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

Abstract

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)₃B₄H₄] 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)₂B₄H₅SFe₃(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 = 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.