A Highly Selective and Robust Co(II)-Based Homogeneous Catalyst for Reduction of CO2 to CO in CH3CN/H2O Solution Driven by Visible Light

Electronic Effect Promoted Visible-Light-Driven CO2-to-CO Conversion in a Water-Containing System

The design of unsaturated nonprecious metal complexes with high catalytic performance for photochemical CO2 reduction is still an important challenge. In this paper, four coordinatively unsaturated Co-salen complexes 1-4 were explored in situ using o-phenylenediamine derivatives and 5-methylsalicylaldehyde as precursors of the ligands in 1-4. It was found that complex 4, bearing a nitro substituent (-NO2) on the aromatic ring of the salen ligand, exhibits the highest photochemical performance for visible-light-driven CO2-to-CO conversion in a water-containing system, with TONCO and CO selectivity values of 5300 and 96%, respectively. DFT calculations and experimental results revealed that the promoted photocatalytic activity of 4 is ascribed to the electron-withdrawing effect of the nitro group in 4 compared to 1-3 (with -CH3, -F, and -H groups, respectively), resulting in a lower reduction potential of active metal centers CoII and lower barriers for CO2 coordination and C-O cleavage steps for 4 than those for catalysts 1-3.

Cobalt-based tripodal complexes as molecular catalysts for photocatalytic CO2 reduction

Construction of artificial photosynthetic systems including CO2 reduction is a promising pathway to produce carbon-neutral fuels and mitigate the greenhouse effect concurrently. However, the exploitation of earth-abundant catalysts for photocatalytic CO2 reduction remains a fundamental challenge, which can be assisted by a systematic summary focusing on a specific catalyst family. Cobalt-based complexes featuring tripodal ligands should merit more insightful discussion and summarization, as they are one of the most examined catalyst families for CO2 photoreduction. In this feature article, the key developments of cobalt-based tripodal complexes as molecular catalysts for light-driven CO2 reduction are discussed to offer an upcoming perspective, analyzing the present progress in electronic/steric tuning through ligand modification and dinuclear design to achieve a synergistic effect, as well as the bottlenecks for further development.

Electronic effects promoted the catalytic activities of binuclear Co(II) complexes for visible-light-driven CO2 reduction in a water-containing system

Under the action of a catalyst, the photoinduced reduction of CO₂ to chemicals and fuels is one of the greenest and environment-friendly approaches for decreasing atmospheric CO₂ emissions. Since the environment was affected by the greenhouse effect, scientists have never stopped exploring efficient photoinduced CO₂ reduction systems, particularly the highly desired non-noble metal complexes. Most of the currently reported complexes based on non-noble metals exhibit low catalytic activity, selectivity, and stability in aqueous systems under the irradiation of visible light. Herein, we report a new binuclear cobalt complex [Co₂(L¹)(OAc)₂](OAc) (Co2L1, HL¹ = 2,6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-methoxyphenol), which accelerates the visible-light-driven conversion of CO₂ to CO in acetonitrile/water (4/1, v/v) nearly 40% more than that for the previously reported [Co₂(L²)(OAc)₂](OAc) (Co2L2, HL² = 2, 6-bis((bis(pyridin-2-ylmethyl)amino)methyl)-4-(tert-butyl)phenol) by our research group. It has an excellent CO selectivity of 98%, and the TON_CO is as high as 5920. Experimental results and DFT calculations showed that the enhanced catalytic performance of Co2L1 is due to the electron-donating effect of a methoxy group (-OCH₃) in Co2L1 compared to a tertiary butyl group (-C(CH₃)₃) in Co2L2, which reduces the energy barrier of the rate-limiting CO₂ coordination step in the visible-light-driven CO₂ reduction process.

Coupling Ruthenium Bipyridyl and Cobalt Imidazolate Units in a Metal-Organic Framework for an Efficient Photosynthetic Overall Reaction in Diluted CO2

