Preparation and properties of the heterometallic cube [Mo3CdS4(H2O)12]4+ and the reaction with H+

Abstract

The orange–brown cadmium-containing heterometallic cube [Mo₃CdS₄(H₂O)₁₂]⁴⁺ has been prepared by reacting [Mo₃S₄(H₂O)₉]⁴⁺ in 2.0 M HCl with Cd²⁺ in the presence of H₃PO₂ as reductant for ∼1 h at 20 °C. Alternatively, it can be obtained by heating a solution of [Mo₃S₄(H₂O)₉]⁴⁺ in 0.5 M Hpts (pts⁻ = p-toluenesulfonate) with Cd metal for ∼1 h at 70 °C. The cube reacts with H⁺ in a process which is inhibited by the replacement of Cl⁻ with pts⁻. Purification by elution from a Dowex cation-exchange column was carried out at low [H⁺] using a mixture of 2.9 M Lipts and 0.1 M Hpts. On titration with [Co(dipic)₂]⁻, the incomplete cube [Mo₃S₄(H₂O)₉]⁴⁺ is reformed, and two equivalents of Co^III oxidant are consumed per cube, consistent with the equation Mo₃CdS₄⁴⁺ + 2Co^III → Mo₃S₄⁴⁺ + Cd²⁺ + 2Co^II. Reaction with [H⁺] in the range 0.5–1.5 M, I = 2.0 M (Lipts) gives H₂, which was determined quantitatively by gas chromatography. The kinetics of the decay Mo₃CdS₄⁴⁺ + 2H⁺ → Mo₃S₄⁴⁺ + Cd²⁺ + H₂ give a rate law k_H[Mo₃CdS₄⁴⁺][H⁺]², with k_H = 4.6 × 10⁻⁴ M⁻² s⁻¹ at 25 °C, I = 2.00 M (Lipts). In 1 M HCl, I = 2.0 M (Lipts), the half-life of 0.3 mM solutions is ∼5 min.