Next Article in Journal
Whole Cell Biocatalysis of 5-Hydroxymethylfurfural for Sustainable Biorefineries
Next Article in Special Issue
Catalytic Hydrogenation of CO2 to Methanol: A Review
Previous Article in Journal
Mechanistic Kinetic Modelling Framework for the Conversion of Waste Crude Glycerol to Value-Added Hydrogen-Rich Gas
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 12
Issue 2
10.3390/catal12020201
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

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

by
Ranita Pal
1,
Manas Ghara
2 and
Pratim Kumar Chattaraj
2,3,*
1
Advanced Technology Development Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
2
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
3
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(2), 201; https://doi.org/10.3390/catal12020201
Submission received: 25 December 2021 / Revised: 24 January 2022 / Accepted: 3 February 2022 / Published: 7 February 2022
(This article belongs to the Special Issue Catalytic Hydrogenation of CO2)
Browse Figures
Versions Notes

Abstract

:
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.

1. Introduction

The application of catalysts in expediting the rate of chemical reactions has very high industrial importance. Most chemical reactions, especially hydrogenation reactions [1,2,3], rely on transition metal (TM) compounds for the catalytic activity. TM coordination complexes acting as homogenous catalysts can be very easily monitored in the solution phase. They can produce higher chemo- and stereoselectivity, making them industrially important especially in the domains of medicine and food. The discrete efficiency of a specific TM complex to direct such transformations is a direct consequence of the presence of partially filled d-orbitals in their valence shell (i.e., a set of electrophilic and nucleophilic frontier orbitals with low energy difference). This allows them to simultaneously interact with an incoming substrate with both the orbitals, hence activating it in the process. Since TMs are associated with high levels of toxicity, low availability, and high cost, efforts are being made towards the use of TM-free catalysts in an attempt to remove these hurdles [4,5]. It was seen that the chemistry of the main group elements often resemblances that of the TMs in terms of their structure and bond characteristics and hence, can react with small molecules such as, CO, H2, C2H4, NH3, etc. under normal conditions [6]. Some examples include the use of singlet carbene (R2C:) [7,8] in the activation of hydrogen and ammonia. The mechanism of the activation process is observed to be similar to that in the oxidative addition of H2 to the TM. In other words, two electron transfer (ET) processes are functioning between the lone pair of electrons and the empty p orbital of carbene with the antibonding (σ*) and the bonding (σ) orbitals of H2 respectively. Heavier congeners of carbene such as silylene [9], germylene [10], stanylene [11], and higher analogs of acetylene, such as ArGeGeAr [12] and ArSnSnAr [13], are also found to activate H2.
Another class of promising catalysts, known as the frustrated Lewis pair (FLP), is introduced based on the idea that a pair of bulky Lewis acid (LA) and Lewis base (LB) cannot form adduct due to steric repulsion. An alternative reaction channel is opened as a result of this steric inhibition produced by the aforementioned FLP [14]. Since the discovery of FLPs by Stephan et al., a variety of combinations of LAs and LBs has been reported both as non-linked (also known as intermolecular FLP) and linked (intramolecular FLP) systems. The LA component includes, from neutral boranes and alanes, to cationic silyliums, phosphoniums, borenium, carbocations, titanocenes, zirconocenes, and others [15,16,17,18,19,20,21,22,23]. Conversely, the LB component includes various amines, imines, pyridines, phosphines, carbenes, ethers, carbanions, silylenes, and the like [24,25,26,27,28,29]. Structures of some inter- and intramolecular FLPs, along with their important roles, are listed in Table 1 and Table 2. A comparison of energetics for H2 activation by TM and FLPs is provided in Table 3 to better understand the effectiveness of FLPs in metal-free H2 activation. FLPs act as good catalyzing agents in reactions involving activation of small molecules such as, CO, CO2, N2O, NO, SO2, alkenes, alkynes, catalytic hydrogenation, and so on [30,31,32,33,34], all of which are discussed later in this article. H2 activation by bridged P/B FLPs, and the role of boron-ligand cooperation (BLC) in activating the same, is explored. Simultaneous activation of H2 and CO2 molecules is also demonstrated in this review. The B–X (X = O, N, S) bond of the FLPs plays a crucial role in activating the molecular hydrogen, where it changes from B+–X− electron-sharing type of interaction to B←X dative bond upon H2 activation.

2. H2 Activation by FLPs

In 2006, Stephan synthesized an ambiphilic molecule and zwitterionic phosphonium hydridoborate [Mes2HPC6F4BH(C6F5)2], where P center acts as the LB and the B center as the LA. On heating at 150 °C, it releases H2 gas producing the ambiphilic phosphinoborane [Mes2PC6F4B(C6F5)2], which in turn forms the original phosphonium hydridoborate compound upon heating with H2 at room temperature [50]. This entire process is essentially the heterolytic cleavage of H2 facilitated by phosphinoborane. A similar process was tried in 2007 with the help of tris-(tertiarybutyl) phosphine (tBu3P) and tris-(pentafluorophenyl) borane (BCF) [36].
There are a few mechanistic paths of H2 activation by FLPs reported in the literature, proposed by various scientists over the years. A mechanistic path, proposed by Welch and Stephan in 2007, describes an initial polarization of H2 occurring as a result of a side-on interaction between H2 and the LA (B(C6F5)3) forming the adduct, H2…B(C6F5)3 [36,51], from which a phosphine LB (PtBu3) abstracts a proton (mechanism shown in Figure 1a). Unfortunately, no such adduct is detected experimentally even at higher H2 pressure (4 atm). Although some computational reports showed the existence of BH5, where H2 is weakly bound to BH3 via an η2 binding mode [52]. Alternatively, H2 may initially interact with the LB followed by the hydride abstraction by the LA. Again, no such H2…LB adduct is detected experimentally. In 2008, another significant mechanistic proposal put forward by Rokob and co-workers [53], on the activation of molecular hydrogen by a phosphine/borane FLP, reinvestigated the interactions of H2 with B(C6F5)3 and PtBu3 separately. Both are found to be repulsive when the components are within a certain distance. From these results, they concluded that another reaction channel should exist for the reactivity of the phosphine/borane pair towards H2. They proposed that the LA and LB components of an FLP are preorganized together to form an “encounter complex” (EC) [54] (Figure 1b), which is stabilized by weak non-covalent interactions (C–H…F hydrogen bond and dispersion interactions). The role of weak dispersion interaction to stabilize the EC was also suggested from NMR spectroscopy [55]. The molecular dynamics simulation study further confirmed the existence of the EC [56]. Afterward, the H2 molecule enters into the reactive pocket of the EC and interacts with both the active centers of the FLP. Now, the Lewis basic phosphorus center of tBu3P donates the lone pair of electron density into the anti-bonding orbital of H2. Conversely, the Lewis acidic B center of B(C6F5)3 accepts electron density into its empty p-orbital from the bonding orbital of H2. As the reaction progresses, this synergistic ET mechanism leads to the continuous weakening of the H-H bond, which ultimately breaks to form P–H and B–H bonds. This mechanism of H2 activation by FLP is termed as the ET model [57] (Figure 2).
In 2010, Grimme et al. [58] proposed a completely different explanation of the H2 activation mechanism by FLP. They suggested that the EC constructed from bulky LA and LB creates an electric field (EF) inside its cavity. On entering this cavity, the H2 molecule becomes polarized by the EF and undergoes a heterolytic cleavage. Therefore, while the entrance of H2 into the FLP cavity is an energy-demanding process, its splitting happens in a barrierless way. This mechanism of H2 activation by FLP is termed the EF model (Figure 2). By re-investigating the mechanism, Rokob et al. predicted a higher acceptance of the ET model compared to the EF model [59]. Recently, Skara et al. [60] reexamined both the ET and EF models of H2 activation and clarified their applicability in two distinct cases. The former is better suited for high energy transition state (TS), having a longer H-H and smaller LB···H2 distance (also known as geometrically “late”). In this case, the electron donation from the Lewis basic center to the σ*(H2) orbital is predominant. The EF model, conversely, is applicable for low energy TS (geometrically “early”) with smaller H-H separation and longer LB···H2 and LA···H2 distances. End-on LA···H2 interaction is predominant in this case.
Moreover, the kinetics and thermodynamic aspects of H2 activation are also investigated and reported in the literature. In particular, a theoretical study performed by Rokob et al. [61] demonstrates the H2 activation by FLP as the overall result of five hypothetical steps as depicted in Figure 3, which includes separating the LA-LB pair, heterolytically cleaving the molecular H2, proton abstraction by LB, hydride abstraction by LA, and stabilization by pairing [LBH]+ and [LAH]− ions. The type of FLP is so chosen so that the LA-LB separation energy (ΔGsep) is low in the first step. The second step, i.e., the heterolytic cleavage of H2 into proton and hydride ions, is endergonic and associated with a free energy of ΔGHH ≈ +128.8 kcal/mol (in toluene; not dependent on the type of FLP used). In most cases, no dative bonds exist between the Lewis centers in equilibrium, and thus the ΔGHH is the only endergonic contribution. In the cases where the dative bonds are present, however, an additional endergonic term (ΔGprep) is required to break such bonds in order to generate free donor and acceptor centers available to receive the H+ and H- ions. The proton attachment by the LB center and the hydride attachment by the LA center are ΔGpa and ΔGha, respectively. The final step involves the ion pair formation with a binding free energy (ΔGstab), which stabilizes the ion pair formed from the separate [LBH]+ and [LAH]− ions. The energy corresponding to this last step remains mostly the same for any LA-LB pair. Thus, the thermodynamics of the overall reaction predominantly depends on the H+ and H− attachments to the LB and LA, respectively. While the former case is backed by experimental data in the form of pKa values (solvent-dependent) [62], no such data are available for the latter [63]. The H− affinity of LA is qualitatively linked to other experimental measurements of Lewis acidity by Heiden and Latham [64].
In a study performed by our group [41], we have explored the influence of boron-ligand cooperation (BLC) in H2 activation and the associated effects on the corresponding activation barrier. The bridged FLP systems considered for the study along with the reaction are provided in Figure 4. System 1 is boroxypyridine, where the oxygen unit at the ortho position of the pyridine ring is replaced by NH and S units to produce Systems 2 and 3, respectively. Systems 4, 5, and 6 are the results of adding –NMe2 moiety at the para position of 1, 2, and 3, respectively.
All the H2 activation processes are exergonic, with Gibbs free energy barrier ranging within 17–25 kcal/mol, where FLP 6 has the lowest barrier. Wiberg bond indices (WBIs) calculated at the bond critical points (BCPs) of the bonds, B–X and C–X (X = O, N and S), along with the changes in the respective bond distances, suggest that on H2 activation, the B–X bond weakens and C–X strengthens. The reason behind this is the change in the nature of the B–X bond from B+–X− in the parent FLPs to B←X dative type in the products. In the cases of the C–X bonds, they develop double bond character. EDA-NOCV performed on the transition state structures support the electron transfer model describing a synchronous transfer of electron density occurring as LB(FLP)→σ*(H2), and σ(H2)→LA(FLP), resulting in the weakening of the H-H bond. A nucleus independent chemical shift (NICS) analysis reveals a reduction in the aromaticity of the pyridine rings upon H2 activation. Hence, the influence of the BLC is demonstrated through the change in the nature of the B–X (X = O, N and S) bonds in all the FLP systems considered for the study. Possible hydrogenation of CO2 is explored with these hydrogenated FLPs, which is discussed later in the article.

