Proton-Coupled Electron Transfer from Tryptophan: A Concerted Mechanism with Water as Proton Acceptor

The mechanism of proton-coupled electron transfer (PCET) from tyrosine in enzymes and synthetic model complexes is under intense discussion, in particular the pH dependence of the PCET rate with water as proton acceptor. Here we report on the intramolecular oxidation kinetics of tryptophan derivatives linked to [Ru(bpy)3]2+ units with water as proton acceptor, using laser flash-quench methods. It is shown that tryptophan oxidation can proceed not only via a stepwise electron–proton transfer (ETPT) mechanism that naturally shows a pH-independent rate, but also via another mechanism with a pH-dependent rate and higher kinetic isotope effect that is assigned to concerted electron–proton transfer (CEP). This is in contrast to current theoretical models, which predict that CEP from tryptophan with water as proton acceptor can never compete with ETPT because of the energetically unfavorable PT part (pKa(Trp•H+) = 4.7 ≫ pKa(H3O+) ≈ −1.5). The moderate pH dependence we observe for CEP cannot be explained by first-order reactions with OH– or the buffers and is similar to what has been demonstrated for intramolecular PCET in [Ru(bpy)3]3+–tyrosine complexes (Sjödin, M.; et al. J. Am. Chem. Soc. 2000, 122, 3932. Irebo, T.; et al. J. Am. Chem. Soc. 2007, 129, 15462). Our results suggest that CEP with water as the proton acceptor proves a general feature of amino acid oxidation, and provide further experimental support for understanding of the PCET process in detail

Proton-Coupled Electron-Transfer Reduction of Dioxygen: The Importance of Precursor Complex Formation between Electron Donor and Proton Donor

The proton-coupled electron transfer (PCET) reaction has drawn extensive attention for its widespread occurrence in chemistry, biology, and materials science. The mechanistic studies via model systems such as tyrosine and phenol oxidation have gradually deepened the understanding of PCET reactions, which was widely accepted and applied to bond activation and transformation. However, direct PCET activation of nonpolar bonds such as the C–H bond, O2, and N2 has yet to be explored. Herein, we report that the interaction between electron donor and proton donor could overcome the barrier of direct O2 activation via a concerted electron–proton transfer mechanism. This work provides a new strategy for developing direct PCET activation of nonpolar bonds

Bioinspired Binickel Catalyst for Carbon Dioxide Reduction: The Importance of Metal–ligand Cooperation

Catalyst design for the efficient CO2 reduction reaction (CO2RR) remains a crucial challenge for the conversion of CO2 to fuels. Natural Ni–Fe carbon monoxide dehydrogenase (NiFe-CODH) achieves reversible conversion of CO2 and CO at nearly thermodynamic equilibrium potential, which provides a template for developing CO2RR catalysts. However, compared with the natural enzyme, most biomimetic synthetic Ni–Fe complexes exhibit negligible CO2RR catalytic activities, which emphasizes the significance of effective bimetallic cooperation for CO2 activation. Enlightened by bimetallic synergy, we herein report a dinickel complex, NiIINiII(bphpp)(AcO)2 (where NiNi(bphpp) is derived from H2bphpp = 2,9-bis(5-tert-butyl-2-hydroxy-3-pyridylphenyl)-1,10-phenanthroline) for electrocatalytic reduction of CO2 to CO, which exhibits a remarkable reactivity approximately 5 times higher than that of the mononuclear Ni catalyst. Electrochemical and computational studies have revealed that the redox-active phenanthroline moiety effectively modulates the electron injection and transfer akin to the [Fe3S4] cluster in NiFe-CODH, and the secondary Ni site facilitates the C–O bond activation and cleavage through electron mediation and Lewis acid characteristics. Our work underscores the significant role of bimetallic cooperation in CO2 reduction catalysis and provides valuable guidance for the rational design of CO2RR catalysts

Proton-Coupled Electron Transfer from Tyrosine: A Strong Rate Dependence on Intramolecular Proton Transfer Distance

Proton-coupled electron transfer (PCET) was examined in a series of biomimetic, covalently linked RuII(bpy)3–tyrosine complexes where the phenolic proton was H-bonded to an internal base (a benzimidazyl or pyridyl group). Photooxidation in laser flash/quench experiments generated the RuIII species, which triggered long-range electron transfer from the tyrosine group concerted with short-range proton transfer to the base. The results give an experimental demonstration of the strong dependence of the rate constant and kinetic isotope effect for this intramolecular PCET reaction on the effective proton transfer distance, as reflected by the experimentally determined proton donor–acceptor distance

Proton-Coupled Electron Transfer from Tyrosine: A Strong Rate Dependence on Intramolecular Proton Transfer Distance

