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.

Engineering Frustrated Lewis Pair Active Sites in Porous Organic Scaffolds for Catalytic CO2 Hydrogenation

Frustrated Lewis pairs (FLPs), featuring reactive combinations of Lewis acids and Lewis bases, have been utilized for myriad metal-free homogeneous catalytic processes. Immobilizing the active Lewis sites to a solid support, especially to porous scaffolds, has shown great potential to ameliorate FLP catalysis by circumventing some of its inherent drawbacks, such as poor product separation and catalyst recyclability. Nevertheless, designing immobilized Lewis pair active sites (LPASs) is challenging due to the requirement of placing the donor and acceptor centers in appropriate geometric arrangements while maintaining the necessary chemical environment to perform catalysis, and clear design rules have not yet been established. In this work, we formulate simple guidelines to build highly active LPASs for direct catalytic hydrogenation of CO2 through a large-scale screening of a diverse library of 25,000 immobilized FLPs. The library is built by introducing boron-containing acidic sites in the vicinity of the existing basic nitrogen sites of the organic linkers of metal-organic frameworks collected in a "top-down" fashion from the CoRE MOF 2019 database. The chemical and geometrical appropriateness of these LPASs for CO2 hydrogenation is determined by evaluating a series of simple descriptors representing the intrinsic strength (acidity and basicity) of the components and their spatial arrangement in the active sites. Analysis of the leading candidates enables the formulation of pragmatic and experimentally relevant design principles which constitute the starting point for further exploration of FLP-based catalysts for the reduction of CO2.

Computational Design of Frustrated Lewis Pairs as a Strategy for Catalytic Hydrogen Activation and Hydrogenation Catalyst

Catalytic hydrogenation is one of the most important reaction types commonly used in chemistry and chemical industry. Recently, there has been significant interest in developing a metal-free hydrogenation catalyst to avoid the problems caused by using heavy transition metal catalysts. On the basis of the advances of metal-free hydrogen activation with frustrated Lewis pairs (FLPs, e.g. tBu₃P/B(C₆F₅)₃) which often uses boron as a Lewis acid center, we computationally explored the prospect for phosphorus(V) and sulfur(VI) as Lewis acid centers to construct FLPs for hydrogen activation and hydrogenation. We found out that the proposed FLPs with P(V)- or S(VI)-centered Lewis acid can also activate H₂ with a mechanism similar to that used by the conventional FLPs. A heterolytic cleavage of H-H is achieved when electrons are donated simultaneously from the σ orbital of H₂ to the empty orbital of the Lewis acid center and from the lone-pair orbital of the Lewis base center to the σ* orbital of H₂. The multiple C-H···F hydrogen bonds further aid the association of the pairs for H₂ activation. Some of our designed FLPs possess kinetics and thermodynamics for developing hydrogenation catalysts. This computational exploration could inspire experimental development of a new type of FLPs with P(V) or S(VI) or a Lewis acid partner for FLPs for reversible H₂ activation.

Effect of Halide Anions on the Electroreduction of CO2 to C2 H4 : A Density Functional Theory Study

The halide anions present in the electrolyte improve the Faradaic efficiencies (FEs) of the multi-hydrocarbon (C₂₊ ) products for the electrochemical reduction of CO₂ over copper (Cu) catalysts. However, the mechanism behind the increased yield of C₂₊ products with the addition of halide anions remains indistinct. In this study, we analysed the mechanism by investigating the electronic structures and computing the relative free energies of intermediates formed from CO₂ to C₂ H₄ on the Cu (100) facet based on density functional theory (DFT) calculations. The results show that formyl *CHO from the hydrogenation reaction of the adsorbed *CO acts as the key intermediate, and the C-C coupling reaction occurs preferentially between *CHO and *CO with the formation of a *CHO-CO intermediate. We then propose a free-energy pathway of C₂ H₄ formation. We find that the presence of halide anions significantly decreases the free energy of the *CHOCH intermediate, and enhances desorption of C₂ H₄ in the order of I^- >Cl^- >Br^- >F^- . Lastly, the obtained results are rationalized through Bader charge analysis.

