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

Riddhi Kumari Riddhi‡ ^a, Francesc Penas-Hidalgo‡ ^b, Hongmei Chen ^b, Elsje Alessandra Quadrelli ^a, Jérôme Canivet *^a, Caroline Mellot-Draznieks *^b and Albert Solé-Daura *^cd
aIRCELYON, UMR 5256, Université LYON 1, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France
^bLaboratoire de Chimie des Processus Biologiques, CNRS UMR 8229, Collège de France, PSL Research University, Sorbonne Université, 75231 Paris Cedex 05, France. E-mail: caroline.mellot-draznieks@college-de-france.fr
^cDepartment de Química Física i Inorgànica, Universitat Rovira i Virgili, Marcel·lí Domingo 1, Tarragona 43007, Spain
^dInstitute of Chemical Research of Catalonia (ICIQ-CERCA), The Barcelona Institute of Science and Technology, Avgda. Països Catalans, 16, 43007 Tarragona, Spain. E-mail: asole@iciq.es

First published on 30th August 2024

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Riddhi Kumari Riddhi

Riddhi Kumari received her BSc from BHU, India and MSc from NIT Warangal, India. She did her Master thesis in LCC, Toulouse under the supervision of Prof. J.-B. Sortais and Dr S. Bastin. Currently, she is a PhD student under the direction of Dr Jérôme Canivet and Dr Alessandra Quadrelli at IRCELYON. She is working on the heterogenization of molecular catalysts into porous supports like metal–organic frameworks and porous organic polymers. A part of her thesis is dedicated to the heterogenization of FLPs in porous solids; and another part focuses on nickel coordination chemistry at bipyridyl sites within photoactive POPs.

Francesc Penas-Hidalgo

Francesc Penas-Hidalgo received his BSc and MSc from Rovira i Virgili University (URV, Spain). He did his Master thesis under the supervision of Dr Albert Solé-Daura and Prof. Josep M. Poblet. Currently, he is pursuing a PhD under the supervision of Dr Albert Solé-Daura and Dr Caroline Mellot-Draznieks at Collège de France, in Paris. He is working on the computational modelling of the reactivity of Frustrated Lewis Pairs (FLPs) towards H2 splitting and CO2 hydrogenation. Moreover, he is computationally exploring other CO2 reduction approaches promoted by polyoxometalates and metalloporphyrins.

Hongmei Chen

Hongmei Chen obtained her PhD in 2021 from the University of Liverpool, joining Prof. Andrew Cooper's group for her PhD studies and working on covalent organic frameworks (COFs) for photocatalytic water splitting. She then started her postdoctoral research at Collège de France on the hydrogenation of CO2 using metal–organic frameworks (MOFs)-based catalysts.

Elsje Alessandra Quadrelli

Alessandra Quadrelli is director of research in chemistry from the French National Centre for Scientific Research, CNRS, at the IRCELYON laboratories. Her research focuses on organometallic mechanisms for grafting catalytically active transition metals or for growing thin films by atomic layer deposition (ALD) on surfaces (Silica, MOFs and silicon wafer substrates). Her current main target reaction is CO2 reduction.

Jérôme Canivet

Jérôme Canivet was appointed CNRS researcher at the IRCELYON in 2010. He works at developing innovative catalytic processes for sustainable fine chemicals and energy. His research topics range from C–C coupling to asymmetry, photocatalysis and green fuels. In 2018, he received the Young Investigator Award from the Catalysis Division of the French Chemical Society for creating trends reducing the gap between homogeneous and heterogeneous catalysis. He further aims at exploiting the confinement of molecular catalytic systems into porous structures for the improvement of their catalytic activity and selectivity, and he is coordinating cooperative projects on this topic.

Caroline Mellot-Draznieks

Caroline Mellot-Draznieks joined the CNRS in 1998 at Institut Lavoisier in Versailles. She then moved to the United Kingdom in 2005, where she held an Advanced EPRSC fellowship at UCL. She is now Director of Research (CNRS) at the Collège de France in Paris. Her research interests are related to nanoporous materials for adsorption and catalysis combining computational chemistry and experimental approaches, with a current major focus on CO2 conversion.

