Synthesis and structure of palladium(II) complexes supported by bis-NHC pincer ligands for the electrochemical activation of CO(2).

A series of bis-NHC pincer complexes of palladium(II) have been prepared and characterized. These pyridyl-spaced dicarbene complexes ([(PDC(R))Pd(MeCN)](PF(6))(2) ) were synthesized with substituents of varying steric bulk at the wingtip positions, which include R = methyl, ethyl, isopropyl, cyclohexyl, mesityl and 2,6-diisopropylphenyl. The synthesis of this library of complexes was accomplished either by direct metallation of the prerequisite pyridyl-spaced bis-imidazolium proligands with Pd(OAc)(2) or via treatment with Ag(2)O to afford the corresponding silver carbenes, which were then transmetallated onto palladium. Solid-state structures for each of the [(PDC(R))Pd(MeCN)](PF(6))(2) derivatives were obtained via X-ray crystallography and allowed for the steric properties of each PDC(R) ligand to be evaluated by two methods. These analyses, which included calculation of the percent buried volume (%V(Bur)) and solid angles of the PDC(R) ligands, served to characterize the steric environment around the palladium center in each of the complexes that was prepared. Finally, voltammetry and controlled potential electrolysis studies were performed to characterize the redox chemistry of the [(PDC(R))Pd(MeCN)](PF(6))(2) derivatives and assess if they could electrocatalyze the reduction of CO(2). The influence of the steric properties of the PDC(R) ligand on the electrochemistry of the resulting complexes [(PDC(R))Pd(MeCN)](PF(6))(2) is also discussed.