Synthesis, Structure, Bridge-Terminal Exchange Kinetics, and

Synthesis, Structure, Bridge-Terminal Exchange Kinetics, and
Published on Web 08/19/2003
Synthesis, Structure, Bridge-Terminal Exchange Kinetics, and Molecular
Orbital Calculations of Pyrazolate-Bridged Digallium Complexes Containing
Bridging Phenyl Groups
Chatu T. Sirimanne, John E. Knox, Mary Jane Heeg, H. Bernhard Schlegel, and Charles H. Winter*
Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202
Received May 21, 2003; E-mail: chw@chem.wayne.edu
Bridging of monoanionic hydrocarbon groups between two metal
centers is well established for the lighter main group elements, and
is particularly broadly appreciated for aluminum.1-4 Major classes
of bridging hydrocarbon ligands in aluminum complexes include
alkane,1 aryl,2 and acetylide and vinyl groups.3 Despite the broad
occurrence of bridging interactions in aluminum chemistry, there
are very few examples of bridging hydrocarbons between the
heavier group 13 elements gallium and indium.4 Structurally
characterized examples for gallium are limited to [Me2Ga(µCtCPh)]2,5a [(η5-C5H5)Fe(µ-η5-C5H4)GaMe2]2,5b and a gallole
dimer containing bridging alkenyl groups.5c For indium, the only
example is [Me2In(µ-2-C4H3S)]2.6 Some striking differences exist
between aluminum and gallium complexes containing similar
ligands. For example, triphenylaluminum is dimeric in the solid
state with two bridging phenyl groups,2a while triphenylgallium is
a monomer in the solid state.7 Trimethylaluminum is dimeric in
the solid state and in solution with a well-known methyl-bridged
structure,1a while the methyl groups in solid trimethylgallium have
only very weak bridge-like interactions between neighboring
gallium atoms.8 Within this perspective, we report the synthesis,
structure, bridge-terminal exchange kinetics, and molecular orbital
calculations of a series of gallium-pyrazolato complexes that
contain bridging phenyl ligands. The observation of bridging phenyl
ligands in these complexes is particularly surprising, since triphenylgallium is monomeric. The results imply that bridging
interactions in the heavier elements may be induced by appropriate
choice of ancillary ligands that dispose two metal centers in close
proximity. Thus, complexes of the heavier group 13 elements with
bridging hydrocarbon ligands are likely to be more accessible than
the current literature may suggest.
Treatment of triphenylgallium (2 equiv) with 3,5-dimethylpyrazole, 3,5-diphenylpyrazole or 3,5-di-tert-butylpyrazole afforded
(C6H5)2Ga(µ-Me2pz)(µ-C6H5)Ga(C6H5)2 (1, 62%), (C6H5)2Ga(µ-Ph2pz)(µ-C6H5)Ga((C6H5)2‚C7H8 (2‚C7H8, 62%), and (C6H5)2Ga(µ-tBu2pz)(µ-C6H5)Ga(C6H5)2 (3, 40%), respectively, as colorless or offwhite crystalline solids (eq 1).
The structural assignments for 1-3 were based on spectral and
analytical data and X-ray crystal structure determinations.9 The 1H
11152
9
J. AM. CHEM. SOC. 2003, 125, 11152-11153
Figure 1. Perspective view of 1. Selected bond lengths (Å) and angles
(deg): Ga(1)-N(1) 1.9670(16), Ga(2)-N(2) 1.9797(16), Ga(1)-C(6)
2.219(2), Ga(1)-C(12) 1.964(2), Ga(1)-C(18) 1.9597(19), Ga(2)-C(6)
2.114(2), Ga(2)-C(24) 1.973(2), Ga(2)-C(30) 1.964(2), N(1)-N(2)
1.376(2), N(1)-Ga(1)-C(6) 100.33(8), N(2)-Ga(2)-C(6) 103.34(8),
Ga(1)-C(6)-Ga(2) 96.88(9).
NMR spectra of 1-3 at ambient temperature showed three broad
multiplets for the bridging phenyl groups and sharper multiplets
for the terminal phenyl groups. The 13C{1H} NMR spectra at
ambient temperature exhibit broad ipso-carbon resonances for the
bridging and terminal phenyl rings between 157.33 and 158.33 ppm
and 146.45-148.26 ppm, respectively.
Figure 1 shows a perspective view of 1 along with selected bond
lengths and angles.9 The molecular structure consists of a dimethylpyrazolato ligand with a diphenylgallium group bonded to each
nitrogen atom. A phenyl group acts as a bridge between the gallium
atoms. The two gallium atoms and the ipso-carbon atom of the
bridging phenyl group define a plane, while the two nitrogen atoms
lie 0.290 and 0.147 Å above and below this plane. The bridging
phenyl group is slightly canted toward Ga(1), giving a distinct
triphenylgallium unit containing Ga(2). The gallium-nitrogen bond
lengths are 1.9670(16) and 1.9797(16) Å. The gallium-carbon bond
lengths lie in the range 1.9597(19)-1.973(2) Å for the terminal
phenyl groups and are 2.114(2) and 2.219(2) Å for the bridging
phenyl group. The geometry about the gallium centers is distorted
tetrahedral. For comparison, crystallographically characterized
gallium complexes with bridging hydrocarbon groups have terminal
gallium-carbon bond lengths that range between 1.95 and 1.99 Å,
bridging gallium-carbon bond lengths between 1.992 and 2.587
Å, and gallium-carbon-gallium angles for bridging hydrocarbon
groups within four-membered rings range between 74.2° and 93.3°.5
The related value for 1 (96.88(9)°) is larger due to geometric
constraints imposed by the Ga2CN2 ring.
