trans-(Cl)-[Ru(5,5′-diamide-2,2′-bipyridine)(CO)2Cl2]: Synthesis, Structure, and Photocatalytic CO2Reduction Activity

Exploring the Impact of Water Content in Solvent Systems on Photochemical CO2 Reduction Catalyzed by Ruthenium Complexes

To achieve artificial photosynthesis, it is crucial to develop a catalytic system for CO2 reduction using water as the electron source. However, photochemical CO2 reduction by homogeneous molecular catalysts has predominantly been conducted in organic solvents. This study investigates the impact of water content on catalytic activity in photochemical CO2 reduction in N,N-dimethylacetamide (DMA), using [Ru(bpy)3]2+ (bpy: 2,2'-bipyridine) as a photosensitizer, 1-benzyl-1,4-dihydronicotinamide (BNAH) as an electron donor, and two ruthenium diimine carbonyl complexes, [Ru(bpy)2(CO)2]2+ and trans(Cl)-[Ru(Ac-5Bpy-NHMe)(CO)2Cl2] (5Bpy: 5'-amino-2,2'-bipyridine-5-carboxylic acid), as catalysts. Increasing water content significantly decreased CO and formic acid production. The similar rates of decrease for both catalysts suggest that water primarily affects the formation efficiency of free one-electron-reduced [Ru(bpy)3]2+, rather than the intrinsic catalytic activity. The reduction in cage-escape efficiency with higher water content underscores the challenges in replacing organic solvents with water in photochemical CO2 reduction.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖(N2S2)]Cl2 (NiN2S2, N2S2=2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1, with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

Overall reaction mechanism of photocatalytic CO2 reduction on a Re(i)-complex catalyst unit of a Ru(ii)-Re(i) supramolecular photocatalyst

Rhenium(i) complexes fac-[ReI(diimine)(CO)3(L)]n+ are mostly used and evaluated as photocatalysts and catalysts in both photochemical and electrochemical systems for CO2 reduction. However, the selective reduction mechanism of CO2 to CO is unclear, although numerous mechanistic studies have been reported. A Ru(ii)-Re(i) supramolecular photocatalyst with fac-[ReI(diimine)(CO)3{OC(O)OCH2CH2NR2}] (R = C2H4OH) as a catalyst unit (RuC2Re) exhibits very high efficiency, selectivity, and durability of CO formation in photocatalytic CO2 reduction reactions. In this work, the reaction mechanism of photocatalytic CO2 reduction using RuC2Re is fully clarified. Time-resolved IR (TR-IR) measurements using rapid-scan FT-IR spectroscopy with laser flash photolysis verify the formation of RuC2Re(COOH) with a carboxylic acid unit, i.e., fac-[ReI(diimine)(CO)3(COOH)], in the photocatalytic reaction solution. Additionally, this important intermediate is detected in an actual photocatalytic reaction using steady state irradiation. Kinetics analysis of the TR-IR spectra and DFT calculations demonstrated the reaction mechanism of the conversion of the one-electron reduced species of RuC2Re with a fac-[ReI(diimine˙-)(CO)3{OC(O)OCH2CH2NR2}]- unit, which was produced via the photochemical reduction of RuC2Re by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), to RuC2Re(COOH). The kinetics of the recovery processes of the starting complex RuC2Re from RuC2Re(COOH) accompanying the release of CO and OH- was also clarified. As a side reaction of RuC2Re(COOH), a long-lived carboxylate-ester complex with a fac-[ReI(diimine)(CO)3(COOC2H4NR2)] unit, which was produced by the nucleophilic attack of TEOA to one of the carbonyl ligands of RuC2Re(CO) with a fac-[ReI(diimine)(CO)4]+ unit, was formed during the photocatalytic reaction. This complex works not only as a precursor in another minor CO formation process but also as an external photosensitiser that photochemically reduces the other complexes i.e., RuC2Re, RuC2Re(COOH), and the intermediate that is reductively converted to RuC2Re(COOH).

