloaddition of alkyne, carbene and carbonylligands

12

• unit of the from a carbonyl products hydroquinone skeleton via

Carbene to play important roles as intermediates in olefin

. Grubbs

et

ruthenium(lI) carbene complexes of the type trans-[Ru=CHCH=CPh2(PPh3)2CI2]' of highly strained cyclic olefins.

1 Cumulene related complexes

Carbene mr::~le)(es with cumulated double bonds of a class of organometallic compound containing a form

(figure 1.6) encompass coordinated to a carbon-rich unsaturated chain. considerable unsaturated carbon due to their physical and chemical nrl"\,nQ'i'T' attracted metal complexes of this structural type have been proposed as one-dimensional molecular wires

16 and exhibit both liquid nonlinear optical properties (NLO)18. length achieved

12

K. H. Dotz in K.H. Dtitz, H. Fischer, P.Hofmann, F.R. U. Schubert, K. Weiss,

Transition Metal Carbene Complexes, VCH Verlag, Weinheim, 1983, p. 218-226.

CP.

S.T.

J. Am. Chem. Soc., 96, 1974, 7808.

LK Johnson and R.H. Grubbs, J. Am. Chem. Soc., 114, 1992, 3974.

16

H.

J.S.

Chern. Int. Ed. Engl., 33, 1994, 547.

D.L.

(b) M.D. Ward, Chem. Soc.

J.M. Tour,

1995,121.

Chern. Int. Ed.

,33,1994,1360.

A.M. P.M. Maitlis, Angew. Chern. Int. Ed.

M. Altmann, U.H.F. Bunz, Angew. Chem. Int. Ed.

Adv. Mater., 7, 1995,248.

1

30,1991,375. (b)

569. (c)

L

Oriol, J.L.

1B

(a) D.W. D. O'Hare, Inorganic Materials, Wiley:

Whittall, M.G. Humphrey, A. Persoons, S. Houbrechts,

(c) W.J.

UK, 1992. (b) I.R.

15, 1

H.J. Byrne, D.J. Cardin, A.P. Davey, J. Mater. Chem., 1, 1991,245.

1935.

so far is the synthesized and characterized in

1 only19. Since then, a few of similar compounds, including group 6 metal complexes, describing appeared

2o

.

However, complexes containing even-numbered carbon chains seem be restricted to the ruthenium-butatrienylidene complex prepared by Lomprey and . Theoretical calculations

22 suggest that the carbon atoms are alternatively electron-poor and electron-rich, on moving chain the metal centre. This is confirmed by the tendency allenylidene complexes to add nucleophiles at C/3, while the protonation of vinilydenes to carbynes reflects ease of addition of electrophiles to Cf324.

MetaliacurTIulene complex

Allenylldene complex

Vinylidene complex

Figure 1.6 Cumulene complexes

Allenylidene

"":::;."C;;"> have known 1976

25 , but thorough investigation these compounds has been hampered by the lack of a preparation method. They are generally synthesized

via

(i) transformation of alkenyl- and alkynyl carbene complexes25b, (ii) coordination of a C3 skeleton dianion, either [C=CC(OR)RY or Li2C3Ph226 or (iii) the most recent and

19

D. A Romero, P.H. Dixneuf, Gazz. Chirn. Ital., 124, 1994,497.

20

(a) D. P.

A

Daridor, L. Toupet, P.H. Dixneuf,

J.

Am. Chern. Soc.,

116,1994,11157.

(b)

G. Roth, H. Fischer, Organometallics, 15, 1996,1139. (c) G. Roth,

H. Fischer, Organometallics, 15, 1996, 5766. (d) R.W. Lass, P. Steinert, J. Wolf, H Werner,

Chem. Eur.

J., 2, 1996, 19. (e) G. Roth, D. M. Gockel, C. Troll, H.

Organometallics,

17, 1998, 1393.

21

J.R. f'l1'YI,nrl::n,

J.P.

Organometallics,

12, 1993,616.

22

N.M. Kostic, R.F. Fenske, Organometallics, 1,1982,974.

23

D. Touchard, N Pirio, P.H.

Organometallics,

14, 1995,4920.

24

C. Kelly, N. M.R. Terry, G.L. Geoffroy, B.S. Haggerty, AL. Rheingold,

Chern. Soc.,

114, 1992,6735.

J.

Arn.

25

(a) H.

Angew. Chern. Int. Ed.

,15,1976,624. (b) E.O.

A Franck, F.H. Kohler, G. Huttner, Angew. Chern. Int. Ed.

26

S. Takahashi, Y. Takai, H. Morimoto, K. Sonogashira,

1984,3.

J.

Chern.

H.J. Kalder,

623.

Chem. Commun.,

Chapter

1:

Introduction

7 straightforward method employed by Selegue and co-workers

27

.

This synthesis comprises the reactions of alkyn-3-0Is with metal complexes ego [RuCpCI(PR

3)2] in polar media to yield hydroxyvinylidene intermediates, which spontaneously dehydrate

in situ

to form allenylidene complexes. Since this discovery, several allenylidene derivatives have been produced using this method, ego iron

28

, rhodium

29 and ruthenium

30 derivatives. The same method is now directed towards the building of bimetallic systems with allenylidene ligands

31 or bridges

32

.

Vinylidene complexes, the most simple form of cumulene ligands, can be prepared in several ways, for instance by using metal acetylide, acyl complexes, vinyl complexes, olefins, carbene complexes or 1-alkynes. An overview of all the synthetic preparations and applications of vinylidene complexes is available in review articles by Stang

33 and Bruce

34

.

Complexes containing these species are likely intermediates in coupling of alkynes to give enynes

35 or butatrienes

36

, and in the synthesis of unsaturated ketones from alkynes and allylic alcohols

37

.

1.3 Binuclear metal complexes bridged by linear unsaturated carbon chains

Complexes in which two transition metals are bridged by linear unsaturated carbon chains have

27

J.P. Selegue, BA Young, S.L. Logan,

Organometallics, 10, 1990,1972.

28

S. Nakanishi, K.1. Gada, S.I. Uchiyama,

Y.

Otsuji,

Bull. Chem. Soc. Jpn., 65,1992,2560.

29

(a) R. Wiedemann, P. Steinert, O. Gevert, H. Werner,

J. Am. Chem. Soc., 118, 1996,

2495. (b) I. Kovacik, M. Laubender, H. Werner, Organometallics, 16, 1997, 5607. (c) D.

Touchard, P. Haquette, A. Daridor, A. Romero, P.H. Dixneuf,

Organometallics, 17, 1998,

3844.

30

T. Braun, P. Steinert, H. Werner,

J. Organomet. Chem., 488,1995, 169.

31

D. Touchard, S. Guesmi, M. Bouchaib, P. Haquette,

Organometallics, 15, 1996, 2579.

A. Daridor, P.H. Dixneuf,

32

H.P. Xia, G. Jia, Organometallics, 16, 1997, 1.

33

P.L. Stang,

Acc. Chem. Res., 15, 1982, 348.

34

M.1. Bruce, Chem. Rev., 91, 1991, 197.

35

C. Bianchini, M. Peruzzini, P. Frediani, J. Am. Chem. Soc., 113, 1991, 5453.

36

Y. Wakatsuki, H. Yamakazi, N. Kumegawa, T. Satoh, J.Y. Satoh,

J.

Am. Chern. Soc.,

113,1991,9604.

37

B.M. Trost, R.J. Kulawiec,

J. Am. Chem. Soc., 114, 1992, 5579.

recently attracted much interest in view of their new material properties

38 and as model systems for surface carbides in heterogeneous catalysis

39

. species, MCnM', can bridge either two similar or two different transition metal fragments.

In complexes where two similar metal fragments are bridged, complexes n==2 are most common. though all three valence bond descriptions (figure 1 have experimental support

39

, most of complexes contain an acetylenic C bridge. cumulenic structure M==C==C=M was observed in a few titanium tantalum complexes.

M-C

=M

Figure 1.7 structures for the MC

2

M species

M

It been experimentally that compounds with an even Cn common than complexes containing an uneven number of in chain. are much more four-4o, and eight-carbon bridged complexes

41 are known and, depending on d n configuration, display either a polyynic (reduced) or a cumulenic (oxidized) structure (figure 1.8). These bimetallic iron chain bridged compounds where were developed by Lapinte and co-worker2.

+ b

Figure 1.8

(a) Polyynic structure and (b) oxidized structure

36

39

M,H.

W.

Chisholm,

Chern. Int. Ed.

,30,1991,673.

B. Niemer, M. Wieser,

Angew, Chern. Int. Ed. Engl.,

1993,923.

115,1993,

40y.

8509.

J,W. W. Weng, A.M. Arif,JA

41

M. Brady, W. Weng, J.A.

J.Am.Chem.

J. Chern. Soc., Chern. Comm., 1994,

42

F. C. Lapinte,

Organometallics,

15, 1996,477.

The complex with the highest number of carbon atoms in the chain,[{ReCp*(PPh

3

)(NO)h(IJ-C

2o)] was recently synthesized by Gladysz and co-workers43. The longest polyyne structurally characterized to is a complex. Two groups simultaneously published structures of this type of complex, a diplatinum complex synthesized by Gladysz et af4 via Hay coupling two C

6 mono-platinum complexes a diiron complex prepared by Akita

Complexes containing two transition metal building blocks, linked by C n bridges (n=1-5), have only recently become accessible

15

.

Synthesis of these compounds, particularly for containing organometallic substituents, turned remarkably easy46. as a new class of one-dimensional molecular

. Very complexes were compounds have been synthesized with an odd of carbons in the C n

.

Only one bridged complex has been structurally characterized, namely [{ReCp*(PPh

3

)(NO)}(IJ-C

3

){MnCp(CO):J]. each chains two possible structures exist (figure 1.9). However, structural and spectroscopic information that the cumulenic forms dominate over the alkyne-carbyne

1.9 Valence structures of bridged chain with different fragments

43

B. Bartil" R. Dembinski, T. Bartik, AM. Arif, JA Gladysz, NewJ. Chem., 21,1997,

44

T.B.

J.C. Bohling, AM. Arif, JA Gladysz, Organometallics, 18, 1999, 3261.

45

A. Sakurai, M. Akita, Y. Mora-aka, Organometallics, 18, 1999, 3241.

46

(a) W. Beck, W. Knauer, C. Robl, Angew. Chem. Int. Ed.

Weng, JA Ramsden, AM. JA

,29, 1990,293. (b) W.

J.

Am. Chem.

Soc., 115, 1993, 3824. (c)

J.W. Seyler, W. Weng, J. Zhou, JA

Organometallics,

12, 1993, 3802.

47

P.

1825.

N.

A. Sgamellotti, C. Floriani, J.

Chem.

Soc., Dalton Trans., 1998,

48

W. Weng, T.

JA

Angew. Chem. Int. Ed.

1994,2199.

Chapter

1:

Introduction

10

Until recently, of n-conjugated metal-bridged trinuclear biscarbene complexes were limitedtothe prepared by Fischer hrll"i,..,Orlcomplex[(CO)5W =C(NMe

2

)C=CC=CHgC=CC=CC(NMe

2

)=W(CO)5] ,

. Since then, the synthesis of and tetrakis( ethynylcarbene) complexes was the lithiated

[C1mM'(Ln)] (m

= any other main group complexes are by nucleophilic substitution of complex [M(COM=C(NMe

2

)C=CLi}] (M

=

W, Cr) for the chlorides in linking atom M' can be a metal, a main group metal or

1.4 Binuclear metal complexes bridged by conjugated ligands

A large number of binuclear complexes with conjugated bridges have been reported. complexes are widely rOl"',,,,..,n for their ability to allow coupling through n delocalization. The such asthe common bridges are

1.10),4,4'-bipyridine

52 nitrogen, including pyrazine, heteroaromatic grou pS53.

5+

Figure 1.10

Sponsler ion recently reported the first delocalized mixed-valence

49

C. Har1baum, H. Chern. Ber.lRecl., 130, 1997, 1063.

50c.

Hartbaum, Roth, H. Fischer, Eur. J.lnorg. Chem., 1

51

C. Creutz, H.

J. Am. Chem. Soc., 95,1 1086.

52

S. Woitellier, J.P.

C. Joachim,

Chern.

, 131,

191.

1

481.

53

S. Boyde, G.F.

54

BA

116, 1994, 2221.

M.D.

W.E. Jones, T.J. Meyer, J. Am. Chern. Soc., 112,1990,7395.

H.N.

M.B. J. Am. Chem. Soc.,

@

I containing (figure 1.11), prepared by the corresponding Fell/Fe" Since then several mixed valence complexes have been synthesized, a Ru"/Ru lll complex prepared by Moreira and co

1.11 Fe"/Felli complexes containing bridges complexes in which two metal-containing fragments are bridged by or

Iigands

57 have enjoyed considerable attention recently since they serve as models for units in related organometallic number fused aromatic rings in the bridging groups

. It was found that increasing the materials, Le. in going from 1

55

I.S. Moreira, D.W. Franco, Inorg. Chem., 33, 1

56

Y. Kim, S. Song, S. Lee, S.W. Lee, K.

Trans., 1998, 1775.

57

R.

Chukwu,

AD.

Hunter, B.D. Santarsiero,

58

R.

McDonald, K.C. Sturge, A.D. Hunter,

L

1607.

T. Yamamoto, J.

Chem. Soc., Dalton

10,1991,2141.

Organometallics, 11, 1992, 893.

Chapter

1:

Introduction

12 to 9,10-C14H859, increased the degree of intermetallic conjugation in complexes having two metal centers directly joined to the aromatic core by metal-carbon a bonds, as has been demonstrated for conjugated organic polymers

6o

.

The two metal centers do indeed interact and they transfer significant electron density to the arene ring. Hunter and co-workers61 prepared the phenylene­ bridged complexes of iron and manganese as well as iron complexes of quinoline and related bridges. The structures of many of the quinoline derivatives were similar to those of related biologically active materials, including herbicides, fungicides and insecticides.

2. Thiophene and related compounds

Thiophenes are well known for their occurrence in fossil fuels o2 and the coordination chemistry of these compounds has received recent attention because of its relevance to the metal-catalyzed hydrodesulfurization of the fossil fuels

63

.

A mechanism for the hydrodesulfurization was proposed by Angelici and co-workers64 on the basis of organometallic model compound and catalytic reactor studies. Thiophenes are aromatic compounds and display coordination properties closer to those of arenes than thioethers. On comparing the ionization potential of thiophene (8.9 eV) and benzene (9.3 e V), it is found that thiophene is slightly more nucleophilic than benzene. Ab initio calculations

65 seem to indicate accumulation of negative charge on the 2- and 5-carbon atoms and a positive charge on the sulfur atom. Coordination can occur through the sulfur atom (8), through the 2- or 5-carbon or both

(r]

1) or through the n-clouds of the C(2)=C(3) or C(4)=C(5) bonds

(fl2) or both (r]4). Whereas r]4coordination implies a sp3-hybridized sulfur atom which is bent out of the plane, the r]5mode involves all the atoms of the ring, including the sulfur atom, and a planar ligand is encountered for complexes of this type. Examples of coordination to all of these sites can be

59

AD. Hunter, D. Ristic-Petrovic, J.L. McLernon, OrganometaJJics, 11, 1992, 864.

60

P.N. Prasad, D.R Ulrich, Non-linear Electroactive Polymers, Plenum, New York, 1988.

61

AD. Hunter, AB. Szigety, Organometallics, 8, 1989,2670.

62

W.L. Orr and C.M. White, Eds., Geochemistry of Sulfur in Fossil fuels, American

Chemical Society, Washington, D.C., 1990.

63

B.C. Gates,

~I.R

Katzer and G.CA Schuit, Chemistry of Catalytic processes; McGraw­

Hill; New York, 1979.

64

J. Chen, L.M. Daniels and RJ. Angelici, J.

Am. Chem. Soc.,

113, 1991,2544.

65

T.B. Rauchfuss, Prog. fnorg. Chem., 39, 1991,259.

found in Iiterature

66

Oligomers of photo-enhanced are compounds of current interest because many of them show activities

67

, while a-polymerization of thiophene electroconductive polythiophenes

68

.

A wide variety of thiophene oligomers crystalline, derivatives have synthesized mainly with the prospect of obtaining precursor molecular and polymers. thiophene polythiophenes have found wide application as potential conducting polymers

69

, electron

"'''/~O''''Tr\''<'

, hydrogenpoor heterocycles

71

, conductors or superconductors

72 and materials with non-linear optical seem to be particularly appealing components to use as bridging ligands for they are likely to permit strong metal-metal interactions over long of their chemistry unfortunately received very attention. One ro.~o.nt

This aspect was the use of 2,5-di(4-pyridyl)thiophene as a bridge between two {Ru(NH

3)5}2+13+ the electrochemical interaction between

It was observed was moderately stronger than that across a 1,4-di(4-pyridyl)butadiene

66

(a) RJ. Angelici,

Coord. Chem. Rev.,

105, 1990,61; (b) TA Waldbach, P.H. van

Rooyen, S.Lotz,

Angew. Chem. /nt. Ed. Eng.,

1993,710; (c) J. Chen, V.G. Young and

RJ. Angelici,

JAm. Chem. Soc.,

117, 1995, 6362; (d) J. Chen and RJ. Angelici,

Organometallics,

1989,3424.

67

(a) J. Lam, H. T. Arnason and L. Hansen,

occurring and Related

Lai, M.C.W. Chan, K-K. Cheung, S-M.

Eds.

and Biology of Naturally-

Amsterdam, 1988. (b) S-W.

C-M. Chi,

Organometallics,

18, 1999, 3991.

68

J. Nakayama and T. Konishi,

Heterocycles,

1988,1731,

69

(a) S. Musmanni and J.P. Ferraris,

J Chem.

D.Y. Son, 18,1999, 1736.

Chem. Comm.,

1993, 172. (b) J. Yao,

70

D. Lorcy, K.D.

Commun.,

1993, 345.

71

K. Yui, H.

Soc. Jpn.,

Y.

1989,1547.

Y. Okuda, J.L. Atwood and M.P.

T. Otsubo, F. Ogura, A.

J Chem. Soc., Chem.

and J. Tanaka,

Bull. Chem.

72

O. Kobayashi,

Sulfur,

43,1989, 187.

73

(a) N.J. Long,

Angew. Chem. Int. Ed. Engl.,

1995,21. (b) RA Ham and D.

Organic Materials for Non-linear Optics,

The Royal Society of Chemistry,

Special Publication No. 91, 1991. (c) I.S.

1999, 1091.

H. YK Chung,

Organometallics, 18,

74

A.C. Ribou, J.P.

Chem.,

33, 1994, 1325.

K. Takahashi, T. Nihira, S. C.W. Spangler,

Aim of this study

The coordination chemistry of thiophene to transition review been have appeared in this regard

66a.

Thiophene derivatives studied been investigated for materials exhibiting NLO properties. It was suggested that incorporation metal centres thiophene is to enhance conducting properties of the molecule

1

.

The coordination of transition metals to thiophene moieties been extended include bithiophene

75

, the most elementary poly thiophene. On the contrary, the coordination chemistry although condensed thiophenes to transition metal moieties has not been are considered to be very promising in the at all,

A convenient, high-yield synthetic method for the preparation of these ring a challenging objective, such a method may encourage more in-depth research studies of these systems.

As molecules as the synthesis of Fischer carbene was contemplated. Although thiophene as ligand have already containing condensed thiophene

Fischer complexes with

, the molybdenum analogue was prepared in this study comparison with the novel condensed thiophene complexes. It is well known that carbonyl groups of molybdenum hexacarbonyl are more labile than those of chromium hexacarbonyl or hexacarbonyl77 and tho,l'ot,,,I'''' lends to unique reactivity and coordination possibilities. Aminolysis result of this study is this to enhance stability, was proposed. The in chapter The synthesis of Fischer complexes of

3,6-dimethylthieno[3,2-b]thiophene and dithieno[3,2 -b:2' ,3'-d]thiophene with group 6 transition metals was devised (figure 1.12). These condensed thiophene units can act as a conjugated n-system to afford communication between the metal functionalities. The results of investigation are addressed in chapters 3

75

S~

Maiorana, A¥ Papagni, E. Ligandro, A.

LJ"""",,,,"\nc

K. Clay, S. Houbrechts, W.

Gazz. Chim. /tal., 125,

1 377.

76

(a) JA Connor,

1

Jones, E.W. Randall,

E.

Rosenberg,

J.

Chern. Soc., Dalton Trans.,

2419. (b) Y.M. Terblans, H.M. S. Lotz,

J.

