Aerobic and Hydrolytic Decomposition of Pseudotetrahedral Nickel Phenolate Complexes

摘要

Pseudotetrahedral nickel(II) phenolate complexesnTpR,MeNi-OAr (TpR,Me = hydrotris(3-R-5-methylpyrazol-n1-yl)borate; R = Ph {1a}, Me {1b}; OAr = O-n2,6-iPr2C6H3) were synthesized as models for nickelsubstitutedncopper amine oxidase apoenzyme, which utilizesnan N3O (i.e., His3Tyr) donor set to activate O2 within its activensite for oxidative modification of the tyrosine residue. Thenbioinspired synthetic complexes 1a,b are stable in dilutenCH2Cl2 solutions under dry anaerobic conditions, but theyndecompose readily upon exposure to O2 and H2O. Aerobic decomposition of 1a yields a range of organic products consistentnwith formation of phenoxyl radical, including 2,6-diisopropyl-1,4-benzoquinone, 3,5,3′,5′-tetraisopropyl-4,4′-diphenodihydroquinone,nand 3,5,3′,5′-tetraisopropyl-4,4′-diphenoquinone, which requires concurrent O2 reduction. The dimeric product complexndi[hydro{bis(3-phenyl-5-methylpyrazol-1-yl)(3-ortho-phenolato-5-methylpyrazol-1-yl)borato}nickel(II)] (2) was obtained bynortho C−H bond hydroxylation of a 3-phenyl ligand substituent on 1a. In contrast, aerobic decomposition of 1b yields a dimericncomplex [TpMe,MeNi]2(μ-CO3) (3) with unmodified ligands. However, a unique organic product was recovered, assigned as 3,4-ndihydro-3,4-dihydroxy-2,6-diisopropylcyclohex-5-enone on the basis of 1H NMR spectroscopy, which is consistent withndihydroxylation (i.e., addition of H2O2) across the meta and para positions of the phenol ring. Initial hydrolysis of 1b yields freenphenol and the known complex [TpMe,MeNi(μ-OH)]2, while hydrolysis of 1a yields an uncharacterized intermediate, whichnsubsequently rearranges to the new sandwich complex [(TpPh,Me)2Ni] (4). Autoxidation of the released phenol under O2 wasnobserved, but the reaction was slow and incomplete. However, both 4 and the in situ hydrolysis intermediate derived from 1anreact with added H2O2 to form 2. A mechanistic scheme is proposed to account for the observed product formation bynconvergent oxygenation and hydrolytic autoxidation pathways, and hypothetical complex intermediates along the former werenmodeled by DFT calculations. All new complexes (i.e., 1a,b and 2−4) were fully characterized by FTIR, 1H NMR, and UV−vis−nNIR spectroscopy and by X-ray crystallography.