Visible light-driven CO2 photoreduction by a Re(I) complex immobilized onto CuO/Nb2O5 heterojunctions

The immobilization of Re(I) complexes onto metal oxide surfaces presents an elegant strategy to enhance their stability and reusability toward photocatalytic CO2 reduction. In this study, the photocatalytic performance of fac-[ClRe(CO)3(dcbH2)], where dcbH2 = 4,4'-dicarboxylic acid-2,2'-bipyridine, anchored onto the surface of 1%m/m CuO/Nb2O5 was investigated. Following adsorption, the turnover number for CO production (TONCO) in DMF/TEOA increased significantly, from ten in solution to 370 under visible light irradiation, surpassing the TONCO observed for the complex onto pristine Nb2O5 or CuO surfaces. The CuO/Nb2O5 heterostructure allows for efficient electron injection by the Re(I) center, promoting efficient charge separation. At same time CuO clusters introduce a new absorption band above 550 nm that contributes for the photoreduction of the reaction intermediates, leading to a more efficient CO evolution and minimization of side reactions.

1,2-Azolylamidino ruthenium(II) complexes with DMSO ligands: electro- and photocatalysts for CO2 reduction

New 1,2-azolylamidino complexes fac-[RuCl(DMSO)3(NHC(R)az*-κ2N,N)]OTf [R = Me (2), Ph (3); az* = pz (pyrazolyl, a), indz (indazolyl, b)] are synthesized via chloride abstraction from their corresponding precursors cis,fac-[RuCl2(DMSO)3(az*H)] (1) after subsequent base-catalyzed coupling of the appropriate nitrile with the 1,2-azole previously coordinated. All the compounds are characterized by 1H NMR, 13C NMR and IR spectroscopy. Those derived from MeCN are also characterized by X-ray diffraction. Electrochemical studies showed several reduction waves in the range of -1.5 to -3 V. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalytic reduction. The catalytic activity expressed as [icat(CO2)/ip(Ar)] ranged from 1.7 to 3.7 for the 1,2-azolylamidino complexes at voltages of ca. -2.7 to -3 V vs. ferrocene/ferrocenium. Controlled potential electrolysis showed rapid decomposition of the Ru catalysts. Photocatalytic CO2 reduction experiments using compounds 1b, 2b and 3b carried out in a CO2-saturated MeCN/TEOA (4 : 1 v/v) solution containing a mixture of the catalyst and [Ru(bipy)3]2+ as the photosensitizer under continuous irradiation (light intensity of 150 mW cm-2 at 25 °C, λ > 300 nm) show that compounds 1b, 2b and 3b allowed CO2 reduction catalysis, producing CO and trace amounts of formate. The combined turnover number for the production of formate and CO is ca. 100 after 8 h and follows the order 1b < 2b ≈ 3b.

Mechanistic Insights into Photochemical CO2 Reduction to CH4 by a Molecular Iron-Porphyrin Catalyst

Iron tetraphenylporphyrin complex modified with four trimethylammonium groups (Fe-p-TMA) is found to be capable of catalyzing the eight-electron eight-proton reduction of CO₂ to CH₄ photochemically in acetonitrile. In the present work, density functional theory (DFT) calculations have been performed to investigate the reaction mechanism and to rationalize the product selectivity. Our results revealed that the initial catalyst Fe-p-TMA ([Cl-Fe(III)-LR₄]⁴⁺, where L = tetraphenylporphyrin ligand with a total charge of -2, and R₄ = four trimethylammonium groups with a total charge of +4) undergoes three reduction steps, accompanied by the dissociation of the chloride ion to form [Fe(II)-L^••2-R₄]²⁺. [Fe(II)-L^••2-R₄]²⁺, bearing a Fe(II) center ferromagnetically coupled with a tetraphenylporphyrin diradical, performs a nucleophilic attack on CO₂ to produce the ¹η-CO₂ adduct [CO₂^•--Fe(II)-L^•-R₄]²⁺. Two intermolecular proton transfer steps then take place at the CO₂ moiety of [CO₂^•--Fe(II)-L^•-R₄]²⁺, resulting in the cleavage of the C-O bond and the formation of the critical intermediate [Fe(II)-CO]⁴⁺ after releasing a water molecule. Subsequently, [Fe(II)-CO]⁴⁺ accepts three electrons and one proton to generate [CHO-Fe(II)-L^•-R₄]²⁺, which finally undergoes a successive four-electron-five-proton reduction to produce methane without forming formaldehyde, methanol, or formate. Notably, the redox non-innocent tetraphenylporphyrin ligand was found to play an important role in CO₂ reduction since it could accept and transfer electron(s) during catalysis, thus keeping the ferrous ion at a relatively high oxidation state. Hydrogen evolution reaction via the formation of Fe-hydride ([Fe(II)-H]³⁺) turns out to endure a higher total barrier than the CO₂ reduction reaction, therefore providing a reasonable explanation for the origin of the product selectivity.