Albert Solé-Daura

Albert Solé-Daura earned his PhD from Rovira i Virgili University (URV, Spain) in 2020. Subsequently, he conducted postdoctoral research at the Collège de France (France), the University of Amsterdam (The Netherlands), the URV as a Margarita Salas fellow, and the Institut Català d’Investigació Química (ICIQ, Spain). Recently, in 2024, he was awarded a “la Caixa” Junior Leader fellowship to initiate his independent research career at ICIQ. His research interests concern the application of computational methods to delve into the mechanisms of catalytic processes, primarily involving by polyoxometalates and Metal–Organic Frameworks.

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1. Introduction

Catalysis plays a key role in the manufacturing processes underlying our chemical industry. Still, there is an urgent need to reduce its environmental impact and to move towards a more sustainable society, which requires a careful reassessment of certain fundamental aspects. Many homogeneous and heterogeneous catalytic methods rely on transition metals as active sites. The use of such potentially scarce resources is coupled with limited recyclability and consequential resource waste in homogeneous systems, or with metal recovery using energy-intensive processes in heterogeneous systems.¹ Simultaneously, the direct conversion of small molecules into valuable synthetic intermediates, particularly employing CO₂ as a readily available and cost-effective C1 reservoir,² gains importance in increasing the sustainability of associated chemical processes.³ Therefore, there is a persistent requirement to develop novel catalytic systems based on readily available, cost-effective elements with wide-ranging applicability, producing high-value compounds for environmentally friendly chemical production.

The advent of molecular frustrated Lewis pairs (FLPs), introduced by Stephan et al. in 2006,⁴ showcases reactivity stemming from steric hindrance between Lewis acid (LA) and base (LB) partners, precluding the formation of the LA–LB adduct.^5–7 FLPs, often referred to as a “catalysis Holy Grail”, have ushered in a new era of sustainable green chemistry. In particular, they allow the transition metal-free activation of small molecules, demonstrating significant potential in hydrogen activation and double bond reduction in fine chemicals, including CO₂ hydrogenation.^8–11

While the field of homogeneous FLPs has gathered substantial knowledge in the past decade,^12,13 the transfer of FLPs’ chemistry to heterogeneous catalysis is rather recent and garnering increasing interest. Besides addressing recyclability issues, the heterogenization of molecular FLPs allows the isolation of active sites within a controlled chemical environment, while preventing undesired interactions and catalyst deactivation. This isolation alters substrate activation interactions on the heterogeneous catalyst, leading to unprecedented reactivities explicable through molecular chemistry mechanisms. The development of heterogeneous FLPs is currently challenging and mainly limited to doped/defective oxide and metal surfaces.^14,15

Incentives for discovering heterogeneous FLPs encompass aspects ranging from molecular-scale control over the distance between FLP's partners, influencing reactivity, to their recyclability, shaping, and scale-up production. Quantum mechanical calculations serve as a powerful tool for the in silico design of molecular architectures, having matured in molecular catalysis for predicting efficient systems, remarkably for asymmetric synthesis.^16–18 However, computational methodologies are at an earlier stage concerning heterogeneous catalysis.^19–22

In addition to the perspectives on FLPs’ chemistry by Stephan et al.,^14,23 recent reviews in the last five years have covered distinct advancements in FLPs’ chemistry by considering either the realm of homogeneous catalysis^24–26 or strategies for their heterogenization into solid materials.^15,27,28 A recent review covered the specific area of the computational chemistry of FLPs, focusing exclusively on molecular systems.²⁹ Therefore, there remains a critical need for a comprehensive analysis of the recent progress and existing challenges in the discovery of FLP-based catalytic systems for CO₂ hydrogenation, encompassing both molecular and solid-state FLPs and integrating recent experimental and theoretical facets.

This review delves into fifteen years of FLPs chemistry development, exploring the chemistry of FLPs based on main group elements from experimental discovery to computational mechanistic dissection. The discussion encompasses the recent experimental advances of FLPs in catalytic processes, tools developed for their heterogenization, and key aspects of structure–activity relationships for CO₂ hydrogenation as determined in silico.

2. Experimentally reported FLPs for CO₂ hydrogenation

Since the pioneering report by Stephan et al. in 2006 with H₂ splitting by a transition metal-free P/B containing molecule,⁴ this field exhibits an ever-expanding range of molecular LB (B, C, N, O, S, Se and Te) and LA (B, Al, Ga, In, C, Si, Sn, N and P) partners in intra- or inter-molecular fashions.^9,30 They demonstrated their large potential for the activation of H₂ and the subsequent reduction of double bonds in fine chemicals,^8,25 including CO₂ hydrogenation.^31–33 We will first summarize here the main experimental findings on FLP-mediated CO₂ hydrogenation using molecular H₂ (Table 1).

