No abstract

Naphthalene oxidation by metal-oxygen intermediates is one of difficult reactions in environmental and biological chemistry. Herein, we report that a MnIV-bis(hydroxo) complex, which was fully characterized by various physicochemical methods, such as UV-vis, ESI-MS, EPR, X-ray and XAS, shows the naphthalene oxidation in the presence of acid to afford 1,4-naphthoquinone. Redox titration of the MnIV-bis(hydroxo) complex exhibits one electron reduction potential of 1.09 V, which is the most positive potential for the previously reported nonheme MnIV-bis(hydroxo) species as well as MnIV-oxo analogues. Kinetic studies including kinetic isotope effect suggest that the naphthalene oxidation by the MnIV-bis(hydroxo) complex in the acid-promoted reaction occurs via a rate-determining electron transfer process.

Redox-inactive metal ions play vital roles in biological O2 activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt­(III) complex bearing a tetradentate macrocyclic ligand, [CoIII(TBDAP)­(O2)]+ (1) (TBDAP = N,N′-di-tert-butyl-2,11-diaza­[3.3]­(2,6)­pyridinophane), toward nitriles that afforded a series of hydroximato­cobalt­(III) complexes, [CoIII(TBDAP)­(R–C­(NO)­O)]+ (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn2+, La3+, Lu3+, and Y3+, the reaction does not form the hydroximato­cobalt­(III) complex but instead gives peroxyimidato­cobalt­(III) complexes, [CoIII(TBDAP)­(R–C­(NH)­O2)]2+ (R = Me (2) and Ph (2 Ph )). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al3+, Sc3+, or H+, 1 is converted to [CoIII(TBDAP)­(O2H)­(MeCN)]2+ (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt­(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co−η1-O2 intermediate through the charge reorganization from a CoII–superoxo to a CoIII–peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.

A mononuclear side-on peroxocobalt(III) complex with a tetradentate macrocyclic ligand, [Co(TBDAP)(O)] (1), shows a novel and facile mode of dioxygenase-like reactivity with nitriles (R-C≡N; R = Me, Et, and Ph) to produce the corresponding mononuclear hydroximatocobalt(III) complexes, [Co(TBDAP)(R-C(═NO)O)], in which the nitrile moiety is oxidized by two oxygen atoms of the peroxo group. The overall reaction proceeds in one-pot under ambient conditions (ca. 1 h, 40 °C). O-Labeling experiments confirm that both oxygen atoms are derived from the peroxo ligand. The structures of all products, hydroximatocobalt(III) complexes, were confirmed by X-ray crystallography and various spectroscopic techniques. Kinetic studies including the Hammett analysis and isotope labeling experiments suggest that the mechanistic mode of 1 for activation of nitriles occurs via a concerted mechanism. This novel reaction would be significantly valuable for expanding the chemistry for nitrile activation and utilization.

Naphthalene oxidation with metal-oxygen intermediates is adifficult reaction in environmental and biological chemistry.H erein, we report that aM n IV bis(hydroxo) complex, which was fully characterized by various physicochemical methods,s uch as ESI-MS,U V/Vis,a nd EPR analysis, X-ray diffraction, and XAS,can be employed for the oxidation of naphthalene in the presence of acid to afford1 ,4-naphthoquinone.R edox titration of the Mn IV bis(hydroxo) complex gave ao ne-electron reduction potential of 1.09 V, which is the most positive potential for all reported nonheme Mn IV bis-(hydroxo) species as well as Mn IV oxoa nalogues.K inetic studies,including kinetic isotope effect analysis,suggest that the naphthalene oxidation occurs through ar ate-determining electron transfer process.