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

Chemistry - A European Journal by Muniyandi Sankaralingam, Mani Balamurugan, Mallayan Palaniandavar, Prabha Vadivelu, Cherumuttathu H. Suresh  /  6h  //  keep unread  //  hide  //  preview 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 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)NiIIO.] 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 the lower cyclohexane oxidation activities of the complexes. Interesting, interesting! Nickel(II) complexes with a strong π-backbonding ligand act as efficient catalysts for the oxidation of alkanes (see figure, m-CPBA=m-chloroperbenzoic acid) by stabilizing the NiO. intermediate, whereas those with a better σ-donor ligand act as less efficient catalysts by destabilizing the reactive intermediate. The computed mechanism for cyclohexane hydroxylation reveals that a high-spin (S=3/2) [(L1/L5)NiIIO.]+ species is the ground state. Visit Website