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2.1. Molecular FLPs reported for non-catalytic and catalytic CO₂ reduction

In this section, we aim to discuss the development of molecular FLPs and the study of their reactivity in biphasic gas–liquid systems from CO₂ activation to CO₂ hydrogenation, leading to unique catalytic systems. Here, FLPs are classified as either structurally intramolecular, i.e. the Lewis base (LB) and Lewis acid (LA) are part of the same molecule, or intermolecular, i.e. the LB and LA partners belong to two independent molecules.

2.2. FLPs heterogenization in porous solids for CO₂ hydrogenation

Although FLP systems are renowned for their metal-free nature and great reactivity and selectivity, their immobilization into porous supports has been driven by the aim to enhance their stability against deactivation. Immobilization would also in principle allow better control of acid–base distances, a key parameter, especially for CO₂ activation.⁶⁸ Furthermore, the heterogenization of FLPs within porous solids should enable their implementation in continuous flow processes, also key for CO₂ valorization at a large scale. While the field of homogeneous FLPs has benefited from the great knowledge gained over the last decade,^12,13 the transfer of FLPs’ chemistry to heterogeneous catalysis is very recent and remains challenging.^14,15 We gather here examples of molecular FLPs heterogenization on various supports for CO₂ and/or H₂ activation (Table 2).

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3. Learnings from computational chemistry

The rational design and development of efficient FLP-based catalysts for CO₂ hydrogenation requires a comprehensive understanding of the intricate relationships between molecular structures and their catalytic functionalities. This entails delving into the catalytic properties of molecular FLPs, while also rationalizing the conformational and environmental effects that govern their integration into the porous framework of materials. In this regard, quantum mechanical calculations have been extensively employed to: (i) obtain atomistic insights into the activation pathways of both H₂ and CO₂; (ii) identify the factors that influence their performances; and (iii) evaluate the potential impact of their immobilization inside porous matrices. This section covers the most relevant computationally-derived advances in this direction, ultimately aiming to predict and drive the synthesis of efficient FLP-based catalytic systems. Sections 3.1 and 3.2 summarize the main findings inferred from studies on molecular FLPs, while Section 3.3 delves into in silico studies on heterogenized FLP systems.

3.1. H₂ splitting in molecular FLPs: a generally accepted initial step

The hydrogenation of CO₂ by FLPs has been often shown to require the initial generation of an active hydrogenating species. This can be achieved via the heterolytic splitting of a H₂ molecule into a hydride and a proton, which are accommodated on the LA and LB centers conforming the FLP, respectively. Beyond CO₂ hydrogenation purposes, the splitting of H₂ by FLPs represents an interesting process per se, playing a central role in the hydrogenation of a variety of unsaturated compounds.²⁶ Besides, the ability to split H₂ under mild conditions confers FLPs the potential to be used as molecular platforms to store H₂ fuel in a safe and easy-to-handle manner. On these grounds, extensive theoretical have been devoted to understanding the reactivity of FLPs towards H₂ and the factors that govern it. This section aims to give a brief overview of the most relevant computationally-derived findings in this sub-area, as well as the current controversies and challenges. For a more comprehensive review of computational studies on H₂ splitting by FLPs, please refer to the ESI.† Also, an extensive array of experimentally tested cases involving H₂ splitting by FLPs has been explored employing computational methods (see ESI† for details). These efforts have contributed to the establishment of a well-defined mechanistic picture depicted in Fig. 16 and are summarized as follows.

On the one hand, intermolecular FLPs proceed via the formation of a non-covalent adduct between the LA and LB partners also referred as an “encounter complex”. This is followed by the insertion of H₂ into the reactive pocket and its splitting to form a hydrogenated [LB–H]⁺⋯[LA–H]⁻ ion pair.

