Photocatalytic CO2 Reduction under Visible-Light Irradiation by Ruthenium CNC Pincer Complexes

Photocatalytic CO₂ reduction using a ruthenium photosensitizer, a sacrificial reagent 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H), and a ruthenium catalyst were carried out. The catalysts contain a pincer ligand, 2,6-bis(alkylimidazol-2-ylidene)pyridine (CNC) and a bipyridine (bpy). The photocatalytic reaction system resulted in HCOOH as a main product (selectivity 70–80 %), with a small amount of CO, and H₂. Comparative experiments (a coordinated ligand (NCMe vs. CO) and substituents (tBu vs. Me) of the CNC ligand in the catalyst) were performed. The turnover number (TON_HCOOH) of carbonyl-ligated catalysts are higher than those of acetonitrile-ligated catalysts, and the carbonyl catalyst with the smaller substituents (Me) reached TON_HCOOH=5634 (24 h), which is the best performance among the experiments.     

Selective Photoelectrochemical Reduction of Aqueous CO2 to CO by Solvated Electrons

Reduction of CO₂ by direct one‐electron activation is extraordinarily difficult because of the ?1.9 V reduction potential of CO₂. Demonstrated herein is reduction of aqueous CO₂ to CO with greater than 90 % product selectivity by direct one‐electron reduction to CO₂^.? by solvated electrons. Illumination of inexpensive diamond substrates with UV light leads to the emission of electrons directly into water, where they form solvated electrons and induce reduction of CO₂ to CO₂^.?. Studies using diamond were supported by studies using aqueous iodide ion (I^?), a chemical source of solvated electrons. Both sources produced CO with high selectivity and minimal formation of H₂. The ability to initiate reduction reactions by emitting electrons directly into solution without surface adsorption enables new pathways which are not accessible using conventional electrochemical or photochemical processes.     

Mn(bipyridyl)(CO)(3) Br]: An Abundant Metal Carbonyl Complex as Efficient Electrocatalyst for CO(2) Reduction

Manganese at work: Carbonyl bipyridyl complexes based on manganese, a non-noble abundant and inexpensive metal, have been proved to be excellent molecular catalysts for the selective electrochemical reduction of CO(2) to CO under mild conditions. Another advantage of manganese complexes over rhenium complexes is that these catalysts operate at markedly less overpotential (0.40?V gain).     

Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium‐Substituted Polyoxometalate

A polyoxometalate of the Keggin structure substituted with Ru^III, ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] in which ⁶Q=(C₆H₁₃)₄N⁺, catalyzed the photoreduction of CO₂ to CO with tertiary amines, preferentially Et₃N, as reducing agents. A study of the coordination of CO₂ to ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] showed that 1) upon addition of CO₂ the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g_x=2.146, g_y=2.100, and g_z=1.935), and 3) the ¹³C NMR spectrum had a broadened peak of bound CO₂ at 105.78 ppm (Δ_1/2=122 Hz). It was concluded that CO₂ coordinates to the Ru^III active site in both the presence and absence of Et₃N to yield ⁶Q₅[Ru^III(CO₂)SiW₁₁O₃₉]. Electrochemical measurements showed the reduction of Ru^III to Ru^II in ⁶Q₅[Ru^III(CO₂)SiW₁₁O₃₉] at ?0.31 V versus SCE, but no such reduction was observed for ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉]. DFT‐calculated geometries optimized at the M06/PC1//PBE/AUG‐PC1//PBE/PC1‐DF level of theory showed that CO₂ is preferably coordinated in a side‐on manner to Ru^III in the polyoxometalate through formation of a Ru? O bond, further stabilized by the interaction of the electrophilic carbon atom of CO₂ to an oxygen atom of the polyoxometalate. The end‐on CO₂ bonding to Ru^III is energetically less favorable but CO₂ is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp² hybridization state. Formation of a O₂C–NMe₃ zwitterion, in turn, causes bending of CO₂ and enhances the carbon sp² hybridization. The synergetic effect of these two interactions stabilizes both Ru–O and C–N interactions and probably determines the promotional effect of an amine on the activation of CO₂ by [Ru^III(H₂O)SiW₁₁O₃₉]^5?. Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO₂ and Et₃N. A mechanistic pathway for photoreduction of CO₂ is suggested based on the experimental and computed results.     

