Premium
Nickel(II) Complexes of Pentadentate N5 Ligands as Catalysts for Alkane Hydroxylation by Using m ‐CPBA as Oxidant: A Combined Experimental and Computational Study
Author(s) -
Sankaralingam Muniyandi,
Balamurugan Mani,
Palaniandavar Mallayan,
Vadivelu Prabha,
Suresh Cherumuttathu H.
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201402391
Subject(s) - chemistry , ethylenediamine , acetonitrile , nickel , ligand (biochemistry) , octahedral molecular geometry , coordination geometry , medicinal chemistry , cyclohexane , octahedron , stereochemistry , crystal structure , crystallography , molecule , inorganic chemistry , hydrogen bond , organic chemistry , biochemistry , receptor
A new family of nickel(II) complexes of the type [Ni(L)(CH 3 CN)](BPh 4 ) 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)(CH 3 CN)](BPh 4 ) 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)(CH 3 CN)](BPh 4 ) 2 by the strongly σ‐bonding but weakly π‐bonding imidazolylmethyl arm as in [Ni(L5)(CH 3 CN)](BPh 4 ) 2 or the sterically demanding 6‐methylpyridylmethyl ([Ni(L3)(CH 3 CN)](BPh 4 ) 2 and the quinolylmethyl arms ([Ni(L4)(CH 3 CN)](BPh 4 ) 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 . ] 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)(OOCOC 6 H 4 Cl)] + ( 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 Ni II leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.