Cyclic Voltammetry of Palladium(II) Complexes with Tridentate Arsine Ligands. Separation of the Two Single-Electron Transfer Steps of the Pd(II) ↔ Pd(0) Interconversion Based on Thermodynamic and Kinetic Discrimination

A detailed electrochemical study of the reduction of the five-coordinate complexes [Pd(mtas)₂]²⁺ and [Pd(ptas)₂]²⁺ (mtas = bis(2-(dimethylarsino)phenyl)methylarsine; ptas = bis(2-(dimethylarsino)phenyl)phenylarsine) has been undertaken in acetonitrile and dichloromethane. When cyclic voltammetry is carried out in acetonitrile at room temperature using conventional scan rates, the main voltammetric feature is a single apparently quasireversible two-electron reduction step. A small peak due to reduction of the Pd(I) dimer [Pd₂(L₃)₂]²⁺ (L₃ = mtas, ptas) formed in a follow-up reaction is also observed. At lower temperatures and faster scan rates, splitting of the two-electron reduction and associated oxidation peak occurs, enabling the thermodynamics and kinetics of the individual one-electron steps to be examined. The intermediate monomeric Pd(I) species are thermodynamically unstable with respect to disproportionation, but slow (relative to the experimental time scale) electrode kinetics for the Pd(I/0) couples and slow kinetics for the disproportionation reaction enable separation of the responses for the Pd(II/I) and Pd(I/0) couples. In dichloromethane, the Pd(I) complexes are thermodynamically stable over potential ranges of ca. 180 and 20 mV for [Pd(ptas)₂]⁺ and [Pd(mtas)₂]⁺, respectively. Examination of the electrochemical data allows rationalization of the trends in stabilities of the Pd(I) species.