CHEMISTRY OF C-TRIMETHYLSILYL-SUBSTITUTED HETEROCARBORANES .10. SYNTHESES, STRUCTURES, AND PROPERTIES OF ANIONIC CHROMIUM(III) AND NEUTRAL CHROMIUM(IV) SANDWICHED METALLACARBORANE COMPLEXES (1,1'-COMMO-CR[2-(SIME3)-3-(R)-2,3-C2B4H4]2)- AND 1,1'-COMMO-CR[2,3-(SIME3)2-2,3-C2B4H4]2 (R = SIME3, ME, AND H)

The reaction between CrCl3 and the THF-solvated double salt Na+Li+2-(SiMe3)-3-(R)-2,3-C2B4H4]2-[R = SiMe3 (I), Me (II), and H (III)] in a molar ratio of 1:2 in benzene, followed by extraction and crystallization of the product from a solution of benzene and THF or TMEDA, produces the sandwiched paramagnetic species Li(THF)4{1,1'-commo-Cr[2,3-(SiMe3)2-2,3-C2B4H4]2} (IV), Li(TMEDA)2{1,1'-commo-Cr[2-(SiMe3)-3-(Me)-2,3-C2B4H4]2} (V), and Li(TMEDA)2{1,1'-commo-Cr[2-(SiMe3)-2,3-C2B4H5]2} (VI) as bright red-orange and air-sensitive crystalline solids in 77, 45, and 64 % yields, respectively. The structural assignments of IV-VI, which were made in part by IR spectra and FAB mass spectral analysis, were confirmed by single-crystal X-ray diffraction studies. The structures reveal that the chromacarborane complexes are all ionic species in which the chromium metal is sandwiched by the two carborane ligands with the slight slippage of the metal toward the cage carbons. Complexes IV and VI crystallize in the monoclinic space group C21c and orthorhombic space group Pccn with a = 19.276 (8) and 11.176 (5) angstrom, b = 14.662 (9) and 14.838 (6) angstrom, c = 18.085 (9) and 23.019 (9) angstrom, beta = 100.53 (3) and 90.00', V = 5025 (4) and 3817 (3) angstrom3, and Z = 4 and 4, respectively. The final refinements of IV and VI converged at R = 0.081 and 0.069 and R(w) = 0.110 and 0.059, respectively. Magnetic susceptibility and EPR spectra of IV indicate the presence of high-spin Cr(III) complexes. The chemical oxidation of IV with PbCl2 produces the novel, diamagnetic, and neutral Cr(IV) sandwich complex 1,1'-commo-Cr[2,3-(SiMe3)2-2,3-C2B4H4]2 (VII), as a dark-red and air-sensitive crystalline solid in 63% yield. Compound VII was characterized on the basis of H-1, B-11, and C-13 NMR spectra, IR and mass spectra, magnetic susceptibility, and also by X-ray diffraction. Compound VII crystallizes in the monoclinic space group P2(1)/n with the following unit cell parameters: a = 9.851 (3) angstrom, b = 13.813 (5) angstrom, c = 11.195 (5) angstrom, beta = 95.66 (3)-degrees, V = 1516 (1) angstrom3, and Z = 4. Full-matrix least-squares refinements of VII converged at R = 0.047, and R. = 0.068 for 1350 observed reflections.