Electron- and hydride-transfer reactivity of an isolable manganese(V)-Oxo complex

The electron-transfer and hydride-transfer properties of an isolated manganese(V)-oxo complex, (TBP ₈Cz)Mn ^V(O) (1) (TBP ₈Cz = octa-tert-butylphenylcorrolazinato) were determined by spectroscopic and kinetic methods. The manganese(V)-oxo complex 1 reacts rapidly with a series of ferrocene derivatives ([Fe(C ₅H ₄Me) ₂], [Fe(C ₅HMe ₄) ₂], and ([Fe(C ₅Me ₅) ₂] = Fc*) to give the direct formation of [(TBP ₈Cz)Mn ^III(OH)] ^- ([2-OH] ^-), a two-electron-reduced product. The stoichiometry of these electron-transfer reactions was found to be (Fc derivative)/1 = 2:1 by spectral titration. The rate constants of electron transfer from ferrocene derivatives to 1 at room temperature in benzonitrile were obtained, and the successful application of Marcus theory allowed for the determination of the reorganization energies (λ) of electron transfer. The λ values of electron transfer from the ferrocene derivatives to 1 are lower than those reported for a manganese(IV)-oxo porphyrin. The presumed one-electron-reduced intermediate, a Mn ^IV complex, was not observed during the reduction of 1. However, a Mn ^IV complex was successfully generated via one-electron oxidation of the Mn ^III precursor complex 2 to give [(TBP ₈Cz)Mn ^IV] ⁺ (3). Complex 3 exhibits a characteristic absorption band at λ _max = 722 nm and an EPR spectrum at 15 K with gmax _′ = 4.68, gmid _′ = 3.28, and gmin _′ = 1.94, with well-resolved ⁵⁵Mn hyperfine coupling, indicative of a d ³ Mn ^IVS = ³/ ₂ ground state. Although electron transfer from [Fe(C ₅H ₄Me) ₂] to 1 is endergonic (uphill), two-electron reduction of 1 is made possible in the presence of proton donors (e.g., CH ₃CO ₂H, CF ₃CH ₂OH, and CH ₃OH). In the case of CH ₃CO ₂H, saturation behavior for the rate constants of electron transfer (k _et) versus acid concentration was observed, providing insight into the critical involvement of H ⁺ in the mechanism of electron transfer. Complex 1 was also shown to be competent to oxidize a series of dihydronicotinamide adenine dinucleotide (NADH) analogues via formal hydride transfer to produce the corresponding NAD ⁺ analogues and [2-OH] ^-. The logarithms of the observed second-order rate constants of hydride transfer (k _H) from NADH analogues to 1 are linearly correlated with those of hydride transfer from the same series of NADH analogues to p-chloranil.