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 deaer ated acetonitrile, and the resulting data were evaluated in light of the Ma rcus theory of electron transfer to determine electron-exchange rate consta nts between manganese(III) and manganese(II) porphyrins. The investigated c ompounds are represented as (P)MnCl, where P = the dianion of dodecaphenylp orphyrin (DPPX; X = H-20, Cl12H8, or F-20) Or tetraphenylporphyrin (TPP). T he electron transfer from semiquinone radical anion derivatives to (P)(MnCl )-Cl-III leads to formation of the corresponding Mn-II complex, [(P)(MnCl)- Cl-II](-). The electron-exchange rate constants derived from the electron-t ransfer rate constants decrease with an increasing degree of nonplanarity o f the porphyrin macrocycle and follow the order: (TPP)MnCl (3.1 x 10(3) 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 o f two molecules of pyridine (py) or DMSO to (DPPH20)MnCl to form [(DPPH20)M n(py)(2)](+) or [(DPPH20)Mn(DMSO)(2)](+) enhances the rate of electron-tran sfer reduction. This indicates that there is a significant decrease in the reorganization energy upon axial ligand coordination df pyridine or DMSO.