Intramolecular boron-boron linkage between polyhedral cages in a commo-metallacarborane. Synthesis and structure of a fluxional metal-boron cluster, [.eta.5-C5(CH3)5]2Co3(CH3)4C4B8H7

Abstract

Abstract : The reaction of Li(+) C5(CH3)5(-), CoCl2, and Na(+)(CH3)2C2B4H5(-) in cold THF forms primarily closo-1,2,3-(eta 5-C5(CH3)5)Co(CH3)2C2B4H4 (I), the triple-decker complex 1,7,2,3-(eta 5-C5(CH3)5)2Co2(CH3)2C2B3H3 (II), a structurally novel complex, (eta 5-C5(CH3)5)2Co3(CH3)4C4B8H7 (III), and traces of other products, in a combined isolated yield of about 50%. Complexes I and II are C5(CH3)5 analogues of C5H5-substituted cobalta-carboranes reported earlier, but III has no previous counterpart. An X-ray diffraction study of III disclosed a structure consisting of two identical (eta 5-C5(CH3)5)Co(CH3)2C2B4H3 units face-coordinated to a third cobalt atom, with a direct B-B bond (1.758(5) A) between the two ligands; the linked boron atoms have terminal hydrogen atoms in the solid-state structure. The 'extra' hydrogen atom was not directly located, but its general position was indicated by a 3.5 deg fold in the molecule which creates a larger inter-ligand pocket on one side than the other. Possible mechanisms and stereochemical implications of the structure and fluxional behavior of III are presented. The chemistry of complex I resembles that of its C5H5-substituted counterpart; thus, degradation of I in basic CH3CN produces nito-1,2,3-(eta 5-C5(CH3)5)Co(CH3)2C2B3H5 in 93% yield.