Chem.,

1998,133.

77

Ch. Eischenbroich, A.

Weinheim, 1992,232.

Organornetallics,

A Concise Introduction, VCH

M = Cr, W, Mo, Mn

L=CO

L3= Cpo CpMe

M= W,Mo

Figure 1.12 Biscarbene complexes of condensed thiophene complexes

The structure of a diiron thiophene complex has been and is shown in figure 1. This compound was prepared, reacting the 2,5-dilithiated thienylene species with two equivalents of

[FeCp(CO)21], for purposes of comparison and the syntheses similar iron complexes with thiophene thienothiophene were thus attempted. The synthesis, stability and reactivity of these complexes are discussed in 5.

(CO){:,pF~FeCp(CO),

S

Figure 1.13 Dii ron thiophene complex

The aspect of this study deals with stability of dinuclear complexes and the involvement of the thiophene rings in stabilizing the metal-carbon bonds. it was anticipated that thiophene rings afford a more rigid ligand, linear oligothiophene spacers would probably stable. In this project, a formal stability study was not conducted but was simply taken to represented by the amount of change in the composition of the in solution. The stability of the individual period of in solid state as well as was compared relatively to one another. were based on electronic and geometrical factors which were deduced from spectral data in solution a favourable entropy structural measured in was not considered and we solid state. it to resulting from the same for comparable

Chapter

1:

Introduction

"'t"""''''''' and was hence '1"''''''''''''''''

16 study represents the initial in our laboratories for the compounds which display properties charge transfer

via

a common

,,...,,,,,,I'\n in material science. investigations will focus on: synthesis and testing of ligand for possible

(i) synthesis of mixed metal complexes with the creating a naturally i:>fJCIIJCI ligand. The rorQ<:lltlr,n of a "push-pull" effect by incorporating electron-poor and electron-rich transition metal in the same molecule is varying the metal atom, different oxidation states of metal and the ligand environment decomposition products in this study, with a monocarbene isolation and an ester functional on sides of the ligand, indicated possibilities of utilizing such

Oi) the synthesis of bis-alkyl and de localization of electron density in

(iii) development of a transfer in n-conjugated systems; complexes and their modification in enhancing system; method for the of the magnitude of

(iv) correlation of spectral structural features with transfer properties.

Carbene com plexes of

Thiophene

1.

General

Recent studies of the interaction between thiophene and transition metals have revealed an intricate and diverse coordination chemistry. Motivation for these studies originated to a large extent from the understanding of the mechanism of catalytic hydrodesulfurization (HDS), the industrial process by which sulfur is removed from fossil fuels

1

.

Thiophenes are amongst the most difficult sulfur-containing compounds targeted in the HDS process to desulfurize and thus of particular interest.

Several types of thiophene (T) coordination modes to transition metals are known. Prior to 1985 only complexes of the fl5-coordination mode were known, but since then complexes exhibiting fl \ fl2- and fl4-coordination modes have been synthesized

2

. Few S-bound complexes have been isolated, for example [Ru(NH3)5T]2+, [FeCp(CO)2Tt, [FeCp(NCMe) 2 (2,5-Me2TW and

[W(CO)3(PCY3hTt In all cases the thiophene is easily replaced by weak ligands since the coordinating nature of the sulfur atom in thiophene is not strong. This weak coordinating ability of the sulfur atom results from delocalization of the sulfur non bonding electrons into the n system of the ring. Arce

et at

reported the synthesis of [OS3(C4H4S)H2(CO)g], a complex exhibiting the fl2­

'B .

C. Gates, J .

R. Katzer, G .

CA Schuit ,

Chemistry of Catalytic Processes;

McGraw-Hili;

New York, 1979.

2

(a) AE. Ogilvy, AE. Skaugest, T.B. Rauchfuss, Organometallics, 8, 1989,2739. (b) R.

Cordone, w.o.

Harman, H. Taube, J. Am. Chem. Soc., 111,1989,5969. (c) R.J. Angelici,

Acc . Chem. Res .

,

21 , 1988, 387.

3

(a) C.G. Kuhn, H. Taube, J . Am. Chem. Soc., 98, 1976,689. (b) J.D. Goodrich, P.N.

Nickias, J .

P . Selegue, Inorg. Chem.

,

26, 1987 , 3424. (c) D. Catheline, D. Astruc, J.

Organomet . Chem.,

272, 1984,417 . (d) H.J. Wasserman, G.J

. Kubas, R.R. Ryan,

J.

Am.

Chem.

Soc., 108, 1986, 2294.

4

AJ. Arce, Y. De Sanctis , AJ. Deeming, J.

Organomet . Chem.,

311, 1986, 371 .

Chapter

2:

Carbene complexes of Thiophene

coordination mode, while the first evidence for r(thiophene complexes were observed by Huckett and Angelici5, who prepared the complex [lrCp"T]. Several115-coordinated complexes can be found in literature, e.g. [Cr(1l5-T)(COht The general stability of these 115-thiophene complexes suggests that this may be the preferred bonding mode to transition metals on catalytic surfaces . All of these modes have been implicated as important intermediates in the HDS process. o s

I

IVI

S

M o

@

S M

S-coordination 112-coordination 114-coordination 1l5-coordination

18

Figure 2.1

Coordination modes of thiophene to one transition metal

Coordination of thiophene to more than one transition metal can also occur at different sites on the thiophene ring. An example where the thiophene is coordinated through the diene to one metal, while the sulfur atom is bonded to another metal, is the complex [Fe(COh{I-l-11 2,S­

DRe(CO)2Cp*f, Angelici

ef

a/ recently synthesized the first thiophene complexes coordinated to three metal centres namely the complexes {1-l3-114,S,S}[lrCp(2,5-Me2T)][M02(CO)4CP2]8 and {1-'3­

114,S,S}[lrCp(2,5-Me

2

T)][Fe2(COh]9, both exhibiting the {1-l3-11

4,S,S}-bonding mode

. In these coordination modes, the planarity of the thiophene ring is destroyed and the sulfur atom is bent out of the plane of the four carbon atoms. This is the reason for the stability of these complexes since, compared to the weak S donor ability of free thiophene, the sulfur atom in 114-thiophene complexes is an excellent donor.

The 111 :115-coordination mode of thiophene has received considerable attention in our laboratories

5

S .

C . Huckett, RJ . Angelici, Organometaflics, 7, 1988, 1491.

6

E .

O . Fischer, K. Ofele, Chem. Ber., 91,1958,2395.

7

M.G. Choi, RJ. Angelici, J. Am. Chem. Soc., 111,1989,8753.

6

J. Chen, RJ. Angelici, Organometallics, 9, 1990, 879 .

9

J. Chen, L.M. Daniels, R.J. Angelici, Polyhedron, 9, 1990,1883.

Chapter

2:

Carbene complexes

of

Thiophene

19 and various bimetallic complexes were synthesized including [Pt(1l1_C4H3S)L

2

{(1J-1l 1:115­

C4H3S)Cr(COh}]10 and [Mn(COMIJ-1l

1

:1l5_C4H3S}Cr(COhf1. The latter complex was found to undergo a metal exchange reaction in solution

12

.

The C-S bond cleavage is observed in a base-catalyzed reaction where the iridium in [lrCp*(1l4-2,5­

Me

2

T)] inserts into a C-S bond of the 1l

4

-thiophene to give the ring-opened iridathiabenzene

[lrCp*(C,S-2,5-Me2T)), a six-membered ring. This iridathiabenzene ring reacts with various metal carbonyls to yield 1l6-coordinated complexes of the type [lrCp*{1l

6

-(C,S-2,5-Me

2

T)} ML3]' (M = W,

Cr, Mo; L = CO, Fc+, Cp)13 . Metal insertion into the C-S bond of thiophene has also been recorded for Pt(O) and Re(O) metal moieties14 . Russian workers have described the preparation of the compound [FeCp(COh(C4H3S)]15, which was reacted with Fe3(CO)12 to yield a trimetallic thiaferrole complex16.

@-M'

M

/ s f8\M'

M

/~j

1l4,S-coordination 1l4,S,S-1J3-coordination

Figure 2.2 Coordination modes of thiophene to more than one transition metal

10

A du Toit , M. Landman, S. Lotz,

J. Chem. Soc., Dalton Trans.,

1997,2955 .

11

TA Waldbach, P.H

. van Rooyen, S. Lotz, Organometallics, 12, 1993,4250.

12

(a) TA Waldbach, P.H. van Rooyen, S. Lotz,

Angew. Chem. Int. Ed. Engl.,

32, 1993,

711. (b) TA Waldbach, R. van Eldik, P.H. van Rooyen, S. Lotz,

Organometallics,

16, 1997,

4056.

13

J. Chen, V.G. Young, R.J . Angelici,

J.

Am.

Chem. Soc.,

117,1995,6362.

14

J.J. Garcia, A Arevalo, V. Montiel, F. del Rio, B. Quiros, H. Adams, P.M. Maitlis,

Organometallics,

16,1997,3216 .

15

AN. Nesmeyanov, N .

E. Kolobova, L.V. Goncharenko, K .

N. Anisimov,

Bull.

A

cad . Sci.

USSR, Div. Chem. Sci. (Engl. Transl.) ,

1976 , 142.

16

AE. Ogilvy, M. Draganjac, T.B. Rauchfuss, S.R

. Wilson,

Organ ometallics

, 7, 1988, 1171.

Chapter

2 :

Carbene complexes

of Thiophene

2. Carbene complexes of arenes and heterocycles

20

Ab initio

calculations indicate that the 2- and 5-positions of thiophene are the most active sites on the ring for nucleophilic attack while the sulfur atom carries a positive charge . It is therefore clear that deprotonation with lithium reagents can be readily accomplished, first abstracting the 2-proton and then the 5-proton in forming the dilithiated species. Since the synthesis of Fischer carbene complexes involves the nucleophilic attack of RLi reagents on carbonyl ligands, the synthesis of thiophene substituted alkoxy carbene complexes seems to be accessible by a combination of these two procedures .

2.1 Monocarbene complexes

The original carbene complex synthesized by Fischer and Maasbbl1

7

, [W(COhC(OMe)Ph] , is also the first example of a carbene complex containing an aromatic substituent. Shortly after this discovery, the structural data of the complexes [W(CO)5C(OMe)Ph] and [Cr(CO) 5 C(OMe)Ph]1 8 were published which clearly showed the sp2-character of the carbene carbon. In addition to the transition metals' synergic d(t2g)-p n-interaction with the carbene carbon, the important role of the heteroatom (X) lone-pair in

P c -P x n-bonding to stabilize the "singlet" carbene carbon was soon recognized1

9

(figure 2 .

3). A phenyl or heteroarene substituent is incorporated into the n­ delocalized network surrounding the carbene carbon and may act as an electron withdrawing or electron donating substituent. Of interest for the latter is the stable [W(CO) 5 C(Ph)ph] carbene complex, which could be synthesized from the complex [W(COhC(OEt)Phf o

.

The complexes

[W(CO) 5 C(OMe)ph] and [Cr(CO) 5 C(OMe)Ph] together with the [MnCp(COhC(OMe)Ph] carbene complex were synthesized to investigate the electrophilic activity of the carbonyl ligands. To determine whether carbene complexes can accommodate dinuclear metal carbonyl complexes,

17

E .

O . Fischer, A. Maasb61 ,

Angew. Chern. ,

76, 1964 , 645 .

18

O.S

. Mills, A.D. Redhouse , J . Chem. Soc. A , 1968 , 642 .

1 9

(a) E.O. Fischer ,

Angew . Chern . Int. Ed. Engl .

,

86, 1974, 651 . (b) F.A. Cotton, C.M.

Lukehart, Prog

. Inorg. Chern.,

16, 1972,487.

2 0

C.P

. Casey,

J. Am. Chem. Soc.,

95 , 1973 , 5833.

Chapter

2 :

Carbene complexes of Thiophene

21

Fischer

et af21

synthesized the dinuclear Mn

2

(CO)g carbene complex [Mn2(CO)gC(OEt) Ph]. The iron carbene complex with phenyl as substituent, [Fe(CO)4C(OEt)Ph] is also known22 , but Fischer-type carbene complexes of iron containing carbonyl ligands have been relatively inaccessible , mostly due to the preferred alkylation of the metal centre instead of the oxygen2 .

..

Figure 2.3 n-delocalized network around carbene carbon in arene carbene complex

Connor and Jones24 synthesized the monocarbene complexes of chromium of the type

[Cr(CO)5C(OR)R'] where R' = 2-thienyl and 2-furyl in order to investigate the influence of different

R-groups on the stability of the empty pz orbital on the carbene carbon . The thienyl carbene complex of tungsten was prepared by Aoki

et af2 5.

Angelici and co-workers2

6 recently reported the synthesis of a cationic 2-thienylidene carbene complex of rhenium, formed by C-H bond activation in a S-thiophene complex (figure 2.4).

base

-w

S

[Re1-D

Figure 2.4 Formation of 2-thienylidene carbene complex

21

E.O. Fischer, E. Offhaus, Chern. Ber.

,

102, 1969,2549 .

22

D.J. Cardin, B . Cetinkaya, M.F. Lappert, Chern. Rev

.,

72, 1972 , 545.

23

(a) M.F. Semmelhack, R. Tamuar, J

. Arn. Chern. Soc.,

105, 1983 , 4099. (b) S . Lotz,

J.L.M

. Dillen, M.M. van Dyk,

J.

Organomet. Chern.,

371,1989,371 .

24

JA Connor, E .

M . Jones,

J.

Chern. Soc.

A, 1971, 1974 .

25

S. Aoki, T . Fujimura, E. Nakamura, J.

Arn. Chern. Soc.,

114,1992,2985.

26

M.J. Robertson, C .

J . White, R.J

. Angelici,

J.

Arn. Chern. Soc.,

116, 1994, 5190 .

. i ll

\ b \

'7 'l....'6

'I

bl\l.

-:?390~3

Chapter

2 : Carbene complexes of Thiophene

22

The precursor [ReCp(NO)(PPh3)thiophene)t undergoes deprotonation by a strong base to give the 2-thienyl complex [ReCp(NO)(PPh 3 )(2-thienyl)) . Re-protonation of the latter with triflic acid does not regenerate the S-thiophene complex but instead protonation occurs at the 3-position to form a thienylidene carbene product.

Monocarbene complexes of 2,2'-bithiophene have been synthesized recently27. These complexes were specifically tailored to contain pentacarbonyl transition metal units, which are known to be strong electron-withdrawing groups, on the one side of a conjugated system . The second thiophene moiety would represent the electron-donor counterpart, to obtain a "push-pull " situation in the molecule . The chromium and tungsten complexes were isolated and subjected to HRS

(Hyper-Rayleigh Scattering) measurements to evaluate the NLO responses of these systems . It was concluded that the preliminary evaluation looked promising , although no later paper on this subject has appeared since .

Raubenheimer

et aP

8

described the synthesis of carbene complexes of iron by the addition of thiazolyl- or isothiazolyl-lithium to [FeCp(COhCI] and the subsequent alkylation or protonation of the products . In the thiazolylidene complexes the N-atom is in an a-position with respect to the coordinated carbene carbon, as is typical for known aminocarbene complexes , while the isothiazolinylidene complexes are unique since the nucleophilic heteroatom is situated y to the coordinated carbon atom.

Me

\

N

[ F e ] = ( )

S

+

[Fe]

(/~

S Me lhiazolyl complex

Isothiazolyl complex

Figure 2.5

Thiazolyl and isothiazolyl carbene complexes

+

2 7

S. Maiorana , A. Papagni , E . Licandro , A. Persoons, K . Clay, S . Houbrechts, W . Porzio,

Gazz .

Chim. Ital .,

125 , 1995 , 377 .

28

J .

G . Toerien, M . Desmet , G .

J . Kruger, H .

G. Raubenheimer,

J.

Organomet . Chem., 479,

1994 , C12.

Chapter

2:

Carbene complexes

of

Thiophene

23

In 1968 Wanzlick and Gfele discovered that heterocyclic carbenes derived from imidazoiium and pyrazolium salts form extraordinarily stable complexes with certain transition metals. The syntheses of the complexes pentacarbonyl(1 ,3-dimethylimidazoline-2-ylidene)chromium(O) A and bis(1 ,3-diphenylimidazoline-2-ylidene)mercury(ll) diperchlorate

B were described

29

,30

(figure 2.6).

They could however not isolate free carbenes. Arduengo opened the access to free, isolable N­ heterocyclic carbenes in 1991

31

.

In these complexes the metal-carbon bond is much less reactive than in Fischer- and Schrock-type carbene complexes. Renewed interest in these complexes have recently been sparked by Hermann

ef al

because of the advantages of N-heterocyclic carbenes as ligands in organometallic catalysts, where they extend the scope of application reached by phosphines.

32

Me

I

N

(

)=C~COb

N

I

Me

A

CsHs

I

N

(1,Hs

I

N

()=H9=<

N

I

CsHs

N

I

CsHs

J

2+

B

Figure 2.6

First N-heterocyclic carbene complexes

2.2 Biscarbene complexes

Fischer

3 employed his classical method of synthesizing carbene complexes to prepare the 1,4­ phenylene biscarbene complexes of chromium and tungsten. These complexes were prepared by reacting p-phenylene dilithium with the relevant metal carbonyl complex. The synthesis of

29

H.W. Wanzlick, H.J. Schonherr,

Angew. Chem. Int. Ed. Engl.,

7, 1968, 141.

30

K. Ofele,

J.

Organomet. Chem.,

12, 1968,42.

31

(a) A.J. Arduengo, R.L.Harlow, M. Kline,

J.

Am. Chem.

Soc., 113, 1991, 361. (b)

A.J.

Arduengo, F. Davidson, H.V.R. Dias, J.R. Goerlich, D. Khasnis, W.J. Marshall, T.K.

Prakasha, J.

Am. Chem.

Soc., 119, 1997, 12742.

32

WA Hermann,

C.

Kocher,

Angew. Chem. Int. Ed. Engl.,

36, 1997, 2162.

33

E.O. Fischer, W. Roll, N. Hoa Tran Huy, K. Ackermann,

Chem. Ber.,

115, 1982,2951.

Chapter

2:

Carbene complexes

of

Thiophene

24 biscarbene complexes where o-phenylene dilithium is utilized to afford 1 ,4-chelating tetracarbonyl biscarbene complexes of chromium, tungsten and molybdenum (figure 2.7) was described in the same article. This carbene study was extended to include the biphenylene Iigand34. Biscarbene complexes of chromium and tungsten were prepared in a similar fashion and an X-ray diffraction study of the chromium complex showed that the molecule was centrosymmetric and the rings strictly coplanar. In a structural study of free biphenylene the rings were found to be twisted35 .

EtO

'C

(CO)¥

---<Q)-

0

c!

M(CO)s

'OE!

M

=

Cr, W

Figure 2.7 Phenylene biscarbene complexes

M=

Cr,

W

C

M(CO)5 i j

"~Et

EtO

\

(CO)'<:©

/

EtO

M

=

Cr, W, Mo

Synthesis of the biscarbene complex of tungsten with anthracene as bridging unie

6 was attempted to try and establish whether the functional group W(CO)5C(OMe) could withdraw electron density from dienes and thus assist in inverse Diels-Alder reactions. The complex was prepared starting from 9,1 O-dibromoanthracene, reacted with tungsten carbonyl and alkylated using trifluoromethane sulfonic acid ester.

MeO W(CO)5

,~

C

Figure 2.8 Bimetallic biscarbene complex with anthracene as bridging unit

34

N. Hoa Tran Huy, P. Lefloch, F. Robert, Y. Jeannin, J. Organomet. Chem., 327, 1987,

211.

35

G. Charbonneau, Y. Delugeard,

Acta Cryst.

8,

33, 1977, 1586.

36

T. Albrecht, J. Sauer, Tetrahedron Lett., 35, 1994,561.

Chapter

2: Carbene complexes of Thiophene 25

Dinuclear biscarbene complexes incorporating thienylene moieties have been synthesized in our laboratories recently. The biscarbene complexes of chromium, tungsten and manganese with thiophene as bridging unit were prepared37 and the crystal structure of the chromium biscarbene complex was determined. The reactions of the complexes in refluxing carbon disulphide, hexane and acetone were investigated. The intention was to investigate the possibility of forming dinuclear biscarbene complexes with coupled olefinic units, resulting in an extended spacer between the metal moieties. No evidence of such reaction products was observed. Instead, the corresponding o-ethylthienyl carboxylate monocarbene complexes and o-ethylthienyl thiocarboxylate monocarbene complexes were isolated, as shown in figure 2.9.

