Effects of Phosphine–Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes

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.