Mapping Active Site Geometry to Activity in Immobilized Frustrated Lewis Pair Catalysts

The immobilization of molecular catalysts imposes spatial constraints on their active site. We reveal that in bifunctional catalysis such constraints can also be utilized as an appealing handle to boost intrinsic activity through judicious control of the active site geometry. To demonstrate this, we develop a pragmatic approach, based on nonlinear scaling relationships, to map the spatial arrangements of the acid-base components of frustrated Lewis pairs (FLPs) to their performance in the catalytic hydrogenation of CO2 . The resulting activity map shows that fixing the donor-acceptor centers at specific distances and locking them into appropriate orientations leads to an unforeseen many-fold increase in the catalytic activity of FLPs compared to their unconstrained counterparts.

Activation of Small Molecules and Hydrogenation of CO2 Catalyzed by Frustrated Lewis Pairs

The chemistry of frustrated Lewis pair (FLP) is widely explored in the activation of small molecules, the hydrogenation of CO2, and unsaturated organic species. A survey of several experimental works on the activation of small molecules by FLPs and the related mechanistic insights into their reactivity from electronic structure theory calculation are provided in the present review, along with the catalytic hydrogenation of CO2. The mechanistic insight into H2 activation is thoroughly discussed, which may provide a guideline to design more efficient FLP for H2 activation. FLPs can activate other small molecules like, CO, NO, CO2, SO2, N2O, alkenes, alkynes, etc. by cooperative action of the Lewis centers of FLPs, as revealed by several computational analyses. The activation barrier of H2 and other small molecules by the FLP can be decreased by utilizing the aromaticity criterion in the FLP as demonstrated by the nucleus independent chemical shift (NICS) analysis. The term boron-ligand cooperation (BLC), which is analogous to the metal-ligand cooperation (MLC), is invoked to describe a distinct class of reactivity of some specific FLPs towards H2 activation.

In Silico Design of Metal-Free Hydrophosphate Catalysts for Hydrogenation of CO2 to Formate

CO2 reduction by H2 using metal-free catalysts is highly challenging. Frustrated Lewis pairs (FLPs) have been considered potential metal-free catalysts for this reaction. However, most FLPs are unstable, which limits...

Effects of Ionic Liquids on the Thermodynamics of Hydrogen Activation by Frustrated Lewis Pairs: A Density Functional Theory Study*

Nowadays, hydrogen activation by frustrated Lewis pairs (FLPs) and their applications are one of the emerging research topics in the field of catalysis. Previous studies have shown that the thermodynamics of this reaction is determined by electronic structures of FLPs and solvents. Herein, we investigated systems consisting of typical FLPs and ionic liquids (ILs), which are well known by their large number of types and excellent solvent effects. The density functional theory (DFT) calculations were performed to study the thermodynamics for H₂ activation by both inter- and intra-molecular FLPs, as well as the individual components. The results show that the computed overall Gibbs free energies in ILs are more negative than that computed in toluene. Through the thermodynamics partitioning, we find that ILs favor the H-H cleavage elemental step over the elemental steps of proton attachment, hydride attachment and zwitterionic stabilization. Moreover, the results show that these effects are strongly dependent on the type of FLPs, where intra-molecular FLPs are more affected compared to the inter-molecular FLPs.

