Preparation and redox properties of the cuboidal molybdenum(III) aqua ion [Mo4S4(H2O)12]4+

Abstract

A procedure for the electrochemical preparation of the orange cuboidal Mo^III aqua ion [Mo₄S₄(H₂O)₁₂]⁴⁺(peak 378 nm; ε= 1 100 M^–1 cm^–1 per Mo₄) from [Mo₄S₄(H₂O)₁₂]⁵⁺(reduction potential 210 mV) is described. The product is air sensitive and at 25 °C 1 mM solutions are oxidised by O₂ to Mo₄S₄⁵⁺ within 5 min. With ClO₄^–(2 M HClO₄) there is a slow conversion of the aqua ion Mo₄S₄⁴⁺ to Mo₄S₄⁵⁺, t_½ca. 12 h at 25 °C. Kinetic studies on the 1 : 1 reductions of seven Co^III complexes have been carried out at 25 °C, I= 2.00 M (LiClO₄), and are independent of [H⁺] in the range 1.0–1.8 M. From linear log–log correlations of rate constants with those for the corresponding [V(H₂O)₆]²⁺ and [Ru(NH₃)₆]²⁺ reductions, it is concluded that the reactions with [Co^III(NH₃)₅X]ⁿ⁺(X = H₂O, Cl^–, Br^–, or I^–) as well as [Co(bipy)₃]³⁺ are outer sphere. The rate constant (0.27 s^–1) for [Co(NH₃)₅(O₂CCH₃)]²⁺ does not similarly correlate, and this reaction is believed to be inner sphere. The reaction with [Co(NH₃)₅F]²⁺ may also be in some part inner sphere. From the rate constant (460 M^–1 s^–1) with [Co(bipy)₃]³⁺(370 mV) as oxidant, a self-exchange rate constant for Mo₄S₄^4+/5+ of 21 M^–1 s^–1 is obtained using the Marcus equations.