3. Catalytic Hydrogenation by FLPs

FLPs can effectively activate molecular hydrogen and hence can be used as a viable alternative catalyst to undergo a metal-free hydrogenation. The general process involves H2 activation followed by sequential H+ and H− transfer from the hydrogenated FLP to the substrate and regeneration of the parent FLP. Catalytic hydrogenation of nitrile, imine, and aziridine using Mes2PC6F4B(C6F5)2 [65,66] is demonstrated by Stephan et al. The same occurs for sterically hindered imines is shown using just an LA, B(C6F5)3, since the imine itself acts as the LB [67]. In the case of less basic imines, the rate of reaction can be increased by adding a small amount of LB P(C6H2Me3)3. In 2014, the groups of Stephan [68] and Ashley [69] separately worked on the hydrogenation of carbonyl compounds to alcohol where B(C6F5)3 and the ether solvent (1,4-dioxane) act as the FLP catalyst. It first splits the H2 molecule and subsequently reduces the carbonyl group. The hydrogenation of aldehydes and ketones to alcohol by B(C6F5)3/cyclodextrine FLP in a non-polar solvent, and the transformation of aryl ketone to a deoxygenated aryl compound, are also demonstrated by Stephan’s group [68,70]. Other polar compounds such as enons, enamines, silyl enol ethers, oximes, etc. can also be hydrogenated by FLPs [71,72,73].
The hydrogenation of non-polar compounds like olefins, however, involves a slightly different process. Here the LB part of the FLP needs to be a weak base so that its conjugate acid (produced on H2 activation) is strong enough to protonate the less reactive olefin [74]. The H− abstraction then occurs from the [HB(C6F5)3]− in the following step. The use of FLPs in a plethora of hydrogenation reactions followed in subsequent years. Alkene hydrogenation by ether/B(C6F5)3 FLP occurs via the generation of [Et2O…H…OEt2]+ and [HB(C6F5)3]− ions, as reported by Hounjet et al. [27]. It is noted that the catalytic hydrogenation of electron-rich olefins by FLP are much easier than that of simple olefins [74]. Conversion of alkyne to cis-alkene using ansa-aminohydroborane [75], aniline to N-cyclohexyl ammonium salt using a H2/B(C6F5)3 pair [76], and anthracene, tetracene, and tetraphene, using Ph2PC6F5/B(C6F5)3 [77], etc., are a few examples of FLP-mediated hydrogenations reported throughout the years. Some N-heterocyclic compounds like acridine, quinoline, and phenanthroline can also be hydrogenated by B(C6F5)3 catalyst [78].
The aforementioned hydrogenation reactions occur by an initial H2 activation followed by H transfer to the substrate. This H+ and H− transfer to the substrate can occur in two different mechanisms. For strong LA components (e.g., B(C6F5)3), substrate activation by protonation or H–bonding interaction is required since the conjugate base is not strong enough to deliver H− on its own. Thus, protonation should occur before the H− transfer [79,80,81]. In such cases, oftentimes, the substrate itself acts as the LB [81]. The other mechanism involves the occurrence of H− transfer before H+ transfer, where substrate activation with another LA is required to effortlessly carry out the hydride transfer [21]. Concerted H+ and H− transfer to the substrate is also a possibility as observed in the hydrogenation of CO2 to HCOOH [82].

4. Catalytic Hydrogenation of CO2

CO2, one of the major greenhouse gases, contributes to the rising global temperature and poses a serious threat to our earth’s atmosphere. Conversion of CO2 into various useful chemical compounds can offer a potential solution to this problem, along with the additional benefit of their utilization for energy and chemical feedstocks. However, the transformation of CO2 is a challenging process owing to its high thermodynamic and kinetic stability. Reduction of CO2 by hydrogen to form methanol is a good way of contributing to renewable resources since it serves as a precursor to many chemicals that are required to generate electricity in fuel cells.
FLP mediated hydrogenation of CO2, by using the hydrogenated FLPs, is a good metal-free catalytic alternative. Ashley et al. [5], in 2009, described the first homogenous hydrogenation of CO2 to CH3OH by initially undergoing heterolytic H2 activation followed by insertion of CO2 into a B–H bond. This general two-step mechanism was followed by other researchers as well [24,83]. In 2010, Menard and Stephan used a P/Al-based FLP to transform CO2 to a methanol derivative [84]. In the same year, Dureen and Stephan experimentally synthesized a four-membered heterocyclic compound containing two B–N bonds, known as boron amidinates [42] (Figure 5). These can take part in various insertion reactions by opening one of the said B–N bonds. By thermolysis, they showed the existence of the transient open-chain isomer of the amidinate, which is responsible for its FLP characteristics. Very recently, the photocatalytic hydrogenation of CO2 to CH3OH [85] and CO2 hydrogenation over magnetic nanoparticles [86] are reported.
Recently, Jiang et al. [88] reported a bridged B/P FLP-mediated CO2 hydrogenation that can follow two mechanistic paths. First being a concerted mechanism of heterolytic cleavage of H2 molecule by the FLP and CO2 hydrogenation, and second where CO2 activation by the FLP is followed by H2 metathesis and reductive elimination of HCOOH in a step-by-step process. However, a computational study [87] performed by our group utilizes FLPs A and B (Figure 5) to hydrogenate CO2 to produce HCOOH, which reveals that the activation of H2 and CO2 occurs simultaneously. It can happen in two possible ways; one where the LB center activates H2 and LA center activates CO2, and the second where the reverse occurs. The natural bond orbital (NBO) and energy density analyses (EDA) performed on the TS corroborate the simultaneous activation theory. The former mechanism (i.e., LB activating H2 and LA activating CO2) has the electron density transferring as HOMO(FLP)→LUMO(H2), HOMO(H2)→LUMO(CO2), and from several occupied MOs of CO2 to LUMO(FLP). The steps of the reaction include the formation of a formate ion attached to the Lewis acidic B center of the FLP followed by a proton transfer from the Lewis basic N center of the FLP to the O center of the formate unit. The first step has a higher barrier which may inhibit the process at ambient conditions, but it can be overcome at higher temperature and pressure. In the case of the other mechanism (i.e., LB activating CO2 and LA activating H2), the catalytic cycle takes two more steps where the COOH unit (attached to the Lewis basic N center of the FLP) reorients itself so as to come closer to the BH moiety to facilitate the proton transfer from the BH to the COOH moiety. Both the catalytic cycles are shown in Figure 6a,b. Heterogenous CO2 reduction to HCOOH is also reported [89] by silica nanopowder supported FLP where the said system forms an FLP-CO2 adduct on the silica surface. The activation of H2 is followed by the conversion of the captured CO2 to HCOOH. Again, substitution of C6H5 groups on B and P centers increases the effectiveness of the whole process compared to C6F5 (too electron-deficient) and C6H11 (too electron-rich) substituents.
In a recent article [90], the CO2 capture by tBu3P/B(C6F5)3 FLP was reported. They studied the whole reaction path to discover that the LA unit plays a more important role in the catalytic action of the FLP, both thermodynamically and kinetically. Whereas the LB unit has a higher impact in the FLP formation. It was thus recommended by this group to select a pair of strong LA and weak LB in designing an FLP for CO2 activation to make the reaction thermodynamically and kinetically feasible.

