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*NbCl(4)] (Cp* = ɳ(5)-C(5)Me(5)) with four equivalents of Li[BH(2)E(3)] (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)(μ(3)-Se)(3)(BH)(μ-Se)(3)] (1) and the homocubane-like cluster [(NbCp*)(3)(μ(3)-Se)(3)(μ-Se)(3)(BH)(μ-Se)] (2). Interestingly, the tri-niobium framework of 1 stabilizes a selenaborate {Se(3)BH}(−) 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 [LiBH(2)S(3)] afforded the trimetallic clusters [(NbCp*)(3)(μ-S)(4){μ-S(2)(BH)}] (3) and [(NbCp*)(3)(μ-S)(4){μ-S(2)(S)}] (4). Both clusters 3 and 4 have an Nb(3)S(6) core, which further stabilizes {BH} and mono-sulfur units, respectively, through bi-chalcogen coordination. All of these species were characterized by (11)B{(1)H}, (1)H, and (13)C{(1)H} NMR spectroscopy, mass spectrometry, infrared (IR) spectroscopy, and single-crystal X-ray crystallography. Moreover, theoretical investigations revealed that the triangular Nb(3) framework is aromatic in nature and plays a vital role in the stabilization of the borate, borane, and chalcogen units.