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(py1)(Cl)2], Hpy1=2-benzylpyridine; [Au(bipyn)(Cl)][PF6], Hbipy1=6-benzyl-2,2′-bipyridine, Hbipy2=6-(1-methylbenzyl)-2,2′-bipyridine, Hbipy3=6-(1,1-dimethylbenzyl)-2,2′-bipyridine, Hbipy4=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(bipy4)(Cl)][PF6]), working with 0.1 mol dm−3 Na[PF6], CH3CN 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(bipy4)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(bipy4)(Cl)][PF6].