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* = eta(5)-C5Me5). The reaction of [(Cp*M)(2)(B2H6)(2)] (M = Ta/Nb) with 02 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 [Ru-3(CO)(12)] led to the isolation of fused metallaborane clusters [(Cp*Nb)(2)(B2H40){Ru(CO)(2)}(2)(B2H4){Ru(CPO)(3)}(2){mu-H}(4)] (3) and [(Cp*Ta)(2)(B3H4O){Ru(CO)(2)}(3){mu(7)-B}{mu-CO}(2){y-H-4}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 mu(7)-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 H-1, C-13, and B-11 nuclear magnetic resonance spectroscopy, and the structural types were unequivocally established by crystallographic analysis of compounds 1, 3, and 4.