Synthesis of metal-rich metallaborane clusters. Evidence for a mechanism involving fragment condensation

Abstract

A mechanistic hypothesis of metallaborane cluster build-up by the condensation of metal and boron containing fragments, which is supported by circumstantial evidence from previous work, suggests improved routes to the synthesis of ferraboranes. This work describes two new approaches consisting of the examination of two precursors with properties consistent with such a hypothesis. The first precursor, a neutral mononuclear dimethyl sulfide substituted iron tetracarbonyl, Fe(CO) 4SMe 2, is a new compound and the high yield synthesis and structural characterization of it are reported. This compound provides a better route to the ferraborane Fe 2(CO) 6B 2H 6 than those presently known and is an isolatable, alternate source of the Fe(CO) 4 fragment. The second precursor, Fe(CO) 3(cco) 2, where cco is η 2- cis-cyclooctene, is a known, ready source of the Fe(CO) 3 fragment. The reaction of Fe(CO) 3(cco) 2 with BH 3 sources at low temperatures results in good yields of known ferraboranes and a product distribution that depends primarily on the ratio of boron to iron in the reactants. Both of these results support a mechanism for metallaborane cluster formation involving rapid metal carbonyl fragment condensation as a principal mechanistic component.