Synthesis, characterization and crystal structure analysis of cobaltaborane and cobaltaheteroborane clusters

Abstract

Cluster expansion reactions of cobaltaboranes were carried out using mono metal-carbonyls, metal halides and dichalcogenide ligands. Thermolysis of an in situ generated intermediate, obtained from the reaction of [Cp*CoCl]₂ (Cp* = C₅Me₅) and [LiBH₄·thf], with three equivalents of [Mo(CO)₃(CH₃CN)₃] followed by the reaction with methyl iodide yielded isocloso-[(Cp*Co)₃B₆H₇Co(CO)₂] (1) and closo-[(Cp*Co)₂B₂H₅Mo₂(CO)₆I] (2). Cluster 1 is ascribed to the isocloso structure based on a 10-vertex bicapped square antiprism geometry. In a similar manner, the reaction of [Cp*CoCl]₂ with [LiBH₄·thf] and the dichalcogenide ligand RS–SR (R = 1-OH-2,6-(tBu)₂-C₆H₂) yielded nido cluster [(Cp*Co)₂B₂H₂S₂] (3). In parallel with the formation of the compounds 1–3, these reactions also yielded known cobaltaboranes [(Cp*Co)₂B₄H₆] (4) and [(Cp*Co)₃B₄H₄] in good yields. After the isolation of compound 4 in good yield, we verified its reactivity with PtBr₂, which yielded closo-[(Cp*Co)₂B₄H₂Br₄] (5). To the best of our knowledge this is the second perhalogenated metallaborane cluster which has been recognized. All the new compounds were characterized by elemental analysis, IR, ¹H, ¹¹B and ¹³C NMR spectroscopy, and the geometric structures were unequivocally established by the X-ray diffraction analysis of compounds 1, 2, 3 and 5. Geometries obtained from the electronic structure calculations employing density functional theory (DFT) are in close agreement with the solid state X-ray structures. In addition, we analyzed the variation in the stability of the model compounds 1′ (1′: Cp analogue of 1, Cp = C₅H₅), [(CpCo)₄B₆H₆] (1a) and [(CpRh)₄B₆H₆] (1b).