Artificial photocatalytic CO₂ reduction, using water as the reductant, is challenging mainly because it is difficult for multiple functional units to cooperate efficiently. Here, we show that the classic photosensitive and H₂O-oxidizing ruthenium bipyridyl units and CO₂-reducing cobalt imidazolate units can be incorporated into a metal-organic framework using a classic organic ligand, imidazo[4,5-f][1,10]phenanthroline. Under visible light without additional sacrificial agents and photosensitizers, the overall conversion of CO₂ and H₂O to CO and O₂ was achieved by the multifunctional photocatalyst in the CH₃CN/H₂O mixed solvent with a high CO production rate of 11.2 μmol g^-1 h^-1 and CO selectivity of ca. 100%. Thanks to its ultramicroporous structure with moderately strong CO₂ adsorption ability, the photocatalyst also exhibited high performances with CO/CH₄ production rates of 5.15/0.62 and 4.26/0.20 μmol g^-1 h^-1 in the gas phase with pure and even diluted CO₂, respectively. Photoluminescence emission spectroscopy and photoelectrochemical tests confirmed that the photosensitive and catalytic units cooperated well to give suitable photocatalytic redox potentials and fast electron-hole separation.

Copper-Based Metal-Organic Framework with a Tetraphenylethylene-Tetrazole Linker for Visible-Light-Driven CO2 Photoconversion

The design of efficient and inexpensive photocatalysts for CO₂ photoreduction under visible light is of great significance for the sustainable development of the entire society. Herein, a copper-based metal-organic framework (MOF) (CUST-804) using a bulky tetraphenylethylene-tetrazole linker is synthesized and successfully used as a photocatalyst for CO₂ reduction. The structural characterizations, as well as the photophysical properties, are investigated systematically. In the heterogeneous catalytic system, CUST-804 exhibits a robust CO production activity up to 2.71 mmol g^-1 h^-1 with excellent recyclability along with a selectivity of 82.8%, which is comparable with those of the reported copper-based MOF system. Theoretical calculations demonstrated that, among three kinds of coordinated model, only the 5-coordinated Cu site is active for CO₂ reduction, in which the *COOH intermediate is stabilized and CO is readily desorbed. The results obtained herein can provide fresh insights into the realization of efficient copper-functionalized crystalline photocatalysts for CO₂ reduction.

Proton Removal Kinetics That Govern the Hydrogen Peroxide Oxidation Activity of Heterogeneous Bioinorganic Platforms

Precise regulation of proton-coupled electron-transfer (PCET) rates holds the key to simultaneously optimizing the turnover frequency and product selectivity of redox reactions that are central to the realization of renewable energy schemes in a sustainable future. In this work, a self-assembled monolayer (SAM) of a Ru complex electrografted onto a glassy carbon (GC) electrode was prepared as a heterogeneous electrocatalytic interface to facilitate the hydrogen peroxide (H₂O₂) oxidation half-cell reaction of a direct hydrogen peroxide/hydrogen peroxide fuel cell. A functional lipid membrane embedded with catalytic amounts of proton carriers was appended on top of the Ru SAM to construct a hybrid bilayer membrane (HBM) platform that can modulate the thermodynamics and kinetics of proton- and electron-transfer steps independently. The performances of the as-prepared Ru SAMs and HBMs toward H₂O₂ oxidation were investigated using electrochemical means, kinetic isotope effect (KIE) studies, and Tafel analyses. Proton carriers featuring borate, phosphate, and nitrile headgroups were found to dictate the transmembrane proton removal rate, thereby controlling the H₂O₂ oxidation activity. The first significance of this work was the expansion of HBM platforms to GC substrates to overcome the limited redox potential window on gold thiol systems, thereby enabling electrochemical investigations of anodic reactions at the SAM-lipid interface. The second highlight of this work was demonstrating for the first time that deprotonation kinetics can be taken advantage of to enhance the electrocatalytic oxidation performance of a metal complex anchored at the SAM-lipid interface of a HBM platform. When the knowledge gaps regarding how PCET steps govern redox pathways are closed, the advances achieved using our unique bioinorganic platform are envisioned to accelerate the understanding and optimization of electrocatalytic processes involving proton- and electron- transfer steps that are fundamental to the development of high-performance energy devices.

Water-Assisted Highly Efficient Photocatalytic Reduction of CO2 to CO with Noble Metal-Free Bis(terpyridine)iron(II) Complexes and an Organic Photosensitizer

Photocatalytic CO₂ reduction reaction is believed to be a promising approach for CO₂ utilization. In this work, a noble metal-free photocatalytic system, composed of bis(terpyridine)iron(II) complexes and an organic thermally activated delayed fluorescence compound, has been developed for selective reduction of CO₂ to CO with a maximum turnover number up to 6320, 99.4% selectivity, and turnover frequency of 127 min^-1 under visible-light irradiation in dimethylformamide/H₂O solution. More than 0.3 mmol CO was generated using 0.05 μmol catalyst after 2 h of light irradiation. The apparent quantum yield was found to be 9.5% at 440 nm (180 mW cm^-2). Control experiments and UV-vis-NIR spectroscopy studies further demonstrated that water strongly promoted the photocatalytic cycle and terpyridine ligands rather than Fe(II) were initially reduced during the photocatalytic process.