Photocatalytic Systems for CO2 Reduction: Metal-Complex Photocatalysts and Their Hybrids with Photofunctional Solid Materials

Photocatalytic CO₂ reduction is a critical objective in the field of artificial photosynthesis because it can potentially make a total solution for global warming and shortage of energy and carbon resources. We have successfully developed various highly efficient, stable, and selective photocatalytic systems for CO₂ reduction using transition metal complexes as both photosensitizers and catalysts. The molecular architectures for constructing selective and efficient photocatalytic systems for CO₂ reduction are discussed herein. As a typical example, a mixed system of a ring-shaped Re(I) trinuclear complex as a photosensitizer and fac-[Re(bpy)(CO)₃{OC₂H₄N(C₂H₄OH)₂}] as a catalyst selectively photocatalyzed CO₂ reduction to CO with the highest quantum yield of 82% and a turnover number (TON) of over 600. Not only rare and noble metals but also earth abundant ones, such as Mn(I), Cu(I), and Fe(II) can be used as central metal cations. In the case using a Cu(I) dinuclear complex as a photosensitizer and fac-Mn(bpy)(CO)₃Br as a catalyst, the total formation quantum yield of CO and HCOOH from CO₂ was 57% and TON_CO+HCOOH exceeded 1300.

Efficient supramolecular photocatalysts for CO₂ reduction, in which photosensitizer and catalyst units are connected through a bridging ligand, were developed for removing a diffusion control on collisions between a photosensitizer and a catalyst. Supramolecular photocatalysts, in which [Ru(N^∧N)₃]²⁺-type photosensitizer and Re(I) or Ru(II) catalyst units are connected to each other with an alkyl chain, efficiently and selectively photocatalyzed CO₂ reduction in solutions. Mechanistic studies using time-resolved IR and electrochemical measurements provided molecular architecture for constructing efficient supramolecular photocatalysts. A Ru(II)–Re(I) supramolecular photocatalyst constructed according to this molecular architecture efficiently photocatalyzed CO₂ reduction even when it was fixed on solid materials. Harnessing this property of the supramolecular photocatalysts, two types of hybrid photocatalytic systems were developed, namely, photocatalysts with light-harvesting capabilities and photoelectrochemical systems for CO₂ reduction.

Introduction of light-harvesting capabilities into molecular photocatalytic systems should be important because the intensity of solar light shone on the earth’s surface is relatively low. Periodic mesoporous organosilica, in which methyl acridone groups are embedded in the silica framework as light harvesters, was combined with a Ru(II)–Re(I) supramolecular photocatalyst with phosphonic acid anchoring groups. In this hybrid, the photons absorbed by approximately 40 methyl acridone groups were transferred to one Ru(II) photosensitizer unit, and then, the photocatalytic CO₂ reduction commenced.

To use water as an abundant electron donor, we developed hybrid photocatalytic systems combining metal-complex photocatalysts with semiconductor photocatalysts that display high photooxidation powers, in which two photons are sequentially absorbed by the metal-complex photosensitizer and the semiconductor, resulting in both high oxidation and reduction power. Various types of dye-sensitized molecular photocathodes comprising the p-type semiconductor electrodes and the supramolecular photocatalysts were developed. Full photoelectrochemical cells combining these dye-sensitized molecular photocathodes and n-type semiconductor photoanodes achieved CO₂ reduction using only visible light as the energy source and water as the reductant. Drastic improvement of dye-sensitized molecular photocathodes is reported.

