Preparation and properties of the high-oxidation-state cuboidal [Mo4S4(H2O)12]6+ ion in aqueous acidic solutions

Abstract

Oxidation of the green mixed-valence Mo^III₃Mo^IV cuboidal ion [Mo₄S₄(H₂O)₁₂]⁵⁺ with a limited (2:1) excess of cis-[VO₂(H₂O)₄]⁺ in 2 M Hpts (Hpts = toluene-p-sulfonic acid) yields two red products. From Dowex cation-exchange chromatography, elution with 2M Hpts, these were identified as [Mo₄S₄(H₂O)₁₂]⁶⁺ and a less strongly held product with the same 6+ core but with lower (≈4+) overall charge. The spectrum of the latter approaches that of [Mo₄S₄(H₂O)₁₂]⁶⁺ as solution pts^– is replaced by ClO₄^– and co-ordinated pts^– is aquated (2–3 h). The reactivity of [Mo₄S₄(H₂O)₁₂]⁶⁺ was studied, including substitution and redox properties, and the tendency of the cube to fragment to the trinuclear molybdenum(IV) ion, [Mo₃S₄(H₂O)₉]⁴⁺, and (initially) monomeric Mo is noted. This property is more pronounced than with [Mo₄S₄(H₂O)₁₂]⁵⁺, and is attributed to the lower core electron count (10 e^–) and higher charge. Substitution of the 6+ ion (H₂O by NCS^–) is substantially faster than for the 4+ and 5+ cubes (factors of up to 10³), and involves in addition a conjugate-base pathway. A statistical factor of two is observed, consistent with two identical centres for substitution on the Mo^III₂Mo^IV₂ cluster. The reaction is monophasic and substitution at the Mo^IV is proposed, in which case there is no detectable substitution at the Mo^III. The stopped-flow reduction of [Mo₄S₄(H₂O)₁₂]⁶⁺ by [Ru(NH₃)₅(H₂O)]²⁺ is first order in the concentration of each reactant, and shows no unusual features. The second phase of reaction corresponds to reduction of [Mo₄S₄(H₂O)₁₂]⁵⁺ to [Mo₄S₄(H₂O)₁₂]⁴⁺. The first but not the second phase includes an [H⁺]^–1-dependent term.