Hypoelectronic metallaboranes: Synthesis, structural characterization and electronic structures of metal-rich cobaltaboranes

Reaction of [Cp*CoCl]2 (Cp* = η5-C5Me5) with [LiBH4· THF] in toluene at -70 C, followed by thermolysis with 2-mercaptobenzothiazole (C7H5NS2) in boiling toluene led to the isolation of a range of cobaltaborane clusters, [(Cp*Co) 2B7H6OMe], 1; [(Cp*Co) 3B8H7R], 2a, b (2a: R = H; 2b: R = Me); [(Cp*Co)3B8H8S], 3 and [(Cp*Co)2B4H4RR′], 4a-d (4a: R, R′ = H; 4b: R = Me, R′ = H; 4c: R = H, R′ = Me and 4d: R, R′ = Me). In parallel to the formation of compounds 1-4, the reaction also yielded known [(Cp*Co)3B4H 4] in good yield. Compound 1 may be considered as 9-vertex hypoelectronic cluster with C1 symmetry, where cobalt atoms occupy the degree 5 vertices. All the dicobaltaboranes 4a-d contains two μ3-H protons and found to be very reactive. As a result, one of them (4a) when reacted with Fe2(CO)9 and sulfur powder yielded, almost immediately, [(Cp*Co)2B 4H5SFe3(CO)9], 5 and [(Cp *Co)2B3H3(μ-CO)Fe(CO) 3], 6. All the new compounds have been characterized in solution by mass, 1H, 11B, 13C NMR spectroscopy and elemental analysis. The structural types were unequivocally established by X-ray crystallographic analysis of compounds 1-6. Density functional theory (DFT) calculations on the model compounds 1′ and 2′ (1′, and 2′ are the Cp analog of 1, and 2a respectively, Cp = C5H 5) yield geometries in agreement with the structure determinations. The existence of large HOMO-LUMO gap of these molecules rationalizes the isocloso description for 2a. Bonding patterns in the structure have been analyzed on the grounds of DFT calculations. © 2013 Elsevier B.V. All rights reserved.