PREPARATION AND PROPERTIES OF THE HIGH-OXIDATION-STATE CUBOIDAL [MO4S4(H2O)12]6+ ION IN AQUEOUS ACIDIC SOLUTIONS

Oxidation of the green mixed-valence Mo(III)3Mo(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 (almost-equal-to 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 10(3)), 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(III)2Mo(IV)2 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.