Deeper Understanding of Mononuclear Manganese(IV)-Oxo Binding Brønsted and Lewis Acids and the Manganese(IV)-Hydroxide Complex

Deepika G. Karmalkar, Mi Sook Seo, Yong Min Lee, Youngsuk Kim, Eunsung Lee, Ritimukta Sarangi, Shunichi Fukuzumi, Wonwoo Nam

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13 Scopus citations

Abstract

Binding of Lewis acidic metal ions and Brønsted acid at the metal-oxo group of high-valent metal-oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal-oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese(IV)-oxo complex binding triflic acid, {[(dpaq)MnIV(O)]-HOTf}+ (1-HOTf). First, 1-HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn-O stretching frequencies of 1-HOTf (e.g., νMn-O at â 793 cm-1) and 1-Mn+ (Mn+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and Mn+ ions, suggesting that H+ and Mn+ bind at the metal-oxo moiety of [(dpaq)MnIV(O)]+. Interestingly, a single-crystal structure of 1-HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn(IV)-oxo complex but a Mn(IV)-hydroxide complex, [(dpaq)MnIV(OH)](OTf)2 (4), with a Mn-O bond distance of 1.8043(19) Å and a Mn-O stretch at 660 cm-1. More interestingly, 4 reverted to 1-HOTf upon dissolution, demonstrating that 1-HOTf and 4 are interconvertible depending on the physical states, such as 1-HOTf in solution and 4 in isolated solid. The reactivity of 1-HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1-Mn+ complexes; an interesting correlation between the Mn-O stretching frequencies of 1-HOTf and 1-Mn+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.

Original languageEnglish
Pages (from-to)16996-17007
Number of pages12
JournalInorganic Chemistry
Volume60
Issue number22
DOIs
StatePublished - 15 Nov 2021

Bibliographical note

Funding Information:
The authors acknowledge the NRF of Korea through CRI (NRF-2021R1A3B1076539 to W.N.) and the Basic Science Research Program (2020R1I1A1A01074630 to Y.-M.L. and 2019R1I1A1A01055822 to M.S.S.). The SSRL Structural Molecular Biology Program is supported by the U.S. Department of Energy Office of Biological and Environmental Research and by the National Institutes of Health, National Institute of General Medical Sciences (P30GM133894). The X-ray diffraction experiment was performed at the Pohang Accelerator Laboratory (Beamline 2D) using synchrotron radiation. The rRaman measurements were taken at the Korea Advanced Institute of Science and Technology (KAIST).

Publisher Copyright:
© 2021 American Chemical Society.

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