Metal-Rich Oxametallaboranes of Group 5 Metals: Synthesis and Structure of a Face-Fused μ₇‑Boride Cluster

Aerobic oxidation of metallaborane compounds is an unexplored field apart from the few reports on accidental oxidation leading to oxametallaboranes. An effective method for the synthesis of group 5 oxametallaboranes has been developed by the oxidation of [(Cp*M)₂(B₂H₆)₂] (M = Ta/Nb) (Cp* = η⁵-C₅Me₅). The reaction of [(Cp*M)₂(B₂H₆)₂] (M = Ta/Nb) with O₂ gas at room temperature yielded oxametallaboranes [(Cp*M)₂(B₄H₁₀O)] (for 1, M = Nb; for 2, M = Ta). Density functional theory calculations signify an increase in the HOMO–LUMO energy gap for 1 and 2 as compared to that for the parent metallaboranes, [(Cp*M)₂(B₂H₆)₂] (M = Ta/Nb). Reaction of 1 and 2 with [Ru₃(CO)₁₂] led to the isolation of fused metallaborane clusters [(Cp*Nb)₂(B₂H₄O)­{Ru­(CO)₂}₂­(B₂H₄)­{Ru­(CO)₃}₂{μ-H}₄] (3) and [(Cp*Ta)₂­(B₃H₄O)­{Ru­(CO)₂}₃­{μ₇-B}­{μ-CO}₂{μ-H}₄] (4). The structure of 3 may be considered as a fusion of five subunits [two tetrahedra (Td), two square pyramids (sqp), and one trigonal bipyramid (tbp)]. One of the key features of cluster 4 is the presence of a μ₇-boride atom that shares three cluster units (one monocapped trigonal prism and two Td). All the compounds have been characterized by mass spectrometry, infrared spectroscopy, and ¹H, ¹³C, and ¹¹B nuclear magnetic resonance spectroscopy, and the structural types were unequivocally established by crystallographic analysis of compounds 1, 3, and 4.