Figure 2.9

Decomposition of biscarbene complexes

Since thiophene derivatives have been largely used in studies for materials exhibiting non-linear optical properties both as spacer and as conjugating linking groups, this prompted the study related to the synthesis of carbene complexes of 2,2'-bithiophene. With this objective in mind, the first biscarbene complex containing two different metal moieties was synthesized (figure 2.10)27.

However, mixtures of reaction products resulted, due to an inefficient method of synthesis, which complicated purification of the products.

EtO

~J--U ~Et

'c

S

II

W(CO)5

S C

II

Cr(CO)5

Figure

2.10

Biscarbene complex containing two different metal moieties

37 Y.M. Terblans, H.M. Roos, S. Lotz, J. Organomet. Chem., 566, 1998, 133.

Chapter

2:

Carbene complexes of Thiophene

3. Synthesis of carbene complexes of Thiophene

26

Once the interesting decomposition patterns of biscarbene complexes of the type [M(CO)5{1J­

T}M(CO)5J (M

=

Cr, W) were recognized

37

, this prompted a further study of the synthesis of analogous complexes of molybdenum. It is well known that the carbonyl ligands of molybdenum pentacarbonyl are much more labile than those of the corresponding tungsten or chromium complexes

38 and vacant coordination sites are generated more readily. These coordination sites are thus available for coordination of heteroatoms and the availability of carbonyls could facilitate carbonyl insertion processes. It was therefore anticipated that, in the light of the decomposition reactions of the biscarbene complexes of thiophene, different reaction patterns were possible for similar molybdenum complexes.

Seeing that the biscarbene complexes of chromium and tungsten have already been synthesized

37

, it was envisaged to prepare the molybdenum analogues of these complexes to compare stabilities of the different products. According to Fischer

9 the stability of carbene complexes increases in the order LMo(CO)5< LCr(CO)5< LW(CO)5' The trend was confirmed in this series of carbene complexes, since the biscarbene complex of molybdenum turned out to be highly unstable while the chromium and tungsten analogues are known to be fairly stable.

The novel molybdenum carbene complexes of thiophene were synthesized using the classical

Fischer method. Thiophene is readily monolithiated as well as dilithiated with n-butyllithium40. The dimetallation of thiophene is done in hexane rather than in THF or ether solvents, since it was found that the ether solvent is attacked under forcing conditions. Addition of a strongly polar co­ solvent, such as HMPT, does not lead to the introduction of a second metal atom, but instead causes ring-opening of the THF. Using hexane as solvent, the n-butyllithiumTMEDA complex is capable of abstracting both the 2- and 5-protons of the thiophene upon adding two equivalents

38

Ch. Elschenbroich,

A.

Salzer,

Organometallics,

A

Concise Introduction,

VCH Verlag,

Weinheim, 1992, 232.

39

K.H. Dotz, H. Fischer, P. Hofmann,

I.R.

Kreissl, U. Schubert, K. Weiss,

Transition Metal

Carbene Complexes;

VCH Verlag Chemie: Weinheim, 1983.

40

L.

Brandsma, H. Verkruijsse,

Preparative Organometallic Chemistry I,

Springer-Verlag,

Berlin/Heidelberg, 1987.

Chapter

2 :

Carbene complexes of Thiophene

27 of the base to one equivalent of thiophene . The first deprotonation occurs at a low temperature

(-10 ° C or lower) while the removal of the second proton is afforded at elevated temperatures .

Upon addition of the metal carbonyl, nucleophilic attack of the dianion of thienylene occurred at carbonyl carbon atoms of two MO(CO)6 complexes to form a dilithium diacylmetallated product. The subsequent quenching of the dilithium salt with triethyloxonium tetrafluoroborate yielded the desired biscarbene complex. In most cases the reactions afforded both the mono- and biscarbene complexes together with a decomposition product and not only the expected biscarbene product.

The general reaction scheme is outlined in figure 2 .

11 . o

+ 28uLiTMEDA

~

Li---ZJ-Li

+

C

4

S S

H

10

Et

O - c / O

S

~MO]

1

EtO

'C

+

----fJ--

OEt

C/

~MO]

[Mot S

2

-

c d lb

Li'~

C

----fJ--

/O-Li'

C

~MO]

[Mot S

EtO

'C

[Mot

----fJ-­

S

3

C/

OEt

~

Figure 2.11 Procedure for preparation of carbene complexes

Reagents:

(a) Hexane, reflux; (b) 2 eq . MO(CO)6' THF, -20°C; (c) 2 eq .

[Et

3

0HBF

4],

CH

2

CI

2

(d) O

2

, acetone

The monocarbene complex, 1, was crystallized from a hexane : dichloromethane (1 : 1) solution and afforded orange-red crystals. The purple biscarbene complex , 2 , was isolated and characterized spectroscopically, but was unstable and decomposed in inert atmosphere after a few hours . In solution decomposition occurred even more rapidly . Complex 3, the decomposition product, was found to be relatively stable and could be fully characterized . Other byproducts from the reaction,

isolated in lower yields, are the purple complex 4, orange-red complex 5, both of which are even stable complex and a yellow organic product, which was found be the bis(ester) complex with two ethoxy substituents on two internal carbon atoms. These complexes were characterized spectroscopically and structures assigned were based primarily on the proton NMR infrared mass spectra. The formation of complexes 4 and 5 is

SU(]oEistE!CI to have ensued

via

the route illustrated in figure 1 The biscarbene complex 2 was found to be highly unstable and thus activated for further modifications. It is proposed that one of the metal fragments of this complex was displaced by a carbonyl group (a). Analogous reactions involving a process have reported

41

.

Deprotonation of a monocarbene complex is accomplished by presence of the TMEDA and nucleophilic attack on this carbonyl centre by the deprotonated monocarbene complex (b) affords complex 4, after protonation. Complex 5, which is analogous to 3, is yielded from the reaction of O

2

(c) with complex 4. (compare figure 2.11) d

EtO

-0-"

'C

I

\ C/

OEt

S

2 a

EtO

~

OEt

'c-<",~>-c/

(CO)5Mcf S

~C~

-';:0

+

EtO'c-O-b

Et

(CO)5Mcf S

c-O-c/O

Et

S

~O

5

c

Figure 2 Proposed mechanism for the formation of

4 and 5

41

(a) K.H. B.

J. Am. Chern. Soc.,

106,1984,434.

Ber.,113,

1980,1449. (b)W.D. Wulff, P.-C. Tang,

Chapter

2 :

Carbene complexes of Thiophene

3.1 Spectroscopic characterization of novel carbene complexes

29

The carbene complexes were characterized using NMR and infrared spectroscopy and mass spectrometry . Due to the instability of complexes

4 and 5, it was impossible to record satisfactory

13C NMR spectra of the complexes, since decomposition occurred in solution . Confirmation of the molecular structure of

1 was obtained from a single crystal X-ray diffraction study .

3.1.1 1H I\IMR spectroscopy

All NMR spectra were recorded in deuterated chloroform as solvent unless otherwise specified .

The

1

H NMR data for complexes 1-3 are summarized in table 2 .

1, while the data for complexes

4 and 5 are tabulated in table 2.2.

Table 2.1

Proton

H3

H4

H5

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-M

OCH

2

CH

3

-O

1H NMR data of complexes 1, 2 and 3

5

Chemical shifts lo, ppm) and Coupling constants

(J,

Hz)

4 3 fJz

/OEt

S

2

C~

"MO(CO ) 6

Et O

' c

3

--0-

OEt

(CO ), M et 2 s c /

~M O(CO).

E tO ,

(CO

C

)5MO~

3

4

--l:Js

/O Et

2 5

C~

"0 S

3

1

2

0

8 .

18 (dd)

7.20 (dd)

7 .

75 (dd)

5.07 (q)

-

1.64 (t)

-

J

4 .

0

1.1

5 .

0

4.1

5.0

1 .

0

7 .

1

-

7.1

-

0

7 .

98 (s)

-

-

5.10 (q)

-

1.67 (t)

-

J

-

-

-

7 .

0

-

7.0

-

0

8.03 (d)

7 .

76 (d)

-

5 .

09 (q)

4.37 (q)

1.67 (t)

1 .

38 (t)

J

4 .

1

4.4

-

7 .

1

7 .

2

7 .

0

7.1

Chapter

2:

Carbene complexes of Thiophene

30

The data support the proposed molecular structures of the complexes. The chemical shifts for the protons of thiophene are at 7.20 ppm for H2 and H5 and at 6.96 ppm for H3 and H442. On comparing these literature values with the corresponding values of the complexes, it is evident that the coordination to a metal fragment has a marked influence on the chemical shifts of the protons.

The protons of the complexes are shifted more downfield than for the uncoordinated thiophene.

Upon coordination to the metal, the carbene moiety causes draining of electron density from the double bonds of the thiophene ring to the electrophilic carbene moiety. Assignments of the thienyl protons are based on assignments made by Gronowitz

43

.

From the NMR data of complex 1 it is observed that the chemical shift of H3 is downfield compared to its position on the spectrum of 2, while the chemical shift of H4 is upfield. This can be explained when considering the resonance structures of the complex as shown in figure 2.13. The TI-resonance effect affords positive charges on H3 and H5 respectively and thus deshielding of these two protons emanates, causing the downfield shift. It is concluded that H3 will shift more downfield than H5 since it is closer to the carbene moiety. The proton H4, however, is not affected by the resonance effect and its chemical shift is therefore comparable with the chemical shift of free thiophene.

Figure 2.13 Draining of electrons in monocarbene complex

In the case of the biscarbene complex, 2, the ring protons are affected by two metal nuclei and the combined withdrawing effect of the two carbene substituents results in an unfavourable electronic effect. Two positive charges are generated on two adjacent carbon atoms, as shown in figure 2.14. On considering complex 3, it is clear that both substituents cause draining of electrons away from the ring but that the influence of the ester group is less profound than the influence of the carbene moiety since the deshielding of H4 is less than for H3.

42

R.J. Abrahams, J. Fischer, P. Loftus, Introduction to NMR Spectroscopy, John Wiley and

Sons, 1988.

43

S. Gronowitz, Adv. Heterocycl. Chern., 1, 1963, 1.

Chapter

2:

Carbene complexes of Thiophene

Figure 2.14 Draining of electrons in biscarbene complexes

Table 2.2

Proton

1H

NMR data of complexes 4 and 5

Chemical shifts (0, ppm) and Coupling constants

(J, Hz)

3 4

OEt

9 10

OEt"~1 ~

/EtO

~

(CO);WS'Y

2

S

5

~

6 b, s

11 c~

"Mo(COls

OEt" c

(CO).Jt.o

~

3 4 OEt c=c

2 S 5 6

9 10 c/

EtO

I

8 S 11

~

OH 0

4

5

H3

H4

H9

H10

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-C

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-C

-OH

°

8 .

03 (d)

7.20 (d)

7 .

33 (d)

8.10 (d)

5 .

04 (q)

5 .

06 (q)

-

4 .

09 (q)

1 .

64 (t)

1 .

65 (t)

-

1 .

51 (t)

6.73 (s)

J

4.4

4.4

4.4

4.4

7.2

7.2

-

7.0

7.2

7.2

-

7.0

-

0

8.03 (d)

7 .

21 (d)

7 .

16 (d)

7.71 (d)

5.03 (q)

4.35 (q)

4.08 (q)

1.64 (t)

1.37 (t)

1.49 (t)

6.57 (s)

7 .

0

7.0

7 .

1

7.0

7.0

-

J

4.4

4.4

4.4

4.4

7 .

1

31

Chapter

2:

Carbene complexes

of Thiophene

32

The methylene regions on the spectra of complexes 4 and 5 are depicted in figure 2.15. The purple colour of 4 is indicative of biscarbene complexes with thiophene spacers and the spectrum of 4 in the methylene region displays two resonances. The intensity of the downfield signal is double that of the upfield signal, indicating two carbene ethoxy units to one additional ethoxy substituent. Notably, the spectrum of complex 4 reveals two overlapping quartet signals for the methylene groups of the alkoxy groups on the two carbene carbons. This can be explained by the different chemical environments of the two methylene groups due to the alcohol functional group on the one and the ethoxy group on the other of the two central carbons, thereby destroying the centre of symmetry present in biscarbene complexes such as complex 2. On the spectrum of complex 5 three signals are present in the methylene region. An additional signal appears at 4.35 ppm and now three signals of equal intensity are found.

:-: ;.~ ~;: ~{

~~~~~~

II I II

~~i

, I I

G~l~~~

~ ~\

I

~~~~~~F .

~

~1-'rP

" --cg~d

TiJli

(a) (b)

Figure 2.15

Methylene proton region on the

1

H NMR spectra of complexes (a) 4 and (b) 5

It is thus clear that one carbene mOiety of 4 has been replaced by an ester functional group to yield 5. Characteristic in this study is the chemical shifts of the methylene protons of the ethoxy groups which were found above 5.0 ppm for carbene moieties, around 4.3 ppm for ester ethoxy functionalities and at 4.0 ppm for ethoxy substituents. Although we favour the structural representation shown for complex 5, we cannot exclude the possibility that the ethoxy and hydroxy substituents may change places to give an alternative structure. However, the large difference in

Chapter

2:

Carbene complexes of Thiophene

33 chemical shifts of the OH resonances on the spectra of 4 and 5 was taken to indicate that the OH substituent is positioned on the carbon atom nearest to the thiophene ring where the carbene moiety is substituted by a ester functional group for complex 5. A "push-pull" effect is introduced in 5, where two different end-groups are present in the molecule.

Figure 2.16 n-delocalization in complex 5

On the spectrum of the organic bis(ester) product, only two quartet signals of equal intensity are observed in the methylene region, one at 4.05 ppm (J

=

7.1 Hz) and one at 4.35 ppm (J

=

7.1 Hz).

These two resonances correspond to the methylene protons of the ethoxy functionality on a centre carbon atom and to a methylene group of an ester end group. The quartet signal associated with the methylene protons of the ethoxy group of the carbene moiety of complexes 4 and 5, is absent on this spectrum. The corresponding methyl signals of these two ethoxy groups are observed at

1.49 and 1.37 ppm, respectively.

On comparing the chemical shift values of complexes 4 and 5 with those of complexes

1,

2 and

3, it is interesting to note that the chemical shift values of the different ethoxy groups are observed at characteristic positions on the spectra of all the compounds. In general the values correspond well, especially on relating the resonances of the monocarbene complex

1 and the decomposition product 3 to the corresponding protons on the spectra of complexes 4 and 5. Complex 5 can be seen as a combination of these two complexes and it is thus not surprising that the values are very comparabie.

3.1.2

13C

NMR spectroscopy

The 13C NMR data of complexes

1-3 are given in table 2.3 while the spectrum for complex

3 is depicted in figure 2.17. Complexes 4 and 5 decomposed during the recording of the spectra. On the 13C NMR spectrum of complex 4 the following peaks (ppm) were observed and assigned:

Chapter

2:

Carbene complexes

of Thiophene 34

212.8,212.6 (CO

trans); 206.2, 206.0 (CO cis); 141.9 (C3); 140.8 (C10); 129.6 (C4); 128.6 (C9);

111.5 (C6); 99.7 (C7); 77.7, 77.2, 68.4 (OCH2CH

3 );

15.5, 15.1, 14.9 (OCH2CH

3).

I

Table 2.3

Carbon

Carbene

C2

C3

C4

C5

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-M

OCH

2

CH

3

-O

M(CO)5

C=O

13C I\IMR data of complexes 1, 2, and 3

I

1

0

307.2

150.1

136.2

128.9

141.4

77.8

-

15.1

-

206.1 (cis)

212.8 (trans)

-

I

Chemical shifts

{o, ~~m}

2

0

312.4

I

154.5

136.9

136.9

154.5

78.3

-

15.0

-

205.6 (cis)

212.9 (trans)

-

3

0

310.8

162.3

139.4

133.0

159.4

78.2

61.9

15.0

14.2

205.7 (cis)

212.7 (trans)

206.1

I

Chemical shift values of carbene carbon atoms fluctuate in a very broad range starting at 200 ppm, for aminocarbene complexes44 , to 400 ppm, for some silicon complexes45 . They depend mainly on the Rand R' groups of the carbene C(R)R' but also on the metal, although to a smaller degree, considering the interaction of the d orbitals on the metal and the pz orbital of the carbene carbon. Deshielding of the carbenium ion is encountered and it can therefore be concluded that carbene carbon atoms in their metal complexes bear a partial positive charge. The chemical shift of carbene ligands as a function of R decreases for complexes of chromium and tungsten as follows: Me> Ph> 1-ferrocenyl> 2-thienyl> 2_furyI24. This decrease is in accordance with the

44

BA

Anderson, W.D. Wulff,

A.

Rahm, J.

Am. Chem. Soc., 115, 1993, 4602.

45 E.O. Fischer, T. Selmayr, F.R. Kreissl, U. Schubert,

Chem. Ber., 110, 1977, 574.

Chapter

2: Carbene complexes of Thiophene

35 lowering of donor properties of these substituents. The carbene carbons are shifted more downfield for biscarbene complexes than for monocarbene or decomposition complexes . This trend was observed on comparing the spectra of complexes 1, 2 and 3 , with respective values of

307.2 ppm, 312.4 ppm and 310.8 ppm . This tendency is not observed for the C2 carbon, since the value observed for complex 3 is further downfield than for complexes 1 and 2. The electron withdraw i ng effect of the ester functionality is suggested to contribute to the deshielding of this carbon . In fact, both carbons C2 and C5 of complexes 5 are observed at higher chemical shifts than the C2 value on the spectra of complexes 1 and 2 . This can be attributed to the polarization effect of the two different end-capped moieties . The chemical shift value obtained for the carbonyl group of the ester moiety is more downfield than the value associated with organic ester carbonyl groups. This is ascribed to the electron-withdrawing influence of the metal fragment on the opposite side of the thiophene ring. The metal carbonyls seem to be little affected by the change in substituents on the carbene carbon.

,

"

,

> ,

"

I

..

''": 'l­

.:;!:

I

-

"'1­

_

.

,

..

~ ....

1

(

\

',:) ,~~

, \ 1

.

,

~ y

",

.~

'r,

:~.

-

" t , ....

,

..;

r ,

~ I

' ­

"

<:;

' c;:

"1

' ~r ,

-,.

. r ,

~

~ go:

.• j

~

-,.

.

I

I

I

I

':.0::­

.

" r ,

,

.

I

-.

" .

\ 9 :-­

-.

I

~F'

-

' ­

~ t' l

' ~ '

I~

.

· i

'-,

I

I

I

I

I

,m

'W

: ' !

--1

I

~

I

.1.

Figure 2.17 13 C NMR spectrum of complex 3

.

1,Ji

, \Y ..

~

~~ ,

......

.

.

,

';

Chapter

2:

Carbene complexes

of Thiophene 36

Chemical shift values for terminal metal carbonyls lie in the range 150 to 240 ppm. Within a group of metals, shielding of the carbonyl nucleus increases with increasing atomic number, for example the carbonyl chemical shifts of Cr(CO)6' MO(CO)6 and W(CO)6 are found at 212, 202 and 192 respectively. In the spectra of the carbene complexes two signals are observed for the metal pentacarbonyl moiety. This is attributed to cis and trans carbonyl ligands.

3.1.3 Infrared Spectroscopy

The C-O stretching vibrational frequencies, in contrast to M-C stretching frequencies can be viewed as being independent from other vibrations in the molecule. In carbene carbonyl complexes, bands caused by veo vibrations occur at lower energies than in the corresponding metal carbonyl complexes. This shows that carbene ligands possess weakern-acceptor properties compared to the carbonyl group.

The infrared data of complexes 1-5 are outlined in table 2.4.

Table 2.4 Infrared data of complexes 1-5

Stretching vibrational frequency

(vr;o, cm-

1

)

Band

A(1)

1

B

A (2)

1

1

2066

2067

1983

1986

1942

1957

2

2064

1985

1943

3

2065

1982

1942

E

1942 1943 1942

1948

First set of values recorded

In dlchloromethane, second set

In hexane

4

2064

1983

1942

1942

5

2065

1982

1943

1943

On the infrared spectra of pentacarbonyl metal carbene complexes, three absorption bands can be distinguished in the terminal carbonyl region,

i.e

two

A1 and one E band, as is expected for C

4V symmetry. When the carbene carbon has a bulky substituent, the E band is sometimes split and

the formally IR-forbidden B band is observed, due to distortion of the equatorial plane of carbonyls.

