ORGANOTRANSITION-METAL METALLACARBORANES .28. CONTROLLED SUBSTITUTION AT CAGE CARBON-ATOMS IN COC2B4 CLUSTERS - LINKAGE OF BIS(COBALTACARBORANE) SANDWICH COMPLEXES VIA CARBON-BOUND BRIDGING GROUPS

The closo-cobaltacarboranes Cp*Co[(Me3Si)2C2B4H4] (Cp* = eta-5-C5Me5) and (eta-5-C5Ph5)Co[(Me3Si)2C2B4H4] were catalytically desilylated by tetrabutylammonium fluoride in wet THF at room temperature, generating the respective (C5R5)Co(C2B4H6) parent complexes (6a, R = Me; 6b, R = Ph). By adjustment of reaction conditions, the monosilyl species Cp*Co[(Me3Si)C2B4H5] (5) was also prepared. The C-H protons in 6a are less acidic toward nucleophiles than are those in 1,2- or 1,7-C2B10H12 (o- or m-carborane); however, reaction of 6a with butyllithium in THF occurred readily to produce Li+Cp*Co(C2B4H5)-, which in turn can be treated with alkyl halides to give mono- or di-C-alkylated derivatives. This was demonstrated by the preparation of the C-ethyl and C-ethyl-C'-benzyl derivatives of 6a and a C-chloro derivative of nido-Cp*CO(C2B3H7). Base-induced removal of the apex BH unit (decapitation) in the closo clusters generated open-faced nido-Cp*Co(RR'C2B3H5) complexes (R, R' = H, alkyl, arylalkyl). Halogenation of the nido compounds afforded the corresponding B(5)-Cl derivatives, which are projected as precursors to tetradecker sandwich complexes via previously described stacking reactions. The application of this chemistry to the synthesis of multicluster systems linked via framework carbons was explored by reactions of 5 and 6a with dibromomethane or o-, m-, or p-dibromoxylcne, which afforded respectively CH2-bridged and xylyl-bridged bis(cobaltacarboranyl) products. Double-decapitation, deprotonation, and metalation of one of the p-xylyl-linked species gave a linked bis(triple-decker sandwich) complex.