5. Activation of Other Small Molecules

Apart from H2 activation, FLPs have proven to be useful in the activation of several other small molecules, such as CO, CO2, N2O, NO, SO2, alkenes, alkynes, and so on, which remain unaffected in the presence of either component of the Lewis pair (Figure 7). A schematic representation of the activation mechanism in π-systems by FLPs and optimized geometries of some transition state structures are provided in Figure 8. Despite its significant thermochemical stability, CO2 reacts with FLP to produce novel carbonic acid derivatives [91]. Reaction with SO2 also occurs similarly [43]. A slightly different reaction occurs with nitrous oxide, which produces an additional compound containing the LB–N=N–O–LA unit [37]. Cooperative addition of FLPs is seen in the cases of CO with an intramolecular P/B FLP (Mes2PCH2CH2B(C6F5)2) [47], t-butylisocyanide with an unsaturated vicinal P/B FLP [48], and in P-ligand C–H bond activation [92]. With NO, FLP forms an adduct producing N-oxyl radical [44], and with olefins [38] and alkynes [87] they form zwitterionic addition products. Alternatively, phosphine-borane FLPs can also deprotonate terminal alkynes to form phosphonium alkynylborates [35,45].
A DFT-based study performed by Trujillo et al. [94] shows the effect of aromaticity in the activation power of FLPs. Replacing a BPh2 group with a borole unit to act as the LA moiety in a geminal P/B FLP, an enhancement in its ability to activate small molecules is theoretically predicted. Our group has investigated the dihydrogen activation of a five-membered P/B FLP (1) reported by Dong et al. [95], along with two other FLPs (2 and 3) modeled with some modification on the former (Figure 9) [96]. The H2 activation brought on by the designed FLPs (2 and 3) turns out to be more favorable, both thermochemically and kinetically, than that by FLP 1. An investigation into the aromaticity of the FLPs, by evaluating NICS (0) and NICS (1) (zz) values at the C4B ring, shows a decrease in the anti-aromatic nature of FLPs 2 and 3 from the reactants to the corresponding TSs where they reach their minimum values. This decrease is steeper in the latter, as an influence of the strong withdrawing –C6F5 groups around the B center. Thus, the anti-aromaticity in FLPs 2 and 3 boost their reactivity by reducing the activation barrier, compared to FLP 1. A similar relation between aromaticity and activation power of FLPs was observed by Zhuang et al. [97] in a DFT study on CO2 activation. Rouf et al. [98] demonstrated an increase in the reaction feasibility of dinitrogen activation by FLPs due to an increase in aromaticity.
Some reactions with FLPs were discovered a few years ago, such as N-sulfinyltolylamine (p-TolNSO) yielding a seven-membered cyclic product where binding occurs via N and O centers of p-TolNSO [46]. The reaction of a phosphine-borane FLP with 1,3-dienes is reported by Ulrich et al. to produce the 1,4-addition product [39]. An 1,2-addition with the C=O group of an α, β-unsaturated aldehyde is reported by Momming et al. [88]. FLP mediated cleavage of B-H bond to produce oxygen-ligated borenium cation is also studied [40]. In the case of intramolecular cyclization involving sterically hindered amine with olefin or acetylene groups, the FLP reactivity increases, as reported by Stephan and Erker [99].
For hydrogenation of terminal alkynes, Liu et al. [100] recently developed an extremely stable polymeric LA (P-BPh3) to tackle the problem of FLP deactivation due to the tight bond formation between the LA and terminal alkynes. The high stability also allows the recycling of the FLP up to 12 times in the catalytic process. Another case of catalyst deactivation occurs in the dehydrogenation of amine-boranes. The solution is reported by Bhattacharjee et al. [101] where they have used a P/B FLP to dehydrogenase dimethylamine borane (DMAB). Its reaction mechanism follows an indirect activation of the B-H bond facilitated by an additional DMAB molecule, followed by deprotonation of the PPh2 unit. FLP mediated amide hydrogenation [102], which uses B(2,6-F2-C6H3)3 with chloride LB, reveals a very important role of the halide. The reaction exhibits high generality, especially extendable to tertiary benzoic acid amides and α-branched carboxamides.
Recently FLPs are utilized in the activation of a single C–F bond in trifluoromethyl group as reported by Young’s group [103]. A combination of phosphine or pyridine base with B(C6F5)3 activates the C–F bond in the trifluoromethyl group to generate difluoromethyl product. In a separate study, the same group [104] further demonstrates the single C–F bond activation in gem-difluoroalkanes by using Al(C6F5)3 instead of B(C6F5)3. We have also performed a theoretical study on the activation of the C–F bond in fluoroalkanes by using a combination of lutidine base with either B(C6F5)3 or Al(C6F5)3 acids [105]. It reveals that the C–F bond activation mediated by Al(C6F5)3/lutidine pair is more favorable than that mediated by B(C6F5)3/lutidine pair. Hence, the Al(C6F5)3 acid is superior to the B(C6F5)3 acid for such unusual bond activation. Last year, Ison and Tubb [106], made a simple yet important calculation for M(C6F5)3 (M = B, Ga, Al) Lewis acids. They evaluated the ratio between orbital interaction energy (EOrb) to the electrostatic and exchange repulsion energy (ESteric) in an energy decomposition analysis (EDA). A correlation between this ratio and the reactivity drawn reveals that only the B(C6F5)3 carries out a catalytic hydrosilylation in ketones. This ratio can be used as a guiding parameter in designing of FLPs in the future. In recent years, FLPs are being increasingly used in polymerization catalysis [107,108,109], polymer chemistry [110,111], and organic synthesis [112,113] as well.

6. Summary

The high thermochemical and kinetic stability of the greenhouse gas, CO2, makes its transformation to useful chemical compounds a challenging task. It requires efficient catalysts, most of which have high levels of toxicity, is costly, and has low availability. To that end, we present a thorough discussion on metal-free catalytic hydrogenation of CO2 by a class of compounds known as frustrated Lewis pair (FLP). They are produced when a pair of Lewis acid and Lewis base are unable to form an adduct due to steric hindrance. We also provide a detailed discussion on the hydrogen activation ability of FLPs, which has been utilized for the catalytic hydrogenation of imines, nitriles, enamines, alkenes, alkynes, ketones, CO2, etc. Apart from the H2 activation, FLPs may activate and react with other small molecules as well; examples include CO2, N2O, CO, NO, SO2, C2H4, C2H2, etc. The mechanisms involved in the activation of molecular hydrogen, activation, and the catalytic hydrogenation of carbon dioxide are thoroughly discussed. The term boron-ligand cooperation (BLC) in analogy to metal ligand cooperation (MLC) is introduced to describe a distinct approach of reactivity for some specific FLPs to activate a chemical bond. The B–X bond (X = O, N and S) present in the concerned FLPs plays an important role in the activation of molecular hydrogen. The B-X bond changes from B+-X− in the parent FLPs to B←X dative type in the products.
The aromaticity/anti-aromaticity of an FLP has been reported to have a relation to its reactivity in activating small molecules on several occasions, and hence, can be used as a guide in designing new and effective FLP systems. The boron-ligand cooperation, applicable at the LA site of an FLP, is established in the case of dihydrogen activation. This boron atom can be replaced by another group with 13 elements to verify if similar cooperation is exhibited, and if so, whether it enhances the reactivity of the new FLP. The ability of FLPs to activate small molecules opens up new avenues in metal-free catalysis, which was previously believed to be the exclusive domain of transition metals. The H2 reactivity of FLP, in particular, continues to be useful in both homo- and heterogenous catalysis. Other than CO2 reduction and small molecules activation, FLPs are being actively researched in surface chemistry owing to the cooperative action of LA and LB centers. They are increasingly utilized in polymerization catalysis, polymer chemistry, and organic synthesis, etc. Application of FLP as a catalyst in the reduction of CO2 can occur either by the concerted or stepwise transfer of both the activated hydrogens, or by simultaneous activation of CO2 and H2 as highlighted through several computational studies. These mechanistic insights may provide experimental researchers with tools to design better catalytic FLPs.

