Trimetallic Cubane-Type Clusters: Transition-Metal Variation as a Probe of the Roots of Hypoelectronic Metallaheteroboranes

In an effort to synthesize chalcogen-rich metallaheteroborane clusters of group 5 metals, thermolysis of [Cp*TaCl 4 ] (Cp* = η 5 -C 5 Me 5 ) with thioborate ligand Li[BH 2 S 3 ] was carried out, affording trimetallic clusters [(Cp*Ta) 3 (μ-S) 3 (μ 3 -S) 3 B(R)], 1-3 (1, R = H; 2, R = SH; and 3, R = Cl). Clusters 1-3 are illustrative examples of cubane-type organotantalum sulfido clusters in which one of the vertices of the cubane is missing. In parallel to the formation of 1-3, the reaction also yielded tetrametallic sulfido cluster [(Cp*Ta) 4 (μ-S) 6 (μ 3 -S)(μ 4 -O)], 6, having an adamantane core structure. Compound 6 is one of the rarest examples containing the μ 4 -oxo unit with a heavier early transition metal, i.e., tantalum. In an effort to isolate selenium analogues of clusters 1-3, we have isolated the trimetallic cluster [(Cp*Ta) 3 (μ-Se) 3 (μ 3 -Se) 3 B(H)], 4, from the thermolytic reaction of [Cp*TaCl 4 ] and Li[BH 2 Se 3 ]. In contrast, the thermolysis of [Cp*TaCl 4 ] with Li[BH 2 Te 3 ] under the same reaction conditions yielded tantalum telluride complex [(Cp*Ta) 2 (μ-Te) 2 ], 5. Compounds 1-4 are hypo-electronic clusters with an electron count of 50 cluster valence electrons. All these compounds have been characterized by 1 H, 11 B{ 1 H}, and 13 C{ 1 H} NMR spectroscopy; infrared spectroscopy; mass spectrometry; and single-crystal X-ray crystallography. The density functional theory calculations were also carried out to provide insight into the bonding and electronic structures of these molecules.