Voltammetric study of an unusual dimer/monomer redox system involving a chromium(III) complex of a tridentate CNN ligand

In an attempt to prepare an alkylidene complex of chromium, the metallacycl e, Cp*Cr(L)(CH2)(2)Si(CH3)(2)(L=TEF), was treated with 2,2'-bipyridine. Ins tead of producing the desired trimethylsilylmethylidine complex, a new comp lex (eta (5) (pentamethylcyclopentadienyl) (2(2",2"-dimethyl-2"-silapropyl) - 1,2-dihydro[2,2']bipyridine- I,3"-diyl)chromium(III), 1) with a methylene bridge between silicon and the C-2 carbon of the bidentate 2,2'-bippridine ligand was formed. The modified bipyridine ligand contains one carbon and two nitrogen donor atoms (CNN ligand). When 1 was treated with [Cp2Fe](+)[P F6](-), oxidative dimerization occurred giving a dimer dication (2) linked at the C-3 carbon atoms of each 2,2'-bipyridine ligand. This dimer was isol ated as the hexafluorophosphate salt. In the present work, the electrochemi cal 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 dimeriza tion of an initially formed transient cation. The dimerization rare constan t was 2 X 10(4) M-1 s(-1) and the equilibrium constant for dimerization mus t exceed 5 X 10(6) M-1. Studies of the cathodic reduction of 2 show that it undergoes an overall two-electron reduction giving mainly 1 as product. Th e reduction apparently proceeds by one-electron reduction to a detectable i ntermediate monocation that dissociates to 1 and the cation of 1 which in t urn is reduced to 1. It was also found that 2 undergoes an irreversible two -electron oxidation, the products of which were not identified. (C) 2001 Th e Electrochemical Society;