Medium Effects Are as Important as Catalyst Design for Selectivity in Electrocatalytic Oxygen Reduction by Iron–Porphyrin Complexes

Several substituted iron–porphyrin complexes were evaluated for oxygen reduction reaction (ORR) electrocatalysis in different homogeneous and heterogeneous media. The selectivity for four-electron reduction to H₂O versus two-electron reduction to H₂O₂ varies substantially from one medium to another for a given catalyst. In many cases, the influence of the medium in which the catalyst is evaluated has a larger effect on the observed selectivity than the factors attributable to chemical modification of the catalyst. For instance, introduction of potential proton relays has variable effects depending on the catalyst medium. Thus, comparisons of selectivity results from supported and soluble molecular ORR electrocatalysts must be interpreted with caution, as selectivity is a property not only of the catalyst, but also of the larger mesoscale environment beyond the catalyst. Still, in all the direct pairwise comparisons in the same medium, the catalysts with potential proton relays have similar or better selectivity for the preferred 4<i>e</i>^– path

Molecular Cobalt Catalysts for O₂ Reduction: Low-Overpotential Production of H₂O₂ and Comparison with Iron-Based Catalysts

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O₂ reduction. Each of these complexes, with Co^III/II reduction potentials that span nearly 400 mV, mediate highly selective two-electron reduction of O₂ to H₂O₂ (93–99%) using decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. Kinetic studies reveal that the rate exhibits a first-order dependence on [Co] and [AcOH], but no dependence on [O₂] or [Fc*]. A linear correlation is observed between log­(TOF) vs <i>E</i>_1/2(Co^III/II) for the different cobalt complexes (TOF = turnover frequency). The thermodynamic potential for O₂ reduction to H₂O₂ was estimated by measuring the H⁺/H₂ open-circuit potential under the reaction conditions. This value provides the basis for direct assessment of the thermodynamic efficiency of the different catalysts and shows that H₂O₂ is formed with overpotentials as low as 90 mV. These results are compared with a recently reported series of Fe-porphyrin complexes, which catalyze four-electron reduction of O₂ to H₂O. The data show that the TOFs of the Co complexes exhibit a shallower dependence on <i>E</i>_1/2(M^III/II) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e^–/4H⁺ O₂ Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

Facile and selective 4e^–/4H⁺ electrochemical reduction of O₂ to H₂O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e^–/4H⁺ electrochemical O₂ reduction to H₂O is possible with rate constants as fast as 10⁵ M^–1 s^–1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O₂ reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O₂ to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O₂ with rate constants >10⁷ M^–1 s^–1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e^–/4H⁺ O₂ Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

Facile and selective 4e^–/4H⁺ electrochemical reduction of O₂ to H₂O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e^–/4H⁺ electrochemical O₂ reduction to H₂O is possible with rate constants as fast as 10⁵ M^–1 s^–1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O₂ reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O₂ to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O₂ with rate constants >10⁷ M^–1 s^–1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e^–/4H⁺ O₂ Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

Facile and selective 4e^–/4H⁺ electrochemical reduction of O₂ to H₂O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e^–/4H⁺ electrochemical O₂ reduction to H₂O is possible with rate constants as fast as 10⁵ M^–1 s^–1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O₂ reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O₂ to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O₂ with rate constants >10⁷ M^–1 s^–1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

Rational Design of Mononuclear Iron Porphyrins for Facile and Selective 4e^–/4H⁺ O₂ Reduction: Activation of O–O Bond by 2nd Sphere Hydrogen Bonding