Author Contributions

P.K.C. came up with the concept and design of the review, and reviewed the final manuscript. R.P. contributed towards the literature survey, analyzed, and wrote the manuscript. M.G. contributed towards the literature survey, wrote the abstract and part of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Department of Science and Technology (J. C. Bose National Fellowship), grant number: SR/S2/JCB-09/2009.

Acknowledgments

P.K.C. would like to thank Maeve Yue for kindly inviting him to contribute an article to the Special Issue, “Catalytic Hydrogenation of CO2” in the MDPI journal, Catalyst. He also thanks DST, New Delhi for the J. C. Bose National Fellowship, grant number SR/S2/JCB-09/2009. R.P. and M.G. thank CSIR for their fellowships, and IIT Kharagpur for the computational facilities.

Conflicts of Interest

The authors declare that they have no conflict of interest regarding the publication of this article, financial, and/or otherwise.

References

  1. Berke, H. Conceptual approach to the reactivity of dihydrogen. Chem. Phys. Chem. 2010, 11, 1837–1849. [Google Scholar] [CrossRef] [PubMed]
  2. de Vries, J.G.; Elsevier, C.J. Handbook of Homogeneous Hydrogenation; WeinheimWiley: Weinheim, Germany, 2007. [Google Scholar]
  3. Siegbahn, P.E.; Tye, J.W.; Hall, M.B. Computational studies of [NiFe] and [FeFe] hydrogenases. Chem. Rev. 2007, 107, 4414–4435. [Google Scholar] [CrossRef] [PubMed]
  4. Spielmann, J.; Buch, F.; Harder, S. Early Main-Group Metal Catalysts for the Hydrogenation of Alkenes with H2. Angew. Chem. 2008, 120, 9576–9580. [Google Scholar] [CrossRef]
  5. Ashley, A.E.; Thompson, A.L.; O’Hare, D. Non-metal-mediated homogeneous hydrogenation of CO2 to CH3OH. Angew. Chem. Int. Ed. 2009, 48, 9839–9843. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  6. Power, P.P. Main-group elements as transition metals. Nature 2010, 463, 171–177. [Google Scholar] [CrossRef] [PubMed]
  7. Hopkinson, M.N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014, 510, 485–496. [Google Scholar] [CrossRef] [PubMed]
  8. Frey, G.D.; Lavallo, V.; Donnadieu, B.; Schoeller, W.W.; Bertrand, G. Facile splitting of hydrogen and ammonia by nucleophilic activation at a single carbon center. Science 2007, 316, 439–441. [Google Scholar] [CrossRef] [Green Version]
  9. Protchenko, A.V.; Birjkumar, K.H.; Dange, D.; Schwarz, A.D.; Vidovic, D.; Jones, C.; Kaltsoyannis, N.; Mountford, P.; Aldridge, S. A stable two-coordinate acyclic silylene. J. Am. Chem. Soc. 2012, 134, 6500–6503. [Google Scholar] [CrossRef]
  10. Peng, Y.; Guo, J.-D.; Ellis, B.D.; Zhu, Z.; Fettinger, J.C.; Nagase, S.; Power, P.P. Reaction of hydrogen or ammonia with unsaturated germanium or tin molecules under ambient conditions: Oxidative addition versus arene elimination. J. Am. Chem. Soc. 2009, 131, 16272–16282. [Google Scholar] [CrossRef]
  11. Peng, Y.; Ellis, B.D.; Wang, X.; Power, P.P. Diarylstannylene activation of hydrogen or ammonia with arene elimination. J. Am. Chem. Soc. 2008, 130, 12268–12269. [Google Scholar] [CrossRef]
  12. Spikes, G.H.; Fettinger, J.C.; Power, P.P. Facile activation of dihydrogen by an unsaturated heavier main group compound. J. Am. Chem. Soc. 2005, 127, 12232–12233. [Google Scholar] [CrossRef]
  13. Peng, Y.; Brynda, M.; Ellis, B.D.; Fettinger, J.C.; Rivard, E.; Power, P.P. Addition of H2 to distannynes under ambient conditions. Chem. Commun. 2008, 6042–6044. [Google Scholar] [CrossRef] [PubMed]
  14. Welch, G.C.; Cabrera, L.; Chase, P.A.; Hollink, E.; Masuda, J.D.; Wei, P.; Stephan, D.W. Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: Facile formation of phosphine-boranes and cationic phosphonium-boranes. Dalton Trans. 2007, 3407–3414. [Google Scholar] [CrossRef] [PubMed]
  15. Farrell, J.M.; Hatnean, J.A.; Stephan, D.W. Activation of hydrogen and hydrogenation catalysis by a borenium cation. J. Am. Chem. Soc. 2012, 134, 15728–15731. [Google Scholar] [CrossRef] [PubMed]
  16. Schäfer, A.; Reißmann, M.; Schäfer, A.; Schmidtmann, M.; Müller, T. Dihydrogen Activation by a Silylium Silylene Frustrated Lewis Pair and the Unexpected Isomerization Reaction of a Protonated Silylene. Chem.–A Eur. J. 2014, 20, 9381–9386. [Google Scholar] [CrossRef]
  17. Ménard, G.; Stephan, D.W. H2 activation and hydride transfer to olefins by Al(C6F5)3-based frustrated Lewis pairs. Angew. Chem. Int. Ed. Engl. 2012, 51, 8272–8275. [Google Scholar] [CrossRef]
  18. vom Stein, T.; Peréz, M.; Dobrovetsky, R.; Winkelhaus, D.; Caputo, C.B.; Stephan, D.W. Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins. Angew. Chem. Int. Ed. 2015, 54, 10178–10182. [Google Scholar] [CrossRef]
  19. Clark, E.R.; Ingleson, M.J. N-Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ-Bond Activation and Catalytic Reductions. Angew. Chem. 2014, 126, 11488–11491. [Google Scholar] [CrossRef]
  20. Metters, O.J.; Forrest, S.J.; Sparkes, H.A.; Manners, I.; Wass, D.F. Small molecule activation by intermolecular Zr (IV)-phosphine frustrated Lewis pairs. J. Am. Chem. Soc. 2016, 138, 1994–2003. [Google Scholar] [CrossRef]
  21. Scott, D.J.; Phillips, N.A.; Sapsford, J.S.; Deacy, A.C.; Fuchter, M.J.; Ashley, A.E. Versatile catalytic hydrogenation using a simple tin (IV) Lewis acid. Angew. Chem. 2016, 128, 14958–14962. [Google Scholar] [CrossRef]
  22. Weicker, S.A.; Stephan, D.W. Main group Lewis acids in frustrated Lewis pair chemistry: Beyond electrophilic boranes. Bull. Chem. Soc. Jpn. 2015, 88, 1003–1016. [Google Scholar] [CrossRef] [Green Version]
  23. Hamilton, H.B.; King, A.M.; Sparkes, H.A.; Pridmore, N.E.; Wass, D.F. Zirconium–Nitrogen Intermolecular Frustrated Lewis Pairs. Inorg. Chem. 2019, 58, 6399–6409. [Google Scholar] [CrossRef] [PubMed]
  24. Sumerin, V.; Schulz, F.; Nieger, M.; Leskelä, M.; Repo, T.; Rieger, B. Facile heterolytic H2 activation by amines and B (C6F5) 3. Angew. Chem. Int. Ed. 2008, 47, 6001–6003. [Google Scholar] [CrossRef] [PubMed]
  25. Chase, P.A.; Stephan, D.W. Hydrogen and Amine Activation by a Frustrated Lewis Pair of a Bulky N-Heterocyclic Carbene and B (C6F5) 3. Angew. Chem. 2008, 120, 7543–7547. [Google Scholar] [CrossRef]
  26. Holschumacher, D.; Bannenberg, T.; Hrib, C.G.; Jones, P.G.; Tamm, M. Heterolytic dihydrogen activation by a frustrated carbene–borane Lewis pair. Angew. Chem. 2008, 120, 7538–7542. [Google Scholar] [CrossRef]
  27. Hounjet, L.J.; Bannwarth, C.; Garon, C.N.; Caputo, C.B.; Grimme, S.; Stephan, D.W. Combinations of ethers and B (C6F5) 3 function as hydrogenation catalysts. Angew. Chem. 2013, 125, 7640–7643. [Google Scholar] [CrossRef]
