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)]⁺ (<b>Ir</b>, 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 <b>Ir</b> efficiently proceeded with 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1<i>H</i>-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 <i>fac-</i>Re­(dmb)­(CO)₃Br (<b>Re</b>). The photocatalytic reduction of CO₂ proceeded efficiently using a mixed system constructed with <b>Ir</b> as a redox photosensitizer and <b>Re</b> as a catalyst, selectively giving CO (Φ_CO = 0.16 using BNAH at λ_ex = 480 nm). <b>Ir</b> was a more suitable photosensitizer for evaluating the activity of the Re catalyst in the photocatalytic reaction compared to [Ru­(dmb)₃]²⁺ (<b>Ru</b>) 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 (<b>Ir</b>–<b>Re</b>), in which the Ir­(III) photosensitizer and the Re­(I) catalyst were connected by a bridging ligand, was newly synthesized. When using BNAH, <b>Ir</b>–<b>Re</b> possessed a greater photocatalytic ability (Φ_CO = 0.21, TON_CO = 130) than the corresponding mixed system of the Ir and Re mononuclear complexes. Using BIH as the reductant, both <b>Ir</b>–<b>Re</b> and the mixed system showed very high photocatalytic activity (Φ_CO = 0.40–0.41, TON_CO = 1700)

Selective Electrocatalysis of a Water-Soluble Rhenium(I) Complex for CO₂ Reduction Using Water As an Electron Donor

Reduction of CO₂ using water as an electron donor could be deemed one of the most important reactions in artificial photosynthesis. In the current study, electrochemical properties of a water-soluble rhenium­(I) tricarbonyl complex with hydroxymethyl groups (<b>[Re­(CH</b>_<b>2</b><b>OH)–OH</b>_<b>2</b><b>]</b>^<b>+</b>) and its electrocatalysis for CO₂ reduction were investigated in aqueous solutions. In comparison with general rhenium­(I) carbonyl complexes, <b>[Re­(CH</b>_<b>2</b><b>OH)–OH</b>_<b>2</b><b>]</b>^<b>+</b> shows high solubility in water even in the presence of both CO₂ and OH^–. In an aqueous solution, <b>[Re­(CH</b>_<b>2</b><b>OH)–OH</b>_<b>2</b><b>]</b>^<b>+</b> exhibited a catalytic current due to CO₂ reduction in a cyclic voltammogram at a positive potential of about 400 mV more than that in a DMF solution. It is suggested that two molecules of a one-electron-reduced Re complex participated in reducing one molecule of CO₂ in the aqueous solution. Electrolysis using <b>[Re­(CH</b>_<b>2</b><b>OH)–OH</b>_<b>2</b><b>]</b>^<b>+</b> at −1.1 V vs NHE in a CO₂-saturated aqueous solution afforded CO and HCOOH with 95 and 4% selectivity, respectively, at pH 6.9; thermodynamically favorable H₂ evolution via water reduction was almost completely suppressed. Even at pH 4.2, the selectivity of CO₂ reduction was still high (CO 84%; HCOOH 3%). A full cell consisting of a cathode unit with <b>[Re­(CH</b>_<b>2</b><b>OH)–OH</b>_<b>2</b><b>]</b>^<b>+</b> for CO₂ reduction and an IrO₂/FTO anode for water oxidation electrocatalytically produced both CO and O₂ as the main products with high efficiency and selectivity

Photochemical Hydrogenation of π‑Conjugated Bridging Ligands in Photofunctional Multinuclear Complexes

Vinylene or ethynylene linkers in the bridging ligands of photofunctional multinuclear complexes synthesized by various coupling reactions, such as the Mizoroki–Heck reaction, olefin metathesis, and Sonogashira coupling, were successfully converted to their corresponding saturated carbon chains using photochemical hydrogenation, which proceeded in an MeCN–pyridine–CF₃COOH (3:1:0.1 v/v/v) mixed solution containing the starting metal complexes and 1,3-dimethyl-2-phenyl-2,3-dihydro-1<i>H</i>-benzo­[<i>d</i>]­imidazole (BIH) as a sacrificial electron donor under visible light irradiation in high yields. Hydrogenation of linkers in a Ru₂–Re trinuclear complex improved the photocatalytic ability for CO₂ reduction

