The access of {NiIV(OH)2} intermediate in Ni(II) mediated oxygen atom transfer to coordinated Phosphine: Combined experimental and computational studies

Abstract

A tetranuclear Ni-complex of [(NiL)4] (1) (whereH2L = (E)-2-(((2-mercaptophenyl)imino)methyl)-6-methoxyphenol) was synthesized and fully characterized. The solvent dependent behaviour of 1 showed the existence of monomeric species which was well explained using several spectroscopic evidences. Compound [NiL(PPh3)] (2) and [LNi(dppe)NiL] (3) were synthesized by the reaction of 1 with respective phosphine in acetone (dppe = (Ph2PCH2CH2PPh2). All Ni-complexes were characterized by x-ray structure analysis and other spectroscopic techniques like 1H NMR, positive-ion ESI etc. The electrochemical responses for all Ni-complexes were explained. The reactivity of 2 and 3 with H2O2 and tBuOOH were explored in homogenous solution which identified that coordinated phosphine group at Ni(II) centre was oxidized and the reaction was preceded via oxygen atom transfer (OAT) reaction. The mechanism of OAT reaction of 2 with H2O2was proposed based on the detection of the intermediates using several possible spectroscopic tools and density functional theory (DFT) calculation approaches which demonstrated the highly exothermic nature of overall reaction (ΔGsol = -85.3 Kcal mol−1) and the reaction pathway involved the oxidative addition of H2O2 at Ni(II) center to form an intermediate Ni(IV)(OH)2 complex (INT1) followed by the reductive elimination of O = PPh3 occurred.