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 MoIII 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+, t½ca. 12 h at 25 °C. Kinetic studies on the 1 : 1 reductions of seven CoIII 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 [CoIII(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 Mo4S44+/5+ of 21 M–1 s–1 is obtained using the Marcus equations.