Unveiling the Catalytic Activity of Nickel(II) Complexes of Pentadentate Ligands in Aromatic Oxidations

Abstract

Selective oxidation of aromatic substrates to phenols is a challenging goal in synthetic chemistry. Herein, we investigate the catalytic activity of nickel(II) complexes of the type [Ni(L)(CH₃CN)](X)₂ (1–4); (X = ClO₄⁻, BPh₄⁻, L = N4Py, BnTPEN, and BnImDPEN) supported by pentadentate ligands in the hydroxylation of aromatic substrates to corresponding phenols using H₂O₂. In benzene oxidation, complex 2 showed a phenol yield of 28% and a turnover number of 560 with 99% selectivity. Also, preferential oxidation of aromatic C─H bonds over aliphatic C─H bonds was noted during the oxidation of substituted benzenes. The kinetic isotope effect value (1.01) supported the involvement of nickel-bound oxygen species in the catalytic cycle rather than hydroxyl radicals. The key source of oxygen in the formed phenol was found to be H₂O₂ based on the isotope-labelling experiments using H₂¹⁸O₂ and H₂¹⁸O. Although the formation of {[(BnTPEN)Ni(OOH)]⁺ + NCCH₃}⁺ intermediate was identified upon reacting complex 2 with H₂O₂, this species did not directly react with benzene to form phenol. Further, DFT studies suggested that the species [(L)Ni^II(O^•)]⁺ derived from O─O homolysis of nickel(II) hydroperoxo intermediate reacts with benzene to produce phenol and the presence of a nickel-based oxidant is the reason behind the excellent selectivity.