  28. Li, H.; Aquino, A.J.; Cordes, D.B.; Hung-Low, F.; Hase, W.L.; Krempner, C. A zwitterionic carbanion frustrated by boranes–Dihydrogen cleavage with weak Lewis acids via an “inverse” frustrated Lewis pair approach. J. Am. Chem. Soc. 2013, 135, 16066–16069. [Google Scholar] [CrossRef]
  29. Mummadi, S.; Unruh, D.K.; Zhao, J.; Li, S.; Krempner, C. “Inverse” frustrated Lewis pairs–activation of dihydrogen with organosuperbases and moderate to weak Lewis acids. J. Am. Chem. Soc. 2016, 138, 3286–3289. [Google Scholar] [CrossRef]
  30. Douglas, W.S. Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen. J. Am. Chem. Soc. 2021, 143, 20002–20014. [Google Scholar]
  31. Sapsford, J.S.; Csókás, D.; Turnell-Ritson, R.C.; Parkin, L.A.; Crawford, A.D.; Pápai, I.; Ashley, A.E. Transition Metal-Free Direct Hydrogenation of Esters via a Frustrated Lewis Pair. ACS Catal. 2021, 11, 9143–9150. [Google Scholar] [CrossRef]
  32. Lambic, N.S.; Sommer, R.D.; Ison, E.A. Tuning Catalytic Activity in the Hydrogenation of Unactivated Olefins with Transition-Metal Oxos as the Lewis Base Component of Frustrated Lewis Pairs. ACS Catal. 2017, 7, 1170–1180. [Google Scholar] [CrossRef]
  33. Zhou, S.; Wan, Q.; Lin, S. Cu/O Frustrated Lewis Pairs on Cu Doped CeO2(111) for Acetylene Hydrogenation: A First-Principles Study. Catalysts 2022, 12, 74. [Google Scholar] [CrossRef]
  34. Liu, X.; Qiu, B.; Yang, X. Bioinspired Design and Computational Prediction of SCS Nickel Pincer Complexes for Hydrogenation of Carbon Dioxide. Catalysts 2020, 10, 319. [Google Scholar] [CrossRef] [Green Version]
  35. Dureen, M.A.; Stephan, D.W. Terminal alkyne activation by frustrated and classical Lewis acid/phosphine pairs. J. Am. Chem. Soc. 2009, 131, 8396–8397. [Google Scholar] [CrossRef]
  36. Welch, G.C.; Stephan, D.W. Facile heterolytic cleavage of dihydrogen by phosphines and boranes. J. Am. Chem. Soc. 2007, 129, 1880–1881. [Google Scholar] [CrossRef]
  37. Otten, E.; Neu, R.C.; Stephan, D.W. Complexation of nitrous oxide by frustrated Lewis pairs. J. Am. Chem. Soc. 2009, 131, 9918–9919. [Google Scholar] [CrossRef] [Green Version]
  38. McCahill, J.S.; Welch, G.C.; Stephan, D.W. Reactivity of "frustrated Lewis pairs": Three-component reactions of phosphines, a borane, and olefins. Angew. Chem. Int. Ed. Engl. 2007, 46, 4968–4971. [Google Scholar] [CrossRef]
  39. Ullrich, M.; Seto, K.S.H.; Lough, A.J.; Stephan, D.W. 1,4-Addition reactions of frustrated Lewis pairs to 1,3-dienes. Chem. Commun. 2009, 2335–2337. [Google Scholar] [CrossRef]
  40. Dureen, M.A.; Lough, A.; Gilbert, T.M.; Stephan, D.W. B–H Activation by frustrated Lewis pairs: Borenium or boryl phosphonium cation? Chem. Commun. 2008, 4303–4305. [Google Scholar] [CrossRef]
  41. Ghara, M.; Pan, S.; Chattaraj, P.K. A theoretical investigation on boron–ligand cooperation to activate molecular hydrogen by a frustrated Lewis pair and subsequent reduction of carbon dioxide. Phys. Chem. Chem. Phys. 2019, 21, 21267–21277. [Google Scholar] [CrossRef]
  42. Dureen, M.A.; Stephan, D.W. Reactions of boron amidinates with CO2 and CO and other small molecules. J. Am. Chem. Soc. 2010, 132, 13559–13568. [Google Scholar] [CrossRef]
  43. Sajid, M.; Klose, A.; Birkmann, B.; Liang, L.; Schirmer, B.; Wiegand, T.; Eckert, H.; Lough, A.J.; Froehlich, R.; Daniliuc, C.G. Reactions of phosphorus/boron frustrated Lewis pairs with SO2. Chem. Sci. 2013, 4, 213–219. [Google Scholar] [CrossRef]
  44. Cardenas, A.J.P.; Culotta, B.J.; Warren, T.H.; Grimme, S.; Stute, A.; Fröhlich, R.; Kehr, G.; Erker, G. Capture of NO by a Frustrated Lewis Pair: A New Type of Persistent N-Oxyl Radical. Angew. Chem. Int. Ed. 2011, 50, 7567–7571. [Google Scholar] [CrossRef]
  45. Mömming, C.M.; Frömel, S.; Kehr, G.; Fröhlich, R.; Grimme, S.; Erker, G. Reactions of an intramolecular frustrated Lewis pair with unsaturated substrates: Evidence for a concerted olefin addition reaction. J. Am. Chem. Soc. 2009, 131, 12280–12289. [Google Scholar] [CrossRef] [PubMed]
  46. Longobardi, L.E.; Wolter, V.; Stephan, D.W. Frustrated Lewis Pair Activation of an N-Sulfinylamine: A Source of Sulfur Monoxide. Angew. Chem. Int. Ed. 2015, 54, 809–812. [Google Scholar] [CrossRef] [PubMed]
  47. Sajid, M.; Lawzer, A.; Dong, W.; Rosorius, C.; Sander, W.; Schirmer, B.; Grimme, S.; Daniliuc, C.G.; Kehr, G.; Erker, G. Carbonylation Reactions of Intramolecular Vicinal Frustrated Phosphane/Borane Lewis Pairs. J. Am. Chem. Soc. 2013, 135, 18567–18574. [Google Scholar] [CrossRef] [PubMed]
  48. Ekkert, O.; Miera, G.G.; Wiegand, T.; Eckert, H.; Schirmer, B.; Petersen, J.L.; Daniliuc, C.G.; Fröhlich, R.; Grimme, S.; Kehr, G. Remarkable coordination behavior of alkyl isocyanides toward unsaturated vicinal frustrated P/B Lewis pairs. Chem. Sci. 2013, 4, 2657–2664. [Google Scholar] [CrossRef]
  49. Appelt, C.; Westenberg, H.; Bertini, F.; Ehlers, A.W.; Slootweg, J.C.; Lammertsma, K.; Uhl, W. Geminal Phosphorus/Aluminum-Based Frustrated Lewis Pairs: C–H versus C≡C Activation and CO2 Fixation. Angew. Chem. Int. Ed. 2011, 50, 3925–3928. [Google Scholar] [CrossRef]
  50. Welch, G.C.; San Juan, R.R.; Masuda, J.D.; Stephan, D.W. Reversible, metal-free hydrogen activation. Science 2006, 314, 1124–1126. [Google Scholar] [CrossRef] [Green Version]
  51. Nori, V.; Pesciaioli, F.; Sinibaldi, A.; Giorgianni, G.; Carlone, A. Boron-Based Lewis Acid Catalysis: Challenges and Perspectives. Catalysts 2022, 12, 5. [Google Scholar] [CrossRef]
  52. Schreiner, P.R.; Schaefer, H.F., III; von Ragué Schleyer, P. The structure and stability of BH5. Does correlation make it a stable molecule? Qualitative changes at high levels of theory. J. Chem. Phys. 1994, 101, 7625–7632. [Google Scholar] [CrossRef]
  53. Rokob, T.A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. Turning frustration into bond activation: A theoretical mechanistic study on heterolytic hydrogen splitting by frustrated Lewis pairs. Angew. Chem. Int. Ed. 2008, 47, 2435–2438. [Google Scholar] [CrossRef] [PubMed]
  54. Jupp, A.R.; Stephan, D.W. New directions for frustrated Lewis pair chemistry. Trends Chem. 2019, 1, 35–48. [Google Scholar] [CrossRef]
  55. Brown, L.C.; Hogg, J.M.; Gilmore, M.; Moura, L.; Imberti, S.; Gärtner, S.; Gunaratne, H.N.; O’Donnell, R.J.; Artioli, N.; Holbrey, J.D. Frustrated Lewis pairs in ionic liquids and molecular solvents–A neutron scattering and NMR study of encounter complexes. Chem. Commun. 2018, 54, 8689–8692. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  56. Bakó, I.; Stirling, A.; Bálint, S.; Pápai, I. Association of frustrated phosphine–borane pairs in toluene: Molecular dynamics simulations. Dalton Trans. 2012, 41, 9023–9025. [Google Scholar] [CrossRef]