The results presented in this Account clearly indicate that we can construct very efficient, selective, and durable photocatalytic systems constructed with the metal-complex photosensitizers and catalysts. The supramolecular-photocatalyst architecture in which the photosensitizer and the catalyst are connected to each other is useful especially on the surface of solid owing to rapid electron transfer from the photosensitizer to the catalyst. On basis of these findings, we successfully constructed hybrid systems of the supramolecular photocatalysts with photoactive solid materials. These hybridizations can add new functions to the metal-complex photocatalytic systems, such as water oxidation and light harvesting.

Effect of Micropores of a Porous Coordination Polymer on the Product Selectivity in RuII Complex-catalyzed CO2 Reduction

To develop an efficient CO2 reduction catalyst, hybridizing a molecular catalyst and a porous coordination polymer (PCP) is a promising strategy because it can combine both advantages of the precise reactivity control of the former and the CO2 adsorption property of the latter. Although several PCP hybrid catalysts have been reported to date, the CO2 sorption behavior and the CO2 reduction reactivity have been investigated separately, and the CO2 enrichment during the catalysis is still unclear. We report CO2 photoreduction under different temperatures and pressures using a PCP-RuII complex hybrid catalyst. The product selectivity (CO or HCOOH) varied depending on the reaction conditions. The altered selectivity could be interpreted in terms of the CO2 capture in the micropores of a PCP.

DFT insights into the photocatalytic reduction of CO2 to CO by Re(I) complexes: the crucial role of the triethanolamine "magic" sacrificial electron donor

The reaction mechanism for the photocatalytic reduction of CO₂ to CO catalyzed by the [Re(en)(CO)₃Cl] complex in the presence of triethanolamine, R₃N (R = CH₂CH₂OH) abbreviated as TEOA, in DMF solution was studied in-depth with the aid of DFT computational protocols by calculating the geometric and free energy reaction profiles for several possible reaction pathways. The reaction pathways studied start with the "real" catalytic species [Re(en)(CO)₃], [Re(en)(CO)₃]^- and/or [Re(en)(CO)₂Cl]^- generated from the excited triplet T1 state upon single and double reductive quenching by a TEOA sacrificial electron donor or photodissociation of a CO ligand. The first step in all the catalytic cycles investigated involves the capture of either CO₂ or the oxidized R₂NCH₂CH₂O˙ radical. In the latter case, the CO₂ molecule is captured (inserted) by the Re-OCH₂CH₂NR₂ bond forming stable intermediates. Next, successive protonations (TEOA also acts as a proton donor) lead to the release of CO either from the energy consuming 2e^- reduction of [Re(en)(CO)₄]⁺ or [Re(en)(CO)₂Cl]⁺ complexes in the CO₂ capture pathways or from the released unstable diprotonated [R₂NCH₂CH₂OC(OH)(OH)]⁺ species regenerating TEOA and the catalyst. The CO₂ insertion reaction pathway is the favorable pathway for the photocatalytic reduction of CO₂ → CO catalyzed by the [Re(en)(CO)₃Cl] complex in the presence of TEOA manifesting its crucial role as an electron and proton donor, capturing CO₂ and releasing CO.

6,6'-Ditriphenylamine-2,2'-bipyridine: Coordination Chemistry and Electrochemical and Photophysical Properties