On the other hand, intramolecular FLPs may either activate H₂ by themselves or dimerize to operate as an intermolecular FLP. The competition between intra- and intermolecular pathways in intramolecular FLPs has not been systematically investigated for a whole range of systems. Thus, setting clear conclusions about the factors that determine their relative likelihood remains challenging. Still, the prevalence of each of these paths might depend on the equilibrium LA⋯LB distance within the intramolecular FLP and on the ability of FLPs to dimerize into stable non-covalent adducts. Up to now, this mechanistic knowledge has been successfully applied to rationalize the H₂ splitting activity of a wide variety of FLPs, including mostly P/B^98–116 (known as thermally-induced FLPs^53,117) N/B pairs,^{29,97,113,115,118–123} and less often carbene/B¹²⁴ and N/TM (TM = Ni, Pt) pairs,¹²⁵ or even that of heterogeneous systems such as hydroxylated indium oxide surfaces.^126–128

Computational efforts have been devoted to identify the key factors that govern the H₂ splitting, aiming at establishing structure–activity relationships and clear design rules. The primary structural requirement of an active FLP is to offer a suitable LA⋯LB distance. For instance, Vankova and co-workers determined that optimal P⋯B distances for H₂ splitting lie within a range of 3 to 5 Å by performing constrained potential-energy surface scans along the P⋯B distance of a P(tBu)₃/B(C₆F₅)₃ FLP.¹²⁹ As for B/N FLPs, Corminbœuf and co-workers identified that the optimal B⋯N distance for H₂ splitting is ca. 2.9 Å. Still, the range of distances in which the energy of the TS varies within an energy range of 5 kcal mol⁻¹ spans from ca. 2.6–3.5 Å.⁶⁸ Thus, when targeting the heterogenization of FLP systems, it is essential to ensure that the positioning of the LA and LB partners within the material does not impose a suboptimal LA⋯LB separation.

Another key factor in FLPs’ reactivity is the electronic properties of the substituents of both LB and LA centers, affecting their basicity and acidity, respectively. A first attempt to set relationships between such molecular features of FLPs and the H₂ splitting free energy (indicating its thermodynamic feasibility) was proposed by Pápai and co-workers.⁵³ They carried out a comprehensive computational investigation using a series of intra- and intermolecular FLPs, which led to several conclusions, which were further supported by other works (see ESI† for details):^{97,114,123,129}

(i) The thermodynamics of the H₂ splitting correlates with the cumulative acid–base strength of the FLP partners, which can be quantified from proton and hydride attachment energies;

(ii) Linked or intramolecular FLPs benefit from a smaller entropic penalty along the reaction coordinate than intermolecular FLPs and thus, require smaller cumulative acid–base strengths to be active;

(iii) Reaction free-energies for H₂ splitting by the analyzed set of intramolecular FLPs was correlated inversely with the LA⋯LB distance. In other words, the shorter the donor–acceptor distance is, the more exergonic the H₂ splitting is.

Ye and Johnson further proposed an original approach to explore computationally the immobilization of intramolecular FLPs for CO₂ hydrogenation in UiO-66, a porous MOF matrix.⁸⁹ They aimed to set the very first structure–activity relationships between the structure of molecular FLPs and their ability to activate H₂ and hydrogenate CO₂ in the H₂/CO₂ gas stream. Here, we focus on their findings on the initial H₂ splitting step, while CO₂ hydrogenation will be discussed in Section 3.3. The authors selected a set of 8 B/N FLPs, which on paper, can be covalently grafted on the terephthalate linkers of UiO-66 (Fig. 17a). These consist of a pending pyrazole functionalized with a BR₂ moiety that leads to a series of B/N FLPs of increasing acidity by varying R. Computing electronic energy profiles for all systems, the authors tried to find Brønsted–Evans–Polanyi (BEP) relationships^130,131 between energetic parameters (reaction energies and barriers) and a set of molecular descriptors. The latter include the acidity of the LA, the basicity of the LB, the electronegativity, chemical hardness and softness of the FLP sites, atomic charges and structural parameters of both the bare FLP and the zwitterionic hydrogenated intermediate.

The H₂ dissociative adsorption energy (i.e. heterolytic splitting of H₂) was found to correlate linearly with the hydride attachment energy (Fig. 17b).⁸⁹ As the LA strength increases, so does the H₂ dissociative adsorption energy. Notably, no correlation was found with the proton attachment energy, in line with the experimental findings of Berke and co-workers.¹³² Still, we can note this computational work only addressed the impact of the substituents at boron (LA) center, resulting in a rather narrow range of proton attachment energies. The analyzed FLPs do not significantly differ in terms of LB basicity, leaving the impact of the latter uncharted.