  57. Hamza, A.; Stirling, A.; András Rokob, T.; Pápai, I. Mechanism of hydrogen activation by frustrated Lewis pairs: A molecular orbital approach. Int. J. Quant. Chem. 2009, 109, 2416–2425. [Google Scholar] [CrossRef]
  58. Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. The mechanism of dihydrogen activation by frustrated Lewis pairs revisited. Angew. Chem. Int. Ed. 2010, 49, 1402–1405. [Google Scholar] [CrossRef]
  59. Rokob, T.A.; Bako, I.; Stirling, A.; Hamza, A.; Papai, I. Reactivity models of hydrogen activation by frustrated Lewis pairs: Synergistic electron transfers or polarization by electric field? J. Am. Chem. Soc. 2013, 135, 4425–4437. [Google Scholar] [CrossRef]
  60. Skara, G.; De Vleeschouwer, F.; Geerlings, P.; De Proft, F.; Pinter, B. Heterolytic Splitting of Molecular Hydrogen by Frustrated and Classical Lewis Pairs: A Unified Reactivity Concept. Sci. Rep. 2017, 7, 16024. [Google Scholar] [CrossRef]
  61. Rokob, T.A.; Hamza, A.; Pápai, I. Rationalizing the reactivity of frustrated Lewis pairs: Thermodynamics of H2 activation and the role of acid–base properties. J. Am. Chem. Soc. 2009, 131, 10701–10710. [Google Scholar] [CrossRef]
  62. Bordwell, F.G. Equilibrium acidities in dimethyl sulfoxide solution. Acc. Chem. Res. 1988, 21, 456–463. [Google Scholar] [CrossRef]
  63. Beckett, M.A.; Strickland, G.C.; Holland, J.R.; Varma, K.S. A convenient nmr method for the measurement of Lewis acidity at boron centres: Correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity. Polymer 1996, 37, 4629–4631. [Google Scholar] [CrossRef]
  64. Heiden, Z.M.; Lathem, A.P. Establishing the hydride donor abilities of main group hydrides. Organometallics 2015, 34, 1818–1827. [Google Scholar] [CrossRef]
  65. Stephan, D.W.; Erker, G. Frustrated Lewis pair chemistry: Development and perspectives. Angew. Chem. Int. Ed. 2015, 54, 6400–6441. [Google Scholar] [CrossRef]
  66. Stephan, D.W.; Erker, G. Frustrated Lewis pairs: Metal-free hydrogen activation and more. Angew. Chem. Int. Ed. 2010, 49, 46–76. [Google Scholar] [CrossRef]
  67. Chase, P.A.; Stephan, D.W. Lewis acid-catalyzed hydrogenation: B (C6F5) 3-mediated reduction of imines and nitriles with H2. Chem. Commun. 2008, 1701–1703. [Google Scholar] [CrossRef]
  68. Mahdi, T.; Stephan, D.W. Enabling catalytic ketone hydrogenation by frustrated Lewis pairs. J. Am. Chem. Soc. 2014, 136, 15809–15812. [Google Scholar] [CrossRef]
  69. Scott, D.J.; Fuchter, M.J.; Ashley, A.E. Nonmetal catalyzed hydrogenation of carbonyl compounds. J. Am. Chem. Soc. 2014, 136, 15813–15816. [Google Scholar] [CrossRef] [Green Version]
  70. Mahdi, T.; Stephan, D.W. Facile protocol for catalytic frustrated Lewis pair hydrogenation and reductive deoxygenation of ketones and aldehydes. Angew. Chem. Int. Ed. 2015, 54, 8511–8514. [Google Scholar] [CrossRef]
  71. Gao, B.; Feng, X.; Meng, W.; Du, H. Asymmetric hydrogenation of ketones and enones with chiral Lewis base derived frustrated Lewis pairs. Angew. Chem. Int. Ed. 2020, 59, 4498–4504. [Google Scholar] [CrossRef]
  72. Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fröhlich, R.; Erker, G. Metal-Free Catalytic Hydrogenation of Enamines, Imines, and Conjugated Phosphinoalkenylboranes. Angew. Chem. Int. Ed. 2008, 47, 7543–7546. [Google Scholar] [CrossRef] [PubMed]
  73. Wei, S.; Du, H. A highly enantioselective hydrogenation of silyl enol ethers catalyzed by chiral frustrated Lewis pairs. J. Am. Chem. Soc. 2014, 136, 12261–12264. [Google Scholar] [CrossRef] [PubMed]
  74. Greb, L.; Oña-Burgos, P.; Schirmer, B.; Grimme, S.; Stephan, D.W.; Paradies, J. Metal-free Catalytic Olefin Hydrogenation: Low-Temperature H2 Activation by Frustrated Lewis Pairs. Angew. Chem. Int. Ed. 2012, 51, 10164–10168. [Google Scholar] [CrossRef] [PubMed]
  75. Chernichenko, K.; Madarász, Á.; Pápai, I.; Nieger, M.; Leskelä, M.; Repo, T. A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes. Nat. Chem. 2013, 5, 718–723. [Google Scholar] [CrossRef] [Green Version]
  76. Mahdi, T.; Heiden, Z.M.; Grimme, S.; Stephan, D.W. Metal-free aromatic hydrogenation: Aniline to cyclohexyl-amine derivatives. J. Am. Chem. Soc. 2012, 134, 4088–4091. [Google Scholar] [CrossRef] [PubMed]
  77. Segawa, Y.; Stephan, D.W. Metal-free hydrogenation catalysis of polycyclic aromatic hydrocarbons. Chem. Commun. 2012, 48, 11963–11965. [Google Scholar] [CrossRef]
  78. Geier, S.J.; Chase, P.A.; Stephan, D.W. Metal-free reductions of N-heterocycles via Lewis acid catalyzed hydrogenation. Chem. Commun. 2010, 46, 4884–4886. [Google Scholar] [CrossRef]
  79. Tussing, S.; Greb, L.; Tamke, S.; Schirmer, B.; Muhle-Goll, C.; Luy, B.; Paradies, J. Autoinduced catalysis and inverse equilibrium isotope effect in the frustrated Lewis pair catalyzed hydrogenation of imines. Chem.–A Eur. J. 2015, 21, 8056–8059. [Google Scholar] [CrossRef]
  80. Greb, L.; Tussing, S.; Schirmer, B.; Ona-Burgos, P.; Kaupmees, K.; Lokov, M.; Leito, I.; Grimme, S.; Paradies, J. Electronic effects of triarylphosphines in metal-free hydrogen activation: A kinetic and computational study. Chem. Sci. 2013, 4, 2788–2796. [Google Scholar] [CrossRef]
  81. Tussing, S.; Kaupmees, K.; Paradies, J. Structure–Reactivity Relationship in the Frustrated Lewis Pair (FLP)-Catalyzed Hydrogenation of Imines. Chem.–A Eur. J. 2016, 22, 7422–7426. [Google Scholar] [CrossRef]
  82. Courtemanche, M.-A.; Pulis, A.P.; Rochette, É.; Légaré, M.-A.; Stephan, D.W.; Fontaine, F.-G. Intramolecular B/N frustrated Lewis pairs and the hydrogenation of carbon dioxide. Chem. Commun. 2015, 51, 9797–9800. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  83. Liu, L.; Vankova, N.; Heine, T. A kinetic study on the reduction of CO2 by frustrated Lewis pairs: From understanding to rational design. Phys. Chem. Chem. Phys. 2016, 18, 3567–3574. [Google Scholar] [CrossRef] [PubMed]
  84. Ménard, G.; Stephan, D.W. Room temperature reduction of CO2 to methanol by Al-based frustrated Lewis pairs and ammonia borane. J. Am. Chem. Soc. 2010, 132, 1796–1797. [Google Scholar] [CrossRef] [PubMed]
  85. Shinde, G.Y.; Mote, A.S.; Gawande, M.B. Recent Advances of Photocatalytic Hydrogenation of CO2 to Methanol. Catalysts 2022, 12, 94. [Google Scholar] [CrossRef]
