Methanol bonding to and electron-transfer reactivity of chloro(tetraphenylporphinato)manganese(III)

Abstract

The ability of axial ligation to modulate redox potentials and electrode half-reactions of metalloporphyrins is illustrated by coordination of methanol to Mn(TPP)Cl (1) and its reduced and oxidized products in 1,2-dichloroethane. Visible spectrophotometry, spectroelectrochemistry, and cyclic and microelectrode voltammetry are used to characterize reactions whereby six-coordinate Mn(III) (Mn(TPP)(CH{sub 3}OH)Cl (3) and Mn(TPP)(CH{sub 3}OH){sub 2}{sup +} (4)) and five-coordinate Mn(II) (Mn(TPP)(CH{sub 3}OH)(6)) species are formed from 1. The axial ligand preferences of these two oxidation states lead to a biphasic behavior in the potential of Mn{sup III/II} reduction. At low CH{sub 3}OH concentrations, five-coordinate 1 is reduced to five-coordinate 6, causing E{sub 1/2} to shift to positive values with increasing (CH{sub 3}OH); at larger methanol concentrations, reduction of 4 to 6 predominates and the shift is in the opposite direction. In addition, the ring- and metal-centered oxidations of 1 at 1.22 and 1.56 V vs Ag/AgCl, respectively, coalesce into a single two-electron process at 1.31 V upon CH{sub 3}OH coordination. The negative shift in the metal-centered oxidation potential is an anticipated consequence of the greater donor strength of two CH{sub 3}OH ligands versus one Cl{sup {minus}} ligand. The positive shift in the ring-centered potential is believed to result from reduced deformationmore » of the porphyrin core and consequent stabilization of the porphyrin HOMO as axial coordination by CH{sub 3}OH returns the Mn atoms to an in-plate position from its significant out-of-plane displacement (0.27 {angstrom}) in 1.« less