Low temperature electrochemistry of metalloporphyrins in dichloromethane: characterization of transient species

Abstract

Cyclic voltammetry in dichloromethane at temperatures down to −90 °C has been used to characterize Fe(TPP)Cl (TPP=tetraphenylporphyrin dianion) and the transient high-spin six-coordinate complex [Fe(TPPXMeIm)Cl] (MeIm= N-methylimidazole). It is shown that low temperature cyclic voltammetry (LTCV) in dichloromethane can give high quality results using standard equipment and electrode systems; IR drop is not a serious problem. At −85 °C the anion Fe(TPP)Cl − slowly dissociates chloride and separate waves can be seen for the subsequent reduction of Fe(TPP)Cl − and Fe(TPP); this is not observed at room temperature. In the presence of excess MeIm, the transient species [Fe(TPP)(MeIm)Cl] decays with a half life of ca. 10 ms at room temperature, but at −90 °C is sufficiently persistent to allow electrochemical characterization. Its reduction occurs at a potential ca. 130 mV negative of that for Fe(TPP)Cl and is chemically irreversible, rapidly converting to Fe(TPP)(MeIm) 2. The utility of low temperature electrochemistry for investigating unstable metalloporphyrins in relatively nonpolar solvents is discussed.