  86. Bibi, M.; Ullah, R.; Sadiq, M.; Sadiq, S.; Khan, I.; Saeed, K.; Zia, M.A.; Iqbal, Z.; Ullah, I.; Ahmad, S.; et al. Catalytic Hydrogenation of Carbon Dioxide over Magnetic Nanoparticles: Modification in Fixed-Bed Reactor. Catalysts 2021, 11, 592. [Google Scholar] [CrossRef]
  87. Ghara, M.; Chattaraj, P.K. A computational study on hydrogenation of CO2, catalyzed by a bridged B/N frustrated Lewis pair. Struct. Chem. 2019, 30, 1067–1077. [Google Scholar] [CrossRef]
  88. Jiang, B.; Zhang, Q.; Dang, L. Theoretical studies on bridged frustrated Lewis pair (FLP) mediated H2 activation and CO2 hydrogenation. Org. Chem. Front. 2018, 5, 1905–1915. [Google Scholar] [CrossRef]
  89. Mentoor, K.; Twigge, L.; Hans Niemantsverdriet, J.W.; Swarts, J.C.; Erasmus, E. Silica Nanopowder Supported Frustrated Lewis Pairs for CO2 Capture and Conversion to Formic Acid. Inorg. Chem. 2021, 60, 55–69. [Google Scholar] [CrossRef]
  90. Liu, L.; Lukose, B.; Ensing, B. A free energy landscape of CO2 capture by frustrated Lewis pairs. ACS Catal. 2018, 8, 3376–3381. [Google Scholar] [CrossRef]
  91. Mömming, C.M.; Otten, E.; Kehr, G.; Fröhlich, R.; Grimme, S.; Stephan, D.W.; Erker, G. Reversible metal-free carbon dioxide binding by frustrated Lewis pairs. Angew. Chem. Int. Ed. 2009, 48, 6643–6646. [Google Scholar] [CrossRef] [Green Version]
  92. Roth, D.; Stirn, J.; Stephan, D.W.; Greb, L. Lewis Superacidic Catecholato Phosphonium Ions: Phosphorus–Ligand Cooperative C–H Bond Activation. J. Am. Chem. Soc. 2021, 143, 15845–15851. [Google Scholar] [CrossRef]
  93. Ghara, M.; Giri, S.; Chattaraj, P.K. Cycloaddition Reactions between H2C = CHR (R = H, CN, CH3) and a Cyclic P/B Frustrated Lewis Pair: A DFT Study. J. Phys. Chem. A 2020, 124, 4455–4462. [Google Scholar] [CrossRef] [PubMed]
  94. Cabrera-Trujillo, J.J.; Fernández, I. Aromaticity can enhance the reactivity of P-donor/borole frustrated Lewis pairs. Chem. Commun. 2019, 55, 675–678. [Google Scholar] [CrossRef] [PubMed]
  95. Dong, S.; Daniliuc, C.G.; Kehr, G.; Erker, G. Formation of Active Cyclic Five-membered Frustrated Phosphane/Borane Lewis Pairs and their Cycloaddition Reactions. Chem. Eur. J. 2020, 26, 745. [Google Scholar] [CrossRef] [PubMed]
  96. Ghara, M.; Chattaraj, P.K. Can a Decrease in Anti-aromaticity Increase the Dihydrogen Activation Ability of a Frustrated Phosphorous/Borane Lewis Pair?: A DFT Study. Theor. Chem. Acc. 2020, 139, 183. [Google Scholar] [CrossRef]
  97. Zhuang, D.; Rouf, A.M.; Li, Y.; Dai, C.; Zhu, J. Aromaticity-promoted CO2 Capture by P/N-Based Frustrated Lewis Pairs: A Theoretical Study. Chem. Asian J. 2020, 15, 266–272. [Google Scholar] [CrossRef]
  98. Rouf, A.M.; Huang, Y.; Dong, S.; Zhu, J. Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation. Inorg. Chem. 2021, 60, 5598–5606. [Google Scholar] [CrossRef]
  99. Voss, T.; Chen, C.; Kehr, G.; Nauha, E.; Erker, G.; Stephan, D.W. Cyclizations via frustrated Lewis pairs: Lewis acid induced intramolecular additions of amines to olefins and alkynes. Chem.–A Eur. J. 2010, 16, 3005–3008. [Google Scholar] [CrossRef]
  100. Liu, Q.; Yang, L.; Yao, C.; Geng, J.; Wu, Y.; Hu, X. Controlling the Lewis Acidity and Polymerizing Effectively Prevent Frustrated Lewis Pairs from Deactivation in the Hydrogenation of Terminal Alkynes. Org. Lett. 2021, 23, 3685–3690. [Google Scholar] [CrossRef]
  101. Bhattacharjee, I.; Bhunya, S.; Paul, A. Frustrated Lewis Acid–Base-Pair-Catalyzed Amine-Borane Dehydrogenation. Inorg. Chem. 2020, 59, 1046–1056. [Google Scholar] [CrossRef]
  102. Sitte, N.A.; Bursch, M.; Grimme, S.; Paradies, J. Frustrated Lewis Pair Catalyzed Hydrogenation of Amides: Halides as Active Lewis Base in the Metal-Free Hydrogen Activation. J. Am. Chem. Soc. 2019, 141, 159–162. [Google Scholar] [CrossRef] [PubMed]
  103. Mandal, D.; Gupta, R.; Jaiswal, A.K.; Young, R.D. Frustrated Lewis-pair-meditated selective single fluoride substitution in trifluoromethyl Groups. J. Am. Chem. Soc. 2020, 142, 2572–2578. [Google Scholar] [CrossRef] [PubMed]
  104. Gupta, R.; Mandal, D.; Jaiswal, A.K.; Young, R.D. FLP-Catalyzed Monoselective C–F Functionalization in Polyfluorocarbons at Geminal or Distal Sites. Org. Lett. 2021, 23, 1915–1920. [Google Scholar] [CrossRef]
  105. Ghara, M.; Giri, S.; Das, P.; Chattaraj, P.K. Possible C-F Bond Activation by B(C6F5)3/Lutidine and Al(C6F5)3/Lutidine Frustrated Lewis Pair: An In Silico Study. J. Chem. Sci. 2022, 134, 1–12. [Google Scholar] [CrossRef]
  106. Ison, E.A.; Tubb, J.L. Energy Decomposition Analysis of Lewis Acid/Base Adducts and Frustrated Lewis Pairs: The Use of EOrb/ESteric Ratios as a Reaction Parameter. Inorg. Chem. 2021, 60, 13797–13805. [Google Scholar] [CrossRef] [PubMed]
  107. McGraw, M.L.; Chen, E.Y.X. Lewis Pair Polymerization: Perspective on a Ten-Year Journey. Macromolecules 2020, 53, 6102–6122. [Google Scholar] [CrossRef]
  108. McGraw, M.L.; Clarke, R.W.; Chen, E.Y. Compounded Sequence Control in Polymerization of One-Pot Mixtures of Highly Reactive Acrylates by Differentiating Lewis Pairs. J. Am. Chem. Soc. 2020, 142, 5969–5973. [Google Scholar] [CrossRef]
  109. Su, Y.; Zhao, Y.; Zhang, H.; Luo, Y.; Xu, X. Rare-Earth Aryloxide/Ylide-Functionalized Phosphine Frustrated Lewis Pairs for the Polymerization of 4-Vinylpyridine and Its Derivatives. Macromolecules 2021, 54, 7724–7731. [Google Scholar] [CrossRef]
  110. Yolsal, U.; Horton, T.A.R.; Wang, M.; Shaver, M.P. Cyclic Ether Triggers for Polymeric Frustrated Lewis Pair Gels. J. Am. Chem. Soc. 2021, 143, 12980–12984. [Google Scholar] [CrossRef] [PubMed]
  111. Yolsal, U.; Wang, M.; Royer, J.R.; Shaver, M.P. Rheological Characterization of Polymeric Frustrated Lewis Pair Networks. Macromolecules 2019, 52, 3417–3425. [Google Scholar] [CrossRef]
  112. Shi, Q.; Chen, Z.; Hu, J. Recent Advances in Organocatalyzed Asymmetric Hydrosilylations. Curr. Org. Chem. 2018, 22, 557580. [Google Scholar] [CrossRef]
  113. Arkhipenko, S.; Whiting, A. Broadening the synthetic organic applications of Frustrated Lewis Pairs. Arkivoc 2017, 2017, 26–40. [Google Scholar] [CrossRef]
Catalysts 12 00201 g001 550
Figure 1. (a) Two proposed intermediates for the reactivity of B(C6F5)3 and tBu3P towards H2. (b) Structure of an “encounter complex” formed by a combination of B(C6F5)3 and PtBu3 pair, where the distance between boron (pink) and phosphorous (yellow) centers are given in angstrom.
Figure 1. (a) Two proposed intermediates for the reactivity of B(C6F5)3 and tBu3P towards H2. (b) Structure of an “encounter complex” formed by a combination of B(C6F5)3 and PtBu3 pair, where the distance between boron (pink) and phosphorous (yellow) centers are given in angstrom.