Another linear BEP relationship was identified between the H₂ splitting energy barrier and the LA chemical hardness.⁸⁹ Increasing the hardness of the LA decreases the barrier for H₂ splitting in accordance with Pearson's theory. We wish to stress here that, unlike the H₂ adsorption energy, this barrier did not correlate with the acidity of the LA. The BEP principle thus does not fully apply to the H₂ splitting process, given the absence of a linear correlation between reaction energies (thermodynamics) and barriers (kinetics). The other BEP relationship found between the H₂ splitting barrier and the bond angle formed by N_b, N_a and B centers in the bare FLP was ascribed to the fact that larger angles induce strain in the FLP, which lowers the barrier. It is also relevant for the present review that the MOF environment was found to marginally affect the adsorption energy of H₂ on the FLP, as similar reaction energies were obtained from both isolated and in-MOF FLP models.

Two years later, Ye et al. revisited the factors that influence the thermodynamics and kinetics of H₂ splitting on FLPs grafted in UiO-66 by enlarging their study to four families of intramolecular FLPs including B/N and B/P pairs attached to structurally distinct molecular scaffolds.⁹¹ Although quantitative linear correlations between H₂ splitting energies and hydride attachment-free energies were found again for each FLP family (with r² > 0.95), as shown above, they did not apply for all FLP families taken together (r² = 0.834). This evidenced that besides hydride attachment-free energies, other factors influence the binding energy of H₂. An in-depth screening of molecular descriptors revealed that it is also proportional to the LA⋯LB distance in the bare FLP and to the variation of the angles concerning the LA/LB sites and the two consecutive atoms of the scaffold in the direction of the FLP partner upon H₂ binding.

Overall, despite the complexity of FLP's reactivity, computational methods have significantly contributed over the years to the generation of valuable knowledge that can be used to guide the experimental design of FLPs to conduct H₂ splitting. Still, several points lack a quantitative unambiguous answer. For instance, the exact structural and electronic parameters that determine whether an intermolecular FLP operates through an intra- or an intermolecular mechanism are yet to be understood. Other aspects of the H₂ splitting by FLPs that remain controversial or unclear nowadays are the relative weights of the impact of electric field polarization and orbital overlap on H₂ splitting, as well as those of LA and LB strengths and how they relate to the FLP's nature and structure (see Fig. S6 and related text in ESI†). Also, although a set of molecular parameters of FLPs have been recognized to influence H₂ splitting kinetics and thermodynamics, generic and quantitative structure–activity relationships are still to be established. Most likely, this is because, besides eventual exceptions, the series of FLPs analyzed in each work are constrained to a single family of FLPs and quite often, account for a rather limited number of structures.

More importantly, within the present context of using FLPs not only for H₂ splitting but also for subsequent hydrogenation of CO₂, balancing the energetics of H₂ splitting with those of the subsequent hydrogenation step is crucial. Specifically, FLPs should bind H₂ strongly enough to make the H₂ splitting step feasible, but not too strong so as to preclude subsequent hydride/proton transfer to an incoming CO₂ molecule. In other words, a too strong Lewis base/acid would produce a too weak Brønsted acid/hydride donor, hampering CO₂ hydrogenation. This adds an extra layer of complexity to the design challenge: the goal is drifted from minimizing energy barriers while favoring thermodynamics for each single elementary step process to identifying the molecular descriptors and the sometimes-overlooked experimental conditions that simultaneously optimize the performance of two distinct processes.

3.2. Molecular FLPs for CO₂ hydrogenation

Carbon dioxide is an exceptionally stable molecule and its activation and conversion are hence not straightforward. The ability of FLPs to heterolytically split H₂ and subsequently hydrogenate organic compounds has prompted the exploration of the potential use of FLPs for the direct hydrogenation of CO₂ with H₂ following the two-step process shown in Scheme 1.

However, CO₂ can be also captured by FLPs as shown in Section 1 and hence, compete with H₂ for FLP sites.¹³ Notably, FLPs often bind CO₂ more strongly than H₂, which may hamper H₂ splitting as the first step of CO₂ hydrogenation because of an additional energy penalty associated to CO₂ release. Still, such FLP–CO₂ adducts are not necessarily detrimental for CO₂ hydrogenation, as they could allow distinct mechanistic scenarios, as discussed in detail below.

