Synthesis, Structure, and Bridge-Terminal Exchange Kinetics of Pyrazolate-Bridged Digallium and Diindium Complexes Containing Bridging Phenyl Groups

Treatment of triphenylgallium with 3,5-dimethylpyrazole, 3,5-diphenylpyrazole, or 3,5-di-tert-butylpyrazole in a 2:1 stoichiometry afforded the phenyl-bridged complexes (C6H5)2Ga(μ-Me2pz)(μ-C6H5)Ga(C6H5)2 (62%), (C6H5)2Ga(μ-Ph2pz)(μ-C6H5)Ga(C6H5)2·C7H8 (62%), and
(C6H5)2Ga(μ-tBu2pz)(μ-C6H5)Ga(C6H5)2 (40%), respectively, as colorless or off-white crystalline
solids. Treatment of triphenylindium with 3,5-di-tert-butylpyrazole in a 2:1 stoichiometry
afforded the phenyl-bridged complex (C6H5)2In(μ-tBu2pz)(μ-C6H5)In(C6H5)2·(C6H14)0.5 (40%).
The molecular structures of (C6H5)2Ga(μ-Ph2pz)(μ-C6H5)Ga(C6H5)2·C7H8, (C6H5)2Ga(μ-tBu2pz)(μ-C6H5)Ga(C6H5)2·(C6H14)0.5, and (C6H5)2In(μ-tBu2pz)(μ-C6H5)In(C6H5)2·(C6H14)0.5 consist
of a 3,5-disubstituted pyrazolato ligand with a diphenylgallio or diphenylindio group bonded
to each nitrogen atom. A phenyl group acts as a bridge between the two metal atoms. By
contrast, treatment of triphenylgallium with 3,5-di-tert-butylpyrazole in a 1:1 stoichiometry
or triphenylindium with 3,5-diphenylpyrazole or 3,5-dimethylpyrazole in 2:1 or 1:1 stoichiometry afforded the dimeric complexes [(C6H5)2Ga(μ-tBu2pz)]2 (63%), [(C6H5)2In(μ-Ph2pz)]2
(40%), and [(C6H5)2In(μ-Me2pz)]2 (92%), respectively, as colorless crystalline solids. The
dimeric nature of these complexes was determined by X-ray crystallography. Treatment of
3,5-di-tert-butylpyrazole with excess trimethylgallium afforded the dimeric complex [Me2Ga(μ-tBu2pz)]2 (82%) as the major product and Me2Ga(μ-tBu2pz)(μ-OCH3)GaMe2 (2.6%) as a
minor product. There was no evidence for the formation of the methyl-bridged complex Me2Ga(μ-tBu2pz)(μ-CH3)GaMe2. The kinetics of bridge-terminal phenyl exchange in (C6H5)2Ga(μ-Me2pz)(μ-C6H5)Ga(C6H5)2, (C6H5)2Ga(μ-Ph2pz)(μ-C6H5)Ga(C6H5)2·C7H8, (C6H5)2Ga(μ-tBu2pz)(μ-C6H5)Ga(C6H5)2, and (C6H5)2In(μ-tBu2pz)(μ-C6H5)In(C6H5)2·(C6H14)0.5 was determined
by 13C NMR spectroscopy and afforded the following range of activation parameters:  ΔH⧧ =
6.0−8.9 kcal/mol, ΔS⧧ = −23.1 to −32.0 eu, and ΔG⧧(298 K) = 15.5−15.8 kcal/mol. The large,
negative values of ΔS⧧ imply ordered transition states relative to the ground state and
rotation along the N−GaPh3 or N−InPh3 vector without metal−nitrogen bond cleavage. The
combined results suggest that the close proximity of the metal atoms is the principal
determinant of the bridging phenyl interactions and that complexes of the heavier group 13
elements with bridging hydrocarbon ligands are likely to be more accessible than the current
state of literature would suggest.