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.