3.3. FLPs heterogenized into porous solids for CO₂ hydrogenation

4. Lessons learned from experimental and computational studies

With tremendous achievements in both the experimental discovery of main group elements FLP systems and the computational understanding of their reactivity, the careful analysis of these two prolific sides of such a subtle chemistry opens opportunities for installing a dialogue between experiments and calculations, exemplified for the CO₂ hydrogenation reaction.

4.1. Finding the sweet spot of reactivity with H₂ and CO₂

H₂ splitting and CO₂ hydrogenation processes generally follow opposite trends with variations of most of the FLPs’ properties, whereby whatever favors one is detrimental to the other. It is thus inevitable to outline parallelisms with the Sabatier principle, widely used in heterogeneous catalysis to describe an optimal catalyst that binds substrate molecules strong enough to enable reactivity but not too much to prevent undesired poisoning issues. Hence, pinpointing optimal FLP catalysts for CO₂ hydrogenation with H₂ demands the identification of sweet spots that balance the feasibility of both H₂ splitting and subsequent CO₂ hydrogenation step.

In this regard, the development of computationally driven strategies to identify sweet spots where the energetics of both steps are balanced to achieve optimal CO₂ hydrogenation activity represents a matter of current research, and some guidance and rules of thumb have been recently reported. Specifically, optimal CO₂ hydrogenation activity has been found to require a moderate cumulative acid/base strength, which can be estimated as the sum of proton and hydride attachment energies to the LA/LB partners (FEPA and FEHA, respectively). Thus far, systematic explorations have been only carried out with intermolecular B/N FLPs, which revealed that the TOF is maximized for LA/LB combinations with FEPA/FEHA ratios ranging linearly from ca. −35/−10 to +25/−35 kcal mol⁻¹.⁴³ Moving away from these ranges leads to too strong pairs, which form too stable zwitterionic hydrogenated FLP intermediates or bind products too strongly, or to too weak pairs, which have difficulties splitting H₂.

Also, intramolecular FLPs allow for some “tricks” to move orthogonally away from these trends, by controlling either the LA⋯LB distances or the rigidity of the molecular scaffolds, as exemplified by the work by Delarmelina et al.¹³⁷ More rigid backbones and longer LA⋯LB distances (of ∼4.4 Å) were found to substantially reduce the free-energy barrier for hydride/proton transfer to CO₂, compared to more flexible scaffolds with LA⋯LB distances of <4 Å. While for H₂ splitting, optimal B⋯N distances were found to lie in a range of 2.6–3.5 Å,⁶⁸ not a single study has claimed yet what is exactly the optimal LA⋯LB distance to promote CO₂ hydrogenation and how it varies as function of the nature of the FLP sites. Still, it is reasonable to assume that optimal performances would be found at the overlap of distance ranges that are suitable for both H₂ splitting and H⁺/H⁻ transfer elementary steps.

In addition, potential poisoning pathways such as competitive binding of species on FLP sites or protodeborylation, are analyzed independently in the literature, but rarely (if ever) in a joint manner to grant a full mechanistic picture of the reaction. Regarding the latter routes, some trends have been found for specific sets of FLPs. These include: shortening of LA⋯LB distances to favor the activation of H₂ by FLPs over that of CO₂; or increasing the steric hinderance around B sites to prevent undesired protodeborylation events.

Moreover, beyond the determination of the adequate energetics balance and the inclusion of poisoning in computational approaches, the calculated data should be translated into experimental parameters such as partial gas pressure and temperature, which remain crucial towards applicable CO₂ hydrogenation systems. For instance, a pioneering system by Corminbœuf and Dyson allows for the FLP-driven CO₂ reduction at 120 °C under 130 bars, specific conditions that were not anticipated from the calculated data but must have required intensive experimental screening.⁴³

4.2. Challenges in turning computational chemistry into target synthesis

Recently, computational chemistry approaches have become an extremely powerful tool for screening the potential use of molecular FLPs for CO₂ hydrogenation, identifying successful LA–LB pairs, which were later verified experimentally. Still, the translation of computational learning into the experimental realm remains far from trivial, especially when tackling the challenge of FLPs’ heterogenization.

