Ligand effects on Ni(II)-catalysed alkane-hydroxylation with m-CPBA.

Nickel(ii) complexes supported by a series of pyridylalkylamine ligands [tris(2-pyridylmethyl)amine (TPA; complexes and ), tris[2-(2-pyridyl)ethyl]amine (TEPA; complexes and ), 6-[N,N-bis(2-pyridylmethyl)aminomethyl]-2,4-di-tert-butylphenol ((Dtbp)Pym2H; complexes and ), 6-[N,N-bis[2-(2-pyridyl)ethyl]aminomethyl]-2,4-di-tert-butylphenol ((Dtbp)Pye2H; complexes and ), N-benzyl-bis(2-pyridylmethyl)amine ((Bz)Pym2; complex ) and N-benzyl-bis[2-(2-pyridyl)ethyl]amine ((Bz)Pye2; complex )] have been synthesized and structurally characterized by X-ray crystallographic analysis [coordinating counter anion (co-ligand) of complexes n (n = 1-6) is AcO(-) and that of complexes n (n = 1-4) is NO(3)(-)]. All complexes, except , were obtained as a mononuclear nickel(ii) complex exhibiting a distorted octahedral geometry, whereas complex was isolated as a dinuclear nickel(ii) complex bridged by two nitrate anions. Catalytic activity of the nickel(ii) complexes were examined in the oxidation of cyclohexane with m-CPBA as an oxidant. In all cases, the oxygenation reaction proceeded catalytically to give cyclohexanol as the major product together with cyclohexanone as the minor product. The complexes containing the pyridylmethylamine (Pym) metal-binding group (, , ) showed higher turnover number (TON) than those containing the pyridylethylamine (Pye) metal-binding group (, , ), whereas the alcohol/ketone (A/K) selectivity was much higher with the latter (Pye system) than the former (Pym system). On the other hand, the existence of the NO(3)(-) co-ligand (, and ) caused a lag phase in the early stage of the catalytic reaction. Electronic and steric effects of the supporting ligands as well as the chemical behavior of the co-ligands on the catalytic activity of the nickel(ii) complexes have been discussed on the basis of their X-ray structures.