Nickel(II) complexes of pentadentate N5 ligands as catalysts for alkane hydroxylation by using m-CPBA as oxidant: a combined experimental and computational study

Abstract

A new family of nickel(II) complexes of the type [Ni(L)(CH3CN)](BPh4)(2), where L= N-methyl-N,N',N'-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N',N'-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N, N'-bis(pyrid-2-ylmethyl)-N'-(quinolin-2-ylmethyl)-ethylenediamine (L4, 4), and N-methyl-N, N'-bis(pyrid-2-ylmethyl)-N'-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L3)( CH3CN)](BPh4)(2) reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni-II/Ni-III redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A = alcohol, K = ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH3CN)](BPh4)(2) by the strongly sigma-bonding but weakly pi-bonding imidazolylmethyl arm as in [Ni(L5)(CH3CN)](BPh4)(2) or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH3CN)](BPh4)(2) and the quinolylmethyl arms ([Ni(L4)( CH3CN)](BPh4)(2), both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni-II-O-center dot] intermediate (ts1(hs), ts2(doublet)), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni-II(L5)(OOCOC6H4Cl)](+) (5a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3 degrees/2 degrees, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the Ni-II leads to a high 3 degrees/2 degrees bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.