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The possible effects of the electron-donating properties of p-hydroxyphenyl groups on the catalytic activity of cobalt porphyrins toward the electro-reduction of O₂ were explored with three cobalt porphyrins having p-hydroxyphenyl substituents in the meso position of the porphyrin ring. The porphyrins were examined as electrocatalysts adsorbed on the surfaces of pyrolytic graphite electrodes in aqueous solution and as homogeneous catalysts dissolved in methanolic solutions. The adsorbed molecules exhibited a higher catalytic activity than those in solution and accomplished the reduction of a portion of the O₂ molecules reaching the electrode to H₂O. However, the four-electron reduction was indirect, with H₂O₂ being formed as an intermediate that was subsequently reduced. Although the introduction of the electron-donating p-hydroxyphenyl substituents on the porphyrin ring altered the catalytic activity of the cobalt porphyrins, the effect was not as great as can be obtained by coordinating ruthenium (II) complexes to π-accepting ligands in the meso position of the porphyrin ring.

The synthesis and electrochemical characterization of ‘crowned’ and ‘dioxocyclam’ metalloporphyrins containing Co(II) or Zn(II) metal ions is presented. Each complex is also characterized by mass spectrometry in addition to a variety of spectroscopic techniques (UV-vis, FTIR, ESR (in the case of Co(II)) and ¹H and ¹³CNMR spectroscopy).

Hematoporphyrin IX, H₂HMP, 8,13-bis(1-hydroxyethyl)-3,7,12,17-tetramethyl-21H,23H-porphine-2,18-dipropionic acid and protoporphyrin IX, H₂PP, 8,13-divinyl-3,7,12,17-tetramethyl-21H,23H-porphine-2,18-dipropionic acid were efficiently immobilized on niobium oxide grafted on a silica gel surface, SiO₂/Nb₂O₅, by the -COO–Nb bond formed between the porphyrin carboxyl groups and the grafted Nb₂O₅. These immobilized porphyrins, SiO₂/Nb₂O₅/H₂HMP and SiO₂/Nb₂O₅/H₂PP, were further reacted with Co(II) in dimethylformamide, resulting in SiO₂/Nb₂O₅/CoHMP and SiO₂/Nb₂O₅/CoPP metallated complexes. The UV-vis spectra of the solid materials showed changes of the Q-bands (a_2u → e_g transition) upon metallation, indicating that by incorporation of Co(II) in the porphyrin ring the local symmetry changed from D_2h to D_4h. These materials, when incorporated in carbon paste electrodes, presented the property of electrocatalyzing O₂ reduction. Rotating disk experiments were performed in order to estimate the number of electrons involved in the process. It was observed that, for both modified electrodes, O₂ was reduced to water in a four-electron process. Amperometric studies showed the potentiality of both modified electrodes as sensors for the determination of dissolved dioxygen. The response time was less than 3 s. A linear response for both systems was obtained between 2 and 12 ppm.

The dehalogenation activities of cobalt porphyrin catalysts with dendritic phenylazomethine units on the meso-positions were studied. Tetrachloroethylene was used as a substrate. By applying samarium iodide as a reducing agent, the tetrachloroethylene conversion into trichloroethylene and dichloroethylene was observed in the presence of the cobalt porphyrin catalyst. The turnover frequency was 1.70 min⁻¹ when a cobalt porphyrin connected with dendritic phenylazomethines (3 generations) was employed as the catalyst. While the turnover frequency was almost the same as that for a non-dendritic porphyrin catalyst (1.57 min⁻¹ by cobalt tetraphenylporphyrin), a significant increase in the turnover frequency was observed when samarium (3.43 min⁻¹) or terbium trifluoromethane sulfonate (8.57 min⁻¹) binds to the phenylazomethine units of the dendrimer. Because these metal ions (Sm³⁺ or Tb³⁺) are positioned in a space between the encapsulated catalytic center and the solvent phase, they can act as electron mediators that relay electrons from the reducing agent (samarium iodide) to the catalytic center (cobalt porphyrin). This facile electron transfer would provide an enhancement in the catalytic activity for the tetrachloroethylene reduction leading to dehalogenation.

Two cobalt porphyrins, (OEP)Co^II and (TPP)Co^II, where OEP and TPP are the dianions of octaethylporphyrin and tetraphenylporphyrin, respectively, were examined as electrocatalysts for the reductive dechlorination of DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane) in N,N′-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). No reaction is observed between DDT and the porphyrin in its Co(II) oxidation state but this is not the case for the reduced Co(I) forms of the porphyrins which electrocatalyze the dechlorination of DDT, giving initially DDD (1,1-bis(4-chlorophenyl)-2,2-dichloroethane), DDE (1,1-bis(4-chlorophenyl)-2, 2-dichloroethylene) and DDMU (1,1-bis(4-chlorophenyl)-2-chloroethylene) as determined by GC-MS analysis of the reaction products. A further dechlorination product, DDOH (2,2-bis(4-chlorophenyl)ethanol), is also formed on longer timescales when using (TPP)Co as the electroreduction catalyst. The effect of porphyrin structure and reaction time on the dechlorination products was examined by GC-MS, cyclic voltammetry, controlled potential electrolysis and UV-visible spectroelectrochemistry and a mechanism for the reductive dechlorination is proposed.

Three face-to-face biscobalt bisporphyrin dyads, including one incorporating a copper(II) ion inside the linker, were synthesized and characterized both spectroscopically and electrochemically in three non-aqueous solvents, dichloromethane, benzonitrile and pyridine. The electrocatalytic reduction of dioxygen with these derivatives on an electrode surface in 1.0 M HClO₄ was also investigated and the results are compared to that obtained with "regular" Pacman biscobalt bisporphyrins under the same experimental conditions. Surprisingly, the tris-metal species (Cu-bisCo) catalyzes the reduction of O₂ mainly via a 2e^- transfer process, leading to H₂O₂, while the bis-metal (bisCo) catalyst produces H₂Ovia a four electron, four proton process.