Highly Efficient and Selective Visible-Light Driven CO2 Reduction by Two Co-Based Catalysts in Aqueous Solution

Photocatalytic CO₂ reduction has been considered as a promising approach to solve energy and environmental problems. Nevertheless, developing inexpensive photocatalysts with high efficiency and selectivity remains a big challenge. In this study, two Co-based complexes [Co₂(L¹)Cl₂] (1-Co) and [Co(L²)Cl] (2-Co) were synthesized by treating two DPA-based (DPA: dipicolylamine) ligands with Co²⁺, respectively. Under visible-light irradiation, the performance of 1-Co as a homogeneous photocatalyst for CO₂ reduction in aqueous media has been explored by using [Ru(phen)₃]²⁺ as a photosensitizer, and triethylolamine (TEOA) as a sacrificial reductant. 1-Co shows high photocatalytic activity for CO₂-to-CO conversion, corresponding to the high TON_CO of 2600 and TOF_CO of 260 h^-1 (TON_CO = turnover number for CO; TOF_CO = turnover frequency for CO). High selectivity of 97% for CO formation is also achieved. The control experiments catalyzed by 2-Co demonstrated that two Co(II) centers in 1-Co may operate independently and activate one CO₂ molecule each. Furthermore, the proposed mechanism of 1-Co for photocatalytic CO₂ reduction has been investigated via electrochemical analysis, a series of quenching experiments, and density functional theory calculations.

Effect of metal ion substitution on the catalytic activity of a pentanuclear metal complex

A pentanuclear cobalt complex that consists of five cobalt ions and six bpp⁻ ligands (Co5, Hbpp = 3,5-bis(2-pyridyl)pyrazole) was newly developed. The obtained complex can catalyze CO₂ reduction under electrochemical and photochemical conditions.

An Unexpected Iron (II)-Based Homogeneous Catalytic System for Highly Efficient CO2-to-CO Conversion under Visible-Light Irradiation

We present two as-synthesized Fe(II)-based molecular catalysts with 1,10-phenanthroline (phen) ligands; Fe(phen)₃Cl₂ (1) and [Fe(phen)₂(CH₃CH₂OH)Cl]Cl (2), and their robust catalytic properties for the conversion of CO₂ to CO in DMF/TEOA (DMF = N,N’-dimethylformamide; TEOA = triethanolamine) solution containing Ru(bpy)₃²⁺ and BIH (1,3-dimethyl-2-phenyl-2,3- dihydro-1H-benzo-[d]-imidazole). High turnover numbers (TONs) of 19,376 were achieved with turnover frequencies (TOFs) of 3.07 s⁻¹ for complex 1 (1.5 × 10⁻⁷ M). A quantum efficiency of 0.38% was observed after 5 h irradiated by 450 nm monochromatic light. The generation rate of CO₂ and H₂ were tuned by optimizing the experimental conditions, resulting in a high CO selectivity of 90%. The remarkable contribution of the photosensitizer to the total TON_CO was found being 19.2% (as shown by tests under similar conditions without catalysts) when BIH was employed as a sacrificial electron donor. The product selectivity in complex 2 reached 95%, and the corresponding TON_CO and TOF_CO were 33,167 and 4.61 s⁻¹ in the same concentration with complex 1 used as catalyst; respectively. This work provides guidance for future designs of simple, highly efficient and selective molecular catalytic systems that facilitate carbon-neutral solar-to-fuel conversion processes

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

Visible-light CO2 photoreduction of polyoxometalate-based hybrids with different cobalt clusters

Two novel high-nuclear cobalt-based polyoxometalates can effectively improve photocatalytic activities for CO₂ photoreduction under visible light.

