Voltammetric Study of an Unusual Dimer/Monomer Redox System Involving a Chromium(III) Complex of a Tridentate CNN Ligand

Abstract

In an attempt to prepare an alkylidene complex of chromium, the metallacycle, was treated with 2,2′-bipyridine. Instead of producing the desired trimethylsilylmethylidine complex, a new complex - (pentamethylcyclopentadienyl)(2(2″,2″-dimethyl-2″-silapropyl)-1,2-dihydro[2,2′]bipyridine-1,3″-diyl)chromium(III), 1) with a methylene bridge between silicon and the carbon of the bidentate 2,2′-bipyridine ligand was formed. The modified bipyridine ligand contains one carbon and two nitrogen donor atoms (CNN ligand). When 1 was treated with oxidative dimerization occurred giving a dimer dication (2) linked at the carbon atoms of each 2,2′-bipyridine ligand. This dimer was isolated as the hexafluorophosphate salt. In the present work, the electrochemical behavior of 1 and 2 has been explored and it has been shown that anodic oxidation of 1, like the chemical oxidation, produces 2 by way of dimerization of an initially formed transient cation. The dimerization rate constant was and the equilibrium constant for dimerization must exceed Studies of the cathodic reduction of 2 show that it undergoes an overall two-electron reduction giving mainly 1 as product. The reduction apparently proceeds by one-electron reduction to a detectable intermediate monocation that dissociates to 1 and the cation of 1 which in turn is reduced to 1. It was also found that 2 undergoes an irreversible two-electron oxidation, the products of which were not identified. © 2001 The Electrochemical Society. All rights reserved.