Mechanistic Insights Into Iron(II) Bis(pyridyl)amine-Bipyridine Skeleton for Selective CO2 Photoreduction

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO₂ reduction. Herein, photocatalytic conversion of CO₂ to CO even at low CO₂ concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO₂ binding over proton. The following 2 H⁺ -assisted Fe-CO formation is rate-determining for selective CO₂ -to-CO reduction. A pendant, proton-shuttling α-OH group (2) initiates PCET for predominant H₂ evolution, while an α-OMe group (3) cancels the selectivity control for either CO or H₂ . The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H₂ ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO₂ RR by photocatalysis.

Enhanced Hydrogen Evolution in Neutral Water Catalyzed by a Cobalt Complex with a Softer Polypyridyl Ligand

To explore the structure-function relationships of cobalt complexes in the catalytic hydrogen evolution reaction (HER), we studied the substitution of a tertiary amine with a softer pyridine group and the inclusion of a conjugated bpy unit in a Co complex with a new pentadentate ligand, 6-[6-(1,1-di-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl]-[2,2']bipyridinyl (Py3Me-Bpy). These modifications resulted in significantly improved stability and activity in both electro- and photocatalytic HER in neutral water. [Co(Py3Me-Bpy)(OH₂ )](PF₆ )₂ catalyzes the electrolytic HER at -1.3 V (vs. SHE) for 20 hours with a turnover number (TON) of 266 300, and photolytic HER for two days with a TON of 15 000 in pH 7 aqueous solutions. The softer ligand scaffold possibly provides increased stability towards the intermediate Co^I species. DFT calculations demonstrate that HER occurs through a general electron transfer/proton transfer/electron transfer/proton transfer pathway, with H₂ released from the protonation of Co^II -H species.

Electro- and Photocatalytic Generation of H2 Using a Distinctive CoII "PN3 P" Pincer Supported Complex with Water or Saturated Saline as a Hydrogen Source

Efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions was achieved using three new Co^II complexes supported by the neutral pincer ligand bis(diphenylphosphino)-2,6-di(methylamino)pyridine ("PN₃ P"). At -1.9 V vs. Fc/Fc⁺ , these catalysts showed 96 % Faradaic efficiency with added water or saturated aqueous saline at rates of up to 316 L(mol cat)^-1 (cm² )^-1  h^-1 using a glassy carbon working electrode. The complex [Co(κ³ -2,6-{Ph₂ PNMe}₂ (NC₅ H₃ )Br₂ ] (1) was also able to photocatalytically reduce water to hydrogen in the presence of a Ru(bpy)₃²⁺ photosensitizer and a reductant.

Graphene-Supported Pyrene-Modified Cobalt Corrole with Axial Triphenylphosphine for Enhanced Hydrogen Evolution in pH 0-14 Aqueous Solutions

A cobalt complex of 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenyl corrole that contains a triphenylphosphine axial ligand (1-PPh₃ ) was synthesized and examined as an electrocatalyst for the hydrogen evolution reaction (HER). If supported on graphene (G), the resulting 1-PPh₃ /G material can catalyze the HER in aqueous solutions over a wide pH range of 0-14 with a high efficiency and durability. The significantly enhanced activity of 1-PPh₃ /G, compared with that of its analogues 1-py/G (the Co-bound axial ligand is pyridine instead of triphenylphosphine) and 2-py/G (Co complex of 5,10,15-tris(pentafluorophenyl)corrole), highlights the effects of the pyrenyl substituent and the triphenylphosphine axial ligand on the HER activity. On one hand, the pyrenyl moiety can increase the π-π interactions between 1 and graphene and thus lead to a fast electron transfer from the electrode to 1. On the other hand, the triphenylphosphine axial ligand can increase the electron density (basicity) of Co and thus make the metal center more reactive to protons at the trans position through a so-called "push effect". This study concerns a significant example that shows the trans effect of the axial ligand on the HER, which has been investigated rarely. The combination of various ligand-design strategies in one molecule has been realized in 1-PPh₃ to achieve a high catalytic HER performance. These factors are valuable to be used in other molecular catalyst systems.

Structure-Activity and Stability Relationships for Cobalt Polypyridyl-Based Hydrogen-Evolving Catalysts in Water

A series of eight new and three known cobalt polypyridyl-based hydrogen-evolving catalysts (HECs) with distinct electronic and structural differences are benchmarked in photocatalytic runs in water. Methylene-bridged bis-bipyridyl is the preferred scaffold, both in terms of stability and rate. For a cobalt complex of the tetradentate methanol-bridged bispyridyl-bipyridyl complex [Co^II Br(tpy)]Br, a detailed mechanistic picture is obtained by combining electrochemistry, spectroscopy, and photocatalysis. In the acidic branch, a proton-coupled electron transfer, assigned to formation of Co^III -H, is found upon reduction of Co^II , in line with a pK_a (Co^III -H) of approximately 7.25. Subsequent reduction (-0.94 V vs. NHE) and protonation close the catalytic cycle. Methoxy substitution on the bipyridyl scaffold results in the expected cathodic shift of the reduction, but fails to change the pK_a (Co^III -H). An analysis of the outcome of the benchmarking in view of this postulated mechanism is given along with an outlook for design criteria for new generations of catalysts.