Most of the in dichloromethane as solvent due to solubility problems in hexane as solvent recorded in

A1

(2) and E bands overlap in the spectra. For the spectra as solvent both bands are visible and the

A1

(2) is characteristically observed as a shoulder on the higher wavenumber side of the

E band.

Mass spectrometry

The fragmentation of complexes 1 are summarized in table 2.5. A molecular ion peak,

M+, was observed on identified. of the complexes. A general fragmentation pattern was ont""t.,.\n

"'·!:>tt.:::•...,.,'" of both complexes

1 and

3 are based on the 98Mo isotope, while the pattern for on one 98Mo isotope and one 96Mo isotope.

The fragmentation

1",."....,"'.0"

2 seems to follow two different routes after the initial stepwise loss of followed by the normal rlo.,r!:.!,"!:.!fl,", route, of the rest of the carbonyls ensues, n!:.!flfQrn with the loss of the ethyl group and then the CO fragment. The second of the ethyl and CO groups fragmentation routes are more possibilities, of which one is shown, and involves the disintegration of the rest of the carbonyls. in figure 18.

two

Table 2.5

Fragmentation nl:ll1lTAP"n of complexes 1, 2 and 3

Complex

1

2

3

Fragment ions (I,

%)

377.8 (17) M+; 349.8 (25) M+ - CO; 321.9 (32) M+ - 2CO; 293.8 (33) M+ - 3CO; 265.8

237.9 (100) M+ - 5CO; 208.9 (76) M+ -5CO -CH

2

CH

3

303.0

M+; 613.2 (10) M+ - 2CO; 585.0 (15) M+ - 3CO; 557.2 (15) M+ -

473.1 (49) M+ - 7CO; 445.2 (73) M+ - 8CO; 417.2 (54) M+ -

360.0 (49) M+ ­ 1OCO - CH

2

CH

3 •

331.1 (54) M+ - 10CO ­

- 11CO - 2CH

2

CH

3 ;

275.0 (100) M+ ­ 12CO - 2CH

2

CH

3

501.2

389.0

, 422.4 (5) M+ - CO; 394.3 (6) M+ - 2CO; 366.3 (5) M+ - 3CO;

CH

Chapter

2:

Carbene complexes of Thiophene

38

EtO n

'c----<..._>-c/

(COhM~

s

OEt

~MO(COh

itt)

I

1-4CO (stepwise) itt)

EtO - i O OEt

' c c/

M~

S

~Mo

1-2Et o~-iO-~o

M~ s

'MO itt)

1-2CO itt)

M o - i O - M o

S l-Et

EtO,c-iO-~o

(CO);/JI~

s

'MO(CO)2 itt)

1-2cO itt)

EtO

'c----<..._>-c~

(CO)M~ n

S

h O

'MO(CO)

I

1-4CO

M o - i O - M O

S

Figure 2.18

Fragmentation pattern of complex 2

Chapter

2 :

Carbene complexes

of Thiophene

3.1.5 X-ray Crystallography

39

A single crystal X-ray diffraction study confirmed the molecular structure of complex 1. Single crystals of the monocarbene complex 1 were afforded from a dichloromethane:hexane (1 : 1) solution . The complex crystallized as orange-red cubic crystals .

Figure 2.19 represent a ball-and-stick plot of the structure . Selected bond lengths and angles are tabulated in table 2.6

.

03

Figure 2.19 8all-and-stick plot of complex 1

The crystal structure of free thiophene was determined by Harshbarger

et

a(+6.

The bond lengths were found to be 1 .

718(4)

A for S-C(2), 1.370(4)

A for C(2)-C(3) and 1.442(2)

A for C(3)-C(4). The bond angles were determined as 92.0(3) ° for C(2)-S-C(5) and 112 .

0(3) ° for S-C(2)-C(3).

46

W.R. Harshbarger, S.H. Bauer, Acta. Cryst., B26 , 1970 , 1010 .

Chapter

2:

Carbene complexes of Thiophene

Table 2.6

I

Mo-C(1)

1

S-C(5)

S-C(2)

0(1 )-C(1)

0(1)-C(6)

C(1)-C(2)

C(2)-C(3)

C(3)-C(4)

C(4)-C(5)

Selected bond lengths and angles of 1

I Bond Lengths (A)

[

2.226(5)

1

C(5)-S-C(2)

1.705(6) C(1 )-0(1 )-C(6)

1.756(5)

1.333(6)

0(1 )-C(1 )-C(2)

0(1 )-C(1 )-Mo

1.450(7)

1.457(8)

1.390(8)

1.-109(8)

1.378(9)

C(2)-C(1 )-Mo

C(3)-C(2)-S

C(2)-C(3)-C( 4)

C(5)-C(-1)-C(3)

C(4)-C(5)-S

I Bond angles n

I

91.9(3)

122.5(4)

106.3(4)

129.4(4)

124.3(4)

109.4(4)

114.1(5)

111.8(5)

112.8(5)

40

The six ligands, five carbonyl groups and one carbene, are arranged octahedrally around the molybdenum centre. The carbonyl ligands in the equatorial plane are staggered relative to the carbene ligand as is manifested by the C1-Mo-C8, C1-Mo-C8A, C1-Mo-C10 and C1-Mo-C10A angles of 88.3(2)°, 88.3(2)°, 95.5(2)° and 95.5(2)° respectively. Two carbonyls are bent towards the carbene carbon and two are bent away from it.

The torsion angle S-C(2)-C(3)-C(4) is 0.169(1)° while the torsion angle C(4)-C(5)-S-C(2) has a value of -1.695(1)° which indicates that the thienyl ring is planar. The thienyl ring, carbene carbon and metal atom are also in the same plane, with the sulfur and oxygen atoms on the same side of the C(1)-C(2) bond. The carbene carbon has sp2-characterwhich is indicated by the Mo-C1-01,

Mo-C1-C2 and 01-C1-C2 angles of 127.6(5)°,125.6(5)° and 106.8(6)° respectively. These bond angles differ from the expected value of 120 ° for a sp2-hybridized carbon atom but are typical for alkoxy carbene complexes. This deformation of the complex may be attributed to steric as well as electronic factors

47

.

On comparing the bond lengths of the thienyl ring in complex 1 with the bond lengths of uncoordinated thiophene, it was found that all of the bonds are longer in the complex except for the C(3)-C(4) bond which is shorter in the complex. The C(carbene)-C(thienyl) bond is also shorter

47

R.J. Goddard, R. Hoffmann, E.D. Jemmis, J. Am.

Chem. Soc.,

102, 1980, 7667.

Chapter

2:

Carbene complexes of Thiophene

41

(1 A57(8)

A) than normal C-C single bonds (1.51 (3) A)19b. The Mo-C(carbene) bond length is comparable with values determined for complexes in literature (figure 2.20) e.g. for

[MO(CO)5C(OEt)p-Tol] I the M-C(carbene) bond length was calculated as 2.189 A

48 and for

[(1-1­

O){Mo(COhC(Ph)OZrCP2)h] II the M-C(carbene) bond length was calculated as 2.195 A49.

Me

Figure 2.20 Structures of reference complexes I and II

The MO-C(O) bond length in MO(CO)6 is 2.06

A.

The average Mo-C(O) bond length in complex 1 was determined as 2.01 A, with the

trans

MO-C(O) bond being the shortest due to poorer n­ acceptor properties of the carbene ligand. For the two reference complexes I and II the corresponding bond lengths are cited as 2.01 A and 2.02 A, respectively. These values are as expected significantly smaller than the value estimated for a Mo-C single bond (2.33 A)50 and indicate double-bond character for the molybdenum-carbon carbonyl bonds. The bond angles of the thienyl ring in complex 1 differ from the angles in uncoordinated thiophene which shows that the thienyl ring is somewhat more distorted in the complex than in free thiophene, indicating ring involvement in stabilizing the carbene carbon.

48

D. Xiaoping, L. Genpei, C. Zhongguo, T. Youqi, C. Jiabi, L. Guixin, X. Weihua, J. Struct.

Chem.,

7,

1988,22.

49

G. Erker, U. Dort, C. Kruger, Y. Tsay, Organometallics, 6,1987,680.

50

Ch. Eischenbroich,

A.

Salzer, Organometallics, A Concise Introduction, VCH Verlag,

Weinheim, 1992,229.

Chapter

2:

Carbene complexes

of Thiophene

4,

Aminocarbene complexes

42

Fischer carbene complexes are often referred to as electrophilic carbene complexes since the carbene carbon is susceptible to nucleophilic attack. Nucleophilic substitution reactions of Fischer carbene complexes with amines were investigated by Connor and Fischer5

1

.

The reaction is similar to the aminolysis of esters to form amides since the M(COkmoiety is electronically similar to a carbonyl oxygen atom. Nucleophilic attack by the nitrogen lone pair on the carbene carbon atom leads to the elimination of alcohol and the formation of the aminocarbene product.

Figure

2.21

Aminolysis reaction of carbene complex

From the early days of carbene chemistry, it was recognized that aminocarbene complexes are more stable than their alkoxy analogues. This was ascribed to greater participation of the nitrogen lone paircompared to oxygen in stabilizing the electrophilic carbene carbon. Indications that amino substituents could stabilize carbene ligands bound to mid-valent group 6 metal centres were later provided by the observations of Kreissl that, while [W(=CC6H4Me-4)Cp(CO)2] reacts with hydrogen chloride to form the acyl complex [W(1l2-C(O)CH2C6H4Me-4)Cp(CO)CI2]52, the same reagent with

[W(=CNEt2)Cp(COh] affords the aminomethylene complex [W(=CHI\lEt2)CpCI(COh]53. Fillipou

54 recently obtained similar results for chromium.

Unfortunately the aminolysis of alkoxycarbene complexes is limited to unhindered primary and, in some cases, secondary amines, and is restricted to those alkoxy carbene complexes accessible from organolithium reagents. An alternative and very efficient method for the preparation of

51

JA Connor,

E.O.

Fischer, J.

Chern. Soc. (A),

1969, 578.

52

F.R. Kreissl, W.J. Sieber, M. Wolfgruber, J. Riede,

Angew. Chern. Int. Ed. Engl., 23,

1984,640.

53

F.R. Kreissl, W.J. Sieber, M. Wolfgruber, J.

Organomet. Chern.,

270, 1984, C45.

54

A.C. Fillipou, D. Wossner, B. Lungwitz, G. Kociokkohn,

Angew. Chern. Int. Ed. Engl., 35,

1996,876.

chromium

,"f'\,,..,,,,rlr\or,o complexes was introduced by Hegedus st

af'5.

It involves the reaction of product in of chlorotrimethylsilane. The reaction is carbonyl group of an amide followed by the

"",n,uu, with an excess of chlorotrimethylsilane affords the hexamethyldisiloxane ensues, as shown in figure 2.22.

Me:$iCl

..

M

=

Cr, n

=

5

M = n

= 4 complexes Figure

2.22

Synthesis of

This method hydrogens

56

. recently

T~lis study to iron aminocarbene complexes lacking a­ to include the preparation of (1-1­ bis(aminocarbene)dimetal complexes chromium and iron

57 and has led to the preparation of a mixed chromium-iron aminocarbene complexes, which principle also be employed to

"ro .... ",,'·o complexes, can in iron alkoxy carbene complexes are not as easy to

Recently the application of substantial differences in complexes have been observed. was undertaken and compared to alkoxy carbene aryl(alkylamino)carbene complexes undergo

55

R Imwinkelried, LS. Hegedus, OrganometaJlics, 7,1988,702.

56

D. Dvorak, Organometa/lics, 14,1995,570.

57

M. Havranek, M. Husak, D. Dvorak, Organometallics, 14, 1995,

58

M.F. Semmelhack,

R.

J.

Am. Chem.

, 1 1 4099.

Chapter

2 :

Carbene complexes of Thiophene

44 thermal reactions with alkynes to give indanones or aminoindenes

5 9

, while N-acylation6o of these complexes produce intermediate products which can be converted to a number of unusual organic compounds. Hegedus

et

a/ investigated the photolytic reactions of aminocarbene complexes to produce amino-p-lactams61 and a-amino acid esters

6 2

4 .

1

Aminolysis reactions of monocarbene complex 1

In order to test the viability of using diamines in linking two carbene fragments in binuclear biscarbene complexes, the reactions of 1 with various amines were investigated . Also, exchanging an alkoxy carbene for an aminocarbene implies changing the bonding properties of the substituents around the carbene carbon and should influence the role of the thienyl substituent.

The molybdenum monocarbene complex 1 was utilized in reactions with amines to yield aminocarbene complexes 6 and 7. The reactions were effected in diethyl ether and the products purified by chromatography on silica gel. Similar behaviour has been described previously for mono- and chelating biscarbene complexes

63

.

The two amines used in the reactions were ammonia (NH 3 ) and 1 ,4-phenylene diamine . The 1,4­ phenylene diamine was chosen to try and substitute the diamine ligand at both active positions to obtain a diaminodicarbene complex in spite of the concern that it may be too bulky for the system. Werner' 4 showed that the mechanism for aminolysis involves more than one amine to activate the carbene carbon. This implies that only small amines will affect this type of reaction.

Nevertheless two equivalents of monocarbene were reacted with one equivalent of 1,4-phenylene diamine, but unfortunately the target product was not formed. Instead only one position was substituted as figure

2 .

23 illustrates .

59

A. Yamashita, Tetrahedron Lett., 27,1986,5915 .

60

R. Aumann , H. Heinan, Chem

. Ber.,

122,1989,1139.

6 1

L.S

. Hegedus, S . D ' Andrea,

J. Org. Chern.,

53, 1988, 3113.

62

L.S

. Hegedus, G . deWeck, S. D'Andrea ,

J.

Arn. Chern. Soc.

,

110,1988,2122.

63

E.O. Fischer , M. Leupold ,

Chern. Ber .

,

105,1972,599 .

64

H. Werner, E .

O . Fischer, B. Heckl, C.G. Kreiter,

J. Organomet. Chem .

,

28 , 1971, 367 .

2 EtO,C--Z)

+

(CO)Jllrf s

1

7

Figure 2.23 Reaction of 1 with 1 ,4-phenylene diamine

Similar were obtained by e t . They reacted carbene complex

[Cr(CO)5C(OCH

3

)CH

3

J with several diamines in order to synthesize diaminodicarbene complexes.

The diamines were benzidine, o-tolidine, 1,3-diaminopropane, 1,4-diaminobutane, 1 diaminopentane, 1,6-diaminohexane 1,10-diaminodecane. While diaminodicarbene complexes were afforded for all the aliphatic diamines, only diamino-monocarbene complexes were obtained for the aromatic diamine compounds. This was in part attributed to the reduced basicity of the amino mOiety caused by the action of the strongly electron-withdrawing aminocarbene group on the amino group through the aromatic system. In aliphatic this effect is much is no present therefore both amino groups are aI.A''';;:>;;:>'IJ'v for bonding. A approach would be to employ small amines in of biscarbene molybdenum complex 2. Reaction of 2 with ammonia in ether aminolysis in a colour change from purple to red, but isolation new products proved troublesome. Many products were formed in low yields, some of which were not stable. Chromatography on silica gel

65

E.a.

s.

J. Organomet. Chem.,

40,1972,367.

failed and products were poorly soluble. This approach was abandoned as it bearing in rnind that precursor 2 is unstable in etheral solutions.

4.2 Spectroscopic characterization of novel aminocarbene complexes

The aminocarbene complexes 6 and 7 were characterized with

1

H NMR-, spectroscopy mass

NMR-, infrared

All NMR spectra were recorded in deuterated chloroform as solvent

4.2.1 1H proton NMR different broad the bond of two complexes are summarized in table 2.7. In the of 6 two for the two NH2 protons. The Z-proton is more downfield than proton peaks appear downfield, which is consistent with double bond.

Mills

et af'6

om,n!:llrll on a structural study to deterrnine the influence oxygen substituent (alkoxy carbene complexes) by a nitrogen substituent complexes). double bond character of both the oxygen bonds

67

.

However, the metal-carbon bond has less double bond in

6 and 7 of competitive back-donation. Since -NH2 is a much better n-donor than it is expected that the introduction of the nitrogen atom will result in greater double bond of the C(carbene)-N bond and less double bond character of the C(carbene)-M bond. bond of the C(carbene)-N is manifested in the deshielding of these two nrntnn

8.4 shift. which 2.8-4.0 ppm on a 1H NMR spectrum, resulting in a downfield same phenomenon is observed in the spectrum of complex 7. In fact, the NH is shifted even more downfield (9.93 ppm) due to the electron-withdrawing nature of the phenyl diamine assist in the of the N-proton. Coupling protons. were to

66

67

O.S. Mills,

A.

O.S. Mills, J. Chern. Soc., Chern. Commun., 1967, 1966.

J. Chern. Soc., Chem. Commun., 1966,814.

Table

Proton

1H

NMR data of complexes 6 and 7

Chemical shifts (0, ppm) and constants (J,

5

6 7

H3

H4

H5

°

7.72 (d)

7.24 (dd)

7.71 (d)

J

4.5

4.5

4.5

4.5

°

7.56 (dd)

7.16 (dd)

7.54 (dd)

J

4.0

1.0

4.0

4.0

4.0

1.0

NHE

1\1 Hz

NH

NH2

Ph(H a)

Ph(H)

8.21 (broad s)

8.36 s)

6.74 (d)

4 (d)

9.0

9.0

1,6 chemical shift of the complexes are observed in the

IIO\llil9a by

7. 1-8.2 ppm with H3 the most and H4. On comparing the chemical shift values for H3 in complexes shift values decrease in the A difference of 0.6 ppm in is observed for the different A possible explanation for this specific charged in complex

1 the thienyl carbon to stabilize it, to compensate for ethoxy group. H3 is deshielded because of shifts more n/"\\'.,n,."o' .....

In complex 6 the amine group is a re,<n,ue'H density to the positively nn"'''''QI'' n-donor property of the from the ring and

"_I'"0'..... n."' .. and lessens the

Chapter

2 :

Carbene complexes of Thiophene

48 contribution from the thienyl ring to stabilize the carbene carbon. Hence the more upfield shift of

H3 in complex 6. In complex 7 the phenylene diamine substituent is even more electron donating than NH 2 because of the presence of the electron donating NH2 substituent on the 4-position of the phenyl ring. This results in the upfield shift of H3 in complex

7 compared to its position on the spectra of complexes 1 and 6, due to a smaller demand on electron stabilization of the thienyl ring .

This pattern is also observed for protons H4 and H5 for all three complexes although the chemical shift differences between these protons on the different spectra are less profound.

4.2.2

13C

NMR spectroscopy

The

13C

NMR data of complexes 6 and 7 are listed in table 2.8.

Table 2.8

Carbon

I

Carbene

C2

C3

C4

C5

C a

(phenyl)

C b

(phenyl)

C(NH0

C(I\IH)

M(CO)5

13C

1\lIVIR data of complex

6 and

7

Chemical shifts (0, ppm)

I

6

0

257 .

5

152 .

2

133 .

1

129.1

133.1

-

-

-

-

206.7 (cis)

213.0 (trans)

I

7

0

258.0

155.9

128.8

128.6

130.1

115.2

128.1

133.9

147 .

6

206 .

6 (cis)

214.0 (trans)

I

On comparing the

13C

NMR data of complexes 6 and 7 with those of complexes 1-3, it is clear that the signals from the carbene carbons in the ethoxy compounds are at lower field than those from

Chapter

2:

Carbene complexes of Thiophene

49 amino complexes. The chemical shift difference is ca 50 ppm . This can be accounted for by the greater degree of C(carbene)-X n-bonding where X

=

N rather than X

=

0 and results in greater shielding of the carbene carbon in aminocarbene complexes .

The carbon atoms of the thienyl ring are little affected by the change in substituents. The chemical shift values for complexes 6 and 7 are slightly more upfield than for the analogous ethoxy carbene complex 1.

4.2.3 Infrared Spectroscopy

The infrared data of complexes 6 and 7 are summarized in table 2.9.

Table 2.9

Infrared data for complexes 6 and 7

Band

A/)

B

A

1

( 2 )

E a

The

A1 ( 1 ) and E bands overlap

Stretching vibrational frequency (v co, cm ·

1

)

6

7

2066

1981

1931 8

1931

2062

1980

1931 8

1931

The structure of carbene complexes may be understood in terms of three limiting forms (A , Band

C) which contribute to the stabilization of the formally electron-deficient carbene carbon (figure

2 .

24). In limiting structure A the substituent R serves as a n-donor, while in limiting structure B substituent Y serves as a n-donor . Limiting structure C is stabilized by n-donation from the metal.

