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

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

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

Spanning Four Mechanistic Regions of Intramolecular Proton-Coupled Electron Transfer in a Ru(bpy)₃²⁺–Tyrosine Complex

Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked [Ru­(bpy)₃]²⁺ photosensitizer in aqueous media has been systematically reinvestigated by laser flash-quench kinetics as a model system for PCET in radical enzymes and in photochemical energy conversion. Previous kinetic studies on Ru–TyrOH molecules (Sjödin et al. <i>J. Am. Chem. Soc.</i> <b>2000</b>, <i>122</i>, 3932; Irebo et al. <i>J. Am. Chem. Soc.</i> <b>2007</b>, <i>129</i>, 15462) have established two mechanisms. Concerted electron–proton (CEP) transfer has been observed when pH < p<i>K</i>_a(TyrOH), which is pH-dependent but not first-order in [OH^–] and not dependent on the buffer concentration when it is sufficiently low (less than ca. 5 mM). In addition, the pH-independent rate constant for electron transfer from tyrosine phenolate (TyrO^–) was reported at pH >10. Here we compare the PCET rates and kinetic isotope effects (<i>k</i>_H/<i>k</i>_D) of four Ru–TyrOH molecules with varying Ru^III/II oxidant strengths over a pH range of 1–12.5. On the basis of these data, two additional mechanistic regimes were observed and identified through analysis of kinetic competition and kinetic isotope effects (KIE): (i) a mechanism dominating at low pH assigned to a stepwise electron-first PCET and (ii) a stepwise proton-first PCET with OH^– as proton acceptor that dominates around pH = 10. The effect of solution pH and electrochemical potential of the Ru^III/II oxidant on the competition between the different mechanisms is discussed. The systems investigated may serve as models for the mechanistic diversity of PCET reactions in general with water (H₂O, OH^–) as primary proton acceptor

Multiple Pathways in the Oxidation of a NADH Analogue

Oxidation of the NADH analogue, <i>N</i>-benzyl-1,4-dihydronicotinamide (BNAH), by the 1e^– acceptor, [Os­(dmb)₃]³⁺, and 2e^–/2H⁺ acceptor, benzoquinone (Q), has been investigated in aqueous solutions over extended pH and buffer concentration ranges by application of a double-mixing stopped-flow technique in order to explore the redox pathways available to this important redox cofactor. Our results indicate that oxidation by quinone is dominated by hydride transfer, and a pathway appears with added acids involving concerted hydride-proton transfer (HPT) in which synchronous transfer of hydride to one O-atom at Q and proton transfer to the second occurs driven by the formation of the stable H₂Q product. Oxidation by [Os­(dmb)₃]³⁺ occurs by outer-sphere electron transfer including a pathway involving ion-pair preassociation of HPO₄^2– with the complex that may also involve a concerted proton transfer

Redox-Active Ligand Assisted Multielectron Catalysis: A Case of Co^III Complex as Water Oxidation Catalyst

Water oxidation is the key step in both natural and artificial photosynthesis to capture solar energy for fuel production. The design of highly efficient and stable molecular catalysts for water oxidation based on nonprecious metals is still a great challenge. In this article, the electrocatalytic oxidation of water by Na­[(L^4–)­Co^III], where L is a substituted tetraamido macrocyclic ligand, was investigated in aqueous solution (pH 7.0). We found that Na­[(L^4–)­Co^III] is a stable and efficient homogeneous catalyst for electrocatalytic water oxidation with 380 mV onset overpotential in 0.1 M phosphate buffer (pH 7.0). Both ligand- and metal-centered redox features are involved in the catalytic cycle. In this cycle, Na­[(L^4–)­Co^III] was first oxidized to [(L^2–)­Co^IIIOH] via a ligand-centered proton-coupled electron transfer process in the presence of water. After further losing an electron and a proton, the resting state, [(L^2–)­Co^IIIOH], was converted to [(L^2–)­Co^IVO]. Density functional theory (DFT) calculations at the B3LYP-D3­(BJ)/6-311++G­(2df,2p)//B3LYP/6-31+G­(d,p) level of theory confirmed the proposed catalytic cycle. According to both experimental and DFT results, phosphate-assisted water nucleophilic attack to [(L^2–)­Co^IVO] played a key role in O–O bond formation

Electrocatalytic Water Oxidation by a Monomeric Amidate-Ligated Fe(III)–Aqua Complex

The six-coordinate Fe^III-aqua complex [Fe^III(dpaq)­(H₂O)]²⁺ (<b>1</b>, dpaq is 2-[bis­(pyridine-2-ylmethyl)]­amino-<i>N</i>-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate–water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe^V(O)²⁺ followed by a reaction first order in catalyst and added water, respectively, with <i>k</i>_o = 0.035(4) M^–1 s^–1 by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O₂ through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst

Role of Proton-Coupled Electron Transfer in the Redox Interconversion between Benzoquinone and Hydroquinone

Benzoquinone/hydroquinone redox interconversion by the reversible Os­(dmb)₃^3+/2+ couple over an extended pH range with added acids and bases has revealed the existence of seven discrete pathways. Application of spectrophotometric monitoring with stopped-flow mixing has been used to explore the role of PCET. The results have revealed a role for phosphoric acid and acetate as proton donor and acceptor in the concerted electron–proton transfer reduction of benzoquinone and oxidation of hydroquinone, respectively

Cu(II) Aliphatic Diamine Complexes for Both Heterogeneous and Homogeneous Water Oxidation Catalysis in Basic and Neutral Solutions

Simply mixing a Cu­(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis, depending on pH. In phosphate buffer at pH 12, the Cu­(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu­(OH)₂ film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm² was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal, following Cu­(II) oxidation to Cu­(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu­(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation via the formation of a Cu­(II) phosphate film during electrolyses. As the pH is decreased, en, with p<i>K</i>_b ≈ 6.7, becomes less coordinating and the precipitation of the Cu­(II) film inhibits water oxidation. The Cu­(II)-based reactivity toward water oxidation is shared by Cu­(II) complexation to the analogous 1,3-propylenediamine (pn) ligand over a wide pH range

Identifying Metal-Oxo/Peroxo Intermediates in Catalytic Water Oxidation by In Situ Electrochemical Mass Spectrometry

Molecular catalysis of water oxidation has been intensively investigated, but its mechanism is still not yet fully understood. This study aims at capturing and identifying key short-lived intermediates directly during the water oxidation catalyzed by a cobalt-tetraamido macrocyclic ligand complex using a newly developed an in situ electrochemical mass spectrometry (EC-MS) method. Two key ligand-centered-oxidation intermediates, [(L2–)CoIIIOH] and [(L2–)CoIIIOOH], were directly observed for the first time, and further confirmed by 18O-labeling and collision-induced dissociation studies. These experimental results further confirmed the rationality of the water nucleophilic attack mechanism for the single-site water oxidation catalysis. This work also demonstrated that such an in situ EC-MS method is a promising analytical tool for redox catalytic processes, not only limited to water oxidation