X-RAY STRUCTURE, EPR, AND MAGNETIC-SUSCEPTIBILITY OF GALLIUM(III) TRIS(3,5-DI-TERT-BUTYL-1,2-SEMIBENZOQUINONATE), A MAIN GROUP TRIRADICAL COMPLEX

The compound Ga(TBSQ.)3 (TBSQ. = 3,5-di-tert-butyl-1,2-benzosemiquinonate anion) has been prepared by two synthetic routes and its structure determined by X-ray analysis. The crystals are rhombohedral, with a = 16.408(6) angstrom, c = 13.898(4) angstrom, V = 3240(2) angstrom3, Z = 3, and space group R3(h); R = 0.050; T = 23-degrees-C. The molecule has a distorted octahedral GaO6 kernel, with bidentate TBSQ.-ligands. The compound and its solutions in noncoordinating solvents are stable for months, even when exposed to air. Frozen solution EPR spectra display evidence for the thermal occupation of both quartet, S = 3/2, and doublet, S = 1/2, states. A triradical should have both states, and comparison of theoretical simulations of the triradical EPR spectrum with the experimental spectrum reveals that the energy separation of the quartet and two doublet states is greater than 0.8 cm-1 and less than 20 cm-1. Ga(TBSQ.)3 reacts with pyridine to give Ga(TBSQ.)(TBC)(PY)2(TBC = 3,5-di-tert-butylcatecholate dianion). Magnetic susceptibility was measured for solid Ga(TBSQ.)3 from 2 to 296 K and for the toluene solution at 296 K. The low-temperature susceptibilities show that the ground state is the doublet states of the triradical, and the high-temperature susceptibilities can only be explained by postulating another thermally accessible doublet state, which is close in energy to the triradical states. This state is postulated to be Ga(TBSQ.)(TBC)(TBQ) (TBQ = 3,5-di-tert-butyl-1,2-benzoquinone).