Two experimental aspects have to be considered when turning computational results into bench-scale trials: the synthetic accessibility of the FLP synthons and the conditions to be applied to reach the targeted FLP's reactivity. Considering the reactivity of computationally designed FLPs for the CO₂ hydrogenation, the conditions reported in the two recent examples by Zhao⁴² and Corminbœuf,⁴³ both claiming for a catalytic FLP system, were 160 °C under 60 bars of CO₂/H₂ and 120 °C under 130 bars, respectively. Far from being trivial, the findings of such harsh conditions to assess the efficiency of carefully designed Lewis pairs can only rely on a large screening of pressures and temperatures, including a range of values not often investigated for molecular catalysis. Thus, the quest for novel experimental FLP systems for CO₂ hydrogenation, whether predicted by preliminary computational studies or not, must encompass from its beginnings these specific ranges of temperatures and pressures, especially when compared to transition-metal-based counterpart systems operating under smoother conditions.¹⁵⁵

Besides the deployment of dedicated organic synthesis of the predicted FLPs, limitations that are related to their heterogenization may emanate from the reactivity/stability of the porous solids (MOF, POP, COF) themselves upon their functionalization with the targeted FLPs. Additional drastic limitations may arise from the catalytic conditions themselves (pressure, temperature, solvent…). In the proposed two-step synthetic pathway devised by Ye and Johnson⁹⁰ for targeting the MOF-heterogenized FLP UiO-67-P-BF₂, the initial step hinges upon a palladium-catalyzed Negishi cross-coupling reaction involving the commercially available 3-iodo-1H-pyrazole and a benzylzinc bromide derivative, which has been reported to yield up to 82%.¹⁵⁶ However, it is worth noting that the synthesis of the 2,4,5-trimethylphenylzinc bromide coupling partner, though commercially procurable, is protected by patents and thus not readily accessible for research groups. Moving on to the second potential step, it entails the oxidation of p-xylene to terephthalic acid following the Amoco process.¹⁵⁷ It is noteworthy that this oxidation process employs acetic acid as a solvent, compressed oxygen at approximately 200 °C, and a combination of cobalt, manganese, and bromide catalysts. These rigorous synthetic conditions raise concerns regarding the potential undesired oxidation of the benzylic position within the linker precursor, consequently leading to the decomposition of the intended motif.¹⁵⁸ Lastly, the borylation of the pyrazole moiety has been reported utilizing a two-step methodology, which involves the quantitative utilization of an additional BCF:carbene adduct.¹⁵⁰ Furthermore, it is important to acknowledge that this step relies on heterogeneous solid–liquid reactivity occurring at the interface of the MOF. It also requires to simultaneously ensure the stability of the HBF₂ reactant against the reactivity of the Zr–OH groups present on the MOF's surface, as well as any pending carboxylic acid defects. This presents a significant challenge due to the potential risk of hydrolysis^159,160 and the subsequent release of highly toxic hydrofluoric acid and fluoroboric acid, particularly given the potential diffusion limitations within the pores of the MOF.

Anticipating these synthetic limitations by computational approaches is extremely challenging. Consequently, the discovery of heterogenized FLP systems currently relies on the “trial-and-error” explorations of porous matrices from a broad range of potential candidates. However, the knowledge gained from both homogenous systems and theoretically derived structure–activity relationships can be used to narrow the range of FLP candidates to be immobilized, thus saving time consuming experimental efforts. Future work may explore potential synergies between the chemistry of organic linkers and their functionalization, and that of metal nodes in MOFs^72,73 towards the construction of heterogeneous FLPs, which are still rather scarcely covered. Current developments in the field of hybrid porous materials hold the potential to make it possible to achieve unequaled FLP-based heterogeneous catalysts with the only limit being the synthetic capacity to prepare their elementary building blocks.

