Nickel(II) Complexes of Pentadentate N5 Ligands as Catalysts for Alkane Hydroxylation by Using m‐CPBA as Oxidant: A Combined Experimental and Computational Study |
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| Authors: | Dr. Muniyandi Sankaralingam Dr. Mani Balamurugan Prof. Dr. Mallayan Palaniandavar Dr. Prabha Vadivelu Dr. Cherumuttathu H. Suresh |
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| Affiliation: | 1. School of Chemistry, Bharathidasan University, Tiruchirappalli—620024, Tamil Nadu (India);2. Department of Chemistry, Central University of Tamil Nadu, Thiruvarur—610001, Tamil Nadu, (India);3. Chemical Sciences and Technology Division, CSIR‐National Institute for Interdisciplinary Science and Technology, Trivandrum—695019. (India) |
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| 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)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 the lower cyclohexane oxidation activities of the complexes. |
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| Keywords: | density functional calculations hydroxylation N ligands nickel Ni O. species |
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