Preparation and properties of the heterometallic cuboidal complex [Mo3CrS4(H2O)12]4+: comparisons with [Mo4S4(H2O)12]4+

A Cr-containing heterometal cuboidal cluster has been prepared for the first time by the reaction of [Cr(H2O)6]2+ with the MoIV3 incomplete cuboidal [Mo3S4(H2O)9]4+ ion. The product has a formula [Mo3CrS4(H2O)12]4+ analogous to [Mo4S4(H2O)12]4+, where the stoichiometries and the products for the hexaaquairon(III) and [Co(dipic)2]–(dipic = pyridine-2,6-dicarboxylate) oxidations are consistent with the 4+ overall charge. Stopped-flow complexation studies with NCS– are assigned as 1:1 NCS– for H2O substitution at the Cr. From the behaviour observed it is concluded that the Cr is in oxidation state III, and is labilised by the three µ3-S ligands. In contrast the MoIII4 ion [Mo4S4(H2O)12]4+ is substituted at a much slower rate, with rate constants similar to those for [Mo(H2O)6]3+, and with no apparent influence of the µ3-S ligands. While substitution on [Mo3CrS4(H2O)12]4+ exhibits an [H+]–1 dependence indicating a conjugate-base pathway (and 2 H2O per Cr), no similar dependence was observed for [Mo4S4(H2O)12]4+. The behaviour thus parallels substitution on [Cr(H2O)6]3+(3d3) and [Mo(H2O)6]3+(4d3) where in the molybdenum case no [H+]–1 dependence is observed. Kinetic studies (25 °C) on the hexaaquairon(III) and [Co(dipic)2]– oxidations of [Mo3CrS4(H2O)12]4+ to give [Mo3S4(H2O)9]4+ and [Cr(H2O)6]3+, and of [Mo4S4(H2O)12]4+ to give [Mo4S4(H2O)12]5+ have also been made.