Photocatalytic CO2 Reduction with Manganese Complexes Bearing a κ2-PN Ligand: Breaking the α-Diimine Hold on Group 7 Catalysts and Switching Selectivity

The fundamental challenge of reducing CO₂ into more valuable energy-containing compounds depends on revealing new catalysts for this process. By removal of the long-standing limitation of α-diimine ligation, which is dominant in photocatalytic complexes in this area, new visible-light, CO₂-reducing photocatalysts based on Mn and Re supported by κ²-PN phosphinoaminopyridine ligands were identified. These catalysts, [M{κ²-(Ph₂P)NH(NC₅H₄)}(CO)₃Br], displayed excellent product selectivity and, by a change of only the metal center, gave a dramatic product switch from CO with M = Mn to HCO₂H with M = Re. The catalyst systems were explored with variation of the ligand, electron donor, solvent, and photosensitizer. The products were definitively traced using ¹³CO₂ as a substrate. Both complexes quenched the excited-state photosensitizer Ru(bpy)₃²⁺*, suggesting oxidative quenching as a potential entry into the catalytic cycle.

Photocatalytic CO2 Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO2, CO, or Proton

The cobalt complexes Co^IIL1(PF₆)₂ (1; L1 = 2,6-bis[2-(2,2'-bipyridin-6'-yl)ethyl]pyridine) and Co^IIL2(PF₆)₂ (2; L2 = 2,6-bis[2-(4-methoxy-2,2'-bipyridin-6'-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO₂ reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO₂ reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E_1/2 = -1.77 and -1.85 V versus Ag/AgNO₃ (or -1.86 and -1.94 V vs Fc^+/0), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)₃]²⁺ as a photosensitizer, tri- p-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H₂ were produced under visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy)₃]⁺. However, bulk electrolysis in a wet CH₃CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co-H] ⁿ⁺ ( n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to produce [Co⁰L2(CO)] with ν_CO = 1894 cm^-1 together with [Co^IIL2]²⁺ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV-vis spectroscopy indicate a slow CO binding rate with the [Co^IL2]⁺ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO₂ is much slower than the photochemical formation of [Ru(bpy)₃]⁺ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy)₃]⁺.

Photocatalytic reduction of CO2 to CO and formate by a novel Co(ii) catalyst containing a cis-oxygen atom: photocatalysis and DFT calculations

A novel CoN₄-complex containing an oxygen atom at cis-coordination site enables to convert CO₂ to CO and formate in a photocatalytic system.

Highly efficient and selective visible-light driven CO2-to-CO conversion by a Co-based cryptate in H2O/CH3CN solution

A Co-based cryptate exhibits highly efficient and selective photocatalytic CO₂-to-CO conversion under either pure CO₂ or 10% CO₂ in a water-containing system.

Visible-light Homogeneous Photocatalytic Conversion of CO2 into CO in Aqueous Solutions with an Iron Catalyst

An iron-substituted tetraphenyl porphyrin bearing positively charged trimethylammonio groups at the para position of each phenyl ring catalyzes the photoinduced conversion of CO₂ . This complex is water soluble and acts as a molecular catalyst to selectively reduce CO₂ into CO under visible-light irradiation in aqueous solutions (acetonitrile/water=1:9 v/v) with the assistance of purpurin, a simple organic photosensitizer. CO is produced with a catalytic selectivity of 95 % and turnover number up to 120, illustrating the possibility of photocatalyzing the reduction of CO₂ in aqueous solution by using visible light, a simple organic sensitizer coupled to an amine as a sacrificial electron donor, and an earth-abundant metal-based molecular catalyst.

Photocatalytic Conversion of CO2 to CO by a Copper(II) Quaterpyridine Complex

The invention of efficient systems for the photocatalytic reduction of CO₂ comprising earth-abundant metal catalysts is a promising approach for the production of solar fuels. One bottleneck is to design highly selective and robust molecular complexes that are able to transform the CO₂ gas. The Cu^II quaterpyridine complex [Cu(qpy)]²⁺ (1) is found to be a highly efficient and selective catalyst for visible-light driven CO₂ reduction in CH₃ CN using [Ru(bpy)₃ ]²⁺ (bpy: bipyridine) as photosensitizer and BIH/TEOA (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole/triethanolamine) as sacrificial reductant. The photocatalytic reaction is greatly enhanced by the presence of H₂ O (1-4 % v/v), and a turnover number of >12 400 for CO production can be achieved with 97 % selectivity, which is among the highest of molecular 3d CO₂ reduction catalysts. Results from Hg poisoning and dynamic light scattering experiments suggest that this photocatalyst is homogenous. To the best of our knowledge, 1 is the first example of molecular Cu-based catalyst for the photoreduction of CO₂ .