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

Reaction of [Cp*CoCl](2) (Cp* = eta(5)-C5Me5) with [LiBH4 center dot THF] in toluene at -70 degrees C, followed by thermolysis with 2-mercaptobenzothiazole (C7H5NS2) in boiling toluene led to the isolation of a range of cobaltaborane clusters, [(Cp*Co)(2)B7H6OMe], 1; [(Cp*Co)(3)B8H7R], 2a, b (2a: R = H; 2b: R = Me); [(Cp*Co)(3)B8H8S], 3 and [(Cp*Co)(2)B4H4RR'], 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)(3)B4H4] in good yield. Compound 1 may be considered as 9-vertex hypoelectronic cluster with C-1 symmetry, where cobalt atoms occupy the degree 5 vertices. All the dicobaltaboranes 4a-d contains two mu(3)-H protons and found to be very reactive. As a result, one of them (4a) when reacted with Fe-2(CO)(9) and sulfur powder yielded, almost immediately, [(Cp*Co)(2)B4H5SFe3(CO)(9)](,) 5 and [(Cp*Co)(2)B3H3(mu-CO)Fe(CO)(3)], 6. All the new compounds have been characterized in solution by mass, H-1, B-11, C-13 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 = C5H5) 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. (C) 2013 Elsevier B. V. All rights reserved.