Aerobic and Hydrolytic Decomposition of Pseudotetrahedral Nickel Phenolate Complexes

Pseudotetrahedral nickel­(II) phenolate complexes TpR,MeNi-OAr (TpR,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
[TpMe,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 [TpMe,MeNi­(μ-OH)]2, while hydrolysis of 1a yields an uncharacterized
intermediate, which subsequently rearranges to the new sandwich complex
[(TpPh,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.