Activation of CO2 by Vanadium Nitrogenase

The reduction of CO₂ into useful products, including hydrocarbon fuels, is an ongoing area of particular interest due to efforts to mitigate buildup of this greenhouse gas. While the industrial Fischer–Tropsch process can facilitate the hydrogenation of CO₂ with H₂ to form short‐chain hydrocarbon products under high temperatures and pressures, a desire to perform these reactions under ambient conditions has inspired the use of biological approaches. Particularly, enzymes offer insight into how to activate and reduce CO₂, but only one enzyme, nitrogenase, can perform the multielectron, multiproton reduction of CO₂ into hydrocarbons. The vanadium‐containing variant, V‐nitrogenase, displays especial reactivity towards the hydrogenation of CO and CO₂. This Focus Review discusses recent progress towards the activation and reduction of CO₂ with three primary V‐nitrogenase systems. These systems span both ATP‐dependent and ATP‐independent processes and utilize approaches with whole cells, isolated proteins, and extracted cofactors.     

Catalytic Reduction of CN−, CO,and CO2 by Nitrogenase Cofactors in Lanthanide‐Driven Reactions

Nitrogenase cofactors can be extracted into an organic solvent to catalyze the reduction of cyanide (CN^?), carbon monoxide (CO), and carbon dioxide (CO₂) without using adenosine triphosphate (ATP), when samarium(II) iodide (SmI₂) and 2,6‐lutidinium triflate (Lut‐H) are employed as a reductant and a proton source, respectively. Driven by SmI₂, the cofactors catalytically reduce CN^? or CO to C₁–C₄ hydrocarbons, and CO₂ to CO and C₁–C₃ hydrocarbons. The C? C coupling from CO₂ indicates a unique Fischer–Tropsch‐like reaction with an atypical carbonaceous substrate, whereas the catalytic turnover of CN^?, CO, and CO₂ by isolated cofactors suggests the possibility to develop nitrogenase‐based electrocatalysts for the production of hydrocarbons from these carbon‐containing compounds.     

Selective Conversion of CO2 into Isocyanate by Low‐Coordinate Iron Complexes

Discovery of the mechanisms for selective transformations of CO₂ into organic compounds is a challenge. Herein, we describe the reaction of low‐coordinate Fe silylamide complexes with CO₂ to give trimethylsilyl isocyanate and the corresponding Fe siloxide complex. Kinetic studies show that this is a two‐stage reaction, and the presence of a single equivalent of THF influences the rates of both steps. Isolation of a thermally unstable intermediate provides mechanistic insight that explains both the effect of THF in this reaction, and the way in which the reaction achieves high selectivity for isocyanate formation.     

Differential Reduction of CO2 by Molybdenum and Vanadium Nitrogenases

The molybdenum and vanadium nitrogenases are two homologous enzymes with distinct structural and catalytic features. Previously, it was demonstrated that the V nitrogenase was nearly 700 times more active than its Mo counterpart in reducing CO to hydrocarbons. Herein, a similar discrepancy between the two nitrogenases in the reduction of CO₂ is reported, with the V nitrogenase being capable of reducing CO₂ to CO, CD₄, C₂D₄, and C₂D₆, and its Mo counterpart only capable of reducing CO₂ to CO. Furthermore, it is shown that the V nitrogenase may direct the formation of CD₄ in part via CO₂‐derived CO, but that it does not catalyze the formation of C₂D₄ and C₂D₆ along this route. The exciting observation of a V nitrogenase‐catalyzed C? C coupling with CO₂ as the origin of the building blocks adds another interesting reaction to the catalytic repertoire of this unique enzyme system. The differential activities of the V and Mo nitrogenases in CO₂ reduction provide an important framework for systematic investigations of this reaction in the future.     

Efficient and Reusable Iron Catalyst to Convert CO2 into Valuable Cyclic Carbonates

The production of cyclic carbonates from CO₂ cycloaddition to epoxides, using the C-scorpionate iron(II) complex [FeCl₂{κ³-HC(pz)₃}] (pz = 1H-pyrazol-1-yl) as a catalyst, is achieved in excellent yields (up to 98%) in a tailor-made ionic liquid (IL) medium under mild conditions (80 °C; 1–8 bar). A favorable synergistic catalytic effect was found in the [FeCl₂{κ³-HC(pz)₃}]/IL system. Notably, in addition to exhibiting remarkable activity, the catalyst is stable during ten consecutive cycles, the first decrease (11%) on the cyclic carbonate yield being observed during the 11th cycle. The use of C-scorpionate complexes in ionic liquids to afford cyclic carbonates is presented herein for the first time.     