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen-Evolution Reaction

A nickel(II) porphyrin Ni‐P (P=porphyrin) bearing four meso‐C₆F₅ groups to improve solubility and activity was used to explore different hydrogen‐evolution‐reaction (HER) mechanisms. Doubly reduced Ni‐P ([Ni‐P]²⁻) was involved in H₂ production from acetic acid, whereas a singly reduced species ([Ni‐P]⁻) initiated HER with stronger trifluoroacetic acid (TFA). High activity and stability of Ni‐P were observed in catalysis, with a remarkable i_c/i_p value of 77 with TFA at a scan rate of 100 mV s⁻¹ and 20 °C. Electrochemical, stopped‐flow, and theoretical studies indicated that a hydride species [H‐Ni‐P] is formed by oxidative protonation of [Ni‐P]⁻. Subsequent rapid bimetallic homolysis to give H₂ and Ni‐P is probably involved in the catalytic cycle. HER cycling through this one‐electron‐reduction and homolysis mechanism has been proposed previously but rarely validated. The present results could thus have broad implications for the design of new exquisite cycles for H₂ generation.

2D Transition-Metal-Dichalcogenide-Nanosheet-Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions

Hydrogen (H2) is one of the most important clean and renewable energy sources for future energy sustainability. Nowadays, photocatalytic and electrocatalytic hydrogen evolution reactions (HERs) from water splitting are considered as two of the most efficient methods to convert sustainable energy to the clean energy carrier, H2. Catalysts based on transition metal dichalcogenides (TMDs) are recognized as greatly promising substitutes for noble-metal-based catalysts for HER. The photocatalytic and electrocatalytic activities of TMD nanosheets for the HER can be further improved after hybridization with many kinds of nanomaterials, such as metals, oxides, sulfides, and carbon materials, through different methods including the in situ reduction method, the hot-injection method, the heating-up method, the hydro(solvo)thermal method, chemical vapor deposition (CVD), and thermal annealing. Here, recent progress in photocatalytic and electrocatalytic HERs using 2D TMD-based composites as catalysts is discussed.

Enhancing electrocatalytic hydrogen evolution by nickel salicylaldimine complexes with alkali metal cations in aqueous media

The pendant, chelating ethers in the second coordination sphere of nickel salicylaldimine complexes bind alkali metals to promote hydrogen evolution electrocatalysis.

Enhanced photocatalytic H2 production on CdS nanorods with simple molecular bidentate cobalt complexes as cocatalysts under visible light

A noble-metal-free photocatalytic hydrogen production system containing a simple bidentate cobalt Schiff base complex as the molecular cocatalyst, CdS nanorods as the photosensitizer, and ascorbic acid as the electron donor is reported in this paper.

Development of a Dinitrosyl Iron Complex Molecular Catalyst into a Hydrogen Evolution Cathode

Despite extensive efforts, the electrocatalytic reduction of water using homogeneous/heterogeneous Fe, Co, Ni, Cu, W, and Mo complexes remains challenging because of issues involving the development of efficient, recyclable, stable, and aqueous-compatible catalysts. In this study, evolution of the de novo designed dinitrosyl iron complex DNIC-PMDTA from a molecular catalyst into a solid-state hydrogen evolution cathode, considering all the parameters to fulfill the electronic and structural requirements of each step of the catalytic cycle, is demonstrated. DNIC-PMDTA reveals electrocatalytic reduction of water at neutral and basic media, whereas its deposit on electrode preserves exceptional longevity, 139 h. This discovery will initiate a systematic study on the assembly of [Fe(NO)2] motif into current collector for mass production of H2, whereas the efficiency remains tailored by its molecular precursor [(L)Fe(NO)2].

A redox hydrogel protects the O2 -sensitive [FeFe]-hydrogenase from Chlamydomonas reinhardtii from oxidative damage

The integration of sensitive catalysts in redox matrices opens up the possibility for their protection from deactivating molecules such as O2 . [FeFe]-hydrogenases are enzymes catalyzing H2 oxidation/production which are irreversibly deactivated by O2 . Therefore, their use under aerobic conditions has never been achieved. Integration of such hydrogenases in viologen-modified hydrogel films allows the enzyme to maintain catalytic current for H2 oxidation in the presence of O2 , demonstrating a protection mechanism independent of reactivation processes. Within the hydrogel, electrons from the hydrogenase-catalyzed H2 oxidation are shuttled to the hydrogel-solution interface for O2 reduction. Hence, the harmful O2 molecules do not reach the hydrogenase. We illustrate the potential applications of this protection concept with a biofuel cell under H2 /O2 mixed feed.

Ligand transformations and efficient proton/water reduction with cobalt catalysts based on pentadentate pyridine-rich environments

A series of cobalt complexes with pentadentate pyridine-rich ligands is studied. An initial Co(II) amine complex 1 is prone to aerial oxidation yielding a Co(III) imine complex 2 that is further converted into an amide complex 4 in presence of adventitious water. Introduction of an N-methyl protecting group to the ligand inhibits this oxidation and gives rise to the Co(II) species 5. Both the Co(III) 4 and Co(II) 5 show electrocatalytic H2 generation in weakly acidic media as well as in water. Mechanisms of catalysis seem to involve the protonation of a Co(II)-H species generated in situ.

A molecular copper catalyst for electrochemical water reduction with a large hydrogen-generation rate constant in aqueous solution

The copper complex [(bztpen)Cu](BF4)2 (bztpen=N-benzyl-N,N',N'-tris(pyridin-2-ylmethyl)ethylenediamine) displays high catalytic activity for electrochemical proton reduction in acidic aqueous solutions, with a calculated hydrogen-generation rate constant (k(obs)) of over 10000 s(-1). A turnover frequency (TOF) of 7000 h(-1) cm(-2) and a Faradaic efficiency of 96% were obtained from a controlled potential electrolysis (CPE) experiment with [(bztpen)Cu](2+) in pH 2.5 buffer solution at -0.90 V versus the standard hydrogen electrode (SHE) over two hours using a glassy carbon electrode. A mechanism involving two proton-coupled reduction steps was proposed for the dihydrogen generation reaction catalyzed by [(bztpen)Cu](2+).