Electrochemical deprotection of a substrate binding site in [Mo2(cp)2(µ-SMe)3(µ-Cl)](cp =η5-C5H5)via chloride-bridge opening. Kinetics of MeCN and ButNC binding at this site

Abstract

The access to a co-ordination site in the quadruply bridged complex [Mo₂(cp)₂(µ-SMe)₃(µ-Cl)](cp =η⁵-C₅H₅) has been found to be redox controlled. Electrochemical one-electron oxidation of the complex unlocks the chloride bridge but the radical cation retains the quadruply bridged geometry of the neutral parent as indicated by cyclic voltammetry and EPR spectroscopy. The chloride bridge opens up in the presence of a substrate (YZ = MeCN, Bu^tNC, Me₂C₆H₃NC or CO), leading to the formation of [Mo₂(cp)₂(µ-SMe)₃Cl(YZ)]˙⁺ derivatives. The site is sensitive to the electronic properties of the substrate, and kinetic studies of the substrate-binding step demonstrated that Bu^tNC reacts faster and is bound more tightly at [Mo₂(cp)₂(µ-SMe)₃Cl]˙⁺ than is MeCN. The reduction of [Mo₂(cp)₂(µ-SMe)₃Cl(YZ)]˙⁺ is reversible for YZ = CO and RNC (R = Bu^t or C₆H₃Me₂) whereas MeCN is lost on reduction. In this case the fact that the chloride ligand is still present at the neighbouring molybdenum centre allows regeneration of the parent complex via bridge reclosure. The reactivity of [Mo₂(cp)₂(µ-SMe)₃(µ-Cl)] in MeCN has also been investigated: instead of the bridge-opening process of the radical cation, the neutral parent loses the chloride bridge in MeCN; the resulting bis(acetonitrile) cation, [Mo₂(cp)₂(µ-SMe)₃(MeCN)₂]⁺, has been isolated and characterized.