Metallaboranes of the early transition metals: direct synthesis and characterization of [{(η⁵-C₅Me₅)Ta}₂ B_n H_m] (n =4, m =10; n =5, m =11), [{(η⁵-C₅Me₅ )Ta}₂ B₅H₁₀(C₆H₄CH₃)], and [{(η⁵-C₅Me₅)TaCl}₂B₅H₁₁]

Abstract

Reaction of [Cp*TaCl₄] (Cp*= η⁵-C₅Me₅) with a sixfold excess of LiBH₄⋅thf followed by BH₃⋅thf in toluene at 100 °C led to the isolation of hydrogen-rich metallaboranes [(Cp*Ta)₂B₄H₁₀] (1), [(Cp*Ta)₂B₅H₁₁] (2), [(Cp*Ta)₂B₅H₁₀(C₆H₄CH₃)] (3), and [(Cp*TaCl)₂B₅H₁₁] (4) in modest yield. Compounds 1–3 are air- and moisture-sensitive but 4 is reasonably stable in air. Their structures are predicted by the electron-counting rules to be a bicapped tetrahedron (1), bicapped trigonal bipyramids (2, 3), and a nido structure based on a closo dodecahedron (4). Yellow tantalaborane 1 has a nido geometry with C_2v symmetry and is isostructural with [(Cp*M)₂B_/4H₈] (M=Cr and Re); whereas 2 and 3 are C_3v-symmetric and isostructural with [(Cp*M)₂B₅H₉] (M=Cr, Mo, W) and [(Cp*ReH)₂B₅Cl₅]. The most remarkable feature of 4 is the presence of a hydride ligand bridging the ditantalum center to form a symmetrical tantalaborane cluster with a long Ta[BOND]Ta bond (3.22 Å). Cluster 4 is a rare example of electronically unsaturated metallaborane containing four TaHB bonds. All these new metallaboranes have been characterized by mass spectrometry, ¹H, ¹¹B, and ¹³C NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of 1–4.