Synthesis, Structure, Bridge−Terminal Exchange Kinetics, and Molecular Orbital Calculations of Pyrazolate-Bridged Digallium Complexes Containing Bridging Phenyl Groups

• Published in: Journal of the American Chemical Society Vol. 125; no. 37; pp. 11152 - 11153
• Main Authors: Sirimanne, Chatu T, Knox, John E, Heeg, Mary Jane, Schlegel, H. Bernhard, Winter, Charles H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 17.09.2003
• Subjects: Al, ga, in, tl, zn, cd, hg derivatives; Chemistry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Organic chemistry; Organic compounds; Organometalloidal and organometallic compounds; Physics; Preparations and properties; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Organic synthesis; Five membered ring; Electronic structure; Molecular structure; Nitrogen heterocycle; Gallium Organic compounds; Dinuclear complex; Organic ligand; Pyrazole derivatives; Electron localization; X ray diffraction; Crystalline structure
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja036256x

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(μ-Me2pz)(μ-C6H5)Ga(C6H5)2 (62%), (C6H5)2Ga(μ-Ph2pz)(μ-C6H5)Ga(C6H5)2·C7H8 (62%), or (C6H5)2Ga(μ-tBu2pz)(μ-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(μ-Me2pz)(μ-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 °C, 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) = 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 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(μ-pz)(μ-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.