Efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the determinants of the realization of a hydrogen-based society, as sluggish OER and ORR are the bottlenecks for the production and utilization of H2, respectively. A Co complex of 5,15-bis­(pentafluorophenyl)-10-(4)-(1-pyrenyl)­phenylcorrole (1) bearing a pyrene substituent was synthesized. When it was immobilized on multiwalled carbon nanotubes (MWCNTs), the 1/MWCNT composite displayed very high electrocatalytic activity and durability for both OER and ORR in aqueous solutions: it catalyzed a direct four-electron reduction of O2 to H2O in 0.5 M H2SO4 with an onset potential of 0.75 V vs normal hydrogen electrode (NHE), and it catalyzed the oxidation of water to O2 in neutral aqueous solution with an onset potential of 1.15 V (vs NHE, η = 330 mV). Control studies using a Co complex of 5,10,15-tris­(pentafluorophenyl)­corrole (2) demonstrated that the enhanced catalytic performance of 1 was due to the strong noncovalent π–π interactions between its pyrene moiety and MWCNTs, which were considered to facilitate the fast electron transfer from the electrode to 1 and also to increase the adhesion of 1 on carbon supports. The noncovalent immobilization of molecular complexes on carbon supports through strong π–π interactions appears to be a simple and straightforward strategy to prepare highly efficient electrocatalytic materials.

Water splitting is promising to realize a hydrogen‐based society. The practical use of molecular water‐splitting catalysts relies on their integration onto electrode materials. We describe herein the immobilization of cobalt corroles on carbon nanotubes (CNTs) by four strategies and compare the performance of the resulting hybrids for H2 and O2 evolution. Co corroles can be covalently attached to CNTs with short conjugated linkers (the hybrid is denoted as H1) or with long alkane chains (H2), or can be grafted to CNTs via strong π–π interactions (H3) or via simple adsorption (H4). An activity trend H1≫H3>H2≈H4 is obtained for H2 and O2 evolution, showing the critical role of electron transfer ability on electrocatalysis. Notably, H1 is the first Janus catalyst for both H2 and O2 evolution reactions in pH 0–14 aqueous solutions. Therefore, this work is significant to show potential uses of electrode materials with well‐designed molecular catalysts in electrocatalysis.

Low-overpotential water oxidation catalyzed by copper(ii) porphyrin to produce O2 in neutral aqueous solution and H2O2 in acidic solution.

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+ ] These authors contributed equally to this work. Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.1002/anie.202002311.Figure 1. a) Proposed homolytic and heterolytic HER pathways. b) Calculated transition state of the bimetallic homolysis for the HÀH bond formation mediated by Ni TPFP. Bond lengths are given in . Angewandte Chemie Communications

A series of cobalt complexes of 5,10,15-tris(pentafluorophenyl)-corrole [Co(tpfc)] (1) with various axial ligands were synthesized and examined as single-site catalysts for water oxidation. The used axial ligands include 4-cyanopyridine (py-CN), pyridine (py), 4-(dimethylamino)pyridine (py-NMe), 4-methoxypyridine (py-OMe), 1-methylimidazole (im-Me), and thiophenolate (thi). Complexes 1-py and 1-py-OMe were structurally characterized. The Co ion in both structures has an almost identical distorted octahedral coordination environment with the four N atoms of tpfc defining the equatorial plane and the two molecules of pyridine (for 1-py) or 4-methoxypyridine (for 1-py-OMe) occupying the axial positions. Electrochemical studies of these Co corroles in acetonitrile showed that they all display two oxidation events and the oxidation waves shift to the cathodic direction with electron-donating axial ligands, a trend that is consistent with increased electron densities on Co ions. All these Co corroles were found to be active for electrocatalytic water oxidation: by using catalyst-coated fluorine-doped tin oxide (FTO) working electrodes, cyclic voltammograms displayed pronounced catalytic waves for water oxidation in 0.1 M pH 7.0 phosphate buffer solutions. The onset overpotentials are in the range of 510 to 580 mV, depending on the electron-donating ability of the trans axial ligands. These results demonstrate that the catalytic activities of Co corroles for water oxidation are considerably affected by the trans axial ligands on Co centers and provide valuable insights into the design of new catalysts for water oxidation.

Water-soluble copper(II) complexes of the dianionic tridentate pincer ligand N,N'-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate (L) are catalysts for water oxidation. In [L-Cu-DMF] (1, DMF = dimethylformamide) and [L-Cu-OAc] (2, OAc = acetate), ligand L binds Cu through three N atoms, which define an equatorial plane. The fourth coordination site of the equatorial plane is occupied by DMF in 1 and by OAc in 2. These two complexes can electrocatalyze water oxidation to evolve O in 0.1 M pH 10 carbonate buffer. Spectroscopic, titration, and crystallographic studies show that both 1 and 2 undergo ligand exchange when they are dissolved in carbonate buffer to give [L-Cu-COH] (3). Complex 3 has a similar structure as those of 1 and 2 except for having a carbonate group at the fourth equatorial position. A catalytic cycle for water oxidation by 3 is proposed based on experimental and theoretical results. The two-electron oxidized form of 3 is the catalytically active species for water oxidation. Importantly, for these two oxidation events, the calculated potential values of E = 1.01 and 1.59 V vs normal hydrogen electrode (NHE) agree well with the experimental values of E = 0.93 and 1.51 V vs NHE in pH 10 carbonate buffer. The potential difference between the two oxidation events is 0.58 V for both experimental and calculated results. With computational evidence, this Cu-bound carbonate group may act as a proton shuttle to remove protons for water activation, a key role resembling intramolecular bases as reported previously.

Asymmetrical Pacman dinuclear Co bisporphyrin shows significantly improved activity and selectivity for catalytic reduction of O2 to water in comparison with corresponding mononuclear Co porphyrins and symmetrical dinuclear Co bisporphyrins.