The kinetics of exchange between the terminal and bridging
phenyl groups of 1-3 (0.106 M) was studied by 13C{1H} NMR
spectroscopy in toluene-d8 between -30 and 30 °C. Details of these
determinations are given in the Supporting Information. Eyring
analysis of the exchange processes gave the following parameters:
10.1021/ja036256x CCC: $25.00 © 2003 American Chemical Society
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Figure 2. Calculated structure of 4, emphasizing (a) overall structure with
selected core bond lengths (Å) and angles (deg), and (b) and (c) orbital
interactions involving the ipso-carbon atom of the bridging phenyl ligand.
1, ∆Hq ) 7.6 ( 0.1 kcal/mol, ∆Sq ) -27.2 ( 2 eu, and ∆Gq(298)
) 15.7 ( 0.1 kcal/mol; 2, ∆Hq ) 6.0 ( 0.1 kcal/mol, ∆Sq ) -32.0
( 2 eu, and ∆Gq(298) ) 15.5 ( 0.1 kcal/mol; 3, ∆Hq ) 8.9 ( 0.1
kcal/mol, ∆Sq ) -23.1 ( 2 eu, and ∆Gq(298) ) 15.8 ( 0.1 kcal/
mol. Analysis of 0.053 M solutions of 1-3 in toluene-d8 afforded
rates and activation parameters that were identical within experimental error, suggesting that the exchange processes are intramolecular. The rates of terminal-bridging phenyl exchange in 1-3
at 25 °C are calculated to be 20.1 s-1 in 1, 24.8 s-1 in 2, and 15.9
s-1 in 3. The large, negative values of ∆Sq imply ordered transition
states relative to the ground states, and rotation along the N-GaPh3
vector without gallium-nitrogen bond cleavage. The activation
parameters and exchange rates at 25 °C are similar, suggesting that
the size of the pyrazolato carbon substituents has only a minor effect
on the exchange process.
To understand the nature of the bonding in 1-3, the model
complex 4 was optimized at the B3LYP/6-311G(d,p) level of
theory.10,11 Model 4 showed a departure from the solid-state
structure of 1 in that the nitrogen and gallium atoms reside in an
approximate plane, while the phenyl ipso-carbon adopts an outof-plane orientation. By contrast, 1 possesses a distorted ring with
the nitrogen atoms favoring an orientation above and below the
Ga2C plane. The differences between the structures of 1 and 4 are
most likely due to steric congestion associated with the phenyl
substituents on the gallium atoms. The out-of-plane phenyl group
orientation in 4 arises from electronic interactions, in which
hybridized orbitals on the phenyl group create delocalized molecular
orbitals as shown in Figure 2, b and c. However, the energy
difference between a planar Ga2N2C ring and one with the bent
carbon atom is only 1.77 kcal/mol, implying that the molecular
orbitals shown in Figure 2 provide little stabilization to the structure.
The isomeric complex 5 was also considered, in which the phenyl
group is terminal and a hydride bridges between the gallium atoms.
Model 5 is 8.1 kcal/mol lower in energy than 4. Thus, a bridging
gallium-hydrogen bond is about 4 kcal/mol stronger than a
gallium-carbon bond in a bridging phenyl ligand.
This work describes the first examples of gallium complexes
containing bridging phenyl ligands. Such interactions are unexpected, due to the small number of structurally characterized heavier
group 13 complexes with bridging hydrocarbon ligands5,6 and the
fact that triphenylgallium is monomeric in the solid state with
terminal phenyl ligands.7 The activation parameters of the bridgeterminal phenyl exchanges in 1-3 are consistent with strict
intramolecular processes, which suggests that the gallium-nitrogen
bonds are sufficiently strong to maintain the structure in solution.
The molecular orbital calculations predict that extended orbital
overlap involving the bridging phenyl carbon atom is weak at best
and thus contributes little to the stabilization of the bridging
interaction. Thus, the nature of the ancillary ligand holding the two
metal atoms together appears to be the key factor that leads to the
unusual bridging phenyl ligands in 1-3. The results of this work
suggest that many other heavier group 13 metal complexes with
bridging hydrocarbon ligands should be available through appropriate choice of supporting ligands. To this end, we have synthesized
and structurally characterized the indium analogue of 3.12 Recent
reports of long, weak Ga-CH2R-Ga contacts (Ga-C > 3 Å) in
solid-state gallium trialkyls8 raise the possibility that even saturated
alkyl groups might be induced to bridge between gallium, indium,
and other heavier main group metal centers.
Acknowledgment. We are grateful to the National Science
Foundation (Grant CHE-9807269 and Special Creativity Extension
thereto to C.H.W.) for support of this research.
Supporting Information Available: Synthetic procedures and
analytical and spectroscopic data for 1-3; tables of final positional
parameters for the calculations (PDF). X-ray crystallographic files for
1 in CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
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JA036256X
J. AM. CHEM. SOC.
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