Mechanistic insights into CO2 reduction to CO by group 5 transition metal monoxide cations

The reduction of carbon dioxide (CO2) by transition-metal oxides in the gas phase serves as a unique model system for understanding transition metal-based catalytic systems in CO2 utilization. In this work, thermochemistry and reaction mechanisms attributed to the two-state reactivity scenario of CO2 reduction by group 5 transition metal monoxide cations are extensively investigated using quantum chemical calculations. The interaction between the VO+ cation with CO2 exhibits an endothermic feature, whereas the reaction involving the TaO+ cation showcases a more pronounced exothermic behavior than the NbO+ cation, in accordance with previously reported reaction rates. Based on in-depth examinations of potential energy surfaces and spin-orbit couplings, it has been revealed that the reaction kinetics of CO2 reduction to CO by the VO+ cation is restricted not only by a significant energy barrier related to the singlet transition state, but also by the limited probability of intersystem crossing. For NbO+ and TaO+ cations, the spin inversion from triplet to singlet pathways becomes the rate-limiting step. The reaction with the TaO+ cation represents a different case from typical two-state reactivity patterns, where the minimum energy crossing point submerged relative to the reactants level stands for the exclusive barrier. A considerably higher probability of intersystem crossing was identified for the reaction of the TaO+ cation with CO2, elucidating the basis for the substantial increase in the rate constant compared to that of the NbO+ cation.

Experimental and computational aspects of molecular frustrated Lewis pairs for CO2 hydrogenation: en route for heterogeneous systems?

Catalysis plays a crucial role in advancing sustainability. The unique reactivity of frustrated Lewis pairs (FLPs) is driving an ever-growing interest in the transition metal-free transformation of small molecules like CO2 into valuable products. In this area, there is a recent growing incentive to heterogenize molecular FLPs into porous solids, merging the benefits of homogeneous and heterogeneous catalysis - high activity, selectivity, and recyclability. Despite the progress, challenges remain in preventing deactivation, poisoning, and simplifying catalyst-product separation. This review explores the expanding field of FLPs in catalysis, covering existing molecular FLPs for CO2 hydrogenation and recent efforts to design heterogeneous porous systems from both experimental and theoretical perspectives. Section 2 discusses experimental examples of CO2 hydrogenation by molecular FLPs, starting with stoichiometric reactions and advancing to catalytic ones. It then examines attempts to immobilize FLPs in porous matrices, including siliceous solids, metal-organic frameworks (MOFs), covalent organic frameworks, and disordered polymers, highlighting current limitations and challenges. Section 3 then reviews computational studies on the mechanistic details of CO2 hydrogenation, focusing on H2 splitting and hydride/proton transfer steps, summarizing efforts to establish structure-activity relationships. It also covers the computational aspects on grafting FLPs inside MOFs. Finally, Section 4 summarizes the main design principles established so far, while addressing the complexities of translating computational approaches into the experimental realm, particularly in heterogeneous systems. This section underscores the need to strengthen the dialogue between theoretical and experimental approaches in this field.

Homogeneous and Heterogeneous Frustrated Lewis Pairs for the Activation and Transformation of CO2

Due to the serious ecological problems caused by the high CO2 content in the atmosphere, reducing atmospheric CO2 has attracted widespread attention from academia and governments. Among the many ways to mitigate CO2 concentration, the capture and comprehensive utilization of CO2 through chemical methods have obvious advantages, whose key is to develop suitable adsorbents and catalysts. Frustrated Lewis pairs (FLPs) are known to bind CO2 through the interaction between unquenched Lewis acid sites/Lewis base sites with the O/C of CO2, simultaneously achieving CO2 capture and activation, which render FLP better potential for CO2 utilization. However, how to construct efficient FLP targeted for CO2 utilization and the mechanism of CO2 activation have not been systematically reported. This review firstly provides a comprehensive summary of the recent advances in the field of CO2 capture, activation, and transformation with the help of FLP, including the construction of homogeneous and heterogeneous FLPs, their interaction with CO2, reaction activity, and mechanism study. We also illustrated the challenges and opportunities faced in this field to shed light on the prospective research.