A New Glimpse into the CO2‐Philicity of Carbonyl Compounds

We report a theoretical study on non‐conventional structures of 1:1 complexes between carbon dioxide and carbonyl compounds. These structures have never been reported before but are relevant for understanding the solubility of carbonyl compounds in supercritical CO₂. The work is based on the results of ab initio calculations at the MP2 and CCSD(T) levels using aug‐cc‐pVDZ and aug‐cc‐pVTZ basis sets. Investigated systems include aldehydes, ketones and esters, together with some fluorinated derivatives. The results are interpreted in terms of natural bond orbital analyses. Harmonic vibrational frequency calculations have also been done in order to compare them with available experimental data. We show for the first time that complexes where CO₂ behaves globally as a Lewis base are stable in the case of ketones and esters, but not in the case of aldehydes, and their stability is similar to that of traditional complexes in which CO₂ behaves as a Lewis acid. This finding considerably modifies the concept of CO₂‐philicity and may have important ramifications in the development of green reactions in supercritical CO₂.     

Efficient CO2 Insertion and Reduction Catalyzed by a Terminal Zinc Hydride Complex

The terminal zinc hydride complex [Tntm]ZnH ( 2 ; Tntm=tris(6‐tert‐butyl‐3‐thiopyridazinyl)methanide) is an efficient hydrosilylation catalyst of CO₂ at room temperature without the need of Lewis acidic additives. The inherent electrophilicity of the system leads to selective formation of the monosilylated product (MeO)₃SiO₂CH (at room temperature with a TOF of 22.2 h^?1 and at 45 °C with a TOF of 66.7 h^?1). In absence of silanes, the intermediate formate complex [Tntm]Zn(O₂CH) ( 3 ) is quantitatively formed within 5 min. All complexes were fully characterized by ¹H and ¹³C NMR spectroscopy and single‐crystal X‐ray diffraction analyses. Density functional theory (DFT) calculations reveal a high positive charge on zinc and the increased preference of the ligand to adopt a κ³‐coordination mode.     

Activating a Low Overpotential CO2 Reduction Mechanism by a Strategic Ligand Modification on a Ruthenium Polypyridyl Catalyst

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu₃tpy)(bpy)(NCCH₃)]²⁺ (tBu₃tpy=4,4′,4′′‐tri‐tert‐butyl‐2,2′:6′,2′′‐terpyridine; bpy=2,2′‐bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO₂ to CO. The methyl group enables CO₂ binding already at the one‐electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO₂ reduction catalysts.     

Photochemical Reduction of Low Concentrations of CO2 in a Porous Coordination Polymer with a Ruthenium(II)–CO Complex

Direct use of low pressures of CO₂ as a C1 source without concentration from gas mixtures is of great interest from an energy‐saving viewpoint. Porous heterogeneous catalysts containing both adsorption and catalytically active sites are promising candidates for such applications. Here, we report a porous coordination polymer (PCP)‐based catalyst, PCP‐Ru^II composite, bearing a Ru^II‐CO complex active for CO₂ reduction. The PCP‐Ru^II composite showed improved CO₂ adsorption behavior at ambient temperature. In the photochemical reduction of CO₂ the PCP‐Ru^II composite produced CO, HCOOH, and H₂. Catalytic activity was comparable with the corresponding homogeneous Ru^II catalyst and ranks among the highest of known PCP‐based catalysts. Furthermore, catalytic activity was maintained even under a 5 % CO₂/Ar gas mixture, revealing a synergistic effect between the adsorption and catalytically active sites within the PCP‐Ru^II composite.     

Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of Ru^II polypyridyl complexes of the structural design [Ru^II(R?tpy)(NN)(CH₃CN)]²⁺ (R?tpy=2,2′:6′,2′′‐terpyridine (R=H) or 4,4′,4′′‐tri‐tert‐butyl‐2,2′:6′,2′′‐terpyridine (R=tBu); NN=2,2′‐bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO₂ to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes’ reactivities. Whereas electron‐donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [Ru^II(tpy)(6‐mbpy)(CH₃CN)]²⁺ (trans‐[ 3 ]²⁺; 6‐mbpy=6‐methyl‐2,2′‐bipyridine) and [Ru^II(tBu?tpy)(6‐mbpy)(CH₃CN)]²⁺ (trans‐[ 4 ]²⁺), in which the methyl group of the 6‐mbpy ligand is trans to the CH₃CN ligand, show electrocatalytic CO₂ reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer–chemical reaction–electron transfer), and is a direct result of steric interactions that facilitate CH₃CN ligand dissociation, CO₂ coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.     

Terpyridine Diphosphine Ruthenium Complexes as Efficient Photocatalysts for the Transfer Hydrogenation of Carbonyl Compounds

The cationic achiral and chiral terpyridine diphosphine ruthenium complexes [RuCl(PP)(tpy)]Cl (PP=dppp ( 1 ), (R,R)-Skewphos ( 2 ) and (S,S)-Skewphos ( 3 )) are easily obtained in 85–88 % yield through a one-pot synthesis from [RuCl₂(PPh₃)₃], the diphosphine and 2,2′:6′,2′′-terpyridine (tpy) in 1-butanol. Treatment of 1 – 3 with NaPF₆ in methanol at RT affords quantitatively the corresponding derivatives [RuCl(PP)(tpy)]PF₆ (PP=dppp ( 1 a ), (R,R)-Skewphos ( 2 a ) and (S,S)-Skewphos ( 3 a )). Reaction of [RuCl₂(PPh₃)₃] with (S,R)-Josiphos or (R)-BINAP in toluene, followed by treatment with tpy in 1-butanol and finally with NaPF₆ in MeOH gives [RuCl(PP)(tpy)]PF₆ (PP=(S,R)-Josiphos ( 4 a ), (R)-BINAP ( 5 a )) isolated in 78 % and 86 % yield, respectively. The chiral derivatives have been isolated as single stereoisomers and 3 a , 4 a have been characterized by single crystal X-ray diffraction studies. The tpy complexes with NaOiPr display high photocatalytic activity in the transfer hydrogenation (TH) of carbonyl compounds using 2-propanol as the only hydrogen donor and visible light at 30 °C, at remarkably high S/C (up to 5000) and TOF values up to 264 h⁻¹. The chiral enantiomers 2 , 2 a and 3 , 3 a induce the asymmetric photocatalytic TH of acetophenone, affording (S)- and (R)-1-phenylethanol with 51 and 52 % ee, respectively, in a MeOH/2-propanol mixture.     

Betaine Catalysis for Hierarchical Reduction of CO2 with Amines and Hydrosilane To Form Formamides,Aminals, and Methylamines

An efficient, sustainable organocatalyst, glycine betaine, was developed for the reductive functionalization of CO₂ with amines and diphenylsilane. Methylamines and formamides were obtained in high yield by tuning the CO₂ pressure and reaction temperature. Based on identification of the key intermediate, that is, the aminal, an alternative mechanism for methylation involving the C⁰ silyl acetal and aminal is proposed. Furthermore, reducing the CO₂ amount afforded aminals with high yield and selectivity. Therefore, betaine catalysis affords products with a diversified energy content that is, formamides, aminals and methylamines, by hierarchical two-, four- and six-electron reduction, respectively, of CO₂ coupled with C−N bond formation.     

Hydrogenation of CO2 to Formic Acid with a Highly Active Ruthenium Acriphos Complex in DMSO and DMSO/Water

The novel [Ru(Acriphos)(PPh₃)(Cl)(PhCO₂)] [ 1 ; Acriphos=4,5‐bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO₂ to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H₂O without the need for amine bases as co‐reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h^?1 were achieved, thus rendering 1 one of the most active catalysts for CO₂ hydrogenations under additive‐free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L^?1) was shown to be limited by product‐dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L^?1, which corresponds to optimized values of TON=16×10³ and TOF_avg≈10³ h^?1.     

