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 Mo(IV)3 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 mu3-S ligands. In contrast the Mo(III)4 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 mu3-S ligands. While substitution on [Mo3CrS4(H2O)12]4+ exhibits an [H+]-1 dependence indicating a conjugate-base pathway (and greater-than-or-equal-to 2 H2O per Cr), no similar dependence was observed for [Mo4S4(H2O)612]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-degrees-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.