Chemistry of C-trimethylsilyl-substituted heterocarboranes .22. Synthetic, spectroscopic, structural, and bonding studies on half- and full-sandwich gallacarboranes of 2,3- and 2,4-C2B4 carborane ligand systems

The reaction of the TMEDA-solvated group 1 salts of the carborane dianions [2,n-(SiMe3)(2)-2,n-C2B4H4](2-) (n = 3, 4) with GaCl3 in a 2:1 carborane-to-CaCl3 molar ratio gave the corresponding full-sandwich gallacarboranes [2,2',n,n'-(SiMe3)(4)-1,1'-commo-Ga(1,2,n-GaC2B4H4)(2)](-) (n = 3 (I), 4 (II) in 36% and 38% yields, respectively. Lowering the carborane-to-GaCl3 molar ratio to 1:1 produced the half-sandwich chlorogallacarboranes 1-(TMEDA)-l-Cl-2,n-(SiMe3)(2)-1,2,n-GaC2B4H4 (n = 3 (III), 4 (IV)) in 51% and 41% yields, respectively. Compounds I-IV were characterized on the basis of H-1, B-11, and C-13 NMR spectra, IR spectra, and single-crystal X-ray analyses. The structures of the gallacarboranes show that the gallium atoms are not equally bonded to the atoms in the C2B3 faces of the carboranes but are slipped toward the more boron rich sides of the faces. Slippages in I and II are such that maximum differences in the Ga-C2B3 distances were 0.34 Angstrom in I and 0.45 Angstrom in II. The distortions were much larger in III and TV such that the carboranes are better described as being eta(3)- and eta(2)-bonded, respectively.