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

Abstract

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.

A palladium–phosphine catalytic system as an active and recycable precatalyst for Suzuki coupling in water

“…A single anodic peak (A L1 ) was observed on the reverse scan. This behavior is similar to that reported previously by Champness et al [37], Batchelor et al [38], Downard et al [39] and our group [40][41][42] in cyclic voltammetric studies of various types of Pd(II) complexes The cathodic peak C M1 at 1.05 V versus the Ag/AgCl reference electrode can be assigned to the formation of a Pd(0) center with a coordination number between 2 and 4 [or alternatively to an equilibrium mixture of several Pd(0) species] [39,[43][44][45]. There are some examples of stable Pd(0) complexes; particularly relevant is [Pd(L) 2 ] 0 (L = 1,2-bis(dimethylarsino)benzene), which is sufficiently stable to be isolated as a solid [46].…”

“…These electrochemical data confirm complex formation between Pd(II) and Y and also show the presence of such Pd(II) in the complex. The overall two-electron reduction of the Pd(II) complexes is expected to take place in separate oneelectron steps [37][38][39], and this might be achieved via manipulation of the thermodynamics or electrode kinetics of either step by correct choice of the experimental conditions. Under our experimental conditions, the Pd(I) complex is unstable.…”

Four-coordinate Pd(II) complexes containing non-symmetric phosphorus ylides: Synthesis, characterization, and catalytic behavior towards Suzuki reaction

“…There were no anodic peaks observed on the reverse scan [68]. This behavior is similar to that reported previously in cyclic voltammetric study of various types of Pd(II) complexes in aprotic solvents [69,70]. This irreversibility may be due to the reaction of a Pd(0) complex with adventitious O 2 or other components of the solution.…”

Silicon–Heteroaromatic [FeFe] Hydrogenase Model Complexes: Insight into Protonation, Electrochemical Properties, and Molecular Structures

“…Cyclic voltammetry at faster scan rates results in a substantial increase of the peak‐to‐peak separation and a significant decrease of the current function ( i p red / v 1/2 ) for the reduction. The change in Δ E p is indicative of slow kinetics of the heterogeneous electron transfers 45–50. Further, the scan rate dependence of the current function could arise from either an EE or an EC rev E mechanism.…”