4.3. Towards catalytic systems

From the computations performed, the first and main assumption made was that the FLP was designed to act as a catalyst, i.e. a species used in an understoichiometric amount to allow a chemical reaction to proceed, often faster, and which remains unchanged at the end of the reaction. However, in most of the systems reported, the final product of the hydrogenation of CO₂ was an adduct with the Lewis acid or base and the organic hydrogenation target (formic acid, methanol,…) was finally released after hydrolysis. For example, in the work by Zhao⁴² and Corminbœuf,⁴³ the catalytic activity referred to the moles of formate salt of the protonated Lewis base per mole of Lewis acid. The two studies reported the use of a large excess of Lewis base of 10000⁴² and 100 equivalents⁴³ compared to the Lewis acid, these systems being however catalytic with respect to the Lewis acid with calculated TONs of 4000 and 24, respectively. As demonstrated by Dyson and Corminbœuf,⁴³ the release of the formate being the limiting step, the use of excess Lewis base is aimed at displacing the equilibrium towards higher CO₂ conversions. These pioneering studies underscore the potential for innovative catalytic systems by highlighting the importance of optimizing the ratio of Lewis acids and bases to achieve efficient CO₂ hydrogenation. The progress made in understanding and fine-tuning these FLP systems pave the way for future research and development in the field.

4.4. Future developments in computational approaches

There have been a few in-depth DFT studies on solids (periodic calculations or cluster-based simulations) using AIMD and free energy reaction profiles for CO₂ hydrogenation. Limitations issues are related to the size of the crystal structure under study. Most studies use UiO-66 or UiO-67 which could be easily handled using their primitive cells. The same approaches might become much more demanding when turning to MOF possessing larger cell sizes and pores (MIL-101, MIL-100 for instance, PCN-777, MOF-808 etc…). Notably, there is currently a blind spot in computational approaches for considering the dynamical behavior of FLPs once grafted or immobilized into the porous host. Such dynamic approaches are however required in order to predict the positioning of the FLPs within the porous host and to restrict the choice of hosts to those allowing the reactivity of the targeted FLPs to be maintained (minimal pore size, confinement effect,¹⁶¹ first coordination sphere interactions with the MOF inorganic sub-network through hydrogen bonding or electrostatics, role of defects). So far, there is no reported theoretical study exploring the participation of the MOF itself in the catalysis as a Lewis partner, typically using open metal sites as acid partners, while facing a grafted basic partner, or vice versa.

The mapping of crossed activity is a significant and achievable milestone. The next frontier is the computational exploration of functionalized MOFs, including the partitioning of multiple LA and LB grafts, as well as their dynamics within porous matrices. With the advanced capabilities of numerous force fields and QM/MM techniques, studying the realistic implantation of such LA/LB pairs is well within reach, promising groundbreaking developments in this field.

5. Conclusions

In conclusion, the last years have seen the tremendous development of molecular FLPs for CO₂ hydrogenation, including both experimental and computational advances, with pioneering achievements. Key insights into reaction mechanisms and the factors influencing the reactivity of FLPs towards H₂ splitting and subsequent CO₂ hydrogenation have been obtained from both experimental and computational studies on molecular FLPs. These have laid the groundwork for developing structure–activity relationships, providing guidelines on how to tune the structural and electronic properties of FLPs to achieve efficient CO₂ hydrogenation. Notably, this knowledge can be applied not only to the rational design of molecular FLPs but also to the identification of promising candidates for incorporation into the structure of porous matrices to develop heterogeneous catalysts. However, while the synergy between experimental findings and computations holds the potential to accelerate the discovery of new catalytic FLP systems, examples that leverage the currently available design rules to successfully develop new FLPs for CO₂ hydrogenation are still rare. While computational evaluation provides a trend for optimal geometry and reactivity, experimental expertise is key to determine the feasibility of the FLPs’ synthesis and to unravel adequate catalytic conditions.

These challenges become even more complex when transitioning to porous solids through the heterogenization of FLPs. This heterogenization should enable precise control over the FLP's geometry, reactivity, and synergistic effects such as enhancing reactant concentrations locally. Recent pioneering examples already installed a dialogue between computations and experimentations but the implementation of computationally designed in experimentally constructed FLP still require intensive efforts for their in catalytic applications. The development of computational methodologies to go even deeper into the predictions of the synthetic and catalytic conditions combined with the proliferation of synthesis methods based on almost infinite variations of compositions and structures of porous hybrid solids is aimed to push the limits of current systems. Given the evolving nature of this research field, the collaborative efforts of synthetic and computational chemistry are poised to yield sustainable FLP-based catalytic systems for direct CO₂ hydrogenation in the foreseeable future.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the French National Agency as part of the program No. ANR-21-CE07-0028 (FLIPS). A.S.-D. also acknowledges the Spanish Ministry of Universities and the European Union – Next Generation EU for financial support through a Margarita Salas grant.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cs00267e

‡ These authors contributed equally to this work.

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