Mechanistic Insights into Hydride-Transfer and Electron-Transfer Reactions by a Manganese(IV)−Oxo Porphyrin Complex

Hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) analogs to a manganese(IV)−oxo porphyrin complex, (TMP)Mn<sup>IV</sup>(O) [TMP = 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)porphyrin], occurs via disproportionation of (TMP)Mn<sup>IV</sup>(O) to [(TMP)Mn<sup>III</sup>]<sup>+</sup> and [(TMP)Mn<sup>V</sup>(O)]<sup>+</sup> that acts as the actual hydride acceptor. In contrast, electron transfer from ferrocene derivatives to (TMP)Mn<sup>IV</sup>(O) occurs directly to afford ferricenium ions and (TMP)Mn<sup>III</sup>(OH) products. The disproportionation rate constant of (TMP)Mn<sup>IV</sup>(O) was determined by the dependence of the observed second-order rate constants on concentrations of NADH analogs to be (8.0 ± 0.6) × 10<sup>6</sup> M<sup>−1</sup> s<sup>−1</sup> in acetonitrile at 298 K. The disproportionation rate constant of (TMP)Mn<sup>IV</sup>(O) in hydride-transfer reactions increases linearly with increasing acid concentration, whereas the rate constant of electron transfer from ferrocene to (TMP)Mn<sup>IV</sup>(O) remains constant irrespective of the acid concentration. The rate constants of electron transfer from a series of ferrocene derivatives to (TMP)Mn<sup>IV</sup>(O) were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energy of electron transfer by the (TMP)Mn<sup>IV</sup>(O) complex.