Electrochemical behaviour of cyclometallated gold(III) complexes. Evidence of transcyclometallation in the fate of electroreduced species

The electrochemical behaviour of a series of neutral and cationic N-C and N-N-C cyclometallated gold(III) species: ([Au(py(1))(Cl)(2)], Hpy(1) = 2-benzylpyridine; [Au(bipy(n))(Cl)][PF6], Hbipy(1) = 6-benzyl-2,2'-bipyridine, Hbipy(2) = 6-(1-methylbenzyl)-2,2'-bipyridine, Hbipy(3) = 6-(1,1-dimethylbenzyl)-2,2'-bipyridine, Hbipy(4) = 6-phenyl-2,2'-bipyridine) has been investigated in different solvent systems using cyclic voltammetry and controlled-potential coulometry. All the species considered show, in cyclic voltammetry, an irreversible one-electron reduction in the potential range -0.9/-1.2 V versus Fc(+/0), using a Pt electrode. On the other hand, exhaustive coulometries have shown the consumption of a number of Faraday per mole within the range 1-2. The very complex decomposition of the electrogenerated species affords elemental gold and significant amounts of different gold by-products. In some situations (e.g. reduction of [Au(bipy(4))(Cl)][PF6]), working with 0.1 mol dm(-3) Na[PF6], CH3C solvent system, it has been possible to characterise the most abundant gold(III) compound, by NMR, FAB-MS, and elemental analysis. The analytical and spectroscopic data provide evidence for the N-C bis-cyclometallated gold(III) derivative (e.g. [Au(bipy(4))(2)][PF6]); the result entails quite an unusual transcyclometallation process. The same compound can also be obtained, albeit in very minor yield, by chemical reduction of [Au(bipy(4))(Cl)][PF6]. (C) 2001 Elsevier Science B.V. All rights reserved.