Structural diversity of mixed polypnictogen complexes: dicationic E2E'2 (E ≠ E' = P, As, Sb, Bi) chains, cycles and cages stabilized by transition metals

Abstract

The reactivity of the tetrahedral dipnictogen complexes [{CpMo(CO)₂}₂(μ,η²:η²-EE')] (E, E' = P, As, Sb, Bi; "Mo₂EE'") towards different one-electron oxidation agents is reported. Oxidation with [Thia][TEF] (Thia⁺ = C₁₂H₈S₂ ⁺; TEF^- = Al{OC(CF₃)₃}₄ ^-) leads to the selective formation of the radical monocations [Mo₂EE']˙⁺, which immediately dimerize to the unprecedented dicationic E₂E'₂ ligand complexes [{CpMo(CO)₂}₄(μ₄,η²:η²:η²:η²-E'EEE')]²⁺ via E-E bond formation. Single crystal X-ray diffraction revealed that, in the case of Mo₂PAs and Mo₂PSb, P-P bond formation occurs yielding zigzag E₂P₂ (E = As (1), Sb (2)) chains, whereas Mo₂SbBi forms a Sb₂Bi₂ (5) cage, Mo₂AsSb an unprecedented As₂Sb₂ unit representing an intermediate stage between a chain- and a cage-type structure, and Mo₂AsBi a novel planar As₂Bi₂ (4a) cycle. Therefore, 1-5 bear the first substituent-free, dicationic hetero-E₄ ligands, stabilized by transition metal fragments. Furthermore, in the case of Mo₂AsSb, the exchange of the counterion causes changes in the molecular structure yielding an unusual, cyclic As₂Sb₂ ligand. The experimental results are corroborated by DFT calculations.