Synthesis, structure, bridge-terminal exchange kinetics, and molecular orbital calculations of pyrazolate-bridged...

Gallium complexes containing bridging phenyl groups were prepared and characterized. 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(mu-Me2pz)(mu-C6H5)Ga(C6H5)2 (62%), (C6H5)2Ga(mu-Ph2pz)(mu-C6H5)Ga(C6H5)2.C7H8 (62%), or (C6H5)2Ga(mu-tBu2pz)(mu-C6H5)Ga(C6H5)2 (40%), respectively, as colorless or off-white crystalline solids. These complexes were characterized by spectral and analytical methods, X-ray crystallography, bridge-terminal exchange kinetics, and molecular orbital calculations for simplified models. The molecular structure of (C6H5)2Ga(mu-Me2pz)(mu-C6H5)Ga(C6H5)2 consists of a dimethylpyrazolato ligand with a diphenylgallium group bonded to each nitrogen atom. A phenyl group acts as a bridge between the two gallium atoms. The kinetics of bridge-terminal phenyl exchange was determined by 13C NMR spectroscopy between -30 and +30 degrees C, and afforded the following range of activation parameters: DeltaH = 6.0-8.9 kcal/mol, DeltaS = -23.1 to -32.0 eu, and DeltaG(298) = 15.5-15.8 kcal/mol. The large, negative values of DeltaS imply ordered transition states relative to the ground state, and rotation along the N-GaPh3 vector without gallium-nitrogen bond cleavage. Molecular orbital calculations were conducted at the B3LYP/6-311G(d,p) level of theory on the simplified model H2Ga(mu-pz)(mu-C6H5)GaH2. The predicted out-of-plane phenyl group orientation arises from electronic interactions, in which hybridized orbitals on the phenyl group create delocalized molecular orbitals. 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 provide little stabilization to the out-of-plane phenyl ligand. The combined results suggest that the close proximity of the gallium 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 the literature would suggest.