Electrocatalytic Reduction of CO2 and H2O with Zn(II) Complexes Through Metal-Ligand Cooperation

Air and water-stable zinc (II) complexes of neutral pincer bis(diphenylphosphino)-2,6-di(amino)pyridine ("PN3P") ligands are reported. These compounds, [Zn(κ2-2,6-{Ph2PNR}2(NC5H3))Br2] (R=Me, 1; R=H, 2), were shown to be capable of electrocatalytic reduction of CO2 at -2.3 V vs. Fc+/0 to selectively yield CO in mixed water/acetonitrile solutions. These complexes also electrocatalytically generate H2 from water in acetonitrile solutions, at the same potential, with Faradaic efficiencies of up to 90 %. DFT computations support a proposed mechanism involving the first reduction of 1 or 2 occurring at the PN3P ligand. Furthermore, computational analysis suggested a mechanism involving metal-ligand cooperation of a Lewis acidic Zn(II) and a basic ligand.

Carbodiimide and Isocyanate Hydroboration by a Cyclic Carbodiphosphorane Catalyst

We report hydroboration of carbodiimide and isocyanate substrates catalyzed by a cyclic carbodiphosphorane catalyst. The cyclic carbodiphosphorane outperformed the other Lewis basic carbon species tested, including other zerovalent carbon compounds, phosphorus ylides, an N-heterocyclic carbene, and an N-heterocyclic olefin. Hydroborations of seven carbodiimides and nine isocyanates were performed at room temperature to form N-boryl formamidine and N-boryl formamide products. Intermolecular competition experiments demonstrated the selective hydroboration of alkyl isocyanates over carbodiimide and ketone substrates. DFT calculations support a proposed mechanism involving activation of pinacolborane by the carbodiphosphorane catalyst, followed by hydride transfer and B-N bond formation.

Advances in CO2 activation by frustrated Lewis pairs: from stoichiometric to catalytic reactions

The rise of CO2 concentrations in the environment due to anthropogenic activities results in global warming and threatens the future of humanity and biodiversity. To address excessive CO2 emissions and its effects on climate change, efforts towards CO2 capture and conversion into value adduct products such as methane, methanol, acetic acid, and carbonates have grown. Frustrated Lewis pairs (FLPs) can activate small molecules, including CO2 and convert it into value added products. This review covers recent progress and mechanistic insights into intra- and inter-molecular FLPs comprised of varying Lewis acids and bases (from groups 13, 14, 15 of the periodic table as well as transition metals) that activate CO2 in stoichiometric and catalytic fashion towards reduced products.

Electrocatalytic reduction of CO2 to CO and HCO2- with Zn(II) complexes displaying cooperative ligand reduction

Air-stable zinc(II) pyridyl phosphine complexes, [Zn(κ²-2,6-{Ph₂PNMe}₂(NC₅H₃))Br₂] (1) and [Zn(κ²-2-{Ph₂PNMe}(NC₅H₃))Br₂] (2) are reported and 1 was capable of electrocatalytic reduction of CO₂ at -2.3 V vs. Fc^+/0 to yield CO/HCO₂H in mixed water/acetonitrile solutions. DFT computations support a proposed mechanism involving electron transfer reactions from a species with the anionic PN³P ligand ("L^-/Zn(II)").

Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis-Azobenzene Photoisomer

The reactivity of the geminal frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H3 CC(O)CH=CH2 , Ph[C(O)]2 Ph, PhN=NPh and Me3 SiCHN2 ), featuring polar or non-polar multiple bonds and/or represent α,β-unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV-induced photoisomerization, which results in the highly selective complexation of cis-azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2- or 1,4-addition products, but to the non-stereoselective (tBu)2 PCH2 -transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X-ray diffraction experiments.

A Zwitterionic Phosphonium Stannate(II) via Hydrogen Splitting by a Sn/P Frustrated Lewis-Pair and Reductive Elimination

The reactivity of the frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was investigated with respect to the activation of elemental hydrogen. The reaction of 1 at elevated hydrogen pressure afforded the intramolecular phosphonium stannate(II) (F5 C2 )2 SnCH2 PH(tBu)2 (3). It was characterized by means of multinuclear NMR spectroscopy and single crystal X-ray diffraction. NMR experiments with the two isotopologues H2 and D2 showed it to be formed via an H2 adduct (F5 C2 )3 HSnCH2 PH(tBu)2 (2) and the subsequent formal reductive elimination of pentafluoroethane; this is supported by DFT calculations. Parahydrogen-induced polarization experiments revealed the formation of a second product of the reaction of 1 with H2 , [HP(tBu)2 Me][Sn(C2 F5 )3 ] (4), in 1 H NMR spectra, whereas 2 was not detected due to its transient nature.

