Synthesis And Electrocatalytic Properties Of Novel Substituted Porphyrin Complexes

Porphyrins are of the tetrapyrrole compounds containing conjugated π structures. Research has shown that porphyrins have very profound values of research and application in the fields of optoelectronic functional materials, medical drugs, chemical catalysis, energy environment and analytical chemistry etc, partly because of their stable π-conjugated system, also on account of their wonderful chemical modifiability. Through changing central metal ions and introducing electron-donating or electron-withdrawing substituents on the meso-and/or beta-positions around the periphery of the porphyrin macrocycle may change the steric and electronic structures and regulate directionally the properties of the compounds, leading to obtain expected novel materials having specific properties.With the rapid development of global economy, energy crisis and environmental pollution problems have become more and more serious, so the development of new clean and efficient energy system is imminent. Fuel cell is considered to be one of the best choices for stable mobile power because of its high energy and environment friendliness. At present, countries all over the world attach great importance to the research, development and application of this new type of energy. The oxygen reduction reaction (ORR) plays a very important role in the electrochemical energy conversion technology of fuel cell, and the selection and use of the electrocatalyst is the key to improve the efficiency of ORR. Over the years, platinum and its alloys have been considered to be the most effective catalyst for the ORR. However, due to the limited resource and expensive, their commercial usage in fuel cell is limited to a great extent. Therefore, in the past twenty years, looking for an inexpensive non noble metal and its oxides, or catalytic non-metal or metal organic compounds to replace Pt as the catalyst of ORR has always been a key subject for scientists to study.It is known that functionalized porphyrins have high catalytic activity on ORR, hence the research and application of this type of compounds has gained more and more attention in the past years. However, the number of metalloporphyrins examined as the ORR catalysts is limited, and the relationship between the structure and performance of the compounds as the catalyst is still not clear and worth to do further study. Therefore, in the first of this dissertation several kinds of novel porphyrins were designed and synthesized. The spectroscopic and electrochemical properties of the newly synthesized compounds were then examined in nonaqueous medial. The influences of central metal ion and substituents at meso-or beta-position of the pophyrins on spectral and electrochemical properties were also investigated. The catalytic activities of the porphyrins for ORR under different experimental conditions were studied emphatically, which revealed the structure-activity relationship and might provid a theoretical and experimental basis for the directed synthesis of fuel cell electrode catalytic materials with good performance.This dissertation mainly includes the following chapters:the first chapter is the introduction, which summarized the general synthetic methods, properties and applications of porphyrins, as well as the current research status of catalytic reduction of molecular oxygen in the world; the second chapter is about electrochemistry and catalytic properties for dioxygen reduction using meso ferrocene-substituted cobalt porphyrins; the third chapter is electrochemistry and spectroelectrochemistry of meso ferrocene-substituted iron and manganese porphyrins; the fourth chapter concerning electrochemistry and catalytic properties of beta nitro-substituted cobalt porphyrins; the fifth chapter is electrochemistry and spectroelectrochemistry of beta nitro-substituted iron and manganese porphyrins.In chapter 2, four cobalt porphyrins containing different number of meso-substituted ferrocenyl groups were synthesized, which can be represented as (Fc)n(CH3Ph)4-nPorCo, where Por is a dianion of the substituted porphyrin, Fc and CHsPh represent ferrocenyl and p-CH3C6H4 groups linked at meso-positions of the macrocycle, and n varies from 0 to 4. The compounds are examined as to their electrochemical properties in N,N’-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate as supporting electrolyte. The results show that each porphyrin undergoes two reversible one-electron reductions and two to six one-electron oxidations in DMF, with the exact number depending upon the number of Fc groups on the compound. Also, each compound was investigated as a catalyst for the electoreduction of dioxygen when adsorbed on a graphite electrode in 1.0 M HClO4. The results indicate that appending ferrocene groups directly to the meso-positions of a porphyrin macrocycle will increase the selectivity of the oxygen reduction. The number of electrons transferred (n) during the catalytic reduction was 2.0 for the three ferrocenyl substituted compounds, consistent with only H2O2 being produced as a product of the reaction. This selectivity of the electrocatalytic oxygen reduction reaction is explained on the basis of steric hindrance by the ferrocene substituents which prevent dimerization of the compound on the surface of electrode.In chapter 3, several iron porphyrins and manganese porphyrins, which having one or two meso-substituted ferrocenyl groups, were synthesized and examined as to their electrochemical and in-situ spectroelectrochemical properties in nonaqueous media. The proposed electron transferred mechanism during the oxidation-reduction processes and the effects of ferrocenyl substituents on the redox potentials and the UV-visible spectra of the compounds were discussed.In chapter 4, two cobalt(Ⅱ) porphyrins containing α β-pyrrole nitro substituent were characterized by cyclic voltammetry, spectroelectrochemistry, linear sweep voltammetry with a rotating disk electrode (RDE) and voltammetry at a rotating ring disk electrode (RRDE). The results were compared with the cobalt porphyrins without nitro group. Two reductions and three oxidations are observed for each compound, with or without nitro group at β-position in CH2Cl2 containing 0.10 M tetra-n-butylammonium perchlorate. The spectroelectrochemical data suggest that the nitro-substituted and non-nitro compounds undergo a same electron transfer process for the oxidations. The first is metal-centered and the next two involve the conjugated π-system of the macrocycle. However, although two reductions can be observed in these two kinds of compounds, their electron transfer mechanisms are different.The first reduction of the compounds without a nitro substituent is metal-centered and leads to formation of a Co(Ⅰ) porphyrin which then reacts with the CH2Cl2 solvent to generate a carbon-bonded CoⅢ-R complex. A further reduction then occurs at more negative potentials to generate an unstable Co(Ⅱ) σ-bonded compound, which is decomposed and oxidized to the initial state of the compound on the electrode surface. In contrast to these reactions, the first reduction of the nitro-substituted porphyrins is macrocycle-centered under the same solution conditions and gives a Co(Ⅱ) porphyrin π-anion radical. This reversible electron transfer process is then followed at more negative potentials by a second reversible one-electron addition to give a Co(Ⅱ) dianion. The results show that the introduction of nitro substituent has a significant effect on the electrochemical behavior of the cobalt porphyrins. In addition, each neutral Co(II) porphyrin was also examined as to its catalytic activity for electoreduction of molecular oxygen when coated on an edge-plane pyrolytic graphite electrode in 1.0 M HClO4. The β-pyrrole nitro-substituted derivatives were shown to be better catalysts than the non-nitro substituted compounds under the utilized experimental conditions.Final, some iron porphyrins and manganese porphyrins, including the compounds having nitro substituent on β-position, were examined in chapter 5 by cyclic voltammetry and in-situ spectroelectrochemistry. The examined compounds are represented as (CH3Ph)4PorM and NO2(CH3Ph)4PorM, where Por is a dianion of the substituted porphyrin and M is a central metal ion, such as Fe, Mn and Co. According to the experimental results, the effects of central metal ions and nitro substitutent on redox properties and spectra of redox potentials were discussed. The electron transfer mechanisms of the compounds were also explored under the given nonaqueous media.