Ligand effects on NiII-catalysed alkane-hydroxylation with m-CPBA

Abstract

Nickel(II) complexes supported by a series of pyridylalkylamine ligands [tris(2-pyridylmethyl)amine (TPA; complexes 1a and 1b), tris[2-(2-pyridyl)ethyl]amine (TEPA; complexes 2a and 2b), 6-[N,N-bis(2-pyridylmethyl)aminomethyl]-2,4-di-tert-butylphenol (^DtbpPym2H; complexes 3a and 3b), 6-[N,N-bis[2-(2-pyridyl)ethyl]aminomethyl]-2,4-di-tert-butylphenol (^DtbpPye2H; complexes 4a and 4b), N-benzyl-bis(2-pyridylmethyl)amine (^BzPym2; complex 5a) and N-benzyl-bis[2-(2-pyridyl)ethyl]amine (^BzPye2; complex 6a)] have been synthesized and structurally characterized by X-ray crystallographic analysis [coordinating counter anion (co-ligand) of complexes na (n = 1–6) is AcO⁻ and that of complexes nb (n = 1–4) is NO₃⁻]. All complexes, except 1b, were obtained as a mononuclear nickel(II) complex exhibiting a distorted octahedral geometry, whereas complex 1b 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 (1a, 3a, 5a) showed higher turnover number (TON) than those containing the pyridylethylamine (Pye) metal-binding group (2a, 4a, 6a), 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 NO₃⁻ co-ligand (1b, 2b and 3b) 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.