Double donation in trigonal planar iron-carbodiphosphorane complexes - a concise study on their spectroscopic and electronic properties

We present the syntheses of trigonal planar coordinated Fe(ii) carbodiphosphorane (CDPR) complexes, starting from iron(ii)-bis(trimethylsilylamide) [Fe{N(SiMe3)2}2] and hexaphenyl-(CDPPh) and sym-dimethyltetraphenyl-carbodiphosphoranes (CDPMe), respectively. Both complexes [CDPPh-Fe{N(SiMe3)2}2] (1) and [CDPMe-Fe{N(SiMe3)2}2] (2) were examined in solution and in the solid state. 1 shows a dissociation equilibrium in solution which we monitored by variable temperature 1H-NMR spectroscopy. Magnetic measurements of 1 and 2 yielded a high spin configuration (S = 2) for both complexes. Quantum chemical calculations were performed to analyze the bonding situation in compound 1.

Towards Extended Zinc Ethylsulfinate Networks by Stepwise Insertion of Sulfur Dioxide into Zn-C Bonds

The ability to utilize polluting gases in efficient metal-mediated transformations is one of the most pressing challenges of modern chemistry. Despite numerous studies on the insertion of SO₂ into M-C bonds, the chemical reaction of SO₂ with organozinc compounds remains little explored. To fill this gap, we report here the systematic study of the reaction of Et₂ Zn towards SO₂ as well as the influence of Lewis bases on the reaction course. Whereas the equimolar reaction provided a novel example of a structurally characterized organozinc ethylsulfinate compound of general formula [(EtSO₂ )ZnEt]_n , the utilization of an excess of SO₂ led to the formation of the zinc(II) bis(ethylsulfinate) compound [(EtSO₂ )₂ Zn]_n . Moreover, we have discovered that the presence of N-donor Lewis bases represents an efficient tool for the preparation of extended zinc ethylsulfinates, which in turn led to the formation of 1D [(EtSO₂ ZnEt)₂ (hmta)]_n and 2D [((EtSO₂ )₂ Zn)₂ (DABCO)]_n ⋅solv (in which solv=THF or toluene, hmta= hexamethylenetetramine, and DABCO=1,4-diazabicyclo[2.2.2]octane) coordination polymers, respectively. The results of DFT calculations on the reactivity of SO₂ towards selected Zn-C reactive species as well as the role of an N-donor Lewis base on the stabilization of the transition states complement the discussion.

In Situ Formation of Frustrated Lewis Pairs in a Water-Tolerant Metal-Organic Framework for the Transformation of CO2

Frustrated Lewis pairs (FLPs) consist of sterically hindered Lewis acids and Lewis bases, which provide high catalytic activity towards non-metal-mediated activation of "inert" small molecules, including CO₂ among others. One critical issue of homogeneous FLPs, however, is their instability upon recycling, leading to catalytic deactivation. Herein, we provide a solution to this issue by incorporating a bulky Lewis acid-functionalized ligand into a water-tolerant metal-organic framework (MOF), named SION-105, and employing Lewis basic diamine substrates for the in situ formation of FLPs within the MOF. Using CO₂ as a C1-feedstock, this combination allows for the efficient transformation of a variety of diamine substrates into benzimidazoles. SION-105 can be easily recycled by washing with MeOH and reused multiple times without losing its identity and catalytic activity, highlighting the advantage of the MOF approach in FLP chemistry.

Photooxidation of triarylphosphines under aerobic conditions in the presence of a gold(iii) complex on cellulose extracted from Carthamus tinctorius immobilized on nanofibrous phosphosilicate

Triarylphosphines were converted to the corresponding oxides via photooxidation as a novel method.

Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ-π coupling mechanism for CO2 decarbonylation

It has been reported that RuH2(η2-H2)2(PCy3)2 (1) could mediate CO2 reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH3 (8), pinBOCHO (9), pinBOCH2OBpin (10), and an unprecedented C2 species pinBOCH2OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ2-O2CH)(CO)(PCy3)2) and 5 (RuH{(μ-H)2Bpin}(CO)(PCy3)2), and the persistent 4 (RuH(κ2-O2CH)(CO)2(PCy3)2) and 6 (RuH2(CO)2(PCy3)2). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH2(CO)(PCy3)2) and characterized the mechanisms leading to the experimentally observed species (3-11). By investigating the experimental system, we learned a new mechanism called σ-π coupling for CO2 decarbonylation. Under this mechanism, CO2 and HBpin first co-coordinate to the Ru center of 13, then σ-π coupling takes place, forming a B-O bond between CO2 and HBpin, Ru-H and Ru-C bonds, and simultaneously breaking the H-Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO2 decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η1-Hη1-H → η2-H2 and η1-Hη1-Bpin → η2-HBpin reductions, which facilitated the oxidative H-Bpin addition or the coordination mode change of CO2 from η1-O to η2-CO for CO2 activation or σ-π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.

Cooperative carbon monoxide to formyl reduction at a trifunctional PBB frustrated Lewis pair

Twofold hydroboration of the bulky Mes*P(vinyl)₂ phosphane with Piers' borane [HB(C₆F₅)₂] followed by C₆F₅/H exchange with 9-BBN generated a reactive P/B/BH FLP structure that cleanly reduced carbon monoxide at the trifunctional frustrated Lewis pair framework to the [B]-formyl stage.

RuBisCO-Inspired CO2 Activation and Transformation by an Iridium(I) Complex

The synthesis of a new iridium(I) complex containing an enamido phosphine anion (dbuP^- ) and its unique reactivity with CO₂ is reported. The complex binds two equivalents of CO₂ and initiates a highly selective reaction cascade. The reaction leads to the reversible cleavage of CO₂ and the enamido ligand as well. Computational analysis points to the existence of a relatively stable Ir-CO₂ complex as a reaction intermediate prior to CO₂ cleavage, which was confirmed experimentally. The observed transformation resembles several aspects of enzymatic CO₂ fixation by RuBisCO.

Site-selective C–H bond carbonylation with CO2 and cobalt-catalysis

Utilization of anthropogenic greenhouse gas CO₂ for catalytic C–C bond formation via conversion to essentially valuable C1 synthons like CO is very challenging.

Metalated Ylides: A New Class of Strong Donor Ligands with Unique Electronic Properties

The development and design of new ligand systems with special donor properties has been essential for crucial advances made in main-group-element and transition-metal chemistry over the years. This Forum Article focuses on metalated ylides as novel ligand systems. These anionic congeners of bisylides possess likewise two lone pairs of electrons at the central carbon atom and can thus function as X,L-type ligands with strong donor abilities. This article highlights recent efforts in the isolation and application of metalated ylides with a focus on work from this laboratory. We summarize structural and electronic properties and their use in organic synthesis as well as main-group-element and transition-metal chemistry.

The broadening reach of frustrated Lewis pair chemistry

The revelation that combinations of Lewis acids and bases for which dative bonding is impeded can activate dihydrogen led to the concept of "frustrated Lewis pairs" (FLPs). Over the past decade, a range of FLP systems and substrate molecules have precipitated a paradigm change in main-group chemistry and metal-free catalysis. The FLP motif has also found application in a growing body of chemical problems in organic synthesis, transition metal and free radical chemistry, materials, enzymatic models, and surface chemistry. The current state of FLP chemistry is assessed herein, and the outlook for the future considered.

Theoretical design and mechanistic study of the metal-free reduction of CO2 to CO

The metal-free silylborane Me₂BSi(CH₂F)₃, screened based on the rules of thumb coming from the summary of a hierarchy of silylboranes, can be used to catalyze the reduction of CO₂ to CO.

Borane-catalyzed indole synthesis through intramolecular hydroamination

Catalytic metal-free intramolecular hydroamination for the synthesis of indoles and tetrahydroisoquinolines was achieved.