A 2,2'-bipyridine with bulky triphenylamine substituents in the 6 and 6' positions of the ligand (6,6'-ditriphenylamine-2,2'-bipyridine, 6,6'-diTPAbpy) was generated. Despite the steric bulk, the ligand readily formed bis(homoleptic) complexes with copper(I) and silver(I) ions. Unfortunately, efforts to use the 6,6'-diTPAbpy system to generate heteroleptic [Cu(6,6'-diTPAbpy)(bpy)]⁺ complexes were unsuccessful with only the [Cu(6,6'-diTPAbpy)₂](PF₆) complex observed. The 6,6'-diTPAbpy ligand could also be reacted with 6-coordinate metal ions that featured small ancillary ligands, namely, the [Re(CO)₃Cl] and [Ru(CO)₂Cl₂] fragments. While the complexes could be formed in good yields, the steric bulk of the TPA units does alter the coordination geometry. This is most readily seen in the [(6,6'-diTPAbpy)Re(CO)₃Cl] complex where the Re(I) ion is forced to sit 23° out of the plane formed by the bpy unit. The electrochemical and photophysical properties of the family of compounds were also examined. 6,6'-diTPAbpy exhibits a strong ILCT absorption band (356 nm, 50 mM^-1 cm^-1) which displays a small increase in intensity for the homoleptic complexes ([Cu(6,6'-diTPAbpy)₂]⁺; 353 nm, 72 mM^-1 cm^-1, [Ag(6,6'-diTPAbpy)₂]⁺; 353 nm, 75 mM^-1 cm^-1), despite containing 2 equiv of the ligand, attributed to an increased dihedral angle between the TPA and bpy moieties. For the 6-coordinate complexes the ILCT band is further decreased in intensity and overlaps with MLCT bands, consistent with a further increased TPA-bpy dihedral angle. Emission from the ¹ILCT state is observed at 436 nm (τ = 4.4 ns) for 6,6'-diTPAbpy and does not shift for the Cu, Ag, and Re complexes, although an additional ³MLCT emission is observed for [Re(6,6'-diTPAbpy)(CO)₃Cl] (640 nm, τ = 13.8 ns). No emission was observed for [Ru(6,6'-diTPAbpy)(CO)₂Cl₂]. Transient absorption measurements revealed the population of a ³ILCT state for the Cu and Ag complexes (τ = 80 ns). All assignments were supported by TD-DFT calculations and resonance Raman spectroscopic measurements.

Polymeric ruthenium precursor as a photoactivated antimicrobial agent

Ruthenium coordination compounds have demonstrated a promising anticancer and antibacterial activity, but their poor water solubility and low stability under physiological conditions may limit their therapeutic applications. Physical encapsulation or covalent conjugation with polymers may overcome these drawbacks, but generally involve multistep reactions and purification processes. In this work, the antibacterial activity of the polymeric precursor dicarbonyldichlororuthenium (II) [Ru(CO)₂Cl₂]_n has been studied against Escherichia coli and Staphylococcus aureus. This Ru-carbonyl precursor shows minimum inhibitory concentration at nanogram per millilitre, which renders it a novel antimicrobial polymer without any organic ligands. Besides, [Ru(CO)₂Cl₂]_n antimicrobial activity is markedly boosted under photoirradiation, which can be ascribed to the enhanced generation of reactive oxygen species under UV irradiation. [Ru(CO)₂Cl₂]_n has been able to inhibit bacterial growth via the disruption of bacterial membranes and triggering upregulation of stress responses as shown in microscopic measurements. The activity of polymeric ruthenium as an antibacterial material is significant even at 6.6 ng/mL while remaining biocompatible to the mammalian cells at much higher concentrations. This study proves that this simple precursor, [Ru(CO)₂Cl₂]_n, can be used as an antimicrobial compound with high activity and a low toxicity profile in the context of need for new antimicrobial agents to fight bacterial infections.

Bioinspired metal complexes for energy-related photocatalytic small molecule transformation

Bioinspired transformation of small-molecules to energy-related feedstocks is an attractive research area to overcome both the environmental issues and the depletion of fossil fuels. The highly effective metalloenzymes in nature provide blueprints for the utilization of bioinspired metal complexes for artificial photosynthesis. Through simpler structural and functional mimics, the representative herein is the pivotal development of several critical small molecule conversions catalyzed by metal complexes, e.g., water oxidation, proton and CO₂ reduction and organic chemical transformation of small molecules. Of great achievement is the establishment of bioinspired metal complexes as catalysts with high stability, specific selectivity and satisfactory efficiency to drive the multiple-electron and multiple-proton processes related to small molecule transformation. Also, potential opportunities and challenges for future development in these appealing areas are highlighted.

