Electrochemical reduction of CO2 catalyzed by [Pd(triphosphine)(solvent)](BF4)2 complexes: Synthetic and mechanistic studies

The free radical addition of phosphorus-hydrogen bonds to carbon-carbon double bonds has been used to prepare a number of new tridentate ligands containing phosphorus. Reactions of these tridentate ligands with [Pd(CH₃CN)₄](BF₄)₂ yield the corresponding [Pd(tridentate)(CH₃CN)](BF₄)₂ complexes. These complexes catalyze the electrochemical reduction of CO₂ to CO in acidic dimethylformamide or acetonitrile solutions if the tridentate ligand is a linear triphosphine ligand. Complexes in which one or more of the phosphorus atoms of the tridentate ligand have been substituted with a nitrogen or sulfur heteroatom do not catalyze the electrochemical reduction of CO₂. Kinetic studies on [Pd(etpC)(CH₃CN)](BPh₄)₂ (where etpC is bis[(dicyclohexylphosphino)ethyl]phenylphosphine) show that, at acid concentrations above 1.0 × 10^-2 M, the reaction is first order in catalyst. First order in CO₂, and independent of acid concentration. At acid concentrations less than 4.0 × 10^-3 M, the catalytic rate is first order in catalyst, second order in acid, and independent of CO₂. The rate is also solvent dependent. A mechanism is proposed to account for these data. Comparison of the rate constants for catalysts with different alkyl and aryl substituents on the terminal phosphorus atoms indicates that the rate of reaction of the palladium(I) intermediates with CO₂ increases with the electron-donating ability of the R groups, and that steric interactions are of less importance. In contrast, the rate constants decrease with increasing steric bulk for substituents on the central phosphorus atoms of the triphosphine ligand. Other relationships between ligand structure and catalyst activity, selectivity, and stability are also discussed. An X-ray diffraction study of the catalytic decomposition product [Pd(etp)]₂(BF₄)₂ (where etp is bis[(diphenylphosphino)-ethyl]phenylphosphine) has been carried out. [Pd(etp)]₂(BF₄)₂ crystallizes in the monoclinic space group P2₁/n with a = 13.842 (6) Å, b = 28.055 (8) Å, c = 19.596 (7) Å, β = 95.80 (3)°, v = 7571 (5) ų, and Z = 4. The structure was refined to R = 0.057 and R_w = 0.0809 for 10352 independent reflections (F > 6σ(F)). This Pd(I) dimer is bridged by two triphosphine ligands. A dihedral angle of 67° exists between the two nearly square planar PdP₃ fragments of [Pd(etp)]₂(BF₄)₂. This dimer can be reoxidized to regenerate the catalytically active complexes.