Atropisomeric Hydrogen Bonding Control for CO2 Binding and Enhancement of Electrocatalytic Reduction at Iron Porphyrins

The manipulation of the second coordination sphere for improving the electrocatalytic CO₂ reduction has led to breakthroughs with hydrogen bonding, local proton source, or electrostatic effects. We have developed two atropisomers of an iron porphyrin complex with two urea functions acting as multiple hydrogen-bonding tweezers to lock the metal-bound CO₂ in a similar fashion found in the carbon monoxide dehydrogenase (CODH) enzyme. The αα topological isomer with the two urea groups on the same side of the porphyrin provides a stronger binding affinity to tether the incoming CO₂ in comparison to the αβ disposition. However, the electrocatalytic activity of the αβ atropisomer outperforms its congener with one of the highest reported turnover frequencies at low overpotential. The strong H/D kinetic isotope effect (KIE) observed for the αα system indicates the existence of a tight water hydrogen-bonding network for proton delivery which is disrupted by addition of an acid source. The small H/D KIE for the αβ isomer and the enhanced electrocatalytic performance on addition of stronger acid indicate the free access of protons to the bound CO₂ on the opposite side of the urea arm.     

CO2吸附活化的研究进展

本文分析讨论了CO₂ 在金属催化剂和金属氧化物催化剂上吸附活化的机理及活化吸附态的反应性能, 提出了CO₂ 作为一种温和氧化剂在化工生产中加以综合利用的有效途径。     

Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell

Molecular catalysts have been shown to have high selectivity for CO₂ electrochemical reduction to CO, but with current densities significantly below those obtained with solid-state materials. By depositing a simple Fe porphyrin mixed with carbon black onto a carbon paper support, it was possible to obtain a catalytic material that could be used in a flow cell for fast and selective conversion of CO₂ to CO. At neutral pH (7.3) a current density as high as 83.7 mA cm⁻² was obtained with a CO selectivity close to 98 %. In basic solution (pH 14), a current density of 27 mA cm⁻² was maintained for 24 h with 99.7 % selectivity for CO at only 50 mV overpotential, leading to a record energy efficiency of 71 %. In addition, a current density for CO production as high as 152 mA cm⁻² (>98 % selectivity) was obtained at a low overpotential of 470 mV, outperforming state-of-the-art noble metal based catalysts.     

铁系催化剂上乙苯与CO2氧化脱氢反应

采用浸渍法和溶胶-凝胶法制备了三种含Mg,Fe和Al量相同的Fe2O3-MgO/γ-Al2O3,Fe2O3/MgAl2O4和MgFe0.1Al1.9O4催化剂,在580°C考察了它们催化乙苯与CO2氧化脱氢反应性能,并采用X射线衍射,表面元素分析,H2-程序升温还原和CO2-程序升温脱附等技术对催化剂体相及表面性质进行了表征.结果表明,催化剂制备方法影响Fe物种的存在形态,进而影响催化剂的稳定性和活性.采用浸渍法制备的Fe2O3/MgAl2O4催化剂含有高度分散的Fe2O3活性物种,该物种具有较好的初活性,但是稳定性较差;而采用溶胶-凝胶法制备的MgFe0.1Al1.9O4催化剂中,Fe物种主要以同晶取代的形式存在于尖晶石骨架中,因而具有较高的乙苯与CO2氧化脱氢催化活性和稳定性.     

Activation of CS2 and CO2 by Silylium Cations

The hydride-bridged silylium cation [Et₃Si−H−SiEt₃]⁺, stabilized by the weakly coordinating [Me₃NB₁₂Cl₁₁]⁻ anion, undergoes, in the presence of excess silane, a series of unexpected consecutive reactions with the valence-isoelectronic molecules CS₂ and CO₂. The final products of the reaction with CS₂ are methane and the previously unknown [(Et₃Si)₃S]⁺ cation. To gain insight into the entire reaction cascade, numerous experiments with varying conditions were performed, intermediate products were intercepted, and their structures were determined by X-ray crystallography. Besides the [(Et₃Si)₃S]⁺ cation as the final product, crystal structures of [(Et₃Si)₂SMe]⁺, [Et₃SiS(H)Me]⁺, and [Et₃SiOC(H)OSiEt₃]⁺ were obtained. Experimental results combined with supporting quantum-chemical calculations in the gas phase and solution allow a detailed understanding of the reaction cascade.     

