Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions

Three series of cobalt tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds have the general formula (T p YPP)Co, butano(T p YPP)Co II, and benzo(T p YPP)Co II, where T p YPP represents the dianion of the meso-substituted porphyrin, Y is a CH 3, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the β- and β′-substituted groups on the four pyrrole rings of the compound. Each porphyrin undergoes one or two reductions depending upon the meso substituent and solvent utilized. Two irreversible reductions are observed for (T p YPP)Co II and butano(T p YPP)Co II in CH 2 Cl 2 containing 0.1 M tetra- n -butylammonium perchlorate; the first leads to the formation of a highly reactive cobalt(I) porphyrin, which can then rapidly react with a solvent to give a Co III CH 2 Cl as the product. Only one reversible reduction is seen for benzo(T p YPP)Co II under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CH 2 Cl 2 . The first oxidation is metal-centered for the (T p YPP)Co and benzo(T p YPP)Co II derivatives, leading to generation of a cobalt(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano(T p YPP)Co II and gives a Co II π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs) when adsorbed on a graphite electrode in 1.0 M HClO 4 . The number of electrons transferred ( n ) during ORRs is 2.0 for the butano(T p YPP)Co II derivatives, consistent with only H 2 O 2 being produced as a product for the reaction with O 2 . However, the reduction of O 2 using the cobalt benzoporphyrins as catalysts gave n values between 2.6 and 3.1 under the same solution conditions, thus producing a mixture of H 2 O and H 2 O 2 as the reduction product. This result indicates that the β and β′ substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.