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.
High-valent transition metal–hydroxide complexes have been proposed as essential intermediates in biological and synthetic catalytic reactions. In this work, we report the single-crystal X-ray structure and spectroscopic characteristics of a mononuclear nonporphyrinic MnIV–(OH) complex, [MnIV(Me3-TPADP)(OH)(OCH2CH3)]2+ (2), using various physicochemical methods. Likewise, [MnIV(Me3-TPADP)(OH)(OCH2CF3)]2+ (3), which is thermally stable at room temperature, was also synthesized and characterized spectroscopically. The MnIV–(OH) adducts are capable of performing oxidation reactions with external organic substrates such as C–H bond activation, sulfoxidation, and epoxidation. Kinetic studies, involving the Hammett correlation and kinetic isotope effect, and product analyses indicate that 2 and 3 exhibit electrophilic oxidative reactivity toward hydrocarbons. Density functional theory calculations support the assigned electronic structure and show that direct C–H bond activation of the MnIV–(OH) species is indeed possible.
A series of cobalt(III)–peroxo complexes, [CoIII(R2-TBDAP)(O2)]+ (1 R2 ; R2 = Cl, H, and OMe), and cobalt(III)–hydroperoxo complexes, [CoIII(R2-TBDAP)(O2H)(CH3CN)]2+ (2 R2 ), bearing electronically tuned tetraazamacrocyclic ligands (R2-TBDAP = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R2-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1 R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O–O bond lengths of 1 Cl [1.398(3) Å] and 1 OMe [1.401(4) Å] were shorter than that of 1 H [1.456(3) Å] due to the different spin states. For 2 R2 , the O–O bond vibration energies of 2 Cl and 2 OMe were identical at 853 cm–1 (856 cm–1 for 2 H ), but their Co–O bond vibration frequencies were observed at 572 cm–1 for 2 Cl and 550 cm–1 for 2 OMe , respectively, by resonance Raman spectroscopy (560 cm–1 for 2 H ). Interestingly, the redox potentials (E 1/2) of 2 R2 increased in the order of 2 OMe (0.19 V) < 2 H (0.24 V) < 2 Cl (0.34 V) according to the electron richness of the R2-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2 R2 showed a reverse trend (k 2: 2 Cl < 2 H < 2 OMe ) with a 13-fold rate enhancement at 2 OMe over 2 Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal–oxygen species with low E 1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co–O bond vibration of 2 OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature–reactivity relationship of metal–oxygen species.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.