Structural evidence

68

,69 led to the assumption that amine substituents are better n-donors than ethoxy substituents. Therefore the contribution of limiting structure A becomes more important where amine substituents are concerned , while metal n-donation is a larger contributing factor in

68

M.Y. Darensbourg , D .

J. Darensbourg ,

Inorg . Chem.,

9,1970,32 .

69

J.A. Connor , J .

P . Lloyd ,

Chem . Rev .

,

1970, 3237 .

the case ethoxy substituents. Aryl substituents are generally poor IT-donors to the as a result carbene complexes with substituents must either have substantial ITdonation from the other substituent or a substantial contribution from limiting form C. with IT-donor substituents is for complexes with poor will low M-C(carbene) bond

(structure

C).

Ef) e

q

R

[M]-C

'y

A

Limiting structures for

B

R

[M]=C

'y c

Transition metal carbonyl bonds can the case of the ethoxy carbene complex, carbon. Backbonding from order. as two resonance structures (figure is necessary to to carbonyl carbon the M-C(carbonyl) bond order and simultaneously increasing the aminocarbene complexes, IT-donation to the carbene substituent stabilizes the ....

.,.'·P"\ono carbon. Backbonding from the carbonyl carbon increases resulting in a higher M-C(carbonyl) bond order and a rto,,..ro,,,"

C(carbonyl)-O bond order. This explains the lower stretching frequencies observed on of 6 7 compared to those of pronounced in the vibration wavenumber for

(2) carbene complexes band.

1-3, which is

In the bond

Figure

M=C=O _ _

Resonance structures for M-C-O

Chapter

2:

Carbene complexes

of Thiophene

4.2.4 Mass spectrometry

51

In the mass spectra of complexes 6 and 7 a molecular ion peak, M+, was obtained for each complex. The fragmentation patterns for both molecules are similar, starting with the initial loss of the carbonyl ligands. Stepwise fragmentation of the carbonyls is observed, followed by the loss of the metal. Fragmentation patterns were based on the g8Mo isotope. The amine substituent and carbene carbon remain bonded to the thiophene ligand. This is contradictory to the fragmentation patterns observed for the ethoxy carbene complexes 1-3 where loss of the ethoxy g roup precedes the loss of the metal moiety.

Table 2.10 gives the most important peaks and fragment ions associated with these in the spectra of the aminocarbene complexes 6 and

7.

Table 2.10 Fragmentation patterns of complexes 6 and 7

I

Complex

I

6

Fragment ions (I,

%)

349.0 (15) M+; 321.0 (10) IW - CO; 292.9 (15) M+ - 2CO; 264.9 (18) M+ - 3CO; 237.0

(29) M+ - 4CO; 209.0 (55) M+ - 5CO; 111.0 (39) C5H5SN+; 83.0 (6) C

4

H

3

S+

7

440.0 (0.04) M+; 412.2 (0.3) M+ - CO; 356.1 (0.4) M+ - 3CO; 328.1 (0.2) M+ -4CO; 300.0

(0.7) M+ - 5CO; 202.1 (100) C

11

H

1O

SN/; 201.1 (61) C

11

H g

SN2\ 108.1 (19) C

5

H

2

SN+

I

Carbene complexes of

Thienothiophene

1. Generai

in thiophenes as with electro-optical molecular devices

1 was recently extended to thieno[3,2-b]thiophenes

2

. unfortunately been impeded by the of methods. use as area

All are known 3.1). Thieno[3,2-b]thiophene (I) was first during his studies on the action of sulfur on octane and conditions dimethylthienothiophene, thiophene and other byproducts were yielded. assumed the formation of 3,4-dimethylthieno[2,3-b]thiophene as the octane isomerization, but Horton

4 correctly identified the product obtained by thieno[3,2-b]thiophene (I). Thieno[2,3-b]thiophene (II) was first of the thienothiophene as to be acid and

Biedermann and Jacobson5

Thieno[3,4-b]thiophene (III), an and Pollack

7 which

II in 1 yield by heating a mixture of citric compound at room temperature

6

, thermal decomposition of 1 was

The fourth isomer, thieno[3,4-c]thiophene (IV), is a condensed aTc,rn"'"r''''''with formally tetracovalent sulfur. Derivatives of this isomer were synthesized by

1

J.M.

Chern. Rev.,

96, 1996, 37.

2

J. Nakayama, H. Dong, K. Sawada, A Ishii, S. Kumakura, Tetrahedron, 52, 1996,471.

3

W. Friedmann, Ber., 49, 1916, 1344.

4

AW.

Horton, J. Org. Chern., 14, 1949, 760.

5

A Biedermann, P. Jacobson, Ber.,

19,1886,2444.

B

H. Wynberg, D.J. Zwanenburg, Tetrahedron Lett., 1967, 761.

7

M.P. N.M.

J. Am. Chern. Soc.,

1 4112.

Chapter

3: Carbene complexes of Thienothiophene and Pollack

8 while the unsubstituted compound could not yet be isolated. s

OJ

10 sJ)

S

\sJ-I

S

III

II

Figure 3.1 Isomers of thienothiophene

cf:Js

IV

Stabilities of the isomers differ considerably. Thieno[3,2-b]thiophene

(I) and thieno[2, 3-b]thiophene

(II) are both stable compounds at room temperature while thieno[3,4-b]thiophene

(III) can only be stored at temperatures below -40°C. Thieno[3,4-c]thienothiophene (IV) was found to be 192 kJ less stable than thieno[3,2-b]thiophene (1)9. Von Rague Schleyer

et afo

calculated the relative energies of the four positional isomers to determine their stability order which was found to be I

> II > III >

IV. This study was conducted to determine the relationship between the thermodynamic stability of the heterocycles and their aromaticity. It was concluded that no direct correlation exists between the two properties, since the most aromatic isomer was found to be isomer IV11. The aromaticity was then found to decrease in the following order:

I

>

II

>

III. Substituted thienothiophenes were prepared with the aim to increase the stability of the compounds. Electron­ withdrawing substituents greatly increased the stability. Thieno[3,4-b]thiophene-2-carboxylic acid and its methyl ester are stable at 20°C

12

.

Electron donor substituents do not have the same effect, since 4,6-dimethylthieno[3,4-b]thiophene is an unstable compound

13

.

From reactivity studies done on heterocycles it is clear that it is difficult to define quantitatively the reactivity of a given

53

8

M.P. Cava, I\I.M. Pollack, J. Am. Chern. Soc., 89, 1967, 3639.

9

D.T. Clark, Tetrahedron Lett., 1967, 5257.

10

G. Subramanian, P. von Rague Schleyer, H. Jiao, Angew. Chem. Int. Ed. Engl., 35, 1996,

2638.

11

C.W.

Bird,

Tetrahedron,

43,1987,4725.

12

V.P. Litvinov, G. Fraenkel, Izv. Akad. Nauk SSSR, Ser. Khim., 1968, 1828.

13 o.

Dann, W. Dimmling, Ber., 87,1954,373.

Chapter

3:

Carbene complexes of Th i enothiophene

54 position

14.

This is due to the high polarisability of the molecule. Reactivity of the thienothiophenes was investigated by Archer and Taylor

1 5

.

Oetritiation in trifluoroacetic acid at 70 0 e has provided the only fully quantitative data for these compounds. This data was compared with data obtained for thiophene under the same conditions . From this data it was found that the a-positions are in each case more reactive than the P-positions, as is the case for thiophene. It was also observed that both positions are more reactive at the

[2,3-b]

isomer (II) than at the

[3,2-b]

isomer (I). The relative reactivities at the a-pos i tion of thieno[2 , 3-b]thiophene (II), thieno[3,2-b]thiophene (I) and thiophene are 7.4

: 7.0

: 1 .

0 .

Examples of compounds containing thienothiophenes as bridging ligands are limited in literature.

In an attempt to enhance the electron density and transmission properties for potential non-linear optical applications of thieno[3 , 2 b]thiophene, the synthesis of 2-arylthieno[3,2-b]thiophene was planned

16

.

Various synthetic routes were proposed for this synthesis and intermediate complexes included the metal complexes 2-tributylstannylthieno[3 , 2-b]thiophene, thieno[3,2-b]thiophen-2­ ylboronic acid and 2-thieno[3,2 b]thienylzinc chloride. These compounds were prepared from lithio precursors . Various other a-substituted complexes have been synthesized in a similar manner

via

this monometallated derivative e.g. selenium and tellurium

1 7 compounds . 2 , 5-0isubstituted thieno[3,2-b]thiophenes were prepared by dilithiation of thieno[3 , 2-b]thiophene followed by the subsequent reaction with electrophiles . Several silyl complexes were synthesized in this fashion

1 8

.

In the same paper the syntheses of 3,6-disubstituted derivatives were reported , prepared via Br-Li exchange reactions involving 3 , 6-dibromothieno[3,2-b]thiophene. Isomeric bis(9 hydroxyf/uoren-9­ yl)thienothiophenes (figure 3 .

2) were discussed as hosts in host-guest clathrate crystals in reactions that are designated as solid-state photosolvolysis

19

.

Guest ethanol molecules in the clathrate crystals reacted photochemically with the diol host compounds to cause photosubstitution

14

H .

B . Amin, R. Taylor ,

J . Chern. Soc., Perkin Trans

2,1978,1053.

1 5

W .

J. Archer, R. Taylor ,

J . Chern. Soc., Perkin Trans

2, 1982,295.

1 6

D . Prim, G . Kirsch , J.

Chern. Soc ., Perkin Trans

1, 1994,2603.

17

M . Blenkle , P . Boldt , C . Brauchle , W . Grahn, I. Ledoux, H. Nerenz, S. Stadler, J . Wichern ,

J . Zyss, J.

Chern. Soc ., Perkin Trans .

2, 1996, 1377.

1 8

L.S

. Fuller, B . Iddon , KA Smith ,

J.

Chern. Soc ., Perkin Trans

1, 1997, 3465 .

1 9

N . Hayashi , Y . Mazaki , K . Kobayashi ,

Tetrahedron Lett. ,

35, 1994, 5883.

Chapter

3:

Carbene complexes of Thienothiophene

in the solid-state.

R = OH, OEt

Figure

3.2

Bis(9-hydroxyf/uoren-9-y/)thieno[3,2-b]thiophene

Unlike thiophene, the coordination chemistry of thieno[3 , 2-b]thiophene and its derivatives in transition metal complexes has not yet been exploited. In our laboratories we are interested in comparing stabilities, properties and structural features of linear chained thiophene (bithiophene and terthiophene) with condensed thiophene (thieno[3,2-b]thiophene and dithieno[3,2­ b]thiophene) as they present themselves in ligands in coordination chemistry. Previously we have found that 3,6-dimethylthieno[3,2-b]thiophene form binuclear

a,

n-complexes where the ligand uses the n-system of one ring to coordinate to a Cr(COh-moiety and the lone-pair of electrons on the

S-atom to bond to a Cr(COkfragmenfo. This utilizes the aromatic electrons of one ring while the other ring is left with a S-coordinated sulfur atom and an olefin .

Cr(CO)5

~

Me

~

(COhC

/

S Me

Figure

3.3 Binuclear a,n-complex of 3 , 6-dimethylthieno[3,2-b]thiophene

2 0

M.

Landman,

M.Sc. thesis,

Novel TT-Heteroarene Complexes of Chromium (0) ,

University of Pretoria, 1997 .

55

Chapter

3 :

Carbene complexes of Thienothiophene

2.

Synthesis of Thienothiophenes

The synthesis of thieno[3,2-b]thiophene was based on the method of Goldfarb and co-workers

21 and was prepared in 50% overall yield. The synthetic route is depicted in figure 3.4.

)oBr o a

- s

Sr S

d

s

Sr

56

e s

OJ s

Figure

3.4 Reagents: (a) Et

2

0, 48% HBr , 75 ° C ; (b) Zn, CH

3

COOH , reflux ; (c) n BuLi, -70 ° C ,

S 8 ' BrCH

2

C0

2

Et; (d) POCI

3

,

DMF; (e) NaOMe, MeOH , heat ; (f) Quinoline , Cu powder, heat

Synthesis of 2,3,5-tribromothiophene was effected according to the method described by

Brandsma and Verkruijsse

22 and involves the reaction of bromine and thiophene in diethyl ether at elevated temperatures . The target product (90%) and HBr were yielded. Reduction of 2 , 3 , 5­

2 1

YAL. Goldfarb, V .

P . Litvinov, S. Ozolin ,

Izv. Akad. Nauk.

SSSR, Ser. Khim., 1965, 510.

22

L . Brandsma, H .

D . Verkruijsse, Synth. Commun .. 18, 1988, 1763.

Chapter

3:

Carbene complexes of Thienothiophene 57

tribromothiophene to 3-bromothiophene

(55%F

was performed using two equivalents of zinc powder in acetic acid . Reaction of 3-bromothiophene with elemental sulfur followed by the addition of ethylbromoacetate yielded ethyl(3-thienothio)acetate (73%). Upon reacting this acetate­ substituted thiophene with phosphorus oxychloride in N , N-dimethylformamide , an aldehyde group was added at the 2-position to afford ethyl(2-formyl-3-thienothio)acetate in this Vilsmeier formylation reaction . Cyclisation occurred under basic conditions and the subsequent decarboxylation using copper powder and quinoline yielded thieno[3 , 2-b]thiophene

21 .

Since no practical short-step synthesis forthieno[3,2-b]thiophene or substituted derivatives thereof were available in literature, the utilization of thienothiophenes in organic synthesis has been limited . Then, in 1994, Choi

et aP4

reported a one-pot synthesis of 3,6-dimethylthieno[3,2­ b]thiophene . The synthesis is based on the discovery made by Teste and Lozac'h25 that the reaction of 2 , 5-dimethyl 3-hexyne-2 , 5-diol with elemental sulfur affords 3,6-dimethylthieno[3,2­ b]thiophene, although the yield was low . Choi

et

a/ re-examined this reaction and varied the reaction conditions until they obtained 3 , 6-dimethylthieno[3 , 2-b]thiophene in a reasonable yield.

They found that by heating a mixture of 2,5-dimethyl-3-hexyne-2,5-diol and sulfur in benzene in an autoclave provided the optimum yield for the target product (figure 3 .

5) . a

-

Me

W

S

S

Me

Figure 3.5 Reagents : (a) Benzene, 200°C, 12h

The use of this ligand was preferred over unsubstituted thieno[3 , 2-b]thiophene in the syntheses of the novel carbene complexes due to the easy preparation method described for this compound.

2 3

S . Gronowitz,

T.

Raznikiewicz,

Org. Synth .

Coli., 5 , 1973, 149 .

24

K .

S . Choi , K. Sawada, H. Dong, M. Hoshino , J. Nakayama , Heterocycles, 38, 1994,143 .

2 5

J. Teste , N . Lozac'h, Bull. Soc. Chim. Fr., 1955 , 422 .

Chapter

3:

Carbene complexes of Thienothiophene

3.

Synthesis of carbene complexes of Thienothiophene and derivatives

58

The dimetallation of thieno[3 , 2-b]thiophene and 3,6-dimethylthieno[3,2-b]thiophene was based on the method described by Bugge

26

. The reactions were carried out in hexane and TMEDA was introduced together with butyllithium to form 2,7-dilithio species at elevated temperatures.

Formation of biscarbene complexes ensued after addition of hexacarbonyl metal complexes and the subsequent quenching with the alkylating agent Et

3

0BF

4 .

Following this procedure, 3,6­ dimethylthieno[3,2-b]thiophene was reacted with the metal complexes Cr(CO)6' W(CO)6, MO(CO)6,

MnCp(COh and Mn(MeCp)(COh to yield the different biscarbene complexes. Again, as for the thiophene analogues, in most cases the monocarbene complexes as well as decomposition products were also isolated and not only the expected biscarbene complexes. Complexes 8-18 were prepared in this manner . The synthetic procedure for the preparation of these complexes is outlined in figure 3.6.

Me

S

~

S

Me

..

Me S

EtO~~

(COhL3M Me

S

EtO

+

'c i j

(COhL3M

Me

8,11 , 14 , 16

Me

9,12 , 15,17,18

C

ML3(COh i j

'~Et

M = Cr, L = CO, 8-10

M=W,L=CO,11-13

M = Mo, L = CO, 14-15

M = Mn, L = Cp, 16-17

M = Mn L= CpMe, 18

EtO

'c i j

(COhL3M

Me

1

02

Me

0

~

'OEt

10 , 13

Figure 3.6 Synthesis of complexes 8-18

Reagents : a (i) 2 eq . n-BuLi (ii) MLiCOh (iii) Et

3

0BF

4

Interesting to note was that the reaction of 3,6-dimethylthieno[3,2-b]thiophene with the

26

A. Bugge ,

Acta Chem. Scand .

,

22, 1968, 63 .

Chapter

3 : Carbene complexes of Thienothiophene 59 manganese metal complexes almost exclusively yielded the biscarbene complexes. In both cases no decomposition product was formed and only for the reaction with [MnCp(COh] could the monocarbene complex be isolated in a low yield. This is in contrast with the rest of the reactions wh ere the monocarbene was usually the main product of the reaction and the biscarbene was formed i n lower yields. In all of the reactions the well known butyl carbene complex

[Cr(CO) 5 C(OEt)Bu] was formed as a result of the excess butyl lithium in the reaction mixture due to insufficient dilithiation of the thienothiophene substrate.

Monocarbene complexes of chromium and tungsten, 8 and 11, were crystallized from hexane:dichloromethane (1: 1) solutions to afford red-orange needles. The biscarbene analogues of these complexes, 9 and 12, yielded purple-black crystals when crystallized from the same solvent mixture. X-ray diffraction studies confirmed the structures of these compounds . The decomposition products, 10 and 13, isolated together with the respective mono- and biscarbene complexes, although in low yield, were characterized spectroscopically and were orange coloured.

For the reaction of molybdenum hexacarbonyl with the dilithiated 3,6-dimethylthieno[3,2­ b]thiophene species , the products were similar to the products obtained from the chromium and tungsten reactions. The red-orange monocarbene complex 14 was isolated together with the purple biscarbene complex 15. Both the yellow monocarbene complex 16 and the purple-brown biscarbene complex 17 were obtained from the reaction of [MnCp(COh] and 3,6­ dimethylthieno[3,2-b]thiophenewhile the purple-brown biscarbene complex 18 formed exclusively in the reaction of the lithiated agent with [Mn(MeCp)(COhl Single crystals of this complex were obtained from a 1 : 1 hexane : dichloromethane solution and subjected to X-ray diffraction studies.

The products isolated from the reaction of [Cr(CO)6] and lithiated thieno[3,2-b]thiophene, instead of the dimethyl analogue, did not resemble those obtained from the similar reaction using 3,6­ dimethylthieno[3,2-b]thiophene. Although the synthetic procedure followed was identical, complexes 19 and 20 were isolated, instead of the expected monocarbene and biscarbene complexes . The structure determinations of complexes 19 and 20 were based on the data collected by the use of NMR spectroscopy, infrared data and mass spectrometry. The structure of complex 19 was confirmed by single crystal X-ray determination. The formation of product 19 can be explained by the reaction of a deprotonated thienothiophene monocarbene complex with a butyl carbene to give an ylide intermediate. Elimination of an ethoxy group ensues . The base

Chapter

3:

Carbene complexes

of

Thienothiophene

60

TMEDA deprotonates the ring at the 7-position of this carbene complex, while the conjugated acid assists in the removal of the ethoxy group from the ylide. Oxidation of the more reactive butyl carbene end of the biscarbene affords the final product 19. All the steps in this proposed reaction route have precedents in literature.

TMEDA

..

s

O E t , c - - Z 5 l e

[TMEDA-HJ'

[cr]~

S

[crf s

S

EtO,c~

S

~cr]

Bu

~)-c

'c-Z~}.--Jl

'OEt

[crf s

..

- EtOj

OEt

S

~··)-c

~cr]

Bu-6-Z~)-J

'OEt

[L]

S e

[Cr]

=

Cr(CO)s

Figure 3.7

19

Proposed mechanism for the formation of

19

Other less likely reaction routes which could also lead to the formation of complex 19, include the synthesis via the formation of a bis-acyllated intermediate. It is assumed that the intermediate, before alkylation, reacts with (i) a third butyl group in a nucleophilic addition reaction, or is (ii) selectively oxidized in the reaction mixture before being attacked by a butyl group. In both cases the products are suggested to have formed by reaction on the bis-acyllated product before

Chapter

3:

Carbene complexes of Thienothiophene

61 alkylation of the reaction mixture was effected. In the first case (figure 3 .