Facile and selective 4e^–/4H⁺ electrochemical reduction of O₂ to H₂O in aqueous medium has been a sought-after goal for several decades. Elegant but synthetically demanding cytochrome c oxidase mimics have demonstrated selective 4e^–/4H⁺ electrochemical O₂ reduction to H₂O is possible with rate constants as fast as 10⁵ M^–1 s^–1 under heterogeneous conditions in aqueous media. Over the past few years, in situ mechanistic investigations on iron porphyrin complexes adsorbed on electrodes have revealed that the rate and selectivity of this multielectron and multiproton process is governed by the reactivity of a ferric hydroperoxide intermediate. The barrier of OO bond cleavage determines the overall rate of O₂ reduction and the site of protonation determines the selectivity. In this report, a series of mononuclear iron porphyrin complexes are rationally designed to achieve efficient OO bond activation and site-selective proton transfer to effect facile and selective electrochemical reduction of O₂ to water. Indeed, these crystallographically characterized complexes accomplish facile and selective reduction of O₂ with rate constants >10⁷ M^–1 s^–1 while retaining >95% selectivity when adsorbed on electrode surfaces (EPG) in water. These oxygen reduction reaction rate constants are 2 orders of magnitude faster than all known heme/Cu complexes and these complexes retain >90% selectivity even under rate determining electron transfer conditions that generally can only be achieved by installing additional redox active groups in the catalyst

Standard Reduction Potentials for Oxygen and Carbon Dioxide Couples in Acetonitrile and <i>N</i>,<i>N</i>‑Dimethylformamide

A variety of next-generation energy processes utilize the electrochemical interconversions of dioxygen and water as the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Reported here are the first estimates of the standard reduction potential of the O₂ + 4<i>e</i>^– + 4H⁺ ⇋ 2H₂O couple in organic solvents. The values are +1.21 V in acetonitrile (MeCN) and +0.60 V in <i>N</i>,<i>N</i>-dimethylformamide (DMF), each versus the ferrocenium/ferrocene couple (Fc^+/0) in the respective solvent (as are all of the potentials reported here). The potentials have been determined using a thermochemical cycle that combines the free energy for transferring water from aqueous solution to organic solvent, −0.43 kcal mol^–1 for MeCN and −1.47 kcal mol^–1 for DMF, and the potential of the H⁺/H₂ couple, – 0.028 V in MeCN and −0.662 V in DMF. The H⁺/H₂ couple in DMF has been directly measured electrochemically using the previously reported procedure for the MeCN value. The thermochemical approach used for the O₂/H₂O couple has been extended to the CO₂/CO and CO₂/CH₄ couples to give values of −0.12 and +0.15 V in MeCN and −0.73 and −0.48 V in DMF, respectively. Extensions to other reduction potentials are discussed. Additionally, the free energy for transfer of protons from water to organic solvent is estimated as +14 kcal mol^–1 for acetonitrile and +0.6 kcal mol^–1 for DMF

Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris­(amido)­amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt­(II) and zinc­(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes <b>[Co­(L)]</b>^<b>–</b> with dioxygen reversibly generates a small amount of a cobalt­(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn­[N­{CH₂CH₂NC­(O)­CH₂N­(CH₂Ph)₂}₃)]⁻ (<b>[Zn­(TN</b>^<b>Bn</b><b>)]</b>^<b>–</b>) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes <b>[M­(L)]</b>^<b>–</b> results in complete proteolysis and formation of the ligands <b>H</b>_<b>3</b><b>L</b>. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative <i>and</i> acidic conditions

Highly Active NiO Photocathodes for H₂O₂ Production Enabled via Outer-Sphere Electron Transfer

Tandem dye-sensitized photoelectrosynthesis cells are promising architectures for the production of solar fuels and commodity chemicals. A key bottleneck in the development of these architectures is the low efficiency of the photocathodes, leading to small current densities. Herein, we report a new design principle for highly active photocathodes that relies on the outer-sphere reduction of a substrate from the dye, generating an unstable radical that proceeds to the desired product. We show that the direct reduction of dioxygen from dye-sensitized nickel oxide (NiO) leads to the production of H₂O₂. In the presence of oxygen and visible light, NiO photocathodes sensitized with commercially available porphyrin, coumarin, and ruthenium dyes exhibit large photocurrents (up to 400 μA/cm²) near the thermodynamic potential for O₂/H₂O₂ in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized NiO over 24 h results in millimolar concentrations of H₂O₂ with essentially 100% faradaic efficiency. To our knowledge, these are among the most active NiO photocathodes reported for multiproton/multielectron transformations. The photoelectrosynthesis proceeds by initial formation of superoxide, which disproportionates to H₂O₂. This disproportionation-driven charge separation circumvents the inherent challenges in separating electron–hole pairs for photocathodes tethered to inner sphere electrocatalysts and enables new applications for photoelectrosynthesis cells