Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals.

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 (2Ph)). 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.

Oxidation of Naphthalene with a Manganese(IV) Bis(hydroxo) Complex in the Presence of Acid.

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

Distinct Reactivity of a Mononuclear Peroxocobalt(III) Species toward Activation of Nitriles.

A mononuclear side-on peroxocobalt(III) complex with a tetradentate macrocyclic ligand, [CoIII(TBDAP)(O2)]+ (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, [CoIII(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). 18O-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.