Substituent effect on the electrochemical reduction of tricarbonylchromium-tropylium tetrafluoroborate in acetonitrile

Abstract

The electrochemical reduction of the tricarbonylchromium alkyl-tropylium tetrafluoroborates (TCC-R−C 7H 6 +BF 4 −; R=Me, Et, Pr i and Bu t) and of the tricarbonylmolybdenum and tungsten tetrafluoroborates (TCM-C 7H 7 +BF 4 −; M=metal) was investigated in MeCN andMeCN+MeCOOH 50:50 V:V. The voltammetric and coulometric results suggest that the process involves the reversible transfer of two electrons to form the complexed cycloheptatrienyl anion. In MeCN, this intermediate couples with the unreduced cation to produce the bis-complexed bitropyl and the rate of dimerization is strongly affected by the steric size of the alkyl group. On the other hand, in MeCN MeCOOH the TCM-cycloheptatrienyl anion undergoes a competitive attack from H + and the starting cation toform TCM-cycloheptatriene and (TCM) 2-bitropyl respectively. The ratio monomer: dimer depends upon both the alkyl group and the nature of the metal.