Silica Nanopowder Supported Frustrated Lewis Pairs for CO2 Capture and Conversion to Formic Acid

Treatment of hydroxylated silica nanopowders S1 and allyl-functionalized silica nanopowders S2 with 3-(diphenylborano)- or 3-bis(pentafluorophenylborano)propyltrimethoxysilane or 2-(diphenylphosphino)- or 2-(dicyclohexylphosphino)ethyltriethoxysilane generates silica nanopowder supported Lewis acids S3 and silica nanopowder supported Lewis bases S4. These surfaces were characterized by ¹³C, ¹¹B, and ³¹P cross-polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR), X-ray photoelectron spectroscopy (XPS), and attenuated total reflection Fourier transform infrared (ATR FTIR). When S3 is combined with solution-phase Lewis bases PR₃ (R = C₆F₅, C₆H₅, mesityl), six associated silica nanopowder supported frustrated Lewis pairs (FLPs) are formed. In another set of six reactions, the interactions between the supported Lewis bases S4 and solution-phase Lewis acids BR₃ with R = C₆F₅, C₆H₅, mesityl produced six more associated supported FLPs. The capture of CO₂ by these FLPs producing FLP-CO₂ Lewis pair adducts S5 and S6 were highlighted by ATR FTIR, and it was found that FLP S5e with R = C₆H₅ on both the supported Lewis acid and solution-phase Lewis base trapped the largest quantities of CO₂ on the silica nanopowder supports. Conversion of CO₂ to HCOOH was achieved by first activating H₂ to generate activated FLP-H₂ surfaces S7 and S9. Addition of CO₂ then generated HCOOH via the silica nanopowder supported FLP-HCOOH adducts S8 and S10. Qualitative identification of HCOOH generation was achieved by ATR FTIR measurements, and surface 10b with R = C₆H₅ proved to be the most successful silica nanopowder surface bound FLP in HCOOH generation. In some cases, diborano formates (-BO(CH)OB-) S11 and S12 were also identified as side products during HCOOH formation. Spectroscopic characterization of purposefully synthesized S11 and S12 included ¹¹B and ³¹P CP MAS NMR.

Halogenated triarylboranes: synthesis, properties and applications in catalysis

Halogenated triarylboranes (BAr₃) have been known for decades, however it has only been since the surge of interest in main group catalysis that their application as strong Lewis acid catalysts has been recognised. This review aims to look past the popular tris(pentafluorophenyl)borane [B(C₆F₅)₃] to the other halogenated triarylboranes, to give a greater breadth of understanding as to how tuning the Lewis acidity of BAr₃ by modifications of the aryl rings can lead to improved reactivity. In this review, a discussion on Lewis acidity determination of boranes is given, the synthesis of these boranes is discussed, and examples of how they are being used for catalysis and frustrated Lewis pair (FLP) chemistry are explained.

Insights into the activation process of CO2 through Dihydrogenation reaction

Based on first principle calculation, activation of CO₂ has been analyzed thoroughly by using different conceptual density functional theory based descriptors like reaction force, reaction force constant, reaction electronic flux, dual descriptor, etc. via dihydrogenation reaction of B₃N₃, H₂ and CO₂. The total reaction is a two-step reaction where initially B₃N₃H₂ is formed from the reaction between B₃N₃ and H₂ and in the second step HCOOH is form due to the reaction of CO₂ by B₃N₃H₂. It has been found that the di-hydrogen reaction for the CO₂ activation is endothermic in nature, which can be changed to exothermic reaction by applying proper external electric field. Movement of H₂ plays an important role in the CO₂ activation process. The reaction force constant, Wiberg bond index and its derivative reveal that the reaction is slightly asynchronous and concerted in nature.

A theoretical investigation on boron–ligand cooperation to activate molecular hydrogen by a frustrated Lewis pair and subsequent reduction of carbon dioxide

Activation of molecular hydrogen by a B/N frustrated Lewis pair.

Chemical reactivity of the frustrated Lewis pairs in borophosphines: a theoretical analysis of their Lewis acidity, Lewis basicity and Fukui function

