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

A series of mononuclear manganese(III)-hydroxo and -aqua complexes, [Mn^III(TBDAP)(OH)₂]⁺ (1), [Mn^III(TBDAP)(OH)(OH₂)]²⁺ (2) and [Mn^III(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 [Mn^II(TBDAP)(OH)(OH₂)]⁺ and [Mn^II(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.