Dinitrosyl Iron Complex [K-18-crown-6-ether][(NO)2Fe(MePyrCO2)]: Intermediate for Capture and Reduction of Carbon Dioxide

Continued efforts are made for the utilization of CO₂ as a C₁ feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro-/photo-)catalysts disclosed that initial step for CO₂ activation involves either nucleophilic insertion or direct reduction of CO₂. In this study, nucleophilic activation of CO₂ by complex [(NO)₂Fe(μ-^MePyr)₂Fe(NO)₂]²⁻ ( 2 , ^MePyr=3-methylpyrazolate) results in the formation of CO₂-captured complex [(NO)₂Fe(^MePyrCO₂)]⁻ ( 2-CO₂ , ^MePyrCO₂=3-methyl-pyrazole-1-carboxylate). Single-crystal structure, spectroscopic, reactivity, and computational study unravels 2-CO₂ as a unique intermediate for reductive transformation of CO₂ promoted by Ca²⁺. Moreover, sequential reaction of 2 with CO₂, Ca(OTf)₂, and KC₈ established a synthetic cycle, 2 → 2-CO₂ → [(NO)₂Fe(μ-^MePyr)₂Fe(NO)₂] ( 1 ) → 2 , for selective conversion of CO₂ into oxalate. Presumably, characterization of the unprecedented intermediate 2-CO₂ may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO₂.     

Recent Advances in Homogeneous Carbonylation Using CO2 as CO Surrogate

Carbon dioxide is a sufficient and important carbon resource, which has been widely used as a C1 building block in synthetic chemistry. Carbonylations with CO are important processes in industry. However, due to the toxicity of CO, its storage and transport are problematic. Attentions are gradually focused on using other safe reagents to be the CO surrogates in carbonylation reactions. This review focuses on the summary of recent developments in using CO₂ as a CO surrogate in homogeneous catalysis. Reductive processes by using H₂, Si‐H, alcohols, etc and redox‐neutral processes are separately summarized.     

Iron Coordination to Hollow Microporous Metal-Free Disalphen Networks: Heterogeneous Iron Catalysts for CO2 Fixation to Cyclic Carbonates

This work shows that a hollow and microporous metal-free N,N′-phenylenebis(salicylideneimine) (salphen) network (H-MSN) can be engineered by Sonogashira coupling of [tetraiodo{di(Zn-salphen)}] building blocks with 1,4-diethynylbenzene in the presence of silica templates and by successive Zn and silica etching. Iron(III) ions could be incorporated into the H-MSN to form hollow and microporous Fe–disalphen networks (H-MFeSN) with enhanced microporosity and surface area. The H-MFeSN showed efficient catalytic performance and recyclability in the CO₂ conversion to cyclic carbonates.     

Magnesium(I) Dimers Bearing Tripodal Diimine–Enolate Ligands: Proficient Reagents for the Controlled Reductive Activation of CO2 and SO2

The first examples of magnesium(I) dimers bearing tripodal ligands, [(Mg{κ³‐N,N′,O‐(ArNCMe)₂(OCCPh₂)CH})₂] [Ar=2,6‐iPr₂C₆H₃ (Dip) 7 , 2,6‐Et₂C₆H₃ (Dep) 8 , or mesityl (Mes) 9 ] have been prepared by post‐synthetic modification of the β‐diketiminato ligands of previously reported magnesium(I) systems, using diphenylketene, O?C?CPh₂. In contrast, related reactions between β‐diketiminato magnesium(I) dimers and the isoelectronic ketenimine, MesN?C?CPh₂, resulted in reductive insertion of the substrate into the Mg?Mg bond of the magnesium(I) reactant, and formation of [{(Nacnac)Mg}₂{μ‐κ²‐N,C‐(Mes)NCCPh₂}] (Nacnac=[(ArNCMe)₂CH]^?; Ar=Dep 10 or Mes 11 ). Reactions of the four‐coordinate magnesium(I) dimer 8 with excess CO₂ are readily controlled, and cleanly give carbonate [(LMg)₂(μ‐κ²:κ²‐CO₃)] 12 (L=[κ³‐N,N′,O‐(DepNCMe)₂(OCCPh₂)CH]^?; thermodynamic product), or oxalate [(LMg)₂(μ‐κ²:κ²‐C₂O₄)] 13 (kinetic product), depending on the reaction temperature. Compound 12 and CO are formed by reductive disproportionation of CO₂, whereas 13 results from reductive coupling of two molecules of the gas. Treatment of 8 with an excess of N₂O cleanly gives the μ‐oxo complex [(LMg)₂(μ‐O)] 14 , which reacts facilely with CO₂ to give 12 . This result presents the possibility that 14 is an intermediate in the formation of 12 from the reaction of 8 and CO₂. In contrast to its reactions with CO₂, 8 reacts with SO₂ over a wide temperature range to give only one product; the first example of a magnesium dithionite complex, [(LMg)₂(μ‐κ²:κ²‐S₂O₄)] 16 , which is formed by reductive coupling of two molecules of SO₂, and is closely related to f‐block metal dithionite complexes derived from similar SO₂ reductive coupling processes. On the whole, this study strengthens previously proposed analogies between the reactivities of magnesium(I) systems and low‐valent f‐block metal complexes, especially with respect to small molecule activations.     

