Nickel(II) Complexes of Pentadentate N5 Ligands as Catalysts for Alkane Hydroxylation by Using m-CPBA as Oxidant: A Combined Experimental and Computational Study

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 NiII/NiIII 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 σ‐bonding but weakly π‐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)NiIIO.] intermediate (ts1hs, ts2doublet), 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 [NiII(L5)(OOCOC6H4Cl)]+ (5 a). Adamantane is oxidized to 1‐adamantanol, 2‐adamantanol, and 2‐adamantanone (3°/2°, 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 NiII leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to t