8) the route prescribes that the thieno[3,2-b]thiophene and the M(CO)5 metal moiety have the ability to withdraw electron density to such an extent that one of the electrophilic carbene carbons is still accessible for nucleophilic attack by an excess of n-BuLi. Such a process comprises the reduction of one of the metal carbonyl units and has not yet been recognized in monocarbene complexes. This type of product was however not isolated for the dimethylthienothiophene ligand and discredits thus this mechanism .

C jMJ

'OLi

C jMJ

,

OEt

+

UiM(CO)51

Figure 3.8 Formation of complex 19

via

a bis-acyllated intermediate

+ LbO

19

Figure 3.9 Formation of complex 19

via

selective oxidation

Alternatively, complex 19 could have been formed by diffusion of O

2 through the silicon tubing, whereby one of the metal moieties is replaced by an oxygen atom. Upon attack of the butyl carbanion on this carbon , Li

2

0 is released and product 19 is formed (figure 3.9). Again such a

Chapter

3 :

Carbene complexes of Thienothiophene

62 mechanism is unlikely as a similar product was not isolated for the reaction of Cr(CO)6 and the dimethyl analogue of this ligand .

The proposed formation of complex 20 emanated from the deprotonation of thienothiophene by the base TMEDA and the subsequent nucleophilic attack of this anion at the carbonyl group of complex 19, followed by the protonation of the oxyanion by the [TMEDA-Hr species (figure 3 .

10) .

Figure 3.10 Formation of complex 20

S

01

8

[TMEDA-Ht

S

+

S

OJr~u

S

OH

I

20

19

3.1 Spectroscopic characterization of novel carbene complexes

Complexes 8-20 were characterized spectroscopically using NMR and IR spectroscopy and mass spectrometry . The structures of compounds 8, 9, 11 , 12, 16, 18 and 19 were confirmed with X-ray crystallography .

3.1.1

1H

NMR spectroscopy

All NIVIR spectra were recorded in deuterated chloroform as solvent unless otherwise specified .

The

1

H NMR data for chromium complexes 8-10 are summarized in table 3.1, the tungsten complexes 11-13 in table 3.2 and the molybdenum complexes 14 and 15 in table 3.3. The data for compounds 16-18, the manganese complexes, are given in table 3.4, while the data of the two thieno[3,2-blthiophene chromium complexes 19 and 20 are reported in table 3.5.

Chapter

3 :

Carbene complexes of Thienothiophene

63

The chemical shifts for the protons of free 3,6-dimethylthieno[3,2-b]thiophene are at 6.92 ppm for

H 2 and H7 and at 2.33 ppm for the methyl substituents on C3 and C6

2

.

The 1H NMR spectrum of th i eno[3 , 2-b]thiophene was recorded in CDCI

3 and two doublets at 7.55 ppm (H2 and H7) and 7.24 ppm (H3 and H6) were observed .

Table

3.1

P roton

H7

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-M

OCH

2

CH

3

-O

Me3

Me6

1H NMR data of complexes 8, 9 and 10

Chemical shifts (0, ppm) and Coupling constants (J, Hz)

0

7.18(q)

5.19 (q)

-

1.71 (t)

-

2.47 (s)

2.41 (d)

1

~ s •

r

~ 3

O EI

5

,<:

~Cr(CO )5

S

8

J

1.2

7 .

0

-

7.1

-

-

1.0

(CO) P~%

EI O

/c

2

~

3 4

V

S

~

OEI c~

~O(C O)5

9

0

-

5 .

19 (q)

-

1.74 (t)

-

2.47 (s)

2.47 (s)

J

-

7 .

1

-

7.1

-

-

-

(CO),Cr

~

~c

EIO/ 2

~

S

3 4

'

V

S

~'

7

/ O EI

~o

10

0

-

5 .

20 (q)

4 .

35 (q)

1.72 (t)

1 .

38 (t)

2.70 (s)

2.43 (s)

J

-

6.9

7 .

2

6.9

7.2

-

-

A quartet peak is observed for the H7 proton on the spectrum of the monocarbene complex 8, while a singlet was expected . The methyl protons of the methyl group C6 resonate as a doublet, indicating

J4H.H coupling of these protons with H7.

The electron withdrawing effect of the carbene moiety is prominent on comparing the chemical shift values of the two methyl groups on C3 and C6 respectively for the three complexes . On the spectrum of the monocarbene complex 8 the methyl group on position 3 is more affected by the influence of the carbene moiety than the methyl sUbstituent on position 6, causing a downfield shift of these protons. On the spectrum of complex 9, the biscarbene complex, both methyl groups are affected by the electrophilic carbene fragment and both methyl groups are observed downfield compared to the values for the unsubstituted ligand. By replacing one carbene moiety with an ester functional group , the cornpetitive electron withdrawing effects cause a significant downfield

Chapter

3 :

Carbene complexes of Thienothiophene

64 shift in the value of the methyl proton on C3, while the methyl group on the carbon adjacent to the ester substituent is also shifted downfield but only marginally so, as is observed on the spectrum of complex 10 . From this it is concluded that the carbene fragment is more electron withdrawing than an ester group .

On the NMR spectrum of complex 10, the decomposition product , evidence for the further decomposition of this product to the bis(ethylester) (figure 3.11) is observed. Signals were observed at 4.34 (4H , q, J

=

7.0),2.34 (6H, s) and 1 .

37 ppm (6H, t, J

=

7.0).

EtO

"C

cf

Me

Me

Figure

3.11 The bis(ethylester) of 3,6-dimethylthieno[3,2-b]thiophene

For the data in table 3 .

2, the monocarbene complex ,

11,

again displays a quartet signal for the

H7 proton due to coupling with the methyl group on C6, which is observed as a doublet, and correlates with the analogous peaks observed on the spectrum of complex 8. It is interesting to note the significant difference in chemical shift values of the OCH

2

CH

3 peak on the spectra of these three tungsten complexes compared to the same peak on the spectra of the three chromium complexes . The peak is observed significantly more upfield for the tungsten complexes than for the chromium complexes, denoting that these chemical shifts are fairly characteristic for a specific metal and unaffected by the other carbene substituent.

H7

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-M

OCH

2

CH

3

-O

Me3

Me6

Chapter

3:

Carbene complexes of Thienothiophene

Table

3.2

1H

NMR data of complexes

11, 12 and

13

Proton

7

,

~

5 ,

5

V

~ 3

5 2

0

7.22 (q)

5.02 (q)

-

1.69 (t)

-

2.48 (s)

2.41 (d)

Chemical shifts (0 , ppm) and Coupling constants (J , Hz)

11

C /

OEI

~(CO) ,

J

1 .

1

7 .

1

-

7 .

1

-

-

1.1

[-~~

Etc! 2

0

-

5 .

04 (q)

-

1 .

72 (t)

-

2.48 (s)

2.48 (s)

3

V

12

5

/OEt

~(COls

J

-

7 .

1

-

7 .

1

-

-

-

(CO)s W~

~c

101 0/

5 5

2

~

3 4

V

5

~ '

1

/

OEI

~O

0

-

5.03 (q)

4 .

36 (q)

1 .

70 (t)

1 .

39 (t)

2.70 (s)

2 .

44 (s)

13

J

-

7.2

7 .

2

7.2

7.2

-

-

65

Also characteristic, is the position of the methylene resonances of the ester groups at ca . 4 .

35 ppm for both complexes

10 and

13, showing that the influence of the different metals is not carried through the rings to the other side of the thienothiophene rings. In figure 3.12 the characteristic positions of the methylene groups as observed for different functional groups in the novel carbene complexes (1-30) are illustrated.

R-C

~M]

R_c:f°

"

o-0I

2

-CI-i:3

"

o-~-CI-i:3

4 .

60-5.25 4.35-4.40

.

Figure

3.12

Characteristic positions of the methylene protons (ppm) on

1

H NM R spectra

The chemical shift values for the methyl protons of the ethoxy substituent are seemingly more insensitive to the different metal substituents and the difference in the values for the tungsten and chromium complexes is less profound.

Chapter

3:

Carbene complexes of Thienofhiophene

Table 3.3

1H

NMR data of complexes 14 and 15

P roton

Chemical shifts (6, ppm) and Coupling constants (J, Hz)

1

~

S ,

V

~ 3

5 S '

O B c /

~ O(CO),

(CO), Mo~

EtC!

S

5

' ~

,

V

~ .

/OE t

1

~~c(C O),

14 15

H7

OCH

2

CH

3

-1V1

OCH

2

CH

3

-M

Me3

Me6

6

7.21 (s)

5.08 (q)

1.69 (t)

2 .

50 (s)

2.41 (s)

J

-

7.0

7 .

1

-

-

6

-

5.13 (q)

1.73 (t)

2.52 (s)

2 .

52 (s)

J

-

7.1

7.1

-

-

66

Table 3.4

Proton

H7

OCH

2

CH

3

-M

OCH

2

CH

3

-M

Me3

Me6

Cp

CpMe

1H

NMR data of complexes 16, 17 and 18

Chemical shifts (0, ppm) and Coupling constants (J , Hz)

0

6.94 (s)

4.67 (q)

1.52 (t)

2 .

34 (s)

2.27 (s)

4 .

67 (s)

1 s ,

~

• 5

V

S

~'

/ O E '

2

~Mn Cp(CO~

16

J

-

6.9

6.9

-

-

-

-

(ooJ,Q>\1 n ~%

BO /

'~

B

~

7

~ c / O

S

~

M nQI( CO}z

17

6

-

4.67 (q)

1.52 (t)

2 .

26 (s)

2.26 (s)

4.67 (s)

-

J

-

7.0

7.0

-

-

-

-

(CO);(C pMe ) Mn"c %

E' O/ 1

~

J 4

r

~

C / O

E

'

S Mn(CpMe )(COh

18

6

-

4 .

74 (q)

1.53 (t)

2.26 (s)

2 .

26 (s)

4.55 (s)

4.43 (s)

1 .

85 (s)

-

J

-

6 .

7

6 .

8

-

-

-

On the spectra of complexes 14 and 15 (table 3

.

3), the methylene protons of these two molybdenum complexes are observed at chemical shift values intermediate of those of the

Chapter

3:

Carbene complexes of Thienothiophene

67 chromium and tungsten complexes, i.e. Cr> Mo > W, following the trend of their relative positions on the periodic table. The values for the methyl protons of the ethoxy substituent of the different complexes are again very comparable.

On comparing the chemical shift values of H7 on the different monocarbene spectra of the data in table 3.4, the value obtained for complex

16

is conspicuously lower than for complexes 8,

11

and 14 and is comparable with the value of 3,6-dimethylthieno[3,2-b]thiophene. This same trend was also observed for a series of carbene metal complexes prepared with thiophene as bridging

Iigand

27

.

The protons of the ethoxy group of all three manganese products are likewise shifted more upfield compared to the chemical shift values of the analogous chromium, tungsten and molybdenum complexes. It can thus be concluded that the coordination of the manganese metal moiety causes less electron draining from the ring system than coordinating a Cr, W or 1\110 metal pentacarbonyl fragment. The presence of an electron donating group such as cyclopentadienyl on the manganese metal fragment can account for the shielding effect observed on the spectra of 16,

17

and 18. The replacement of three carbonyl groups, which are electron-withdrawing, with a cyclopentadienyl ligand on the metal fragment, increases the contribution of the metal moiety in stabilizing the electrophilic carbene carbon and lessens the role of either the ring system or the ethoxy group.

On the spectrum of 18, two singlet resonances are observed for the Cp protons while only one singlet is found on the spectra of

16

and 17, since the Cp group of

18

is substituted with a methyl group on one position and thus gives rise to two non-equivalent groups of two protons. The electron donating effect of this methyl group is evident from the upfield shift of the Cp protons on the spectrum of

18

compared to those of unsubstituted Cpo

On comparing the data for complexes 19 and 20 in table 3.5, the methylene protons are observed as quartets at 5.21 ppm and 5.14 ppm, respectively. The OH-signal is not observed on the spectrum of complex 20. The protons on the ring system of the two complexes correspond well with one another and the values for the second thienothiophene unit in complex 20 are comparable to the data for OMTT. The positions of the butyl groups on the respective spectra are

27

Y.M. Terblans, H.M. Roos, S. Lotz, J.

Organomet. Chern.,

566, 1998, 133.

Chapter

3 :

Carbene complexes of Thienothiophene

almost identical.

Table 3.5

Proton

1H

NMR data of complexes 19 and 20

Chemical shifts (0, ppm) and Coupling constants (J, Hz)

O~~ s u f 7 c/

OEt

6 S 2

~Cr(CO)5

I

'Mb

~

h2 h

6

~ ~ s o:):)):c

#Cr]

~

I

7 S 4 3

'~Et

13 S

'0

9 Su

19 20

H3

H6

H9

H12

H13

OCH

2

CH

3

-M

OCH

2

CH

3

-M

C(O)CH

2

CH

2

CH

2

CH

3

C(O)CH

2

CH

2

CH

2

CH

3

C(O)CH

2

CH

2

CH

2

CH

3

C(O)CH

2

CH

2

CH

2

CH

3

3.1.2

13C

NMR spectroscopy

°

8 .

33 (5)

7.82 (5)

-

-

-

5.21 (q)

1.69 (t)

2 .

93 (t)

1.74 (t)

1.42 (t)

0 .

95 (t)

7.0

7.3

7.5

7.5

7 .

3

-

-

-

7.0

J

-

-

°

8.36 (5)

7.88

(5)

6 .

99 (5)

7.59 (d)

7.27 (d)

5.14(q)

1.65 (t)

2.91 (t)

1.75 (t)

1.43 (t)

0.94 (t)

-

5.1

5.1

7.1

7.1

J

-

-

7.3

7.3

7.3

7 .

3

68

The

13C

NMR data of chromium complexes 8-10 are summarized in table 3.6, the tungsten and molybdenum complexes 11-15 in table 3.7

. The data for compounds 16-18, the manganese complexes, are given in table 3.8, while the data of the two thieno[3,2-b]thiophene chromium complexes 19 and 20 are reported in table 3 .

9. Assignment of the spectra of the complexes as well as the uncoordinated DMTT was based on

13C

NMR studies of selenolo[3,2-b]selenophene

2 8

28

S. Gronowitz,

T.

Frejd, A.-B . Hbrnfeldt, Chemica Scripta, 5, 1974,236.

C arbene

C2

C3

C 4

C5

C6

C7

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-M

OCH

2

CH

3

-O

Me3

Me6

M(COh

8

0

318.7

1 50.1

130.8

144 .

7

142.6

129.5

128 .

2

76.4

-

15.6

-

18 .

6

14.7

216 .

8 (cis)

222.8 (trans)

Chapter

3 :

Carbene complexes

of

Thienothiophene

Table 3.6

13C

NMR data of complexes 8, 9 and 10

Carbon I I

Chemical shifts (0 , ppm)

9

I

0 n.o.

167.7

131.9

149.8

149 .

8

131.9

167 .

7

76 .

6

-

15 .

5

-

17 .

6

17.6

216 .

3 (cis)

223.6 (trans)

10

0

317.9

163 .

6

130 .

6

1 42.7

142.0

129 .

7

163 .

1

76.4

61.4

15 .

6

14.4

17.7

15.0

216 .

7 (cis)

223 .

3 (trans)

I

69

Due to the poor solubility of the biscarbene complex in deuterated chloroform, the spectrum of complex 9 is of an unsatisfactory quality and the carbene peak was not observed . Use of different solvents did not , however, improve the situation, since the complex was even less soluble in acetone , dichloromethane and benzene . This same problem was also encountered for the biscarbene complex of tungsten, complex 12 , and again the carbene peak was absent on the spectrum.

The chemical shift values (ppm) for the carbon atoms of uncoordinated 3,6-dimethylthieno[3,2­ b]thiophene are at 121 .

8 (C2 and C7), 130.1 (C4 and C5), 140.0 (C3 and C6) and 14 .

6 (Mef The

1 3 C NMR spectrum of thieno[3 , 2-b]thiophene was recorded in CDCI3and the chemical shift values

(ppm) observed for the different resonance peaks were assigned as follows: 131.7 (C2 and C7),

Chapter

3:

C arbene complexes

of Thienofhiophene

70

11 9 .

7 (C3 and C6) and 139.7 (C4 and C5). On the spectrum of complex 10, the decomposition product, evidence for the existence of the bis(ethylester) complex was observed, as was the case for the

1

H NMR spectrum. It is therefore suggested that the decomposition product undergoes further decomposition in solution to form this compound by substitution of the second metal fragment by an oxygen atom. Resonance peaks were found at the following chemical shift values

(pp m ): 162.8 (C(O)OEt), 138.5 (C2 and C7), 128.4 (C3 , C6) and 130.5 (C4 and C5) . This product i s also present on the spectrum of the decomposition product of tungsten, complex 13 .

Carbene carbon resonance peaks are found at characteristic values for the different metals . The values fall within the range of 317 to 319 ppm for the chromium complexes, while the molybdenum complexes display values slightly lower at 306 to 309 ppm and the carbene carbon resonances are observed at 292 to 297 ppm on the tungsten spectra . This same sequence is noted for the position of the M(CO)5 peaks on the different spectra, i.e. the Cr(CO)5 chemical shift values are at the highest chemical shift values and the W(CO)5 resonance peaks at the lowest chemical shift values, correlating with their positions on the periodic table . An increase in the atomic number of metals in the same period leads to an increase in the shielding of the carbonyl groups . In each case the difference in chemical shift value (L\6), on changing the metal from chromium to molybdenum to tungsten, is approximately 10 ppm . The methylene carbons of the different metal spectra reveal the opposite trend. The methylene carbons of the ethoxy group of the carbene on the spectra of the chromium complexes are observed upfield from those of molybdenum while those of the tungsten complexes are found downfield . The methylene carbons of the chromium complexes are thus the most shielded. The sequence for the methylene protons is therefore W

>

Mo

>

Cr . This phenomenon can be best described by considering two resonance structures I and II (figure 3 .

13) . Resonance structure I is more important for biscarbene complexes than structure II. Important , however, is the contribution of structure II to the final molecule, causing small differences in the chemical shifts of the methylene protons . The contribution of structure II is more significant for the tungsten complexes compared to those of chromium . Structure II shows the deshielding of the ethoxy groups and consequently shielding of the carbene-metal mOiety, evident in the upfield shift in M(COh values . The methyl carbons of the ethoxy groups , however, seem to be less affected by the different resonance structures and fall within the range of 15 .

2 to

15 .

6 ppm for all the complexes.

Chapter

3 :

Carbene complexes of Thienathiaphene

M e

EtO

'C

[M(

Me

C

#M]

'OEt

~

..

ffi

EtO~

~C

[M]/

Me

8

71

II

Figure

3.13 Resonance structures I and II

Table

3.7

13C

NMR data of complexes 11-15

Carbon

Carbene

C2

C3

C4

C5

C6

C7

OCH

2

CH

3

-M

OCH

2

CH

3

-O

OCH

2

CH

3

-M

OCH

2

CH

3

-O

Me3

Me6

M(COh

I

144.1

131.0

128 .

8

79 .

0

-

15.4

-

18.9

14.6

11

0

292 .

6

150.2

132.1

144.1

197.8 (cis)

201 .

9 (trans)

Chemical shifts (0, ppm)

I

12

I

13

I

14

I

15

0

0 0 0 n .

o . 296 .

3 308.4 306.6

164 .

9

123 .

5

149 .

3

162 .

8

130.0

144.7

141.3

129.7

149.6

131 .

4

143 .

7

143 .

2

154 .

2

136 .

1

140 .

2

149 .

3

123.5

164.9

79 .

6

-

15 .

3

-

18.4

18.4

159.3

79.5

61 .

5

15 .

2

14.2

18.4

14.8

130 .

9

128 .

8

78.2

-

15 .

5

-

18 .

8

14 .

7

140.2

136.1

154.2

78.8

-

15.5

-

18.2

18 .

2

197 .

3 (cis)

202 .

2 (trans)

197.8 (cis)

202.3 (trans)

206 .

1 (cis)

212.6 (trans)

205.7 (cis)

212.7 (trans)

I

Chapter

3 :

Carbene complexes of Thienothiophene

72

It is interesting to note that the chemical shift value of C2 , the ipso carbon atom coordinated to the carbene moiety, is conspicuously lower for the three monocarbene complexes than for the biscarbene and decomposition products. The value for C2 varies from 150.1 to 155 .