Proton-coupled electron transfer (PCET) was examined in a series of biomimetic, covalently linked RuII(bpy)3–tyrosine complexes where the phenolic proton was H-bonded to an internal base (a benzimidazyl or pyridyl group). Photooxidation in laser flash/quench experiments generated the RuIII species, which triggered long-range electron transfer from the tyrosine group concerted with short-range proton transfer to the base. The results give an experimental demonstration of the strong dependence of the rate constant and kinetic isotope effect for this intramolecular PCET reaction on the effective proton transfer distance, as reflected by the experimentally determined proton donor–acceptor distance

Electrocatalytic Water Oxidation with a Copper(II) Polypeptide Complex

A self-assembly-formed tri­glycyl­glycine macro­cyclic ligand (TGG^4–) complex of Cu­(II), [(TGG^4–)­Cu^II–OH₂]^2–, efficiently catalyzes water oxidation in a phosphate buffer at pH 11 at room temperature by a well-defined mechanism. In the mechanism, initial oxidation to Cu­(III) is followed by further oxidation to a formal “Cu­(IV)” with formation of a peroxide intermediate, which undergoes further oxidation to release oxygen and close the catalytic cycle. The catalyst exhibits high stability and activity toward water oxidation under these conditions with a high turnover frequency of 33 s^–1

Bioinspired Trinuclear Copper Catalyst for Water Oxidation with a Turnover Frequency up to 20000 s^–1

Solar-powered water splitting is a dream reaction for constructing an artificial photosynthetic system for producing solar fuels. Natural photosystem II is a prototype template for research on artificial solar energy conversion by oxidizing water into molecular oxygen and supplying four electrons for fuel production. Although a range of synthetic molecular water oxidation catalysts have been developed, the understanding of O–O bond formation in this multielectron and multiproton catalytic process is limited, and thus water oxidation is still a big challenge. Herein, we report a trinuclear copper cluster that displays outstanding reactivity toward catalytic water oxidation inspired by multicopper oxidases (MCOs), which provides efficient catalytic four-electron reduction of O2 to water. This synthetic mimic exhibits a turnover frequency of 20000 s–1 in sodium bicarbonate solution, which is about 150 and 15 times higher than that of the mononuclear Cu catalyst (F–N2O2Cu, 131.6 s–1) and binuclear Cu2 complex (HappCu2, 1375 s–1), respectively. This work shows that the cooperation between multiple metals is an effective strategy to regulate the formation of O–O bond in water oxidation catalysis

Redox-Active Ligand Assisted Multielectron Catalysis: A Case of Electrocatalyzed CO₂‑to-CO Conversion

The selective reduction of carbon dioxide remains a significant challenge due to the complex multielectron/proton transfer process, which results in a high kinetic barrier and the production of diverse products. Inspired by the electrostatic and H-bonding interactions observed in the second sphere of the [NiFe]-CODH enzyme, researchers have extensively explored these interactions to regulate proton transfer, stabilize intermediates, and ultimately improve the performance of catalytic CO2 reduction. In this work, a series of cobalt(II) tetraphenylporphyrins with varying numbers of redox-active nitro groups were synthesized and evaluated as CO2 reduction electrocatalysts. Analyses of the redox properties of these complexes revealed a consistent relationship between the number of nitro groups and the corresponding accepted electron number of the ligand at −1.59 V vs. Fc+/0. Among the catalysts tested, TNPPCo with four nitro groups exhibited the most efficient catalytic activity with a turnover frequency of 4.9 × 104 s–1 and a catalytic onset potential 820 mV more positive than that of the parent TPPCo. Furthermore, the turnover frequencies of the catalysts increased with a higher number of nitro groups. These results demonstrate the promising design strategy of incorporating multielectron redox-active ligands into CO2 reduction catalysts to enhance catalytic performance

Photocatalytic Hydrogen Production with Conjugated Polymers as Photosensitizers

Artificial photosynthesis is a chemical process that aims to capture energy from sunlight to produce solar fuels. Light absorption by a robust and efficient photosensitizer is one of the key steps in solar energy conversion. However, common photosensitizers, including [Ru­(bpy)₃]²⁺ (RuP), remain far from the ideal. In this work, we exploited the performance of conjugated polymers (CPs) as photosensitizers in photodriven hydrogen evolution in aqueous solution (pH 6). Interestingly, CPs, such as poly­(fluorene-<i>co</i>-phenylene) derivative (429 mmol_H₂·g_CP^–1·h^–1), exhibit steady and high reactivity toward hydrogen evolution; this performance can rival that of a phosphonated RuP under the same conditions, indicating that CPs are promising metal-free photosensitizers for future applications in photocatalysis