Hydrogen evolution catalyzed by cobalt diimine-dioxime complexes

Artificial photosynthesis: closing remarks

This paper derives from my closing remarks lecture at the 198^th Faraday Discussion meeting on Artificial Photosynthesis, Kyoto, Japan, February 28–March 2. The meeting had sessions on biological approaches and fundamental processes, molecular catalysts, inorganic assembly catalysts, and integration of systems for demonstrating realistic devices. The field has had much progress since the previous Faraday Discussion on Artificial Photosynthesis in Edinburgh, UK, in 2011. This paper is a personal account of recent discussions and developments in the field, as reflected in and discussed during the meeting. First it discusses the general directions of artificial photosynthesis and some considerations for a future solar fuels technology. Then it comments on some scientific directions in the area of the meeting.

Tracking Co(I) Intermediate in Operando in Photocatalytic Hydrogen Evolution by X-ray Transient Absorption Spectroscopy and DFT Calculation

X-ray transient absorption spectroscopy (XTA) and optical transient spectroscopy (OTA) were used to probe the Co(I) intermediate generated in situ from an aqueous photocatalytic hydrogen evolution system, with [Ru^II(bpy)₃]Cl₂·6H₂O as the photosensitizer, ascorbic acid/ascorbate as the electron donor, and the Co-polypyridyl complex ([Co^II(DPA-Bpy)Cl]Cl) as the precatalyst. Upon exposure to light, the XTA measured at Co K-edge visualizes the grow and decay of the Co(I) intermediate, and reveals its Co-N bond contraction of 0.09 ± 0.03 Å. Density functional theory (DFT) calculations support the bond contraction and illustrate that the metal-to-ligand π back-bonding greatly stabilizes the penta-coordinated Co(I) intermediate, which provides easy photon access. To the best of our knowledge, this is the first example of capturing the penta-coordinated Co(I) intermediate in operando with bond contraction by XTA, thereby providing new insights for fundamental understanding of structure-function relationship of cobalt-based molecular catalysts.

Catalytic reduction of proton, oxygen and carbon dioxide with cobalt macrocyclic complexes

The conversion of solar energy into chemical energy by the reduction of small molecules provides a promising solution for the effective energy storage and transport. In this manuscript, we have highlighted our recent researches on the catalysis of cobalt-macrocycle complexes for the reduction of O2, proton and CO2. We have successfully clarified the reaction mechanisms of catalytic O2 reduction with cobalt phthalocyanine (Co[Formula: see text](Pc)) and cobalt chlorin (Co[Formula: see text](Ch)) based on detailed kinetic study under homogeneous conditions. The presence of proton-accepting moieties on these macrocyclic ligands enhances the electron-accepting ability, leading to the efficient catalytic two-electron reduction of O2 to produce hydrogen peroxide (H2O[Formula: see text] with high stability and less overpotential in acidic solutions. When Co[Formula: see text](Ch) is adsorbed on multi-walled carbon nanotubes (MWCNTs) and employed as an electrocatalyst, CO2 was successfully reduced to form CO with a Faradaic efficiency of 89% at an applied potential of -1.1 V vs. NHE in an aqueous solution. Finally, photocatalytic H2 evolution was attained from ascorbic acid with Co[Formula: see text](Ch) as a catalyst and [Ru(bpy)3][Formula: see text] (bpy [Formula: see text] 2,2[Formula: see text]-bipyridine) as a photocatalyst via a one-photon two-electron process.

Wafer-sized multifunctional polyimine-based two-dimensional conjugated polymers with high mechanical stiffness

One of the key challenges in two-dimensional (2D) materials is to go beyond graphene, a prototype 2D polymer (2DP), and to synthesize its organic analogues with structural control at the atomic- or molecular-level. Here we show the successful preparation of porphyrin-containing monolayer and multilayer 2DPs through Schiff-base polycondensation reaction at an air-water and liquid-liquid interface, respectively. Both the monolayer and multilayer 2DPs have crystalline structures as indicated by selected area electron diffraction. The monolayer 2DP has a thickness of∼0.7 nm with a lateral size of 4-inch wafer, and it has a Young's modulus of 267±30 GPa. Notably, the monolayer 2DP functions as an active semiconducting layer in a thin film transistor, while the multilayer 2DP from cobalt-porphyrin monomer efficiently catalyses hydrogen generation from water. This work presents an advance in the synthesis of novel 2D materials for electronics and energy-related applications.

Covalent Design for Dye-Sensitized H2-Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst

Dye-sensitized photoelectrochemical cells (DS-PECs) for water splitting hold promise for the large-scale storage of solar energy in the form of (solar) fuels, owing to the low cost and ease to process of their constitutive photoelectrode materials. The efficiency of such systems ultimately depends on our capacity to promote unidirectional light-driven electron transfer from the electrode substrate to a catalytic moiety. We report here on the first noble-metal free and covalent dye-catalyst assembly able to achieve photoelectrochemical visible light-driven H2 evolution in mildly acidic aqueous conditions when grafted onto p-type NiO electrode substrate.