Unique Solvent Effects on Visible-Light CO₂ Reduction over Ruthenium(II)-Complex/Carbon Nitride Hybrid Photocatalysts

Photocatalytic CO₂ reduction using hybrids of carbon nitride (C₃N₄) and a Ru­(II) complex under visible light was studied with respect to reaction solvent. Three different Ru­(II) complexes, <i>trans</i>(Cl)-[Ru­(X₂bpy) (CO)₂Cl₂] (X₂bpy = 2,2′-bipyridine with substituents X in the 4-positions, X = COOH, PO₃H₂, or CH₂PO₃H₂), were employed as promoters and will be abbreviated as <b>RuC</b> (X = COOH), <b>RuP</b> (X = PO₃H₂), and <b>RuCP</b> (X = CH₂PO₃H₂). When C₃N₄ modified with a larger amount of <b>RuCP</b> (>7.8 μmol g^–1) was employed as a photocatalyst in a solvent having a relatively high donor number (e.g., <i>N,N</i>-dimethylacetamide (DMA), <i>N,N</i>-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO)) with the aid of triethanolamine (TEOA) as an electron donor, the hybrid photocatalyst exhibited high performance for CO₂ reduction, producing CO and HCOOH with relatively high CO selectivity (40–70%). On the other hand, HCOOH was the major product when <b>RuC</b>/C₃N₄ or <b>RuP</b>/C₃N₄ was employed regardless of the loading amount of the Ru­(II) complex and the reaction solvent. Results of photocatalytic reactions and UV–visible diffuse reflectance spectroscopy indicated that polymeric Ru species, which were formed in situ from <b>RuCP</b> on C₃N₄ under irradiation in a solvent having a high donor number, were active catalysts for CO formation. Nonsacrificial CO₂ reduction using <b>RuP</b>/C₃N₄ was accomplished in a DMA solution containing methanol as an electron donor, which means that visible light energy was stored as chemical energy in the form of CO and formaldehyde (Δ<i>G</i>° = +67.6 kJ mol^–1). This study demonstrated the first successful example of an energy conversion scheme using carbon nitride through photocatalytic CO₂ reduction

Photocatalytic CO₂ Reduction Using Cu(I) Photosensitizers with a Fe(II) Catalyst

Photocatalytic systems developed from complexes with only abundant metals, i.e., Cu^I(dmp)­(P)₂⁺ (dmp =2,9-dimethyl-1,10-phenanthroline; P = phosphine ligand) as a redox photosensitizer and Fe^II(dmp)₂(NCS)₂ as a catalyst, produced CO as the main product by visible light irradiation. The best photocatalysis was obtained using a Cu^I complex with a tetradentate dmp ligand tethering two phosphine groups, where the turnover number and quantum yield of CO formation were 273 and 6.7%, respectively

Photocatalytic CO₂ Reduction Using Cu(I) Photosensitizers with a Fe(II) Catalyst

Photocatalytic systems developed from complexes with only abundant metals, i.e., Cu^I(dmp)­(P)₂⁺ (dmp =2,9-dimethyl-1,10-phenanthroline; P = phosphine ligand) as a redox photosensitizer and Fe^II(dmp)₂(NCS)₂ as a catalyst, produced CO as the main product by visible light irradiation. The best photocatalysis was obtained using a Cu^I complex with a tetradentate dmp ligand tethering two phosphine groups, where the turnover number and quantum yield of CO formation were 273 and 6.7%, respectively

Selective Formic Acid Production via CO₂ Reduction with Visible Light Using a Hybrid of a Perovskite Tantalum Oxynitride and a Binuclear Ruthenium(II) Complex

