Trimetallic Chalcogenide Species: Synthesis, Structures, and Bonding

In an attempt to isolate boron-containing tri-niobium polychalcogenide species, we have carried out prolonged thermolysis reactions of [Cp*NbCl₄] (Cp* = ɳ⁵-C₅Me₅) with four equivalents of Li[BH₂E₃] (E = Se or S). In the case of the heavier chalcogen (Se), the reaction led to the isolation of the tri-niobium cubane-like cluster [(NbCp*)₃(μ₃-Se)₃(BH)(μ-Se)₃] (1) and the homocubane-like cluster [(NbCp*)₃(μ₃-Se)₃(μ-Se)₃(BH)(μ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se₃BH}^- ligand. A selenium atom is further introduced between boron and one of the selenium atoms of 1 to yield cluster 2. On the other hand, the reaction with the sulfur-containing borate adduct [LiBH₂S₃] afforded the trimetallic clusters [(NbCp*)₃(μ-S)₄{μ-S₂(BH)}] (3) and [(NbCp*)₃(μ-S)₄{μ-S₂(S)}] (4). Both clusters 3 and 4 have an Nb₃S₆ core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by ¹¹B{¹H}, ¹H, and ¹³C{¹H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb₃ framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.