Photochemical hydrogen production and cobaloximes: the influence of the cobalt axial N-ligand on the system stability

We report on the first systematic study of cobaloxime-based hydrogen photoproduction in mixed pH 7 aqueous/acetonitrile solutions and demonstrate that H2 evolution can be tuned through electronic modifications of the axial cobalt ligand or through introduction of TiO2 nanoparticles. The photocatalytic systems consist of various cobaloxime catalysts [Co(dmgH)2(L)Cl] (L = nitrogen-based axial ligands) and a water soluble porphyrin photosensitizer. They were assayed in the presence of triethanolamine as a sacrificial electron donor. Optimal turnover numbers related to the photosensitizer are obtained with electron-rich axial ligands such as imidazole derivatives (1131 TONs with N-methyl imidazole). Lower stabilities are observed with various pyridine axial ligands (443 TONs for para-methylpyridine), especially for those containing electron-acceptor substituents. Interestingly, when L is para-carboxylatopyridine the activity of the system is increased from 40 to 223 TONs in the presence of TiO2 nanoparticles.

Direct Observation by Rapid-Scan FT-IR Spectroscopy of Two-Electron-Reduced Intermediate of Tetraaza Catalyst [Co(II)N4H(MeCN)](2+) Converting CO2 to CO

In the search for the two-electron-reduced intermediate of the tetraaza catalyst [Co(II)N4H(MeCN)](2+) (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene) for CO2 reduction and elementary steps that result in the formation of CO product, rapid-scan FT-IR spectroscopy of the visible-light-sensitized catalysis, using Ir(ppy)3 in wet acetonitrile (CD3CN) solution, led to the observation of two sequential intermediates. The initially formed one-electron-reduced [Co(I)N4H](+)-CO2 adduct was converted by the second electron to a transient [Co(I)N4H](+)-CO2(-) complex that spontaneously converted CO2 to CO in a rate-limiting step on the second time scale in the dark under regeneration of the catalyst (room temperature). The macrocycle IR spectra of the [Co(I)N4H](+)-CO2(-) complex and the preceding one-electron [Co(I)N4H](+)-CO2 intermediate show close similarity but distinct differences in the carboxylate modes, indicating that the second electron resides mainly on the CO2 ligand. Vibrational assignments are corroborated by (13)C isotopic labeling. The structure and stability of the two-electron-reduced intermediate derived from the time-resolved IR study are in good agreement with recent predictions by DFT electronic structure calculations. This is the first observation of an intermediate of a molecular catalyst for CO2 reduction during the bond-breaking step producing CO. The reaction pathway for the Co tetraaza catalyst uncovered here suggests that the competition between CO2 reduction and proton reduction of a macrocyclic multi-electron catalyst is steered toward CO2 activation if the second electron is directly captured by an adduct of CO2 and the one-electron-reduced catalyst intermediate.

Modular Homogeneous Chromophore-Catalyst Assemblies

Photosynthetic reaction center (RC) proteins convert incident solar energy to chemical energy through a network of molecular cofactors which have been evolutionarily tuned to couple efficient light-harvesting, directional electron transfer, and long-lived charge separation with secondary reaction sequences. These molecular cofactors are embedded within a complex protein environment which precisely positions each cofactor in optimal geometries along efficient electron transfer pathways with localized protein environments facilitating sequential and accumulative charge transfer. By contrast, it is difficult to approach a similar level of structural complexity in synthetic architectures for solar energy conversion. However, by using appropriate self-assembly strategies, we anticipate that molecular modules, which are independently synthesized and optimized for either light-harvesting or redox catalysis, can be organized into spatial arrangements that functionally mimic natural photosynthesis. In this Account, we describe a modular approach to new structural designs for artificial photosynthesis which is largely inspired by photosynthetic RC proteins. We focus on recent work from our lab which uses molecular modules for light-harvesting or proton reduction catalysis in different coordination geometries and different platforms, spanning from discrete supramolecular assemblies to molecule-nanoparticle hybrids to protein-based biohybrids. Molecular modules are particularly amenable to high-resolution characterization of the ground and excited state of each module using a variety of physical techniques; such spectroscopic interrogation helps our understanding of primary artificial photosynthetic mechanisms. In particular, we discuss the use of transient optical spectroscopy, EPR, and X-ray scattering techniques to elucidate dynamic structural behavior and light-induced kinetics and the impact on photocatalytic mechanism. Two different coordination geometries of supramolecular photocatalyst based on the [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine) light-harvesting module with cobaloxime-based catalyst module are compared, with progress in stabilizing photoinduced charge separation identified. These same modules embedded in the small electron transfer protein ferredoxin exhibit much longer charge-separation, enabled by stepwise electron transfer through the native [2Fe-2S] cofactor. We anticipate that the use of interchangeable, molecular modules which can interact in different coordination geometries or within entirely different structural platforms will provide important fundamental insights into the effect of environment on parameters such as electron transfer and charge separation, and ultimately drive more efficient designs for artificial photosynthesis.

