CHEMISTRY OF C-TRIMETHYLSILYL-SUBSTITUTED MAIN-GROUP HETEROCARBORANES .8. SYNTHESIS OF CLOSO-GALLACARBORANES AND THEIR REACTIVITY TOWARD A BIS(BIDENTATE) LEWIS BASE, 2,2'-BIPYRIMIDINE - CRYSTAL-STRUCTURES OF CLOSO-1-(TERT-C4H9)-1-GA-2,3-(SIME3)2-2,3-C2B4H4 AND 1,1'-(2,2'-C8H6N4)[1-(TERT-C4H9)-1-GA-2,3-(SIME3)2-2,3-C2B4H4]2

The reaction of [Ga(t-C4H9)Cl2]2 with the nido-carborane dianionic double salts Na+Li+[2-(SiMe3)-3-(R)-2,3-C2B4H4]2- (R = SiMe3, Me, H) in a molar ratio of 1:2 in THF produced closo-1-(t-C4H9)-1-Ga-2,3-(SiMe3)2-2,3-C2B4H4 (I), closo-1-(t-C4H9)-1-Ga-2-(SiMe3)-3-(Me)-2,3-C2B4H4 (II), and closo-1-(t-C4H9)-1-Ga-2-(SiMe3)-2,3-C2B4H5 (III) as moderately air-sensitive colorless oily liquids, in 71, 83, and 62% yields, respectively. In addition, the corresponding neutral nido-carborane precursor 2-(SiMe3)-3-(R)-2,3-C2B4H6 was produced in yields ranging from 14 to 36%. The closo-gallacarboranes I-III were characterized on the basis of H-1, B-11, and C-13 pulse Fourier transform NMR, IR, and mass spectroscopy. The low-temperature, single-crystal X-ray analysis of I shows that the gallacarborane has a closo geometry in which the gallium atom occupies an apical vertex in a distorted pentagonal bipyramid. The gallium-bound tert-butyl group makes a pronounced tilt toward the cage carbons above the C2B3 face with angles of about 21 and 24-degrees to the axis that bisects the apical boron and gallium and the axis that bisects the apical gallium and the centroid of the C2B3 face, respectively. The closo-gallacarborane I crystallized in the triclinic space group P1BAR with a = 9.160 (2) angstrom, b = 9.763 (3) angstrom, c = 12.961 (4) angstrom, alpha = 96.84 (2)-degrees, beta = 106.56 (2)-degrees, gamma = 111.20 (2)-degrees, V = 1003.3 (5) angstrom 3, and Z = 2. Full-matrix least-squares refinements of I converged at R = 0.047 and R(w) = 0.063. The instantaneous reaction between closo-gallacarboranes and 2,2'-bipyrimidine produced the donor-acceptor complexes 1,1'-(2,2'-C8H6N4)[1-(t-C4H9)-1-Ga-2,3-(SiMe3)2-2,3-C2B4H4]2 (IV), 1,1'-(2,2'-C8H6N4)[1-(t-C4H9)-1-Ga-2-(SiMe3)-3-(Me)-2,3-C2B4H4]2 (V), and 1,1'-(2,2'-C8H6N4)[1-(t-C4H9)-1-Ga-2-(SiMe3)-2,3-C2B4H5]2 (VI) in 74-86% yields. The complexes IV-VI were characterized by H-1, B-11, and C-13 NMR, IR, and mass spectroscopy. A single-crystal X-ray analysis was also performed on complex IV. The structure shows that the 2,2'-bipyrimidine base bridges the two gallacarboranes with a center of inversion between the carbons that link the two pyrimidine bases. The apical gallium in each cage is slipped severely from the centroidal position above the C2B3 face in such a way that it could be considered as eta-1 bonded to the unique boron. Consequently, the Ga-bound tert-butyl group is significantly more tilted from the Ga-C2B3 centroid axis toward the cage carbons by about 56.3-degrees than in the precursor, despite the steric hindrance of the bulky trimethylsilyl groups on the cage carbons. Complex IV crystallized in the monoclinic space group C2/c with a = 24.732 (10) angstrom, b = 14.190 (4) angstrom, c = 15.797 (5) angstrom, beta = 117.97 (3)-degrees, V = 4896 (2) angstrom 3, and Z = 4. Full-matrix least-squares refinement of IV converged at R = 0.051 and R(w) = 0.061.