Oxidative <i>versus</i> basic asynchronous hydrogen atom transfer reactions of Mn(<scp>iii</scp>)-hydroxo and Mn(<scp>iii</scp>)-aqua complexes

Zhang, Jisheng; Lee, Yong Min; Seo, Mi Sook; Kim, Youngsuk; Lee, Eunsung; Fukuzumi, Shunichi; Nam, Wonwoo

doi:10.1039/d2qi00741j

Cited by 8 publications

(6 citation statements)

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“…Our previous investigations of a terminal Co III –oxo complex, PhB( t Bu Im) 3 Co III O, revealed rates of C–H activation that more strongly correlate with substrate p K a rather than BDFE, consistent with an imbalanced CPET reaction in favor of basic (PT) reactivity. ,,,,− Based on these results, we have been interested in synthesizing metal–oxo complexes with greater basicity to explore the frontier between imbalanced CPET reactivity and stepwise PTET reactivity. We recently reported an adamantyl (Ad)-substituted Co III –oxo complex, PhB( Ad Im) 3 Co III O, which, given the greater electron-donating properties of the Ad groups, is more basic than our previous Co III –oxo complex .…”

Section: Introductionmentioning

confidence: 91%

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)–Oxo Complexes

et al. 2023

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Transition metal−oxo complexes are key intermediates in a variety of oxidative transformations, notably C−H bond activation. The relative rate of C−H bond activation mediated by transition metal−oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton−electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal−oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C−H bonds with the terminal Co III −oxo complex PhB( tBu Im) 3 Co III O. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB( Ad Im) 3 Co III O, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB( tBu Im) 3 Co III O with C−H substrates, and O−H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer−electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.

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Section: Introductionmentioning

confidence: 91%

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)–Oxo Complexes

et al. 2023

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“…For instance, water oxidation within the oxygen-evolving complex (OEC) of photosystem II occurs through five oxidation states via multiple proton-coupled electron transfers (PCETs). − In each oxidation state, manganese–oxygen complexes, such as Mn–aqua, −hydroxo, and −oxyl/oxo species, are proposed to be involved. Thus, a comprehensive exploration of manganese–aqua, −hydroxo, and −oxyl/oxo species within OECs is essential. , Although significant efforts have been focused on synthesizing and characterizing manganese–oxo, −hydroxo, and −aqua species, the stepwise processes within OECs require further fundamental insights, including an understanding of the thermodynamics of manganese–oxygen species. − Recent theoretical investigations by Baik’s group have focused on water oxidation using a manganese catalyst bearing a macrocycle in aqueous solutions . Density functional theory (DFT) calculations unveiled a sequential pathway from the aqua complex to the peroxide intermediate via Mn–aqua, −hydroxo, and −oxyl species.…”

Section: Introductionmentioning

confidence: 99%

“…31 Another strategy to increase the thermodynamic driving force is to raise the redox potential (E red ) value. 11,12,35 However, such modulation of reactivity for manganese− hydroxo species remains largely unexplored.…”

Section: ■ Introductionmentioning

confidence: 99%

Controlling Redox Potential of a Manganese(III)–Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

Lee,

Moon,

Cho

2024

J. Am. Chem. Soc.

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A series of mononuclear manganese(III)−hydroxo and −aqua complexes, [Mn III (TBDAP)(OH) 2 ] + (1), [Mn III (TBDAP)(OH)(OH 2 )] 2+ (2) and [Mn III (TBDAP)(OH 2 ) 2 ] 3+ (3), were prepared from a manganese(II) precursor and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials (E 1/2 ) in the order of 1 (−0.15 V) < 2 (0.56 V) < 3 (1.11 V), while pK a values exhibited a reverse trend in the order of 3 (3.87) < 2 (11.84). Employing the Bordwell Equation, the O−H bond dissociation free energies (BDFE) of [Mn II (TBDAP)(OH)(OH 2 )] + and [Mn II (TBDAP)(OH 2 ) 2 ] 2+ , related to the driving force of 1 and 2 in hydrogen atom transfer (HAT), were determined as 75.3 and 77.3 kcal mol −1 , respectively. It was found that the thermodynamic driving force of 2 in HAT becomes greater than that of 1 as the redox potential of 2 increases through protonation from 1 to 2. Kinetic studies on electrophilic reactions using a variety of substrates revealed that 1 is only weakly reactive with O−H bonds, whereas 2 can activate aliphatic C−H bonds in addition to O−H bonds. The reaction rates increased by 1.4 × 10 4 -fold for the O−H bonds by 2 over 1, which was explained by the difference in BDFE and the tunneling effect. Furthermore, 3, possessing the highest redox potential value, was found to undergo an aromatic C−H bond activation reaction under mild conditions. These results provide valuable insights into enhancing electrophilic reactivity by modulating the redox potential of manganese(III)−hydroxo and −aqua complexes through protonation.

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“…56,59,[67][68][69][70][71][72][73] Our previous investigations of a terminal Co III -oxo complex, PhB( tBu Im)3Co III O, revealed rates of C-H activation that more strongly correlate with substrate pKa rather than BDFE, consistent with an imbalanced CPET reaction in favor of basic (PT) reactivity. 40,57,59,66,[74][75][76] Based on these results, we have been interested in synthesizing metaloxo complexes with greater basicity to explore the frontier between imbalanced CPET reactivity and stepwise PTET reactivity. We recently reported an adamantyl (Ad) substituted Co III -oxo complex, PhB( Ad Im)3Co III O, which, given the greater electron donating properties of the Ad groups, is more basic than our previous Co III -oxo complex.…”

Section: Introductionmentioning

confidence: 99%

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-atom Abstraction by Co(III)-oxo Complexes

Zhao

Goetz

Schneider

et al. 2022

Preprint

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Transition metal-oxo complexes are key intermediates in a variety of oxidative transformations, notably C–H bond activation. The rela-tive rate of C–H bond activation mediated by transition metal-oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton-electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox po-tentials of the substrate/metal-oxo may dominate in some cases. In this context we have found basicity-governed concerted activation of C–H bonds with the terminal CoIII-oxo complex PhB(tBuIm)CoIIIO. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analo-gous, more basic complex, PhB(AdIm)CoIIIO, and studied its reactiv-ity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB(tBuIm)CoIIIO with C–H substrates and O–H activation of phenol substrates displays mechanistic crossover to stepwise PTET reactivity. Analysis of the thermodynamics of PT and ET reveal a distinct thermodynamic crossing point between concerted and stepwise reactivity. Further-more, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover which results in slower prod-uct formation.

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Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(iii)-hydroxo and Mn(iii)-aqua complexes

Abstract: Hydrogen atom transfer (HAT) of metal-oxygen intermediates such as metal-oxo, -hydroxo and -superoxo species have so far been studied extensively. However, HAT reactions of metal-aqua complexes have yet to be...

Cited by 8 publications

References 100 publications

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)–Oxo Complexes

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)–Oxo Complexes

Controlling Redox Potential of a Manganese(III)–Bis(hydroxo) Complex through Protonation and the Hydrogen-Atom Transfer Reactivity

Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-atom Abstraction by Co(III)-oxo Complexes

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