Synthesis and Electrochemical Studies of Nickel beta-Diketonate Complexes Incorporating Asymmetric Diimine Ligands

The reaction of ppa(X) {(4-X-phenyl)-pyridin-2-ylmethylene-amine; X = H, Me, Et, OMe, F, Cl, Br, and I} with [Ni(beta-diketonate) (2)(H(2)O)(2)] {beta-diketonate = 1,3-diphenylpropanedionate (dbm), 2,2,6,6-tetramethyl-3,5-heptadionate (tmhd), or hexafluoroacetylacetonate (hfac)} yields a series of nickel complexes. X-ray crystallography reveals octahedral coordinated nickel centres with a cis arrangement of the beta-diketonate ligands. The beta-diketonate ligands adopt 'planar' or 'bent' coordination modes, whereas the aryl ring of the ppa(X) ligand is twisted with respect to the pyridylimine unit. The electrochemical behaviour of the complexes reveals quasi-reversible or irreversible one-electron oxidation to Ni(III) in the case of the [Ni(tmhd)(2)(ppaX)] and [Ni(dbm)(2)(ppa(X))] complexes, respectively. The peak potential for oxidation is dependent on the type of beta-diketonate ligand but essentially independent of the substituent, X, on the ppa(X) ligand. The [Ni(beta-diketonate)(2)(ppa(X))] complexes (X = F, Cl, Br, and I) also undergo ligand based reduction.