Catalysts 12 00201 g001
Catalysts 12 00201 g002 550
Figure 2. Schematic figure of the electron transfer (ET) and electric field (EF) model-based interpretations of FLP-mediated H2 activation.
Figure 2. Schematic figure of the electron transfer (ET) and electric field (EF) model-based interpretations of FLP-mediated H2 activation.
Catalysts 12 00201 g002
Catalysts 12 00201 g003 550
Figure 3. Partitioning scheme of the reaction of free energy to interpret the thermodynamic requirement of H2 activation by FLP.
Figure 3. Partitioning scheme of the reaction of free energy to interpret the thermodynamic requirement of H2 activation by FLP.
Catalysts 12 00201 g003
Catalysts 12 00201 g004 550
Figure 4. Chemical structure of the bridged FLP systems and the studied reaction.
Figure 4. Chemical structure of the bridged FLP systems and the studied reaction.
Catalysts 12 00201 g004
Catalysts 12 00201 g005 550
Figure 5. Schematic representation of two isomers of boron amidinate FLPs A and B. (Reprinted from ref. [87] with permission from Springer Nature. Copyright © 2022, Springer Science Business Media, LLC).
Figure 5. Schematic representation of two isomers of boron amidinate FLPs A and B. (Reprinted from ref. [87] with permission from Springer Nature. Copyright © 2022, Springer Science Business Media, LLC).
Catalysts 12 00201 g005
Catalysts 12 00201 g006 550
Figure 6. Proposed mechanistic cycles for hydrogenation of CO2 catalyzed by FLPs A and B, where Lewis base of FLP activates (a) H2 and (b) CO2. (Reprinted from ref. [87] with permission from Springer Nature. Copyright © 2022, Springer Science Business Media, LLC).
Figure 6. Proposed mechanistic cycles for hydrogenation of CO2 catalyzed by FLPs A and B, where Lewis base of FLP activates (a) H2 and (b) CO2. (Reprinted from ref. [87] with permission from Springer Nature. Copyright © 2022, Springer Science Business Media, LLC).
Catalysts 12 00201 g006
Catalysts 12 00201 g007 550
Figure 7. Some examples of small molecule activation by FLPs.
Figure 7. Some examples of small molecule activation by FLPs.
Catalysts 12 00201 g007
Catalysts 12 00201 g008 550
Figure 8. Schematic representation of activation of π-systems by FLPs, and optimized geometries of the transition states (TS) for the activations of CO2 (TS1), ethylene (TS2), cyanoethylene (TS3a and TS3b), and propylene (TS4a and TS4b) with the P/B FLP. The bond distances are provided in Å unit. (Adapted from ref. [93] with permission from American Chemical Society. Copyright © 2022, American Chemical Society).
Figure 8. Schematic representation of activation of π-systems by FLPs, and optimized geometries of the transition states (TS) for the activations of CO2 (TS1), ethylene (TS2), cyanoethylene (TS3a and TS3b), and propylene (TS4a and TS4b) with the P/B FLP. The bond distances are provided in Å unit. (Adapted from ref. [93] with permission from American Chemical Society. Copyright © 2022, American Chemical Society).
Catalysts 12 00201 g008
Catalysts 12 00201 g009 550
Figure 9. Schematic diagrams and optimized geometries of the FLPs and the related TSs for H2 activation. Bond distances are provided in Å unit. (Adapted from ref. [96] with permission from Springer Nature. Copyright © 2022, Springer-Verlag GmbH Germany).
Figure 9. Schematic diagrams and optimized geometries of the FLPs and the related TSs for H2 activation. Bond distances are provided in Å unit. (Adapted from ref. [96] with permission from Springer Nature. Copyright © 2022, Springer-Verlag GmbH Germany).
Catalysts 12 00201 g009
Table
Table 1. Structures of the Lewis acid and Lewis base units of some intermolecular FLPs and their roles.
Table 1. Structures of the Lewis acid and Lewis base units of some intermolecular FLPs and their roles.
Serial No.Intermolecular FLPSmall Molecules ActivatedReference Number
DonorAcceptor
1PtBu3 Catalysts 12 00201 i001H2[15]
2 Catalysts 12 00201 i002(Me5C6)3Si+H2[16]
3PtBu3Al(C6F5)3H2, Alkyne[17,35]
4 Catalysts 12 00201 i003 Catalysts 12 00201 i004H2[18]
5 Catalysts 12 00201 i005 Catalysts 12 00201 i006H2[19]
6PtBu3 Catalysts 12 00201 i007H2, CO2, THF, Phenylacetylene[20]
7 Catalysts 12 00201 i008 Catalysts 12 00201 i009H2[21]
8 Catalysts 12 00201 i010B(C6F5)3H2[24]
9 Catalysts 12 00201 i011B(C6F5)3H2[25]
10 Catalysts 12 00201 i012BPh3H2[28]
11PtBu3B(C6F5)3H2, N2O, Ethylene,
Alkyne, 1,3-Diene, B-H bond		[35,36,37,38,39,40]
Table
Table 2. Structures of some intramolecular FLPs and their role.
Table 2. Structures of some intramolecular FLPs and their role.
Serial No.Intramolecular FLPSmall Molecules ActivatedReference Number
1 Catalysts 12 00201 i013H2[14]
2 Catalysts 12 00201 i014H2[41]
3 Catalysts 12 00201 i015CO, CO2, RNC, PhCCH, MeCN[42]
4 Catalysts 12 00201 i016SO2, NO, Olefin, N-Sulfinylamine[43,44,45,46]
5 Catalysts 12 00201 i017CO[47]
6 Catalysts 12 00201 i018RNC[48]
7 Catalysts 12 00201 i019Acetylene, CO2[49]
Table
Table 3. A comparison of energetics for H2 activation by TM and FLPs.
Table 3. A comparison of energetics for H2 activation by TM and FLPs.
H2 Cleavage by Transition MetalH2 Cleavage by FLP
ComplexFree Energy Barrier of H2 ActivationFLPFree Energy Barrier of H2 Activation
Catalysts 12 00201 i02016.8 Kcal/mol at SMD(Water)-M06/6-311++G(2df,p)//M06/6-31+G(d,p) level Catalysts 12 00201 i02121.7 Kcal/mol at SMD(Benzene)-ωB97XD/6-31++G(d,p) level
Catalysts 12 00201 i02232.3 Kcal/mol at SMD(Methanol)-M06/6-31++G(d,p) level Catalysts 12 00201 i02328.9 Kcal/mol at PCM(toluene)-M06-2X/def2-TZVP//M06-2X/def2-SVP level
Catalysts 12 00201 i0245.9 Kcal/mol at SMD(Water)-M06/6-31++G(d,p) level Catalysts 12 00201 i02520.7 Kcal/mol at PCM(CH2Cl2)-B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Pal, R.; Ghara, M.; Chattaraj, P.K. Activation of Small Molecules and Hydrogenation of CO2 Catalyzed by Frustrated Lewis Pairs. Catalysts 2022, 12, 201. https://doi.org/10.3390/catal12020201

AMA Style

Pal R, Ghara M, Chattaraj PK. Activation of Small Molecules and Hydrogenation of CO2 Catalyzed by Frustrated Lewis Pairs. Catalysts. 2022; 12(2):201. https://doi.org/10.3390/catal12020201

Chicago/Turabian Style

Pal, Ranita, Manas Ghara, and Pratim Kumar Chattaraj. 2022. "Activation of Small Molecules and Hydrogenation of CO2 Catalyzed by Frustrated Lewis Pairs" Catalysts 12, no. 2: 201. https://doi.org/10.3390/catal12020201

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop