Preparation, structure and properties of three [MoxW4–xS4(H2O)12]5+ (x = 1–3) and [MoW3Se4(H2O)12]5+ cuboidal complexes alongside [Mo4S4(H2O)12]5+ and [Mo4Se4(H2O)12]5+

Abstract

The preparation of [MoW₃S₄(H₂O)₁₂]⁵⁺, [Mo₂W₂S₄(H₂O)₁₂]⁵⁺, [Mo₃WS₄(H₂O)₁₂]⁵⁺ and [MoW₃Se₄(H₂O)₁₂]⁵⁺ from trinuclear incomplete cuboidal complexes [W₃S₄(H₂O)₉]⁴⁺, [MoW₂S₄(H₂O)₉]⁴⁺, [Mo₂WS₄(H₂O)₉]⁴⁺ and [W₃Se₄(H₂O)₉]⁴⁺ respectively has been achieved by reaction with [Mo₂Cl₈]^4–. The structures of the 5+ cube [MoW₃S₄(H₂O)₁₂][pts]₅·Hpts·16H₂O (pts^– = p-toluenesulfonate) and [Me₂NH₂]₆[MoW₃S₄(NCS)₁₂]·0.5H₂O (6+ cube) have been determined by X-ray diffraction. Reversible behaviour is observed in cyclic voltammetry on the 5+ cubes, and reduction potentials (E°′ vs. NHE) for the 6+/5+ and 5+/4+ couples have been determined. The cubes are more strongly reducing as the number of W atoms is increased with E°^′/mV values for [MoW₃S₄(H₂O)₁₂]^6+/5+ (258), [MoW₃S₄(H₂O)₁₂]^5+/4+ (–395) significantly smaller than values previously reported for [Mo₄S₄(H₂O)₁₂]^6+/5+ (860) and [Mo₄S₄(H₂O)₁₂]^5+/4+ (210). Peaks λ/nm (ε/M^–1 cm^–1 per cube) from UV/VIS/NIR spectra in 2.0 M Hpts shift from 635(435), 1100(122) for [Mo₄S₄(H₂O)₁₂]⁵⁺ to higher energy transitions at 522(660), 850(200) for [MoW₃S₄(H₂O)₁₂]⁵⁺. Oxidation of the 5+ cubes with for example [Fe(H₂O)₆]³⁺ gives first the 6+ cube which then decays with fragmentation to trinuclear products always with loss of W. While oxidation to the 6+ cube depends on reduction potentials, a different order is observed and other factors are important in the decay process.