The chemical reactivity of a set of borophosphines of the general formula R₂B-G-PY₂, where G is the connector group between the Lewis acidic site, a borane group, and the Lewis basic site, a phosphine fragment, is theoretically investigated through their Lewis acidity and Lewis basicity, as well as the location of the Fukui function and the shape of the molecular electrostatic potential. The role of some global reactivity descriptors, like the vertical ionization potential, I, and the vertical electron affinity, A, is also analyzed in order to gain a deeper insight on the intrinsic chemical reactivity of these borophosphines. We also use the energies involved in the formation of the adducts between the borophosphine and the ions H^- and H⁺ to estimate the Lewis acidity and Lewis basicity, respectively; by their nature, these energies represent local reactivity descriptors. Some of these borophosphines are able to activate the covalent bond in the hydrogen molecule. Possible paths for the hydrogen release reaction from the zwitterion R[Formula: see text]HB-G-PH[Formula: see text] are studied using the mentioned quantities, suggesting that an intramolecular hydride shift mechanism seems to be more favorable than a proton migration process. The acceptor Fukui function f⁺(r) proved to be useful to identify the acidic molecular sites for the interaction with the hydride ion and the relative stability of the corresponding adducts is related to the relative values of this function.

Theoretical studies on bridged frustrated Lewis pair (FLP) mediated H2 activation and CO2 hydrogenation

DFT calculations show that H₂ and CO₂ activation by bridged phosphane/borane frustrated Lewis pairs (FLPs) experiences a one-step concerted mechanism with small reaction barriers.

Formylation or methylation: what determines the chemoselectivity of the reaction of amine, CO2, and hydrosilane catalyzed by 1,3,2-diazaphospholene?

DFT computations have been performed to gain insight into the mechanisms of formylation/methylation of amines (e.g. methylaniline (1a)/2,2,4,4-tetramethylpiperidine (2a)) with CO2 and hydrosilane ([Si]H2, [Si] = Ph2Si), catalyzed by 1,3,2-diazaphospholene ([NHP]H). Different from the generally proposed sequential mechanism for the methylation of amine with CO2, i.e. methylation proceeds via formylation, followed by further reduction of formamide to give an N-methylated amine, the study characterized a competition mechanism between formylation and methylation. The chemoselectivity originates from the competition between the amine and [NHP]H hydride to attack the formyloxy carbon of [Si](OCHO)2 (the insertion product of CO2 into [Si]H2). When the attack of an amine (e.g.1a) wins, the transformation affords formamide (1b) but would otherwise (e.g.2a) result in an N-methylated amine (2c). The reduction of formamide by [Si]H2 or [NHP]H is highly unfavorable kinetically, thus we call attention to the sequential mechanism for understanding the methylation of amine with CO2. In addition, the study has the following key mechanistic findings. The activation of CO2 by [NHP]H establishes an equilibrium: [NHP]H + CO2 ⇄ [NHP]OCHO ⇄ [NHP]+ + HCO2-. The ions play catalytic roles to promote formylation via HCO2- or methylation via[NHP]+ . In 1a formylation, HCO2- initiates the reaction, giving 1b and silanol byproducts. However, after the initiation, the silanol byproducts acting as hydrogen transfer shuttles are more effective than HCO2- to promote formylation. In 2a methylation, [NHP]+ promotes the generation of the key species, formaldehyde and a carbocation species (IM17+ ). Our experimental study corroborates our computed mechanisms.

Be12O12 Nano-cage as a Promising Catalyst for CO2 Hydrogenation

An efficient conversion of CO₂ into valuable fuels and chemicals has been hotly pursued recently. Here, for the first time, we have explored a series of M₁₂x₁₂ nano-cages (M = B, Al, Be, Mg; X = N, P, O) for catalysis of CO₂ to HCOOH. Two steps are identified in the hydrogenation process, namely, H₂ activation to 2H*, and then 2H* transfer to CO₂ forming HCOOH, where the barriers of two H* transfer are lower than that of the H₂ activation reaction. Among the studied cages, Be₁₂O₁₂ is found to have the lowest barrier in the whole reaction process, showing two kinds of reaction mechanisms for 2H* (simultaneous transfer and a step-wise transfer with a quite low barrier). Moreover, the H₂ activation energy barrier can be further reduced by introducing Al, Ga, Li, and Na to B₁₂N₁₂ cage. This study would provide some new ideas for the design of efficient cluster catalysts for CO₂ reduction.