Rapid and efficient electrochemical CO reduction is an ongoing challenge for the production of sustainable fuels and chemicals. In this work, electrochemical CO reduction is investigated using metalloporphyrin catalysts (metal = Mn, Fe, Co, Ni, Cu) that feature one hydroxyphenyl group, and three other phenyl groups, in the porphyrin heterocycle (5-(2-hydroxyphenyl)-10,15,20-triphenylporphyrin, TPOH). These complexes, which are minimal versions of related complexes bearing up to eight proton relays, were designed to allow more straightforward determination of the role of the 2-hydroxylphenyl functional group. The iron-substituted version of TPOH supports robust reduction of CO in acetonitrile solvent, where carbon monoxide is the only detected product. Addition of weak Brønsted acids (1 M water or 8 mM phenol) gives rise to almost 100-fold enhancement in turnover frequency. Surprisingly, the iron analogue is a poor catalyst when the solvent is changed to dimethylformamide. These results lead to the proposal of a model where the hydroxyphenyl group behaves as a local proton source, a hydrogen bond donor to CO-bound intermediates, and a hydrogen bonding partner to Brønsted acids. The observations from this model suggest improvements for existing electrocatalytic CO reduction systems.

The development of catalytic systems that selectively reduce O2 to water is needed to continue the advancement of fuel cell technologies. As an alternative to platinum catalysts, derivatives of iron (Fe) and cobalt (Co) porphyrin molecular catalysts provide one benchmark for catalyst design, but incorporation of these catalysts into heterogeneous platforms remains a challenge. Co-porphyrins can be heterogeneous O2 reduction catalysts when immobilized on to edge plane graphite (EPG) electrodes, but their selectivity for the desired four-electron reduction of O2 to H2O is often poor. Herein, we demonstrate substantial improvements in the O2 reduction selectivity using a Co-porphyrin that incorporates a 2-pyridyl group at one of the meso-positions of a Co-tetraarylporphyrin (cobalt­(II) 5-(2-pyridyl)-10,15,20-triphenylporphyrin, CoTPPy). The properties of CoTPPy immobilized on EPG were investigated using cyclic voltammetry, rotating disk, and rotating ring-disk electrochemistry. The presence of a single 2-pyridyl group in the CoTPPy gives rise to the four-electron reduction of O2, as opposed to the two-electron reduction commonly associated with cobalt porphyrins. Detailed electrochemical studies of CoTPPy and related Co and Fe porphyrins are described. Use of Co instead of Fe improves overpotentials by over 200 mV with a factor of 2 increase in maximum turnover frequency (TOFmax). This work demonstrates that a simple change in catalyst structure can dramatically change the selectivity for O2 reduction.

Oxygen reduction in acidic aqueous solution mediated by a series of asymmetric iron (III)-tetra(aryl)porphyrins adsorbed to basal- and edge- plane graphite electrodes is investigated. The asymmetric iron porphyrin systems bear phenyl groups at three meso positions and either a 2-pyridyl, a 2-benzoic acid, or a 2-hydroxyphenyl group at the remaining meso position. The presence of the three unmodified phenyl groups makes the compounds insoluble in water, enabling catalyst retention during electrochemical experiments. Resonance Raman data demonstrate that catalyst layers are maintained, but can undergo modification after prolonged catalysis in the presence of O2 . The introduction of a single proton relay group at the fourth meso position makes the asymmetric iron porphyrins markedly more robust catalysts; these molecules support higher sustained current densities than the parent iron tetraphenylporphyrin. Iron porphyrins bearing a 2-pyridyl group are the most active catalysts and operate at stable current densities ≥1 mA cm(-2) for over 5 h. Comparative analysis of the catalysts with different proton relays also is reported.

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Organometallic Ru­(II)-cymene complexes linked to ferrocene (Fc) via nitrogen heterocycles have been synthesized and studied as cytotoxic agents. These compounds are analogues of Ru­(II)-arene piano-stool anticancer complexes such as RAPTA-C. The Ru center was coordinated by pyridine, imidazole, and piperidine with 0-, 1-, or 2-carbon bridges to Fc to give six bimetallic, dinuclear compounds, and the properties of these complexes were compared with their non-Fc-functionalized parent compounds. Crystal structures for five of the compounds, their Ru-cymene parent compounds, and an unusual trinuclear compound were determined. Cyclic voltammetry was used to determine the formal MIII/II potentials of each metal center of the Ru-cymene-Fc complexes, with distinct one-electron waves observed in each case. The Fc-functionalized complexes were found to exhibit good cytotoxicity against HT29 human colon adenocarcinoma cells, whereas the parent compounds were inactive. Similarly, antibacterial activity from the Ru-cymene-Fc compounds was observed against Bacillus subtilis, but not from the unfunctionalized complexes. In both cases, the IC50 values correlated quantitatively with the Fc+/0 reduction potentials. This is consistent with more facile oxidation to give ferrocenium, and subsequent generation of toxic reactive oxygen species, leading to greater cytotoxicity. The antioxidant properties of the complexes were quantified by a 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay. EC50 values indicate that linking of the Ru and Fc centers promotes antioxidant activity.