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

A series of mononuclear manganese­(III)–hydroxo and −aqua complexes, [MnIII(TBDAP)­(OH)2]+ (1), [MnIII(TBDAP)­(OH)­(OH2)]2+ (2) and [MnIII(TBDAP)­(OH2)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 [MnII(TBDAP)­(OH)­(OH2)]+ and [MnII(TBDAP)­(OH2)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 × 104-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.