Light-Activated Carbon Monoxide Prodrugs Based on Bipyridyl Dicarbonyl Ruthenium(II) Complexes

Two photoactivatable dicarbonyl ruthenium(II) complexes based on an amide-functionalised bipyridine scaffold (4-position) equipped with an alkyne functionality or a green-fluorescent BODIPY (boron-dipyrromethene) dye have been prepared and used to investigate their light-induced decarbonylation. UV/Vis, FTIR and 13 C NMR spectroscopies as well as gas chromatography and multivariate curve resolution alternating least-squares analysis (MCR-ALS) were used to elucidate the mechanism of the decarbonylation process. Release of the first CO molecule occurs very quickly, while release of the second CO molecule proceeds more slowly. In vitro studies using two cell lines A431 (human squamous carcinoma) and HEK293 (human embryonic kidney cells) have been carried out in order to characterise the anti-proliferative and anti-apoptotic activities. The BODIPY-labelled compound allows for monitoring the cellular uptake, showing fast internalisation kinetics and accumulation at the endoplasmic reticulum and mitochondria.

Plasmonic Heterostructure Functionalized with a Carbene-Linked Molecular Catalyst for Sustainable and Selective Carbon Dioxide Reduction

Hybridization of homogeneous catalytic sites with a photoelectrode is an attractive approach to highly selective and tunable photocatalysis using heterogeneous platforms. However, weak and unclear surface chemistry often leads to the dissociation and irregular orientation of catalytic centers, restricting long-term usability with high selectivity. Well-defined and robust ligands that can persist under harsh photocatalytic conditions are essential for the success of hybrid-type photocatalysis. Here, we introduce N-heterocyclic carbene as a durable linker for the immobilization of a Rubpy complex-based CO₂ reduction site (cis-dichloro-(4,4'-diphosphonato-Rubpy)(p-cymene) (RuCY)) on a p-type gallium nitride/gold nanoparticle (p-GaN/AuNP) heterostructure. The p-GaN/AuNPs/RuCY photocathode was coupled with a hematite photoanode to drive photoelectrochemical CO₂ reduction along with water oxidation. Highly selective CO₂ reduction into formates, up to 98.2%, was achieved utilizing plasmonic hot electrons accumulated on AuNPs. The turnover frequency was 1.46 min^-1 with a faradic efficiency of 96.8% under visible light illumination (243 mW·cm^-2). This work demonstrates that the N-heterocyclic carbene-mediated surface functionalization with homogeneous catalytic sites is a promising approach to increase the sustainability and usability of hybrid catalysts.

Coordination Chemistry of Ru(II) Complexes of an Asymmetric Bipyridine Analogue: Synergistic Effects of Supporting Ligand and Coordination Geometry on Reactivities

The reactivities of transition metal coordination compounds are often controlled by the environment around the coordination sphere. For ruthenium(II) complexes, differences in polypyridyl supporting ligands affect some types of reactivity despite identical coordination geometries. To evaluate the synergistic effects of (i) the supporting ligands, and (ii) the coordination geometry, a series of dicarbonyl-ruthenium(II) complexes that contain both asymmetric and symmetric bidentate polypyridyl ligands were synthesized. Molecular structures of the complexes were determined by X-ray crystallography to distinguish their steric configuration. Structural, computational, and electrochemical analysis revealed some differences between the isomers. Photo- and thermal reactions indicated that the reactivities of the complexes were significantly affected by both their structures and the ligands involved.

An Ir(III) Complex Photosensitizer With Strong Visible Light Absorption for Photocatalytic CO2 Reduction

A cyclometalated iridium(III) complex having 2-(pyren-1-yl)-4-methylquinoline ligands [Ir(pyr)] has a strong absorption band in the visible region (ε_444nm = 67,000 M⁻¹ cm⁻¹) but does not act as a photosensitizer for photochemical reduction reactions in the presence of triethylamine as an electron donor. Here, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) was used instead of the amine, demonstrating that BI(OH)H efficiently quenched the excited state of Ir(pyr) and can undergo the photochemical carbon dioxide (CO₂) reduction catalyzed by trans(Cl)-Ru(dmb)(CO)₂Cl₂ (dmb = 4,4′-dimethyl-2,2′-bipyridine, Ru) to produce formate as the main product. We also synthesized a binuclear complex combining Ir(pyr) and Ruvia an ethylene bridge and investigated its photochemical CO₂ reduction activity in the presence of BI(OH)H.

Coordination chemistry in the design of heterogeneous photocatalysts

Heterogeneous catalysts have been widely used for photocatalysis, which is a highly important process for energy conversion, owing to their merits such as easy separation of catalysts from the reaction products and applicability to continuous chemical industry and recyclability. Yet, homogenous photocatalysis receives tremendous attention as it can offer a higher activity and selectivity with atomically dispersed catalytic sites and tunable light absorption. For this reason, there is a major trend to combine the advantages of both homogeneous and heterogeneous photocatalysts, in which coordination chemistry plays a role as the bridge. In this article, we aim to provide the first systematic review to give a clear picture of the recent progress from taking advantage of coordination chemistry. We specifically summarize the role of coordination chemistry as a versatile tool to engineer catalytically active sites, tune light harvesting and maneuver charge kinetics in heterogeneous photocatalysis. We then elaborate on the common fundamentals behind various materials systems, together with key spectroscopic characterization techniques and remaining challenges in this field. The typical applications of coordination chemistry in heterogeneous photocatalysis, including proton reduction, water oxidation, carbon dioxide reduction and organic reactions, are highlighted.

Studies of Carbon Monoxide Release from Ruthenium(II) Bipyridine Carbonyl Complexes upon UV-Light Exposure

The UV-light-induced CO release characteristics of a series of ruthenium(II) carbonyl complexes of the form trans-Cl[RuLCl₂(CO)₂] (L = 4,4'-dimethyl-2,2'-bipyridine, 4'-methyl-2,2'-bipyridine-4-carboxylic acid, or 2,2'-bipyridine-4,4'-dicarboxylic acid) have been elucidated using a combination of UV-vis absorbance and Fourier transform infrared spectroscopies, multivariate curve resolution alternating least-squares analysis, and density functional theory calculations. In acetonitrile, photolysis appears to proceed via a serial three-step mechanism involving the sequential formation of [RuL(CO)(CH₃CN)Cl₂], [RuL(CH₃CN)₂Cl₂], and [RuL(CH₃CN)₃Cl]⁺. Release of the first CO molecule occurs quickly (k₁ ≫ 3 min^-1), while release of the second CO molecule proceeds at a much more modest rate (k₂ = 0.099-0.17 min^-1) and is slowed by the presence of electron-withdrawing carboxyl substituents on the bipyridine ligand. In aqueous media (1% dimethyl sulfoxide in H₂O), the two photodecarbonylation steps proceed much more slowly (k₁ = 0.46-1.3 min^-1 and k₂ = 0.026-0.035 min^-1, respectively) and the influence of the carboxyl groups is less pronounced. These results have implications for the design of new light-responsive CO-releasing molecules ("photoCORMs") intended for future medical use.

Ruthenium(II) carbonyl compounds with the 4'-chloro-2,2':6',2''-terpyridine ligand

Two ruthenium carbonyl complexes with the 4'-chloro-2,2':6',2''-terpyridine ligand (tpy-Cl, C15H10ClN3), i.e. [RuCl(tpy-Cl)(CO)2][RuCl3(CO)3] (I) [systematic name: cis-di-carbonyl-chlorido(4'-chloro-2,2':6',2''-terpyridine-κ3N)ruthenium(II) fac-tricarbonyltri-chlorido-ruthenate(II)], and [RuCl2(tpy-Cl)(CO)2] (II) [cis-dicarbonyl-trans-di-chlorido(4'-chloro-2,2':6',2''-terpyridine-κ2N1,N1')ruthenium(II)], were synthesized and characterized by single-crystal X-ray diffraction. The RuII atoms in both centrosymmetric structures (I) and (II) display similar, slightly distorted octa-hedral coordination spheres. The coordination sphere in the complex cation in compound (I) is defined by three N atoms of the tridentate tpy-Cl ligand, two carbonyl carbon atoms and one chlorido ligand; the charge is balanced by an octa-hedral [Ru(CO)3Cl3]- counter-anion. In the neutral compound (II), the tpy-Cl ligand coordinates to the metal only through two of its N atoms. The coordination sphere of the RuII atom is completed by two carbonyl and two chlorido ligands. In the crystal structures of both (I) and (II), weak C-H⋯Cl inter-actions are observed.

Development of hybrid photocatalysts constructed with a metal complex and graphitic carbon nitride for visible-light-driven CO2 reduction

The research progress of metal-complex/C₃N₄ hybrid photocatalysts for CO₂ reduction made by our group is highlighted.

Temperature dependence of photocatalytic CO2reduction by trans(Cl)–Ru(bpy)(CO)2Cl2: activation energy difference between CO and formate production

The temperature dependence of photocatalytic CO₂reduction bytrans(Cl)–Ru(bpy)(CO)₂Cl₂(bpy: 2,2′-bipyridine) has been researched in ethanol (EtOH)/N,N-dimethylacetamide (DMA) solutions containing [Ru(bpy)₃]²⁺(a photosensitizer) and 1-benzyl-1,4-dihydronicotinamide (BNAH, an electron donor). The catalytic system efficiently reduces CO₂to carbon monoxide (CO) with formate (HCOO⁻) as a minor product. The mechanism of the catalysis consists of the electron-relay cycle and the catalytic cycle: in the former cycle the photochemically generated reduced species of the photosensitizer injects an electron to the catalyst, and in the latter the catalyst reduces CO₂. At a low concentration of the catalyst (5.0 μM), where the catalytic cycle is rate-determining, the temperature dependence of CO/HCOO⁻is also dependent on the EtOH contents: the selectivity of CO/HCOO⁻decreases in 20% and 40%-EtOH/DMA with increasing temperature, while it increases in 60%-EtOH/DMA. The temperature dependence of the CO/HCOO⁻selectivity indicates that the difference in activation energy (ΔΔG^‡) between CO and HCOO⁻production is estimated asca.3.06 kJ mol⁻¹in 40%-EtOH/DMA at 298 K.

Iridium(III) 1-Phenylisoquinoline Complexes as a Photosensitizer for Photocatalytic CO2 Reduction: A Mixed System with a Re(I) Catalyst and a Supramolecular Photocatalyst

An Ir(III) complex with 1-phenylisoquinoline (piq) ligands [Ir(piq)2(dmb)](+) (Ir, dmb = 4,4'-dimethyl-2,2'-bipyridine) exhibited strong absorption in the visible region, and the lifetime of its excited state was very long (τ = 2.8 μs). Photochemical reduction of Ir efficiently proceeded with 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as reductants, giving the one-electron-reduced species (OERS), which was stable in solution at ambient temperature. The OERS of the Ir complex possessed strong reductive power, sufficient to supply an electron to fac-Re(dmb)(CO)3Br (Re). The photocatalytic reduction of CO2 proceeded efficiently using a mixed system constructed with Ir as a redox photosensitizer and Re as a catalyst, selectively giving CO (ΦCO = 0.16 using BNAH at λex = 480 nm). Ir was a more suitable photosensitizer for evaluating the activity of the Re catalyst in the photocatalytic reaction compared to [Ru(dmb)3](2+) (Ru) because the Ir complex was more stable in the photocatalytic reaction, and its decomposition products did not function as catalysts for CO2 reduction while the decomposition products of the Ru complex functioned as catalysts for the reduction of CO2 to HCOOH, inducing a drastic perturbation of the product distribution. A supramolecular photocatalyst (Ir-Re), in which the Ir(III) photosensitizer and the Re(I) catalyst were connected by a bridging ligand, was newly synthesized. When using BNAH, Ir-Re possessed a greater photocatalytic ability (ΦCO = 0.21, TONCO = 130) than the corresponding mixed system of the Ir and Re mononuclear complexes. Using BIH as the reductant, both Ir-Re and the mixed system showed very high photocatalytic activity (ΦCO = 0.40-0.41, TONCO = 1700).

Highly active and selective photochemical reduction of CO2 to CO using molecular-defined cyclopentadienone iron complexes

Highly active and selective visible-light-driven CO₂ reduction to CO catalyzed by well-defined cyclopentadienone iron complexes.

Synthesis and properties of new mononuclear Ru(ii)-based photocatalysts containing 4,4′-diphenyl-2,2′-bipyridyl ligands

Almost colourless trans-Ru^IICl₂(N^N)(CO)₂ (N^N = a derivative of 4,4′-diphenyl-2,2′-bipyridyl) complexes are reasonably effective photocatalytic oxidants in combination with a photosensitizer and sacrificial oxidant.