3 ppm for the monocarbene complexes while the corresponding value for the biscarbene and decomposition complexes ranges between 154.2 and 167.5 ppm. The reason for this occurrence is attributed to the electron withdrawing effect of the carbene-metal fragment (figure 3.14). In the case of the monocarbene complexes, only one carbene moiety is coordinated to the ring system, whereas the coordination of two electron withdrawing substituents, as is the case for the biscarbene and decomposition products , enhances the draining of electrons from the ligand. Therefore the deshielding effect on C2 (and C7) is more substantial for the biscarbene complexes and decomposition complexes than for the monocarbene compounds. vs

Monocarbene complex Biscarbene complex

Figure 3.14

Electron draining in monocarbene and biscarbene complexes

For the data of complexes 14 and 15 in table 3 .

7, the methyl substituents on the ring system is notably affected by the coordination of the ligand to an electron withdrawing substituent. This is evident from the large chemical shift difference between Me3 and Me6 on the spectrum of complex 14, the monocarbene product. A difference of 4 ppm is observed in that Me3, the methyl group adjacent to the carbene substituent, is shifted downfield compared to Me3 of the uncoordinated ligand. The chemical shift value for Me6, 14.7 ppm, is comparable with the value obtained for Me6 of the uncoordinated ligand, which was found to be 14.6 ppm. This methyl substituent (Me6) can thus serve as a probe to determine the influence of the carbene substituent on the second ring of the condensed ring system . It is therefore concluded that the impact of an electron withdrawing substituent only affects the ring directly coordinated to it.

Chapter

3 :

Carbene complexes

of Thienothiophene

Table 3.8

13C

NMR data of complexes 16, 17 and 18

Carbon

C arbene

C2

C3

C4

C5

C6

C7

OCH

2

CH

3

-M

OCH

2

CH

3

-M

Me3

Me6

Cp

I

16

0

321 .

2

155 .

3

130.3

141 .

0

141 .

5

122 .

0

123 .

1

72.3

15 .

4

15 .

2

14 .

7

86 .

3

Chemical shifts (0 , ppm)

I

17

I

0

320 .1

165.1

123 .

0

143 .

3

143 .

3

123 .

0

165 .

1

72 .

7

15.4

15 .

1

15 .

1

86 .

5

18

0

319 .

2

167 .

5

122 .

0

142 .

9

142 .

9

122 .

0

167 .

5

72 .

9

15 .

6

15.4

15.4

85 .

0

86 .

5

13.7

231 .

7

73

I

CpMe

M(COh

-

231.3

-

231.2

Two single peaks were observed for the Cp carbons on the spectrum of complex 16, indicating two groups of non-equivalent carbon atoms . The quaternary carbon, bonded to the methyl substituent, was not observed. The M(COh peak is observed downfield from the values obtained for IVI(COh (M

=

Cr, W , Mo) peaks observed on the spectra of similar complexes.

The resonances on the 13 C NMR spectrum of complex 20 confirm the presence of two thienothiophene rings in this complex , while the data obtained correlates well with the data of complex 19 . These complexes are thus closely related and the environments of the carbon atoms are similar. The values obtained for the chemical shifts of the second thienothiophene unit are comparable to those of the free ligand.

155.1

-

-

-

-

75.7

43.0

25.5

22.8

13.9

15.1

217.5 (cis)

223.5 (trans)

194.5

Chapter

3:

Carbene complexes of Thienothiophene

Table 3.9

13C

NMR data of complexes 19 and 20

Carbon

I

74

19

0

317.0

164.0

146.8, 139.4, 134.4

Chemical shifts (0, ppm)

I

20

0

317.9

164.1

146.8, 139.4, 139.2, 134.3, 132.5

I

C7

C8

C9

C12

Carbene

C2

C3, C4, C5, C6,

C10,C11

C13

OCH

2

CH

3

-M

CH

2

CH

2

CH

2

CH

3

CH

2

CH

2

CH

2

CH

3

CH

2

CH

2

CH

2

CH

3

CH

2

CH

2

CH

2

CH

3

OCH

2

CH

3

-M

IVI(CO)5

C=O

155.1

146.1

139.2

120.1

124.3

75.7

38.7

27.0

22.4

13.8

15.1

217.5 (cis)

223.7 (trans)

-

3.1.3 Infrared Spectroscopy

The infrared data of complexes 8-15 are outlined in table 3.10, while table 3.12 contains the infrared data of complexes 16, 17 and 18. The data of complexes 19 and 20 are given in table

3.13.

The spectrum of complex 17 is depicted in figure 3.15.

Chapter

3: Carbene complexes of Thienothiophene

Table

3.10

Infrared dataa of complexes

8-15 in carbonyl region

Band

A/

1

)

8

2055

2056

~

9

2053

2054

I

I i

I

Stretching vibrational frequency (veo, cm­

1

)

10

2057

11

2063

2065

12

2062

B

1979

1986

1986

1984

1984 1989

1982

1983

A

1

(2) 1938

1952

1941

1956

1940 1933

1949

1938

E 1938 1941 1940

1933 1938

1942 1948

1939 aFlrst set of values recorded

In dlchloromethane, second set

In hexane

13

2065

1985

1936

1936

14

2064

1983

1941

1941

15

2063

1988

1946

1946

75

Table

3.11 Literature carbonyl stretching frequency values

Band

Complex

Cr(COhC(OEt)T

A

1

(1)

2063

B

1987

A

1

(2)

1961

Cr(COhC(OEt)Ph 2062a 1963 1954

W(COhC(OEt)T 2070

1977 1952

W(COhC(OMe)Ph 2079 b

1992 a Spectrum recorded in pentane; b

Spectrum recorded in nujol

1953

E

1950

1942

1944

1953

Reference

29

31

30

32

The experimental values obtained for the various complexes correlate well with values reported in literature (table 3.11). On comparing the stretching frequencies of the monocarbene complex

[Cr(CO)5C(OEt)Tf9 with those obtained for the monocarbene complex 8, it was concluded that the higher electron density is found on the metal nuclei with thienothiophene bridging ligands. This is confirmed by comparing the values obtained for complex [W(CO)5C(OEt)T]3o with the stretching frequencies of complex

11.

The lower wavenumbers observed for complexes

8 and

11, indicate stronger metal-carbonyl back bonding and thus a decrease in C-O bond order. This implies weaker

29

JA Connor, E.M. Jones, J. Chem. Soc. A, 1971, 1974.

30

S. Aoki, T. Fujimura, E. Nakamura, J. Am. Chern. Soc., 114, 1992,2985.

Chapter

3 :

Carbene complexes of Thienothiophene

76 metal to carbene carbon n-bonding. Alternatively, the substituents on the carbene carbon may pla y a greater role in stabilizing the electrophilic carbene carbon. The values reported for the monocarbene complexes of chromium and tungsten contain i ng a phenyl substituent ,

[Cr(CO)sC(OEt)ph] 3 1 and [W(CO)sC(OMe)Ph]32, are even higher than for the thienyl carbene complexes, implying that the phenyl unit is a poorer donor of electron density compared to the thienyl or , consequently, the thienothienyl substituent.

Figure 3.15 Carbonyl region on the infrared spectrum of complex 17

Table 3.12 Infrared data a of complexes 16-18 in carbonyl region

Band

16

A ' 1961 , 1933

A"

1893 a Values recorded

In dlchloromethane

, 1871

Stretching vibrat i onal frequency (v -:; o, cm · ' )

17

1962,1932

1896, 1872

18

1958 , 1929

1892 , 1868

On the infrared spectra of all three compounds the carbonyl stretching frequencies are observed as four strong bands, instead of the usual two bands expected for these complexes. It is thus

3'

M .

Y . Darensbourg, D .

J . Darensbourg ,

Inorg . Chern.,

9 , 1970 , 32.

32

E.O. Fischer, A. Maasbol, Chern. Ber., 100,1967,2445.

Chapter

3:

Carbene complexes of Thienoth i ophene

77 concluded that more than one isomer exist in solution . The plane of the substituents of the carbon has two possible orientations relative to the cyclopentadienyl ring: coplanar or perpendicular.

Restricted rotation around the carbene-metal bond leads to the existence of two separate conformational isomers and two different positions for the Cp ring relative to the ethoxy group and th e t hiophene ring system (figure 3 .

16) . Literature reports disclose the same occurrence on the

IR spectra of [MnCp(COh{C(OMe)Ph}] 33, [MnCp(COh{C(OEt)Ph}]34, [Cr('16-Ar)(CO) 2 {C(OMe)Ph}p 5 and [MnCp(CO)2{'12-C2H4}P 6.

~Me

S

"Mn"

oo

/ o c ,

",,,KUj;

Me

OEt vs

07

Me

X

00"'./

~C oc

I

OEt

~ ~ e and

Figure

3.16

Conformational isomers of complex

16

Table

3.13

Infrared data a of complexes

19 and

20 in carbonyl region

Band

A,

(2)

B

A ,")

E a Values recorded

In dlchloromethane

Stretching vibrational frequency (v r.o

, cm ·' )

19

20

2058

1978

1943

1943

2056

1981

1938

1938

33

E.O

. Fischer, R.L. Clough and P . Stuckler, J.

Organomet. Chem.,

120,1976, C6 .

3 4

E.O. Fischer , E .

W . Meineke , F.R. Kreissl, Chem. Ber., 110, 1977,1140.

3 5

H.-J. Beck, E.O. Fischer , C .

G. Kreiter, J.

Organomet. Chem.,

26,1971, C41.

3 6

H. Alt, M. Herberhold, C .

G. Kreiter , H. Strack, J.

Organomet. Chem .

,

77 , 1974 , 353.

Chapter

3 :

Carbene complexes of Thienothiophene

3.

1 .4 Mass spectrometry

78

The fragmentation patterns of complexes

8-19 are reported in table 3 .

14. The molecular ion peak,

M+ , was observed for all the complexes except on the mass spectrum of complex 9 . On the spectrum of complex 15 only the molecular ion peak could be assigned because of the lack of a distinguishable fragmentation pattern .

Table 3.14 Fragmentation patterns of complexes 8-19

I

Complex

8

I

Fragment ions (I,

%)

415 .

9 (4) M + ; 387.9 (18) M+ - CO; 359 .

9 (17) M+ - 2CO; 331 .

9 (8) M+ - 3CO; 303 .

9 (35)

M+ - 4CO; 275 .

9 (100) M+ - 5CO; 246 .

9 (78) M+ - 5CO -CH

2

CH

3 ;

218 .

9 (79) M + - 6CO­

CH

2

CH

3

;

167.1 (35) C a

H

7

S/

I

9

635 .

8 (24) M+ - CO; 551.8 (15) M + - 4CO ; 523 .

7 (79) M + - 5CO; 495 .

8 (49) M + - 6CO ;

467.9 (73) M + -7CO; 411.9 (54) M + - 9CO; 383 .

9 (73) M + -10CO ; 355 .

0 (49) M + -10CO

- CH

2

CH

3

10

11

12

13

14

488.2 (2) M + ; 460.2 (10) M+ - CO; 432 .

1 (6) M+ - 2CO; 404.0 (2) M+ - 3CO; 376.1 (21)

M+ - 4CO ; 348 .

1 (93) M+ - 5CO; 319.1 (30) M+ - 5CO - CH

2

CH

3 ;

291.0 (10) M + - 6CO -

CH

2

CH

3

547 .

8 (19) M + , 519.7 (36) M + - CO; 491 .

8 (17) M + - 2CO; 463.8 (18) M+ - 3CO; 435 .

8

(30) M + - 4CO; 407.8 (100) M+ - 5CO; 378 .

8 (37) M+ - 5CO - CH

2

CH

3 ;

350.9 (71) M +­

6CO - CH

2

CH

3

;

167.1 (33) C a

H

7

S

2

+

931 .

9 (22) M+; 902.9 (26) M+ - CO; 873 .

8 (15) M+ - 2CO; 818.4 (14) M + - 4CO ; 787.8

(14) M+ - 5CO; 760.2 (72) M+ - 6CO ; 732 .

9 (43) M + - 7CO ; 703 .

8 (48) M+ - 8CO ; 675 .

9

(36) M + - 9CO; 647.9 (45) M + -10CO ; 618 .

9 (35) M + - 10CO - CH

2

CH

3

;

590 .

9 (44) M + -

11 CO ­ CH

2

CH

3

;

562.9 (40) M + - 11 CO - 2CH

2

CH

3 ;

534 .

9 (50) M+ - 12CO - 2CH

2

CH

3

620.2 (1) M + ; 592 .

1 (2) M+ - CO ; 564 .

3 (1) M + - 2CO ; 536 .

1 (1) M + - 3CO ; 508.3 (2) M + -

4CO ; 480 .

2 (9) M+ - 5CO ; 451 .

2 (2) M + - 5CO - CH

2

CH

3

;

423.0 (4) M+ - 6CO ­ CH

2

CH

3

;

407 .

1 (2) M+ - 6CO CH

2

CH

3

-

0

462.2 (4) M + ; 434 .

2 (9) M + - CO; 406 .

2 (6) M+ - 2CO; 378.1 (8) M+ - 3CO; 350 .

1 (8) M+ -

4CO ; 322.1 (41) M+ - 5CO; 293.1 (6) M+ - 5CO - CH

2

CH

3 ;

265.1 (15) M+ - 6CO ­

CH

2

CH

3 ;

167.1 (32) C a

H

7

S/

752 .

2 (3) M+ 15

Ch a pter

3:

Carbene complexes of Thienofhiophene

79

I

Complex

I

16

Fragment ions (I,

%)

400.0 (5) M+; 371 .

0 (1) M+ - CO ; 344 .

0 (57) M+ - 2CO ; 315 .

0 (13) M+ - 2CO - CH

2

CH

3;

286.9 (100) M+ - 3CO - CH

2

CH

3;

221 .

9 (37) M+ - 3CO - CH

2

CH

3

- Cp; 167.0 (34) C

S

H

7

S

2

+

I

17

632 .

7 (12) M + ; 604.7 (1) M+ - CO; 576 .

7 (81) M+ - 2CO ; 548 .

7 (1) M+ - 3CO; 520.6 (100)

M+ - 4CO ; 491 .

5 (15) M + - 4CO - CH

2

CH3 ; 463 .

5 (66) M + - 5CO - CH

2

CH 3; 434.4 (15) M+

- 5CO - 2CH

2

CH

3

; 406.3 (44) M+ - 6CO - 2CH

2

CH

3;

341 .

2 (13) M+ - 6CO - 2CH

2

CH

3

-

Cp; 287 .

2 (15) M + - 6CO - 2CH

2

CH

3 -

Cp - Mn; 222 .

1 (5) M+ - 6CO - 2CH

2

CH

3 -

2Cp -

Mn; 167 .

1 (9) C

S

H

7

S/

18

19

660.5 (7) M+ ; 604.4 (73) M+ - 2CO; 548.3 (100) M+ - 4CO; 519 .

2 (11) M + - 4CO­

CH

2

CH

3

;

491.2 (36) M+ - 5CO - CH

2

CH

3

; 462.2 (6) M + - 5CO - 2CH

2

CH

3

;

434 .

5 (25) M+ ­

6CO - 2CH

2

CH

3

472.4 (1) M + ; 444.3 (1) M+ - CO ; 416.3 (1) M+ - 2CO; 388.3 (1) M+ - 3CO; 360.2 (4) M + ­

4CO ; 332 .

3 (27) M+ 5CO ; 303 .

2 (7) M+ - 5CO -

CH~CH 3 ;

275 .

1 (7) M+ - 6CO - CH

2

CH

3

All the complexes follow the same basic fragmentation pattern, which starts with the stepwise loss of the carbonyl groups . This is followed by the elimination of the ethyl fragment of the ethoxy group after which the remaining CO mOiety is lost. In the case of the biscarbene complexes the loss of one ethyl group and one CO group precedes the loss of the second ethyl group and the second CO fragment. This is obvious on considering the pattern of the two manganese biscarbene complexes 17 and 18 .

The existence of the bis(ethylester) thienothiophene complex, observed on the NMR spectra of complexes 10 and 13, is confirmed by the presence of its molecular ion peak, M+

=

312, on the mass spectra of both compounds .

3.1.5 X-ray Crystallography

Single crystal X-ray diffraction studies were employed to confirm the molecular structures of complexes 8, 9, 11, 12, 16 , 18 and 19 . Single crystals of these carbene complexes were obtained from dichloromethane:hexane (1 : 1) solutions. Complex 8 crystallized as orange needles while complex 9 gave deep-purple needle-like crystals. The orange-red needle-like crystals were characterized as complex 11 and the deep-purple tungsten biscarbene complex 12 gave small,

Chapter

3:

Carbene complexes of Thienothiophene

80 needle-like crystals. Complex 16 gave yellow-brown cubic crystals . Complex 18 crystallized as black crystals. The crystals of complex 19 were cubic and had a purple-red colour. Figures 3.17 to 3 .

22 and figure 3 .

24 represent ball-and-stick plots of these respective structures. Selected bond leng th s and angles are tabulated in tables 3.15, 3.17, 3.19, 3.20, 3.22, 3.23 and 3.25

.

3.1.5.1 Crystal structure of complex 8

02

Figure 3.17 8all-and-stick plot of complex 8

The crystal structure of uncoordinated thieno[3,2-b]thiophene was determined by Cox

et af3

7

and the bond lengths were found to be as follows: S(1 )-C(2) 1.72

A,

C(2)-C(3) 1.36

A,

C(3)-C(4) 1.41

A,

C(4)-C(5) 1.36

A and C(5)-S(1) 1.74

A.

The bond angles were determined as C(2)-S(1)-C(5)

91.2°, S(1)-C(2)-C(3) 116 .

5 ° , C(2)-C(3)-C(4) 111 .

7 ° , C(3)-C(4)-C(5) 114 .

3 ° and C(4)-C(5)-S(1)

110.2

° .

37

E.G. Cox, R.J.J.H . Gillot, GA Jeffrey,

Acta Crystallogr.,

2,1949,356.

Chapter

3:

Carbene complexes

of Thienofhiophene

Table 3.15 Selected bond lengths and angles of complex 8

I

C r( 1) -C(1 )

8

I

Bond lengths (A)

I

8

2.081(4) C(7)-S(2)-C(4)

1.722(4) C(S)-S(1 )-C(2)

S(1)-C(S)

S(1 )C(2)

S(2)-C(7 )

S(2)-C(4)

0(1)-C(1)

1.7S1(4}

1 .721 (S)

1.729(4)

1.317(S)

C(1)-0(1)-C(10)

0(1 )-C(1 )-C(2)

0(1 )-C(1 )-Cr(1)

C(2)-C(1 )-Cr(1)

0(1)-C(10)

C(1)-C(2)

C(2)-C(3)

C(3)-C(4)

C(4}-C(S)

C(S)-C(6)

C(6)-C(7)

1.4S7(S)

1.469(6)

1.413(6)

1.411 (6)

1 .

377(6)

1.341 (7)

1.431 (6)

C(3)-C(2)-S(1 )

C(2)C(3)-C(4)

C(S)-C(4)-C(3)

C(S)-C(4)-S(2)

C(3)-C(4)-S(2)

C(6)-C(7)-S(2)

C(7)-C(6)-C(S)

C(4)-C(S)-C(6)

C(4 )-C(S)-S(1)

81

I

Bond angles

( 0 )

I

89 .

S(2)

92.1 (2)

123.6(3)

104.9(3)

128 .

6(3)

126.S(3)

111.S(3)

109.8(4)

116 .

3(4)

111.0(3)

132.7(3)

116.4(4)

108 .

S(4)

114.6(4)

110.3(3)

From the structure of complex 8 it is noted that the heteroaromatic ring, the carbene carbon and the metal are coplanar. The metal fragment is orientated towards the sulfur atom and away from the methyl substituent on the ring. The Cr metal is approximately octahedrally arranged with two of the cis-carbonyl ligands bending away from the carbene carbon . This is indicated by the bond angles of 94.S(1t for C(12)A-Cr(1)-C(1) . On considering the positions of the ethyl group of the ethoxy substituent relative to the metal fragment in the solid state, the Z-isomer of this complex along the carbene-O bond is observed. This is in accordance with the observation in literature that the Z-isomer is by far the most popular arrangement in crystal structures of ethoxy- or methoxy­ substituted carbene complexes of octahedrally coordinated metals

38

.

On comparing the bond lengtl1s of the ring system in the structure of complex 8 with those of free

38

(a) O.S. Mills, A.D . Redhouse,

J.

Chern. Soc. A, 1968, 642 . (b) G. Huttner, B. Krieg,

Chern. Ber., 10S, 1972, 67. (c) U . Schubert, Organometallics, 1, 1982, 108S.

