Nickel(II) complexes containing chelating N-heterocyclic carbene-phosphine ligands ([NiL2](BPh4)2, for which L = [MeIm(CH2)2PR2], MeIm = 1- methylimidizolylidene, and R = Ph or Et) have been synthesized for the purpose of studying how this class of ligand affects the electrochemical and thermodynamic properties compared to the nickel bis-diphosphine analogues. The nickel complexes were synthesized and then characterized by X-ray crystallography, electrochemical methods, and thermodynamic studies, including DFT calculations. On the basis of the reduction potentials (E°), substitution of an NHC for one of the phosphines in a diphoshine ligand resulted in negative shifts in potential by 0.6 to 1.2 V relative to the corresponding nickel bis-diphosphine complexes. From computational studies of the nickel hydride complex of the phenyl-substituted phosphine-carbene ligand, the hydride donor ability was determined to improve by 32 kcal/mol relative to the estimated hydride donor ability for the analogous nickel complex of the chelating diphosphine ligand 1,3-bis(diphenylphosphino)propane. The free energy for addition of H2 is presented, and the implications for catalysis are discussed. These quantitative results highlight the substantial effect that NHC ligands can have on the electronic properties of the metal complexes.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry