Base control of electron-transfer reactions of manganese(III) porphyrins

Homogeneous electron-transfer kinetics for the reduction of four different manganese(III) porphyrins using different reductants were examined in deaerated acetonitrile, and the resulting data were evaluated in light of the Marcus theory of electron transfer to determine electron-exchange rate constants between manganese(III) and manganese(II) porphyrins. The investigated compounds are represented as (P)MnCl, where P = the dianion of dodecaphenylporphyrin (DPPX; X = H₂₀, Cl₁₂H₈, or F₂₀) or tetraphenylporphyrin (TPP). The electron transfer from semiquinone radical anion derivatives to (P)Mn(III)Cl leads to formation of the corresponding Mn(II) complex, [(P)Mn(II)Cl]^-. The electron-exchange rate constants derived from the electron-transfer rate constants decrease with an increasing degree of nonplanarity of the porphyrin macrocycle and follow the order: (TPP)MnCl (3.1 x 10³ M^-1 · s^-1) > (DPPH₂₀)MnCl (1.1 X 10^-2 M^-1 · s^-1) > (DPPCl₁₂H₈)MnCl (3.5 X 10^-4 M^-1 · s^-1) > (DPPF₂₀)MnCl (4.3 x 10^{- 6} M^-1 · s^-1). The coordination of two molecules of pyridine (py) or DMSO to (DPPH₂₀)MnCl to form [(DPPH₂₀)Mn(py)2]⁺ or [(DPPH₂₀)Mn(DMSO)₂]⁺ enhances the rate of electrontransfer reduction. This indicates that there is a significant decrease in the reorganization energy upon axial ligand coordination of pyridine or DMSO.