Chapter

3:

Carbene complexes of Thienothiophene

82 thieno[3,2-b]thiophene, all of the bonds are longer (or equal) in the complex except for the C(5)­

C(6) bond which is shorter . These bond lengths are all in the range between the characteristic bond d i stances of a C(Sp 2 )-C(Sp2) single bond (1.46 A) and a C(Sp2)-C(Sp 2 ) double bond (1 .

32 A)39 .

The bonds are therefore more delocalized in the complex than for the free ligand. This effect is more apparent in the thiophene r i ng fragment directly coordinated to the carbene moiety and diminishes on going to the second conjugated ring fragment.

The metal-carbene carbon bond length is 2.08(1) A, while the arene-carbene carbon bond is

1.47(1) A . The bond between oxygen and the carbene carbon is 1.32(1) A and the distance between the oxygen and the ethyl group is 1.46(1)

A.

These values are very similar to literature values obtained for other monocarbene complexes of chromium pentacarbonyl (table 3 .

16) of the type [Cr(CO) s C(OR)R'].

Table 3.16 Literature values

Complex

Cr(CO) s C(OMe)Ph

Cr(CO)sC(OEt)Me

Cr(CO)sC(OH)Ph

Cr(CO) s C[OSi(SiMe

3

) 3 J1-furyl

M-C ~ ><hp n p

2.04(3)

2 .

05(1)

2.05(1)

2 .

03(1)

Bond length (A)

Cc>< h pnp-O

1.33(2)

1.31(1)

1.32(1 )

1 .

32(1 )

O-R

1.46

-

-

-

C~~'hpn'

-R'

1.47(4)

1 .

51(1)

-

1.45(1 )

Reference

38(a)

40

4 1

42

The Cr-carbene bond of 2 .

08(1) A, compared to a Cr(O)-C single bond of 2 .

21 A and a Cr-C double bond of 1.91 A in Cr(CO)6 ' together with the C ca r bene -O bond distance of 1 .

32(1) A , compared to the values found in diethyl ether (1.43 A) and acetone (1.23 A)4 3 , indicate partial double bond

3 9

F.H. Allen, O. Kennard, D.G. Watson ,

L.

Brammer, A.G

. Orpen, R. Taylor,

J .

Chern.

Soc .

Perkin Trans

2, 1987, S1.

4 0

K.H. Dotz, H. Fischer, P .

Hofmann, F .

R. Kreissl, U. Schubert , K . Weiss,

Transition Metal

Carbene Complexes,

VCH Verlag, Weinheim , 1983, p.94

.

41

R.J. Klinger , J .

C. Huffman, J .

K . Kochi,

Inorg. Chem .

,

20 ,

1981,

34.

42

U. Schubert, M . Wiener, F .

H. Kohler,

Chern. Ber .

,

112 , 1979, 708 .

4 3

A .

W . Parkins, R.C

. Poller,

An Introduction to Organometallic Chemistry,

Macmillan

Publishers , London , 1986 , p .

53.

Chapter

3 :

Carbene complexes of Thienoth i ophene

83 character for both bonds . The carbene plane is staggered with the cis-carbonyl ligands and the

trans carbonyl

C-Cr bond length is 0 .

04

A shorter than the average cis-carbonyl C-Cr bond length.

3.1.5

.

2 Crystal structure of complex 9

Figure 3.18 8all-and-stick plot of complex 9

The arene ring system, carbene carbons and metals in the structure of complex 9 are coplanar and the molecule is centrosymmetric. The planar nature of this arene spacer is ideal for n­ conjugation and metal metal communication through this bridging ligand . The two Cr(COh metal fragments are found on opposite sides of the spacer ligand, positioned towards the sulfur atom and away from the methyl substituent on the ring, similar to the structure of complex 8 . This is in contrast with the structure of the biscarbene complex of chromium pentacarbonyl, constituting a thienylene space~7 .

In this complex , the metal fragments are found on the same side of the thiophene ring and on the side opposite to the sulfur atom in the ring. However , for the biscarbene complex of chromium pentacarbonyl with biphenylene as bridging ligand, the metal moieties are again on opposite sides of the axis connecting the two carbene carbon atoms

44

.

The cis-carbonyl

4 4

N . Hoa Tran Huy, P . Lefloch , F. Robert, J. Jeannin ,

J.

Organomet .

Chern., 327, 1987 ,

211 .

Chapter

3:

Carbene complexes

of Thienothiophene

84 groups in 9 are staggered relative to the carbene carbon. The

trans-carbonyl

C-Cr bond distance is 0.02

A shorter than the average cis-carbonyl C-Cr bond length.

The bond lengths of the ring system are all longer or similar to those of free thieno[3,2-b]thiophene except for C(4A)-S, which is shorter. The suggested delocalization in the ring system is promoted by the observed bond distances, which are all very similar, and correspond to distances between characteristic C(Sp2) single bonds and C(Sp2) double bonds. In contrast to the observation made for complex 8, the delocalization in this complex occurs throughout the whole arene system, aided by the influence of the second metal fragment, which is absent in complex 8.

The bond angles of the thieno[3,2-b]thiophene substituent for complex 9 differ marginally from those of the free DMTT, indicating a slight distortion of the arene ring due to metal coordination.

Table 3.17 Selected bond lengths and angles of complex 9

I

Cr-C(1)

9

I

Bond lengths

(A)

I

9

2.068(3) C(3)-C(2)-S

S-C(4A)

S-C(2)

C(1)-C(2)

C(2)-C(3)

C(3)-C(4)

C(4A)-C(4)

O(6)-C(1)

1.721 (2)

1.745(3)

1.473(4)

1.393(4)

1.415(4)

1.378(5)

1.325(3)

1.449(3)

C(2)-C(3)-C(4)

C(4)-C(4A)-S

C(4A)-S-C(2)

O(6)-C(1 )-C(2)

O(6)-C(1 )-Cr

C(2)-C(1 )-Cr

O(6)-C(11)

I

Bond angles

(0) I

113.1 (2)

109.3(2)

110.8(2)

91.1(1)

105.0(2)

129.2(2)

125.7(2)

The Cr-CCarbene distance of 2.07(1)

A falls within the range of Cr-CCarbene distances (2.00-2.10

A) reported for alkoxy carbene complexes of chromium and can be compared with literature values of similar biscarbene complexes given in table 3.18.

Chapter

3:

Carbene complexes of Thienothiophene

Table 3.18 Literature values

Complex

[( Cr(CO)5C(OEt)hT]

[{Cr(CO)5C(OEt}2 biphenylene]

M-C ca r bene

2 .

04(1)

2 .

05(1)

Bond length (A)

C car be n e-O

1 .

32(1 )

1 .

32(1 )

O-R

1.45(1)

-

85

C c ar b en e -R '

1.47(1)

1.49(1 )

Reference

27

44

3 .

1.5.3 Crystal structure of complex 11

O '2A

02

Figure 3.19

Ball-and-stick plot of complex

11

It is interesting to note that the C(2)-S bond length is conspicuously longer for all of the complexes than the rest of the C-S bonds . The lengthening of all the C-S bonds compared to those in the free ligand implies that the sulfur atoms are less involved in the delocalization through the ring system in the complexes . The involvement of the sulfur atom in the aromaticity of the free ligand is much more significant. The C(2)-S bond in the complexes is the most affected by this result because of the coordination of the carbene moiety at C2 .

Chapter

3:

Carbene complexes of Thienothiophene

Table 3.19 Selected bond lengths and angles of complex 11

I

W -C ( 1)

11

I

Bond lengths

(A)

2.219(6)

I

11

C(7)-S(2)-C(4)

1 .

727(6) C(5)-S(1)-C(2)

S( 1 )-C(5)

1 .

771(7) 0(1)-C(1)-C(2)

S(1)-C(2)

1.733(8 ) 0(1 )-C(1 )-W

S(2)-C(7)

S(2)C(4 )

0(1)-C( 1 )

0(1)-C(10)

C(1)-C(2)

C(2)-C(3)

C(3)-C(4)

C(4)-C(7)

C(5)-C(6)

C(6)-C(7)

1.746(7)

1.345(8)

1 .

456(9)

1.460(9)

1.402(8)

1 .

395(10)

1.399(10)

1.428(11 )

1 .

398(10)

C(2)-C(1 )-W

C(3)-C(2)-S(1 )

C(2)-C(3)-C(4)

C(5)-C( 4 )-C(3)

C(5)-C(4)-S(2)

C(3)-C(4 )-S(2)

C(4)-C(5)-C(6)

C(4)-C(5)-S(1 )

C(7)-C(6)-C(5)

C(6) C(7)-S(2)

86

I

Bond angles

C)

I

92 .

0(3)

91.3(3)

104 .

5(5)

128.1(5 )

127 .

3(5)

111 .

1(5)

110 .

9(6)

116 .

3(6)

110 .

0(6)

133 .

7(5)

115 .

6(6)

109 .

6(6)

109 .

0(8)

114 .

2(6)

3 .

1.5.4 Crystal structure of complex 12

05

Figure 3.20 8all-and-stick plot of complex 12

Chapter

3 :

Carbene complexes of Thienothiophene

Table 3 .

20 Selected bond lengths and angles of complex 12

I

W C (1)

12

S-C(4A)

S-C ( 2 )

C(1) C(2)

C(2)-C ( 3)

C(3)-C(4)

C(4A)-C(4)

0(1)-C(1)

I

Bond lengths (A)

I

2 .

221 (8)

12

C(3)-C(2)-S

1 .

728(7) C(2)-C(3)-C(4)

C(4)-C(4A)-S

1.

761 (7 )

1.460(10)

1.399(10)

C(4A)-S-C(2)

0(1 )-C(1 )-C(2)

0(1)-C(1)-W

1.427(9 )

1.390(15 )

1 .

342(9)

C(2)-C(1 )-W

0(1)-C(6) 1.452(9)

87

I

Bond angles n

1 13.2(5 )

118.0(5)

109.4(6)

90.9(3)

105.9(6)

128 .

2(5)

125 .

7(5)

I

The structure of complex 12 is very similar to the structure of complex 9 . Delocalization through the ring is again implied by the lengthening of the arene ring carbon-carbon bond distances. The metal-carbene bond length is now, however, 2.22(1)

A for the larger tungsten metal atom, compared to the value of 2.07(1)

A for the smaller chromium atom . The average carbonyl-metal bond length is also 0.15

A longer in this structure compared to the structures containing chromium metal atoms (complexes 8 and 9) .

The methyl substituents on the condensed ligand is coplanar to the carbene plane for all four structures (complexes 8, 9, 11 and 12).

In table 3.2'1 literature values are reported for mono- and biscarbene complexes of tungsten . The bond distances for complexes 11 and 12 are very comparable with these values, although the metal-carbene value for complex 12 is significantly longer (2.22(1) A) than the average bond distance (2 .

13 A) reported for the analogous complexes in this table. This trend was also observed for the chromium complexes 8 and 9 , signifying that back bonding from the metal to the carbene carbon is reduced and that the thienothiophene fragment is a better n-donor than phenyl , thiophene or anthracene in comparable complexes .

Chapter

3:

Carbene complexes of Thienothiophene

Table 3.21 literature values

Complex

W( CO)5C(OMe)Ph

W(CO)5C(Ph)Ph

M-Ccarbene

2.05

2 .

15(1)

Bond length

(A)

O-R

Cca rb ene-O

1 .

31

-

1.43

-

[ {W(CO>SC(OEt)h T )

[{W(CO)sC(OMe)}2(anthracendiyl)]

2.15(1)

2.16(1)

1 .

33(1 )

1 .

31(1)

88

1.47(1)

-

Ccarbene-R'

1.49

1.45(2) ,

1.51 (2)

1.46(1 )

1.50(1 )

Reference

45

46

47

48

3.1.5.5 Crystal structure of complex 16

Figure 3.21 8all-and-stick plot of complex 16

45 O.S. Mills, A.D. Redhouse,

Angew. Chern. Int . Ed . Engl. ,

4 , 1965,1142 .

46 C.P. Casey, T.J. Burkhardt, CA Bunnel , J .

C. Calabrese , J.

Am. Chern. Soc. ,

99, 1977,

2127.

47 Y.M. Terblans, PhD thesis,

Pretoria,

1996.

Thiophene Bimetallic Carbene Complexes,

University of

48 T . Albrecht, J. Sauer,

Tetrahedron Lett.,

35, 1994, 561.

Chapter

3 :

Carbene complexes of Thienothiophene

89

Table 3.22 Selected bond lengths and angles of complex 16

I

Mn-C(1)

16

I

Bond lengths (A)

I

16

I

Bond angles

( 0 )

I

C(S)-S(1 )-C(2) 1 .

944(6) 91 .

3(3)

1.721 (6) C(7)-S(2)-C(4) 90 .

1 (3)

S( 1 )-C(S)

1.76S(6) C(1 )-0(1 )-C(1 0) 122.8(S)

S( 1 ) -C(2 )

0(1 )-C(1 )-C(2)

S(2)-C(7) 1.734(6) 106 .

2(S)

S(2)-C(4) 1.737(8) 0(1)-C(1) Mn 127 .

8(4)

O(1)-C(1)

0(1)-C(10)

C(1)-C(2)

C(2)-C(3)

C(3)-C(4)

C(4)-C(7)

C(S)-C(6)

C(6)-C(7)

1 .

344(7)

1.4S4(8)

1.460(8)

1.410(8)

1.430(9)

1.382(9)

1.447(9)

1.373(10)

C(2)-C(1 )-Mn

C(3)-C(2)-S(1 )

C(2)-C(3)-C( 4)

C(S)-C(4)-C(3)

C(S)-C(4)-S(2)

C(3)-C( 4)-S (2)

C(4)-C(S)-C(6)

C(4)-C(S)-S(1)

C(7)-C (6)-C (S)

C(6)-C(7)-S(2)

126 .

0(4)

112.3(4)

109.4(S)

11S.4(S)

111 .

6(S)

132 .

9(S)

114 .

S(S)

111.S(S)

108 .

S(6)

11S .

3(S)

The steric effect of the methyl substituent on the ring system is evident in considering the angles

C(1)-C(2)-C(3) and C(1)-C(2)-S(1) around the sp 2 -hybridized ring carbon C(2). These angles are

129.6(5) ° and 112.3(4) ° , respectively, and are an indication of the degree of distortion . Seeing that in the ring system the carbene carbon and the metal atom are all in the same plane , it is clear that the ethoxy substituent on the carbene carbon and the methyl group on the heteroaromatic ring are competing for the same space, which is sterically unfavourable . The molecule is therefore distorted to accommodate both of these groups, which causes the deviation in bond angles . The heteroaromatic ring is bent towards the metal moiety. It has previously been suggested that the methyl substituent on the ring is the reason for the metal orientation towards the sulfur atom and not the opposite conformation as is observed in complexes with thiophene27 and unsubstituted thienothiophene spacer units (see complex 19). Bond lengths in this complex are comparable to literature values reported for similar complexes (table 3 .

24), although the carbene-metal bond is slightly longer than the average value determined in literature .

Chapter

3:

Carbene complexes of Thienothiophene

3 .

1.5.6

Crystal structure of complex 18

90

Figure 3.22

8all-and-stick plot of complex

18

From figure 3.22 it is clear that the conformation of the complex is such that the carbene plane

(formed by the metal, the carbene carbon and the two substituents Rand R') is perpendicular to the mirror plane of the IVlnCp(CO)2 fragment. Although this conformation is electronically the more unfavourable

38c

, it is sterically preferred. The two conformations differ by the relative orientation of the carbene plane and the mirror plane of the metal fragment: co-planar or perpendicular (figure

3.23) .

~

OCCO

/R

Mn=C

j

..

.."

'R' vs

~

..

.,R

Mn=C'

j

..

.." ........

R'

OC CO

Figure 3.23

Conformations of MnCp(CO)2 carbene complexes

The geometry around the manganese centre is pseudotetrahedral and the cyclopentadienyl ring is parallel to the carbene plane with the orientation of the methyl substituent cOinciding with the

Mn-carbene bond.

Chapter

3:

Carbene complexes of Thienothiophene

Table 3.23 Selected bond lengths and angles of complex 18

I

Mn C ( 1)

18

I

Bond lengths

(A)

1 .

922(5)

I

18

C(3)-C(2)-S

S-C(4A)

S-C(2)

C ( 1)-C(2)

1 .

733(5)

1.761 (5)

1.485(7)

1.394(7)

C(2)-C(3)

C(3)-C(4)

C(2) C(3)-C(4)

C(4)-C(4A)-S

C(4A)-S-C(2)

0(1 )-C(1 )-C(2)

C(4A)-C(4)

0(1 )-C(1)

1.428(7)

1 .

388(10)

1.338(6)

0(1 )-C(1 )-Mn

C(2)-C(1 )-Mn

0(1)-C(6) 1.465(6)

9 1 i

Bond angles

( 0 )

112.9(3)

109.7(4)

110 .

8(5)

91.2(2)

105.1(4)

129.9(3)

124.8(4)

I

The Mn-carbene bond of 1.922(5)

A is somewhat longer than the average value obtained for literature examples of mono- and biscarbene complexes containing a manganese metal fragment

(see table 3 .

24) . This value is , however , considerably shorter than the metal-carbene bond distances observed in the analogous chromium and tungsten complexes . The bond angles around the carbene carbon seem to be insensitive to change in the metal.

Table 3.24 Literature values

Complex

MnCp(COhC(OMe)menthyl

MnCp(COhC(OEt)Ph

MnCp(COhC(Ph)Ph

[1J0 {MnCp(CO)

2

CPh hl

M-C ca r b en e

1 .

89(2)

1 .

87(1 )

1 .

89(2)

1 .

85(2),

1 .

88(5)

Bond length

(A)

C carbene -O

1.33(2)

1.36(2)

-

-

C carben e-R'

1 .

53(2)

1.54(2)

-

-

Reference

4 9

3 8 c

38c

50

49

S . Fontana, U . Schubert, E.O

. Fischer , J.

Organomet. Chem. ,

146, 1978, 39.

50

E .

O . Fischer , J . Chen, U . Schubert,

Z.

Naturforsch. , Tei/ B,

37 , 1982, 1284.

Chapter

3:

Carbene complexes of Thienofhiophene

3.1 .

5.7 Crystal structure of complex 19

92

Figure 3.24 8all-and-stick plot of complex 19

Table 3.25 Selected bond lengths and angles of complex 19

I

19

Cr-C(1)

I

Bond lengths

(A)

I

2.059(3)

1 .

729(3)

19

C(7)-S(2)-C(4)

C(5)-S(1 )-C(2)

S(1)-C(5)

S(1)-C(2)

S(2)-C(4)

S(2)-C(7)

0(1)-C(1)

0(1)-C(13)

C(1)-C(2)

C(2)-C(3)

C(3)-C(4)

C(4)-C(5)

C(5)-C(6)

C(6)-C(7)

C(7)-C(8)

1 .

765(3)

1.733(3)

1.752(3)

1.335(3)

1.465(3)

1.464(4)

1 .

384(4)

1.408(4)

1.399(4)

1.417(4)

1 .

381(4)

1.477(4)

C(1 )-0(1 )-C(13)

0(1 )-C(1 )-C(2)

0(1 )-C(1 )-Cr

C(2)-C(1 )-Cr

C(3)-C(2)-S(1 )

C(2)-C(3)-C(4)

C(5) C(4)-C(3)

C(5)-C(4)-S(2)

C(3)-C(4)-S(2)

C(6)-C(7)-S(2)

C(7)-C(6)-C(5)

C(4)-C(5)-C(6)

C(4) C(5)-S(1)

I

Bond angles

C)

I

90 .

68(13)

90 .

96(13)

122 .

8(2)

105.4(2)

130 .

0(2)

124.20(19)

111.7(2)

112.1 (2)

113 .

6(2)

111.4(2)

134 .

8(2)

113 .

1 (2)

111 .

1 (3)

113.8(2)

111.4(2)

Chapter

3:

Carbene complexes

of

Thienothiophene

93

The ring system in this structure is again planar and lies within the plane of the carbene carbon and the metal atom and now includes the carbonyl group at C7. The metal-carbene distance is very similar to those encountered for complexes 8 and 9 . Interesting to note is that the orientation of the metal fragment is opposite to all the structures containing the 3,6-dimethylthieno[3,2­ b]thiophene unit and is directed away from the sulfur atom in the ring system, on the opposite side of the ring . This phenomenon may be attributed to the absence of the methyl substituent on the ring, eliminating any steric reason for positioning itself away from the sulfur atom . This is also the preferred orientation for the metal moiety in thienyl ethoxy carbene complexes

27

.

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