Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State

Yao-Cheng Tian; Yang Jiang; Yen-Hao Lin; Peng Zhang; Chun-Chieh Wang; Shengfa Ye; Way-Zen Lee

doi:10.1021/jacsau.2c00268

Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State

JACS Au. 2022 Aug 11;2(8):1899-1909. doi: 10.1021/jacsau.2c00268. eCollection 2022 Aug 22.

Authors

Yao-Cheng Tian¹, Yang Jiang², Yen-Hao Lin¹, Peng Zhang^{2

3}, Chun-Chieh Wang¹, Shengfa Ye², Way-Zen Lee^{1

4}

Affiliations

¹ Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan.
² State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
³ University of Chinese Academy of Sciences, Beijing 100049, China.
⁴ Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan.

Abstract

Systematic investigations on H atom transfer (HAT) thermodynamics of metal O₂ adducts is of fundamental importance for the design of transition metal catalysts for substrate oxidation and/or oxygenation directly using O₂. Such work should help elucidate underlying electronic-structure features that govern the OO-H bond dissociation free energies (BDFEs) of metal-hydroperoxo species, which can be used to quantitatively appraise the HAT activity of the corresponding metal-superoxo complexes. Herein, the BDFEs of two homologous Co^III- and Mn^III-hydroperoxo complexes, 3-Co and 3-Mn, were calculated to be 79.3 and 81.5 kcal/mol, respectively, employing the Bordwell relationship based on experimentally determined pK _a values and redox potentials of the one-electron-oxidized forms, 4-Co and 4-Mn. To further verify these values, we tested the HAT capability of their superoxo congeners, 2-Co and 2-Mn, toward three different substrates possessing varying O-H BDFEs. Specifically, both metal-superoxo species are capable of activating the O-H bond of 4-oxo-TEMPOH with an O-H BDFE of 68.9 kcal/mol, only 2-Mn is able to abstract a H atom from 2,4-di-tert-butylphenol with an O-H BDFE of 80.9 kcal/mol, and neither of them can react with 3,5-dimethylphenol with an O-H BDFE of 85.6 kcal/mol. Further computational investigations suggested that it is the high spin state of the Mn^III center in 3-Mn that renders its OO-H BDFE higher than that of 3-Co, which features a low-spin Co^III center. The present work underscores the role of the metal spin state being as crucial as the oxidation state in modulating BDFEs.