Chemistry of<i>C</i>-Trimethylsilyl-Substituted Heterocarboranes. 22. Synthetic, Spectroscopic, Structural, and Bonding Studies on Half- and Full-Sandwich Gallacarboranes of 2,3- and 2,4-C₂B₄Carborane Ligand Systems

Abstract

The reaction of the TMEDA-solvated group 1 salts of the carborane dianions [2, n -(SiMe 3 ) 2 -2, n -C 2 B 4 H 4 ] 2 - ( n = 3, 4) with GaCl 3 in a 2:1 carborane-to-GaCl 3 molar ratio gave the corresponding full-sandwich gallacarboranes [2,2‘, n, n ‘-(SiMe 3 ) 4 -1,1‘- commo -Ga(1,2, n -GaC 2 B 4 H 4 ) 2 ] - ( n = 3 ( I ), 4 ( II )) in 36% and 38% yields, respectively. Lowering the carborane-to-GaCl 3 molar ratio to 1:1 produced the half-sandwich chlorogallacarboranes 1-(TMEDA)-1-Cl-2, n -(SiMe 3 ) 2 -1,2, n -GaC 2 B 4 H 4 ( n = 3 ( III ), 4 ( IV )) in 51% and 41% yields, respectively. Compounds I − IV were characterized on the basis of 1 H, 11 B, and 13 C 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 C 2 B 3 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−C 2 B 3 distances were 0.34 Å in I and 0.45 Å in II . The distortions were much larger in III and IV such that the carboranes are better described as being η 3 - and η 2 -bonded, respectively.