Fine tuning of metallaborane geometries: chemistry of metallaboranes of early transition metals derived from metal halides and monoborane reagents

Abstract

Reaction of [Cp_nMCl_4−x] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl₄] (Cp=η⁵-C₅H₅, Cp*=η⁵-C₅Me₅), with [LiBH₄⋅thf] at −70 °C followed by thermolysis at 85 °C in the presence of [BH₃⋅thf] yielded the hydrogen-rich metallaboranes [(CpM)₂ (B₂H₆)₂] (1: M=V; 2: M=Nb) and [(Cp*Ta)₂(B₂H₆)₂] (3) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal–metal bond bridged by two hexahydroborate (B₂H₆) groups forming a symmetrical complex. Addition of [BH₃⋅thf] to 3 results in formation of a metallaborane [(Cp*Ta)₂B₄H₈ (μ-BH₄)] (4) containing a tetrahydroborate ligand, [BH₄]−, bound exo to the bicapped tetrahedral cage [(Cp*Ta)₂B₄H₈] by two Ta-H-B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B—H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, ¹H, ¹¹B, and ¹³C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1–4.