Solution properties of the cuboidal mixed-metal cluster [Mo3NiS4(H2O)10]4+ including complexing (at Ni) and redox properties

Abstract

The green-blue complex [Mo3NiS4(H2O)10]4+ has been prepared by NaBH4 reduction of a 1:10 solution of [Mo3S4(H2O)9]4+ and NiCl2·6H2O in 0.5 M HCl, and purified by Dowex 50W-X2 cation-exchange chromatography. Solutions (0.1–2.0 mM) in 2 M HClO4 obey Beer's law, consistent with the single cube formulation. Complexation equilibration reactions, Mo3NiS44++ X–⇌ Mo3NiS4X3+, studied by stopped-flow spectrophotometry (25°C), give forward and back rate constants for X–= Cl– of kf= 9.4 M–1 s–1, kb= 0.097 s–1, and for X–= NCS– of kf= 45 M–1 s–1, kb= 0.062 s–1. Complexation, assigned to substitution at the tetrahedrally ligated (and metal–metal bonded) nickel centre, is unusually slow, and slower than substitution on [Ni(H2O)6]2+. Redox reactions, e.g. Mo3NiS44++ 2CoIII→ Mo3S44++ Ni2++ 2CoII, with [Co(dipic)2]–(dipic = pyridine-2,6-dicarboxylate) and [Fe(H2O)6]3+ as oxidants, have also been studied. Rate laws are in both cases first order in the concentration of each reactant. Whereas at 25°C the [Co(dipic)2]– reaction is [H+]-independent and outer sphere (0.35 M–1 s–1), the reaction with [Fe(H2O)6]3+ gives a dependence ka+kb[H+]–1, with ka= 5.4 × 10–4 M–1 s–1 and kb= 5.3 × 10–4 s–1, assigned to outer- and inner-sphere mechanisms respectively. All studies were in aqueous perchloric acid solutions, I= 2.00 M (LiClO4).