A Poly(cobaloxime)/Carbon Nanotube Electrode: Freestanding Buckypaper with Polymer-Enhanced H2-Evolution Performance

A freestanding H2-evolution electrode consisting of a copolymer-embedded cobaloxime integrated into a multiwall carbon nanotube matrix by π-π interactions is reported. This electrode is straightforward to assemble and displays high activity towards hydrogen evolution in near-neutral pH solution under inert and aerobic conditions, with a cobalt-based turnover number (TON(Co)) of up to 420. An analogous electrode with a monomeric cobaloxime showed less activity with a TON(Co) of only 80. These results suggest that, in addition to the high surface area of the porous network of the buckypaper, the polymeric scaffold provides a stabilizing environment to the catalyst, leading to further enhancement in catalytic performance. We have therefore established that the use of a multifunctional copolymeric architecture is a viable strategy to enhance the performance of molecular electrocatalysts.

Experimental and Theoretical Insight into Electrocatalytic Hydrogen Evolution with Nickel Bis(aryldithiolene) Complexes as Catalysts

A series of neutral and monoanionic nickel dithiolene complexes with p-methoxyphenyl-substituted 1,2-dithiolene ligands have been prepared and characterized with physicochemical methods. Two of the complexes, the monoanion of the symmetric [Ni{S2C2(Ph-p-OCH3)2}2] (3(-)) with NBu4(+) as a counterion and the neutral asymmetric [Ni{S2C2(Ph)(Ph-p-OCH3)}2] (2), have been structurally characterized by single-crystal X-ray crystallography. All complexes have been employed as proton-reducing catalysts in N,N-dimethylformamide with trifluoroacetic acid as the proton source. The complexes are active catalysts with good faradaic yields, reaching 83% for 2 but relatively high overpotential requirements (0.91 and 1.55 V measured at the middle of the catalytic wave for two processes observed depending on the different routes of the mechanism). The similarity of the experimental data regardless of whether the neutral or anionic form of the complexes is used indicates that the neutral form acts as a precatalyst. On the basis of detailed density functional theory calculations, the proposed mechanism reveals two different main routes after protonation of the dianion of the catalyst in accordance with the experimental data, indicating the role of the concentration of the acid and the influence of the methoxy groups. Protonation at sulfur seems be more favorable than that at the metal, which is in marked contrast with the catalytic mechanism proposed for analogous cobalt dithiolene complexes.

Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting

Water electrolysis is considered as the most promising technology for hydrogen production. Much research has been devoted to developing efficient electrocatalysts for hydrogen production via the hydrogen evolution reaction (HER) and oxygen production via the oxygen evolution reaction (OER). The optimum electrocatalysts can drive down the energy costs needed for water splitting via lowering the overpotential. A number of cobalt (Co)-based materials have been developed over past years as non-noble-metal heterogeneous electrocatalysts for HER and OER. Recent progress in this field is summarized here, especially highlighting several important bifunctional catalysts. Various approaches to improve or optimize the electrocatalysts are introduced. Finally, the current existing challenges and the future working directions for enhancing the performance of Co-implicated electrocatalysts are proposed.

Molecular engineered nanomaterials for catalytic hydrogen evolution and oxidation

Surface functionalization allows the immobilization of molecular catalysts for hydrogen evolution and uptake onto conducting materials and yields electrodes based on earth-abundant elements as alternative to the use of platinum catalysts.

Cyano-decorated ligands: a powerful alternative to fluorination for tuning the photochemical properties of cyclometalated Ir(iii) complexes

Two bis-cyclometalated Ir(iii) photocatalysts were synthesized with nitrile moieties which imparted remarkable photophysical properties over conventional fluorinated ligands.

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. Herein nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C-H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H2. Support for the theoretically predicted mechanism is provided by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.

Reengineering cyt b562 for hydrogen production: A facile route to artificial hydrogenases

Bioinspired, protein-based molecular catalysts utilizing base metals at the active are emerging as a promising avenue to sustainable hydrogen production. The protein matrix modulates the intrinsic reactivity of organometallic active sites by tuning second-sphere and long-range interactions. Here, we show that swapping Co-Protoporphyrin IX for Fe-Protoporphyrin IX in cytochrome b562 results in an efficient catalyst for photoinduced proton reduction to molecular hydrogen. Further, the activity of wild type Co-cyt b562 can be modulated by a factor of 2.5 by exchanging the coordinating methionine with alanine or aspartic acid. The observed turnover numbers (TON) range between 125 and 305, and correlate well with the redox potential of the Co-cyt b562 mutants. The photosensitized system catalyzes proton reduction with high efficiency even under an aerobic atmosphere, implicating its use for biotechnological applications. This article is part of a Special Issue entitled Biodesign for Bioenergetics--the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Enhanced photocatalytic hydrogen production by introducing the carboxylic acid group into cobaloxime catalysts

[Co(iii)(dmgH)₂(py-m-CH₂CH₂COOH)Cl] showed a much improved photocatalytic H₂ production activity compared to Co(iii)(dmgH)₂(py)Cl], and the COOH group may serve as a proton relay to account for its promising performance.