Geminal bis-borane formation by borane Lewis acid induced cyclopropyl rearrangement and its frustrated Lewis pair reaction with carbon dioxide

The borylated tetrahydroborole obtained by the reaction of cyclopropylacetylene with Piers' borane adds carbon dioxide under frustrated Lewis pair conditions.

Cyclopropylacetylene reacts with two molar equivalents of Piers' borane [HB(C₆F₅)₂] under mild conditions by an addition/rearrangement sequence with cyclopropyl ring opening to give a mixture of two α-B(C₆F₅)₂ substituted tetrahydroboroles. This compound forms an active frustrated Lewis pair with P^tBu₃ that heterolytically splits dihydrogen and adds carbon dioxide as a geminal chelate bis-boryl component. The respective reactions of the two-fold HB(C₆F₅)₂ addition to Ph-CH₂CH₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH were studied as a geminal Lewis acid reference. Most of the products were characterized by X-ray diffraction.

Carbon Dioxide Transformation in Imidazolium Salts: Hydroaminomethylation Catalyzed by Ru-Complexes

The catalytic species generated by dissolving Ru3 (CO)12 in the ionic liquids 1-n-butyl-3-methyl-imidazolium chloride or 1-n-butyl-2,3-dimethyl-imidazolium chloride are efficient multifunctional catalysts for: (a) reverse water-gas shift, (b) hydroformylation of alkenes, and (c) reductive amination of aldehydes. Thus the reaction of alkenes with primary or secondary amines (alkene/amine, 1:1) under CO2 /H2 (1:1) affords the hydroaminomethylations products in high alkene conversions (up to 99 %) and selectivities (up to 96 %). The reaction proceeds under relatively mild reaction conditions (120 °C, 60 bar=6 MPa) and affords selectively secondary and tertiary amines. The presence of amine strongly reduces the alkene hydrogenation competitive pathway usually observed in the hydroformylation of terminal alkenes by Ru complexes. The catalytic system is also highly active for the reductive amination of aldehydes and ketones yielding amines in high yields (>90 %).

N-Heterocyclic Olefins as Robust Organocatalyst for the Chemical Conversion of Carbon Dioxide to Value-Added Chemicals

In this report, the activity of N-heterocyclic olefins (NHOs) as a newly emerging class of organocatalyst is investigated for the chemical fixation of carbon dioxide through reactions with aziridines to form oxazolidinones and the N-formylation of amines with polymethylhydrosiloxane (PMHS) or 9-borabicyclo[3.3.1]nonane (9-BBN) as the reducing agent under mild conditions. The exocyclic carbon atoms of NHOs are highly nucleophilic owing to the electron-donating ability of the two nitrogen atoms. This high nucleophilicity of the NHOs activates CO2 molecules to form zwitterionic NHO-carboxylate (NHO-CO2 ) adducts, which are active in formylation reactions as well as the carboxylation of aziridines to oxazolidinones.

Selective Catalytic Synthesis Using the Combination of Carbon Dioxide and Hydrogen: Catalytic Chess at the Interface of Energy and Chemistry

The present Review highlights the challenges and opportunities when using the combination CO2 /H2 as a C1 synthon in catalytic reactions and processes. The transformations are classified according to the reduction level and the bond-forming processes, covering the value chain from high volume basic chemicals to complex molecules, including biologically active substances. Whereas some of these concepts can facilitate the transition of the energy system by harvesting renewable energy into chemical products, others provide options to reduce the environmental impact of chemical production already in today's petrochemical-based industry. Interdisciplinary fundamental research from chemists and chemical engineers can make important contributions to sustainable development at the interface of the energetic and chemical value chain. The present Review invites the reader to enjoy this exciting area of "catalytic chess" and maybe even to start playing some games in her or his laboratory.

Facilitated carbon dioxide reduction using a Zn(II) complex

Two new Zn(II) complexes have been prepared and evaluated for their capacity to activate and reduce CO2. The electrochemical properties of dichlorobis[diphenyl-(2-pyridyl)phosphine-κ(1)-N]zinc(II) [corrected]. and dichloro[diphenyl-(2-pyridyl)phosphine-κ(1)-N]zinc(II) 2 are compared using cyclic voltammetry. Electrochemical results indicate that 2 leads to a facilitated CO2 reduction to evolve CO at a glassy carbon electrode.