Aerobic and hydrolytic decomposition of pseudotetrahedral nickel phenolate complexes

Pseudotetrahedral nickel(II) phenolate complexes Tp^R,MeNi-OAr (Tp^R,Me = hydrotris(3-R-5-methylpyrazol-1-yl)borate; R = Ph {1a}, Me {1b}; OAr = O-2,6-ⁱPr₂C₆H₃) were synthesized as models for nickel-substituted copper amine oxidase apoenzyme, which utilizes an N₃O (i.e., His₃Tyr) donor set to activate O₂ within its active site for oxidative modification of the tyrosine residue. The bioinspired synthetic complexes 1a,b are stable in dilute CH₂Cl₂ solutions under dry anaerobic conditions, but they decompose readily upon exposure to O₂ and H₂O. Aerobic decomposition of 1a yields a range of organic products consistent with formation of phenoxyl radical, including 2,6-diisopropyl-1,4-benzoquinone, 3,5,3′,5′-tetraisopropyl-4,4′-diphenodihydroquinone, and 3,5,3′,5′-tetraisopropyl-4,4′-diphenoquinone, which requires concurrent O₂ reduction. The dimeric product complex di[hydro{bis(3-phenyl-5-methylpyrazol-1-yl)(3-ortho-phenolato-5-methylpyrazol-1- yl)borato}nickel(II)] (2) was obtained by ortho C-H bond hydroxylation of a 3-phenyl ligand substituent on 1a. In contrast, aerobic decomposition of 1b yields a dimeric complex [Tp^Me,MeNi]₂(μ-CO₃) (3) with unmodified ligands. However, a unique organic product was recovered, assigned as 3,4-dihydro-3,4-dihydroxy-2,6-diisopropylcyclohex-5-enone on the basis of ¹H NMR spectroscopy, which is consistent with dihydroxylation (i.e., addition of H₂O₂) across the meta and para positions of the phenol ring. Initial hydrolysis of 1b yields free phenol and the known complex [Tp^Me,MeNi(μ-OH)]₂, while hydrolysis of 1a yields an uncharacterized intermediate, which subsequently rearranges to the new sandwich complex [(Tp^Ph,Me)₂Ni] (4). Autoxidation of the released phenol under O₂ was observed, but the reaction was slow and incomplete. However, both 4 and the in situ hydrolysis intermediate derived from 1a react with added H₂O₂ to form 2. A mechanistic scheme is proposed to account for the observed product formation by convergent oxygenation and hydrolytic autoxidation pathways, and hypothetical complex intermediates along the former were modeled by DFT calculations. All new complexes (i.e., 1a,b and 2-4) were fully characterized by FTIR, ¹H NMR, and UV-vis-NIR spectroscopy and by X-ray crystallography.