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.     

Stoichiometric Reduction of CO(2) to CO by Aluminum-Based Frustrated Lewis Pairs

Reducing frustration: The reaction of Mes(3) P(CO(2) )(AlI(3) )(2) in the presence of a CO(2) atmosphere results in the formation of Mes(3) P(CO(2) )(O(AlI(2) )(2) )(AlI(3) ) and [Mes(3) PI][AlI(4) ] (Mes=2,4,6-Me(3) C(6) H(2) ) with the evolution of CO.     

Thermodynamic and Catalytic Insights into Non-Reductive Transformation of CO2 with Amines into Organic Urea Derivatives

Carbon dioxide (CO₂) utilization as a carbonyl source is an attractive and promising approach to yielding value-added organic urea derivatives, which are currently produced with toxic reagents such as phosgene and carbon monoxide, along with the contribution to mitigating global warming. However, the direct intermolecular reaction between CO₂ and amines into organic urea derivatives has thermodynamic limitations, and such obstacles need to be considered well in order to establish efficient reaction systems. Herein, this review describes the thermodynamic aspects for producing several organic urea compounds, viz., N,N’-dibutylurea, N,N’-di(tert-butyl)urea, 2-imidazolidinone (ethylene urea), N,N’-dimethyl-2-imidazolidinone, tetrahydro-2-pyrimidinone (propylene urea), and N,N’-diphenylurea, based on the results of computational calculations. Besides, a variety of the state-of-the-art reaction systems with/without catalyst for synthesizing such organic urea compounds operated under pressurized CO₂ have been summarized and discussed to make not only advantages but also disadvantages clear. We have also overviewed the very recently reported approaches that employ alkylcarbamic acids as substrates and instead does not require external CO₂. The thermodynamic and catalytic insights garnered here could be a fruitful guideline for fairly assessing each reaction system and further improving the efficiency of CO₂ utilization as a carbonyl source.     

Dinitrosyl Iron Complex [K‐18‐crown‐6‐ether][(NO)2Fe(MePyrCO2)]: Intermediate for Capture and Reduction of Carbon Dioxide

Continued efforts are made for the utilization of CO₂ as a C₁ feedstock for regeneration of valuable chemicals and fuels. Mechanistic study of molecular (electro‐/photo‐)catalysts disclosed that initial step for CO₂ activation involves either nucleophilic insertion or direct reduction of CO₂. In this study, nucleophilic activation of CO₂ by complex [(NO)₂Fe(μ‐^MePyr)₂Fe(NO)₂]^2? ( 2 , ^MePyr=3‐methylpyrazolate) results in the formation of CO₂‐captured complex [(NO)₂Fe(^MePyrCO₂)]^? ( 2‐CO₂ , ^MePyrCO₂=3‐methyl‐pyrazole‐1‐carboxylate). Single‐crystal structure, spectroscopic, reactivity, and computational study unravels 2‐CO₂ as a unique intermediate for reductive transformation of CO₂ promoted by Ca²⁺. Moreover, sequential reaction of 2 with CO₂, Ca(OTf)₂, and KC₈ established a synthetic cycle, 2 → 2‐CO₂ → [(NO)₂Fe(μ‐^MePyr)₂Fe(NO)₂] ( 1 ) → 2 , for selective conversion of CO₂ into oxalate. Presumably, characterization of the unprecedented intermediate 2‐CO₂ may open an avenue for systematic evaluation of the effects of alternative Lewis acids on reduction of CO₂.