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

Abstract

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. © 2011 American Chemical Society.