A hybrid material consisting of CaTaO₂N (a perovskite oxynitride semiconductor having a band gap of 2.5 eV) and a binuclear Ru­(II) complex photocatalytically produced HCOOH via CO₂ reduction with high selectivity (>99%) under visible light (λ > 400 nm). Results of photocatalytic reactions, spectroscopic measurements, and electron microscopy observations indicated that the reaction was driven according to a two-step photoexcitation of CaTaO₂N and the Ru photosensitizer unit, where Ag nanoparticles loaded on CaTaO₂N with optimal distribution mediated interfacial electron transfer due to reductive quenching

Hybrids of a Ruthenium(II) Polypyridyl Complex and a Metal Oxide Nanosheet for Dye-Sensitized Hydrogen Evolution with Visible Light: Effects of the Energy Structure on Photocatalytic Activity

Hybrid materials consisting of a ruthenium­(II) polypyridyl complex and a Dion–Jacobson type perovskite oxide nanosheet were studied as photocatalysts for dye-sensitized H₂ evolution under visible light with respect to the energy structure of the hybrids. Three Ru­(II) complexes, Ru^II{(4,4′-X₂-bpy)₂(4,4′-(CH₂PO₃H₂)₂-bpy)} (X = H, CH₃, CF₃; bpy = 2,2′-bipyridine), were used as redox photosensitizers. HCa_2–<i>x</i>Sr_<i>x</i>Nb₃O₁₀ (0 ≤ <i>x</i> ≤ 2) and HCa₂Nb_3–<i>y</i>Ta_<i>y</i>O₁₀ (0 ≤ <i>y</i> ≤ 1.5) nanosheet aggregates, having a tunable conduction band potential (<i>E</i>_CB), were employed as the building block. Nanosheets that possess more negative <i>E</i>_CB value were found to be preferable for the dye-sensitized H₂ evolution, unless electron injection from the excited-state sensitizer to the conduction band of a nanosheet is hindered. Among the combinations tested, the highest activity was obtained when an HCa₂Nb₃O₁₀ nanosheet was sensitized by Ru^II{(4,4′-(CH₃)₂-bpy)₂(4,4′-(CH₂PO₃H₂)₂-bpy)}, exhibiting a maximum apparent quantum yield of ca. 10% at 460 nm and a turnover number of ca. 3800 (for 20 h). This study highlighted that it is possible to maximize the performance of dye-sensitized H₂ evolution on a sensitizer/semiconductor hybrid by refining the <i>E</i>_CB value of a semiconductor and the oxidation potential of the excited state of a photosensitizer

Synergetic Effect of Ligand Modification of a Ru(II) Complex Catalyst and Ag Loading for Constructing a Highly Active Hybrid Photocatalyst Using C₃N₄ for CO₂ Reduction

Hybrid photocatalysts comprising a semiconductor and a metal complex catalyst are promising for the efficient, visible-light-driven chemical conversion of CO2 to useful fuels. As a typical example, a hybrid photocatalyst comprising Ag-loaded carbon nitride (C3N4) and trans-(Cl)-[Ru{4,4′-(PO3H2)2-2,2′-bipyridine}(CO)2Cl2] (RuP), RuP/Ag/C3N4, has been developed. However, the effects of the reduction potential of RuP on the photocatalytic CO2 reduction activity of hybrid systems have not yet been investigated. This study utilized a ligand-modified Ru catalyst (RuCP) to construct a new hybrid photocatalyst (RuCP/Ag/C3N4) in which a methylene spacer was inserted into RuCP between the bipyridine ligand and the phosphonic acid anchor groups on RuP to obtain a more negative reduction potential for the Ru catalyst. RuCP/Ag/C3N4 yielded formic acid with 2.5-fold turnover frequency and 6.5-fold turnover number (>12,000, used RuCP base) in comparison to RuP/Ag/C3N4 under visible light irradiation. The superior photocatalytic activity of RuCP/Ag/C3N4 was attributed to the faster CO2 reduction reaction on RuCP owing to its more negative reduction potential. The faster CO2 reduction on RuCP was facilitated by Ag loading on C3N4, which accumulated photoexcited electrons on Ag-loaded C3N4 and improved electron supply to the RuCP. This finding sheds light on a new approach for the development of hybrid photocatalysts for CO2 reduction