STRUCTURAL AND BONDING INVESTIGATION OF AN UNBRIDGED DONOR-ACCEPTOR COMPLEX 1-(2,2'-BIPYRIMIDINE)-1-(T-BUTYL)-2,3-BIS(TRIMETHYLSILYL)-2,3-DICARBA-1-GALLA-CLOSO-HEPTABORANE(6)

The reaction between the closo-gallacarborane, 1-(t-C4H9)-1-Ga-2,3-SiMe3)2-2,3-C2B4H4 (1), and 2,2'-bipyrimidine in a molar ratio of 1:1 in dry benzene produced the unbridged donor-acceptor complex 1-(2,2'-C8H6N4)-1-(t-C4H9)-1-Ga-2,3-(SiMe3)2-2,3-C2B4H4 (2) as a pale-yellow crystalline solid in 81% yield. The spectroscopic data of 2 are almost identical to those of the bridged species 1,1'-(2,2'-C8H6N4)-[1-(t-C4H9)-1-Ga-2,3-(SiMe3)2-2,3-C2B4H4]2 (3). However, the single-crystal X-ray structure of 2 reveals that the complex consists of only one distorted gallacarborane unit and a bipyrimidine molecule, that acts as a bidentate ligand. The apical gallium in 2 is displaced further away f rom the centroidal position above the carborane than it is in 1. Despite the steric hindrance imposed by the bulky trimethylsilyl groups on the cage carbons, the Ga-C(t-butyl) bond is tilted toward the cage carbon atoms. Complex 2 crystallizes in the monoclinic space group P2(1) with a = 11.279 (3), b = 10.060 (3), c = 11.911 (3) angstrom, beta = 94.94 (2)-degrees, V = 1346.5 (6) angstrom3, Z = 2, D(x) = 1.24 g cm-3, lambda(MoK(alpha)) = 0.71069 angstrom, mu(MoK(alpha)) = 11.80 cm-1, F(000) = 528, and T = 220 K. Full-matrix least-squares refinement for 2 converged at P = 0.036 and Rw = 0.045 for 3032 observed reflections. Molecular orbital calculations showed that antibonding interactions between the gallium and carborane cage carbons, induced by base complexation, are a contributing factor to the gallium's slip distortion that is found in 2.