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Ni(II)/H 2 O 2 Reactivity in Bis[(pyridin-2-yl)methyl]amine Tridentate Ligand System. Aromatic Hydroxylation Reaction by Bis(μ-oxo)dinickel(III) Complex

2009; American Chemical Society; Volume: 48; Issue: 11 Linguagem: Inglês

10.1021/ic900059m

1520-510X

A. Kunishita, Yoshitaka Doi, Minoru Kubo, Takashi Ogura, Hideki Sugimoto, Shinobu Itoh,

Magnetism in coordination complexes

The nickel(II) complexes 1X supported by bis[(pyridin-2-yl)methyl]benzylamine tridentate ligands carrying m-substituted phenyl groups (X = OMe, Me, H, Cl, NO2) at the 6-position of pyridine donor groups (LX, N,N-bis[(6-m-substituted-phenylpyridin-2-yl)methyl]benzylamine) have been synthesized and characterized. The X-ray crystallographic analyses have revealed that [NiII(LH)(CH3CN)(H2O)](ClO4)2 (1H), [NiII(LOMe)(CH3CN)(MeOH)](ClO4)2 (1OMe), [NiII(LMe)(CH3CN)(H2O)](ClO4)2 (1Me), and [NiII(LCl)(CH3CN)(H2O)](ClO4)2 (1Cl) have a five-coordinate square pyramidal geometry, whereas [NiII(LNO2)(CH3CN)2(H2O)](ClO4)2 (1NO2) exhibits a six-coordinate octahedral geometry having an additional CH3CN co-ligand. 1H NMR spectra of the nickel(II) complexes 1X in CD3CN have indicated that all the complexes have a high spin ground state. The nickel(II) complexes 1X react with hydrogen peroxide (H2O2) in acetone to give bis(μ-oxo)dinickel(III) complexes 2X exhibiting a characteristic UV−vis absorption band at ∼420 nm. In the case of 2H, a resonance Raman band ascribable to a Ni2O2 core vibration was observed at 611 cm−1 that shifted to 586 cm−1 upon H218O2. The bis(μ-oxo)dinickel(III) intermediates 2X undergo an efficient aromatic ligand hydroxylation reaction, producing a mononuclear nickel(II)-phenolate complexes 4X via a putative intermediate (μ-phenoxo)(μ-hydroxo)dinickel(II) (3X). The kinetic studies on the aromatic ligand hydroxylation process including m-substituent effects (Hammett analysis) and kinetic deuterium isotope effects (KIE) have indicated that the reaction of 2X to 3X involves an electrophilic aromatic substitution mechanism, where C−O bond formation and C−H bond cleavage occur in a concerted manner. Intermediate 3H was detected by ESI-MS during the course of the reaction, and the final product 4H was characterized by elemental analysis, ESI−MS, and X-ray crystallographic analysis.