Formation of metallaboranes from the group IV transition metals and pentaborane(9): crystal and molecular structure of [(Cp(2)Zr)(2)B(5)H(8)][B(11)H(14)].

The reactions between [(C(5)H(5))(2)MCl(2)] (where M = Ti, Zr, Hf) and Li[B(5)H(8)] in a variety of solvents have been investigated. In the case of Zr, a pale orange solid, mu-(Cp(2)ClZr)B(5)H(8) (1), is formed in 70% yield. Compound 1 exists as a B(5)H(9) cage with a Cp(2)ClZr moiety replacing a bridging H atom. The variable temperature NMR spectra of 1 reveal two fluxional processes, one (DeltaG(++) = 54 kJ mol(-1)) which renders a plane of symmetry in the molecule and a higher temperature one (DeltaG(++) = 48 kJ mol(-1)) which renders all the basal B atoms equivalent. Dynamic processes are suggested to account for these observations. Passage of a CH(2)Cl(2) solution of 1 through silica gel affords 2, [(Cp(2)Zr)(2)B(5)H(8)][B(11)H(14)], a yellow, air-stable, crystalline solid, in 14% yield. The cation in 2, [(Cp(2)Zr)(2)B(5)H(8)](+), consists of a distorted spiro[2.2]pentane-like B(5) moiety comprising two B(3) triangles sharing a naked boron vertex. The two triangles are twisted 73 degrees with respect to each other, and the two [Cp(2)Zr] groups bond in a trihapto arrangement to the two opposite B-B-B edges. Each exterior B-Zr edge is H-bridged, and the B atoms possess terminal hydrogens. Reactions of Cp(2)HfCl(2) with Li[B(5)H(8)] lead to the formation of the analogue of 2, [(Cp(2)Hf)(2)B(5)H(8)][B(11)H(14)] (3). The precursor to 3, that is, the Hf analogue of 1, is not observed. Reaction between Li[B(5)H(8)] and Cp(2)TiCl(2) afforded no identifiable products, but reaction with CpTiCl(3) resulted in cage coupling and the formation of B(10)H(14).