Electrochemistry of Fe(IV) and Mn(IV) corroles containing meso-dichlorophenyl substituents and the use of these compounds as catalysts for the electroreduction of dioxygen in acid media

Abstract

Two meso-dichlorophenyl substituted metallocorroles were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in dichloromethane, benzonitrile, and pyridine containing 0.1 M tetra-n -butylammonium perchlorate (TBAP) as supporting electrolyte. The examined compounds are represented as (Cl 2 Ph) 3 CorFe IV Cl and (Cl 2 Ph) 3 CorMn IV Cl where (Cl 2 Ph) 3 Cor is the trianion of 5,10,15-tri(2,4-dichlorophenyl)corrole. Each metallocorrole was examined as to its catalytic activity for the electoreduction of dioxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO 4 . Cyclic voltammetry combined with linear sweep voltammetry at a rotating disk electrode (RDE) and a rotating ring disk electrode (RRDE) was utilized to evaluate the catalytic activity for the electroreduction of O 2 . The main O 2 reduction product is hydrogen peroxide under the given experimental conditions.

Electrochemistry of Corroles in Nonaqueous Media

“…Almost two dozen papers (refs , , , , − , , , , , , and − ) have been published on the electrochemistry of manganese corroles since the initial electrochemical characterization of (OEC)Mn in 1998 . Some of the published papers have elucidated the redox properties of air stable compounds with a Mn­(III) center (refs , , , − , , , , and − ), while others have involved characterization of synthesized corroles with Mn­(IV) (refs , , , , , , , , and − ) or Mn­(V) (refs , , and ) central metal ions. Most of the elucidated redox processes have been oxidations, in large part because of the quite negative potential needed to access the singly reduced form of the compounds.…”

“…In the initial study of (OEC)Mn by Kadish, Vogel, and their co-workers, it was stated that the first oxidation of (OEC)Mn “unambiguously leads to a Mn­(IV) complex as ascertained by the ESR spectrum” . Assignments of a Mn­(IV) oxidation state were also made in a follow-up study of (OEC)Mn by Ou and co-workers, as well as in a number of later studies from other laboratories involving a characterization of manganese corroles with different skeletal structures (see refs , , , , , , , , and − ). However, if the examined (OEC)Mn derivative were actually present in solution as a Mn­(II) π-cation radical, i.e., (OEC +• )­Mn II , then the reductions and oxidations in nonaqueous media might both involve the conjugated macrocycle and lead to an initial formation of [(OEC)­Mn II ] − and [(OEC 2+ )­Mn II ] + prior to generation of a final Mn­(III) and Mn­(IV) corrole, respectively.…”

“…The electrochemistry of iron corroles has attracted much attention over the last 2 decades (see refs , , , , , , , , , , and − ) due in part to their similarity to iron porphyrins, in part to their rich electrochemistry, and in part to their wide range of potential applications. Some reports on iron corrole electrochemistry have been limited to describing only the first reduction and first oxidation of the compounds, while others have demonstrated that iron corroles can undergo multiple reductions and oxidations in nonaqueous media, some of which involve the central metal ion and some of which involve the macrocycle.…”

“…Some reports on iron corrole electrochemistry have been limited to describing only the first reduction and first oxidation of the compounds, while others have demonstrated that iron corroles can undergo multiple reductions and oxidations in nonaqueous media, some of which involve the central metal ion and some of which involve the macrocycle. The exact number of redox reactions exhibited by a given iron corrole will depend upon the number and type of meso- and β-pyrrole substituents (see refs , , , , , , and ), the type and number of axial ligands (see refs , , , , , , , , , and − ), the oxidation state of the central metal ion in the initial compound (see refs , , , , , , , , , and − ), and the solution conditions (see refs , , ).…”

Electrochemistry of Bis(pyridine)cobalt (Nitrophenyl)corroles in Nonaqueous Media

“…Metallocorroles are now an often-studied group of macrocycles because of their unique spectral and electrochemical properties, as well as their potential applications as catalysts for a variety of reactions. − A rich redox reactivity can be observed for transition-metal corroles, where both metal- and ring-centered reactions have been characterized under a variety of solution conditions. The most-often-studied derivatives have contained cobalt, ,− iron, − or manganese ,,,− central metal ions, which have been characterized in both high and low oxidation states. A conversion between the different oxidation states of the corrole is easy to achieve using electrochemical techniques and has been best demonstrated in the case of cobalt derivatives, where the potentials for reduction or oxidation will systematically vary with changes in the number and types of electron-donating or electron-withdrawing substituents on the macrocycle. ,,,,,,, …”