2,2 -联吡啶钌配合物催化CO2制备环状碳酸酯机理研究

研究了2,2 -联吡啶钌配合物RuCl₃(2,2 -bipy)(CH₃OH)与十六烷基三甲基氯化铵(CTAC)组成的催化体系催化二氧化碳与不同的环氧化合物进行环加成反应制备环状碳酸酯. 在此基础上, 利用电喷雾质谱(ESI-MS)对RuCl₃(2,2 -bipy)(CH₃OH)/CTAC催化CO₂与环氧丙烷(PO)反应制备碳酸丙烯酯(PC)进行了研究, 检测到了反应中间态配合物RuCl₃(2,2 -bipy)(PO)与RuCl₃(2,2 -bipy)(PC), 为该反应机理研究提供了实验证据. 研究结果表明, RuCl₃(2,2 -bipy)(CH₃OH)/CTAC催化体系催化CO₂与环氧化合物的反应首先是通过环氧化合物与RuCl₃(2,2 -bipy)(CH₃OH)中的甲醇分子发生配体交换引发的, 经CTAC中的氯离子进攻环氧化合物开环、二氧化碳插入Ru—O键、分子内关环及消去生成环状碳酸酯.     

Selective Production of Acetone in the Electrochemical Reduction of CO2 Catalyzed by a Ru–Naphthyridine Complex

The controlled potential electrolysis of [Ru(bpy)(napy)₂(CO)₂](BF₄)₂ ( 1 ; bpy=2,2′‐bipyridine, napy=1,8‐naphthyridine) in the presence of LiBF₄ in CO₂‐saturated DMSO at −1.65 V (vs. Ag/Ag⁺) produced CO and Li₂CO₃ [Eq. (a)], while similar electrolysis in the presence of (CH₃)₄NBF₄ resulted in formation of acetone together with (CH₃)₃N and {(CH₃)₄N}₂CO₃ [Eq. (b)]. This represents the first almost selective generation of acetone upon electrochemical reduction of CO₂. The selectivity is ascribed to depression of reductive cleavage of the Ru−CO bond of 1 due to an attack of the nonbonded nitrogen atom of napy at the carbonyl carbon atom.     

Photocatalytic Reduction of CO2 over Heterostructure Semiconductors into Value‐Added Chemicals

Photoreduction of CO₂, which utilizes solar energy to convert CO₂ into hydrocarbons, can be an effective means to overcome the increasing energy crisis and mitigate the rising emissions of greenhouse gas. This article covers recent advances in the CO₂ photoreduction over heterostructure‐based photocatalysts. The fundamentals of CO₂ photoreduction and classification of the heterostructured photocatalysts are discussed first, followed by the latest work on the CO₂ photoreduction over heterostructured photocatalysts in terms of the classification of the coupling semiconductors. Finally, a brief summary and a perspective on the challenges in this area are presented.     

Bridge‐Localized HOMO‐Binding Character of Divinylanthracene‐Bridged Dinuclear Ruthenium Carbonyl Complexes: Spectroscopic,Spectroelectrochemical, and Computational Studies

The electronic properties of four divinylanthracene‐bridged diruthenium carbonyl complexes [{RuCl(CO)(PMe₃)₃}₂(μ? CH?CHArCH?CH)] (Ar=9,10‐anthracene ( 1 ), 1,5‐anthracene ( 2 ), 2,6‐anthracene ( 3 ), 1,8‐anthracene ( 4 )) obtained by molecular spectroscopic methods (IR, UV/Vis/near‐IR, and EPR spectroscopy) and DFT calculations are reported. IR spectroelectrochemical studies have revealed that these complexes are first oxidized at the noninnocent bridging ligand, which is in line with the very small ν(C?O) wavenumber shift that accompanies this process and also supported by DFT calculations. Because of poor conjugation in complex 1 , except oxidized 1⁺ , the electronic absorption spectra of complexes 2⁺ , 3⁺ , and 4⁺ all display the characteristic near‐IR band envelopes that have been deconvoluted into three Gaussian sub‐bands. Two of the sub‐bands belong mainly to metal‐to‐ligand charge‐transfer (MLCT) transitions according to results from time‐dependent DFT calculations. EPR spectroscopy of chemically generated 1⁺ – 4⁺ proves largely ligand‐centered spin density, again in accordance with IR spectra and DFT calculations results.