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 (TpYPP)Co, butano(TpYPP)Co-II, and benzo-(TpYPP)Co-II, where TpYPP represents the dianion of the meso-substituted porphyrin, Y is a CH3, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the beta- and beta'-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 (TpYPP)-Co-II and butano(TpYPP)Con in CH2Cl2 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 (CoCHCl)-C-III-Cl-2 as the product. Only one reversible reduction is seen for benzo(TpYPP)Con under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt(II) porphyrin pi-anion radical. Three oxidations can be observed for each examined compound in CH2Cl2. The first oxidation is metal-centered for the (TpYPP)Co and benzo(TpYPP)Con derivatives, leading to generation of a cobalt(III) porphyrin with an intact pi-ring system, but this redox process is ring-centered in the case of butano(TpYPP)Con and gives a Con pi-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 HClO4. The number of electrons transferred (n) during ORRs is 2.0 for the butano(TpYPP)Co-II derivatives, consistent with only H2O2 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 H2O and H2O2 as the reduction product. This result indicates that the beta and beta' substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.