Electrochemistry of dimeric organopalladium(II) complexes containing bridging [pyridin-2-yl(phenyl)methyl-C,N]− and [bis(pyridin-2-yl)phenylmethyl-C,N,N′]− groups

Abstract

The electrochemistry of the palladium(II) dimers [Pd(CHPhpy)(L)Cl]2 (L = 4-methylpyridine (4-Mepy) (1a), 3,4-dimethylpyridine (3,4-Me2py) (1b), 3,5-dimethylpyridine (3,5-Me2py) (1c), 2-benzylpyridine (2-Bzpy) (1d) and [Pd(CPhpy2)Cl]2 (2) has been investigated in dichloromethane and acetonitrile. Under cyclic voltammetric conditions, complexes 1 were found to undergo a chemically reversible one electron oxidation at moderate scan rates in dichloromethane and fast scan rates in acetonitrile. The reversible potential of this process is essentially solvent independent. A second, solvent dependent, irreversible one electron oxidation is observed at more positive potentials. The following equation summarises the short time domain data: [Pd(II)Pd(II)][Pd(III)Pd(II)]+[Pd(III)Pd(III)]2+[L-Pd(III)Pd(III)-L]n+→Products In the presence of a coordinating ligand, the first oxidation becomes a two electron process on the synthetic timescale presumably leading to a [Pd(III)Pd(III)] dimer with a metal-metal interaction while the coordinating ligand occupies the vacant axial site on each palladium atom. Thus, long time domain electrochemical experiments occur via a different mechanism. For the formation of both one electron and two electron oxidation products, it is possible that oxidation of the ligand rather than metal centres has occurred. If Pd(III) centres are formed then metal-metal bond formation may occur to give bond orders of 0.5 for [Pd(III)Pd(II)]+ and 1.0 for [Pd(III)Pd(III)]2+. Complex 2 exhibits essentially the same electrochemical behaviour as complexes 1 except that the one electron oxidative product [Pd(III)Pd(II)]+ is more stable. Thus, evidence for oxidation state +III for organopalladium complexes has been obtained, but no further oxidation to Pd(IV) was observed.