Activation of Small Molecules and Hydrogenation of CO 2 Catalyzed by Frustrated Lewis Pairs

: The chemistry of frustrated Lewis pair (FLP) is widely explored in the activation of small molecules, the hydrogenation of CO 2 , 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 CO 2 . The mechanistic insight into H 2 activation is thoroughly discussed, which may provide a guideline to design more efﬁcient FLP for H 2 activation. FLPs can activate other small molecules like, CO, NO, CO 2 , SO 2 , N 2 O, alkenes, alkynes, etc. by cooperative action of the Lewis centers of FLPs, as revealed by several computational analyses. The activation barrier of H 2 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 H 2 activation.


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, H 2 , C 2 H 4 , NH 3 , etc. under normal conditions [6].Some examples include the use of singlet carbene (R 2 C:) [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 H 2 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 H 2 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 H 2 .
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 Tables 1 and 2. A comparison of energetics for H 2 activation by TM and FLPs is provided in Table 3 to better understand the effectiveness of FLPs in metal-free H 2 activation.FLPs act as good catalyzing agents in reactions involving activation of small molecules such as, CO, CO 2 , N 2 O, NO, SO 2 , alkenes, alkynes, catalytic hydrogenation, and so on [30][31][32][33][34], all of which are discussed later in this article.H 2 activation by bridged P/B FLPs, and the role of boronligand cooperation (BLC) in activating the same, is explored.Simultaneous activation of H 2 and CO 2 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 H 2 activation.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 Tables 1 and 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.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 Tables 1 and 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.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 Tables 1 and 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.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 Tables 1 and 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.

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 η 2 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

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 η 2 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

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 η 2 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

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 η 2 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  [36].
There are a few mechanistic paths of H 2 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 H 2 occurring as a result of a side-on interaction between H 2 and the LA (B(C 6 F 5 ) 3 ) forming the adduct, H 2 . . .B(C 6 F 5 ) 3 [36,51], from which a phosphine LB (PtBu 3 ) abstracts a proton (mechanism shown in Figure 1a).Unfortunately, no such adduct is detected experimentally even at higher H 2 pressure (4 atm).Although some computational reports showed the existence of BH 5 , where H 2 is weakly bound to BH 3 via an η 2 binding mode [52].Alternatively, H 2 may initially interact with the LB followed by the hydride abstraction by the LA.Again, no such H 2 . . .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 H 2 with B(C 6 F 5 ) 3 and PtBu 3 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 H 2 .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 H 2 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 tBu 3 P donates the lone pair of electron density into the anti-bonding orbital of H 2 .Conversely, the Lewis acidic B center of B(C 6 F 5 ) 3 accepts electron density into its empty p-orbital from the bonding orbital of H 2 .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 H 2 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 H 2 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 H 2 molecule becomes polarized by the EF and undergoes a heterolytic cleavage.Therefore, while the entrance of H 2 into the FLP cavity is an energy-demanding process, its splitting happens in a barrierless way.This mechanism of H 2 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 H 2 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•••H 2 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•••H 2 and LA•••H 2 distances.End-on LA•••H 2 interaction is predominant in this case.
Moreover, the kinetics and thermodynamic aspects of H 2 activation are also investigated and reported in the literature.In particular, a theoretical study performed by Rokob et al. [61] demonstrates the H 2 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 H 2 , 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 (∆G sep ) is low in the first step.The second step, i.e., the heterolytic cleavage of H 2 into proton and hydride ions, is endergonic and associated with a free energy of ∆G HH ≈ +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 ∆G HH is the only endergonic contribution.In the cases where the dative bonds are present, however, an additional endergonic term (∆G prep ) 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 ∆G pa and ∆G ha , respectively.The final step involves the ion pair formation with a binding free energy (∆G stab ), 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 p Ka 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 boronligand 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.In a study performed by our group [41], we have explored the influence of boronligand cooperation (BLC) in H 2 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 -NMe 2 moiety at the para position of 1, 2, and 3, respectively.
All the H 2 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 H 2 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 CO 2 is explored with these hydrogenated FLPs, which is discussed later in the article.
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.

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 H 2 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 Mes 2 PC 6 F 4 B(C 6 F 5 ) 2 [65,66] is demonstrated by Stephan et al.The same occurs for sterically hindered imines is shown using just an LA, B(C 6 F 5 ) 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(C 6 H 2 Me 3 ) 3 .In 2014, the groups of Stephan [68] and Ashley [69] separately worked on the hydrogenation of carbonyl compounds to alcohol where B(C 6 F 5 ) 3 and the ether solvent (1,4-dioxane) act as the FLP catalyst.It first splits the H 2 molecule and subsequently reduces the carbonyl group.The hydrogenation of aldehydes and ketones to alcohol by B(C 6 F 5 ) 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 H 2 activation) is strong enough to protonate the less reactive olefin [74].The H − abstraction then occurs from the [HB(C 6 F 5 ) 3 ] − in the following step.The use of FLPs in a plethora of hydrogenation reactions followed in subsequent years.Alkene hydrogenation by ether/B(C 6 F 5 ) 3 FLP occurs via the generation of [Et 2 O . . .H . . .OEt 2 ] + and [HB(C 6 F 5 ) 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 H 2 /B(C 6 F 5 ) 3 pair [76], and anthracene, tetracene, and tetraphene, using Ph 2 PC 6 F 5 /B(C 6 F 5 ) 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(C 6 F 5 ) 3 catalyst [78].
The aforementioned hydrogenation reactions occur by an initial H 2 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(C 6 F 5 ) 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 CO 2 to HCOOH [82].

Catalytic Hydrogenation of CO 2
CO 2, one of the major greenhouse gases, contributes to the rising global temperature and poses a serious threat to our earth's atmosphere.Conversion of CO 2 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 CO 2 is a challenging process owing to its high thermodynamic and kinetic stability.Reduction of CO 2 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 CO 2, by using the hydrogenated FLPs, is a good metal-free catalytic alternative.Ashley et al. [5], in 2009, described the first homogenous hydrogenation of CO 2 to CH 3 OH by initially undergoing heterolytic H 2 activation followed by insertion of CO 2 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 CO 2 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 CO 2 to CH 3 OH [85] and CO 2 hydrogenation over magnetic nanoparticles [86] are reported.Concerted H + and H − transfer to the substrate is also a possibility as observed in the hydrogenation of CO2 to HCOOH [82].

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/Albased 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 Recently, Jiang et al. [88] reported a bridged B/P FLP-mediated CO 2 hydrogenation that can follow two mechanistic paths.First being a concerted mechanism of heterolytic cleavage of H 2 molecule by the FLP and CO 2 hydrogenation, and second where CO 2 activation by the FLP is followed by H 2 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 CO 2 to produce HCOOH, which reveals that the activation of H 2 and CO 2 occurs simultaneously.It can happen in two possible ways; one where the LB center activates H 2 and LA center activates CO 2 , 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 H 2 and LA activating CO 2 ) has the electron density transferring as HOMO (FLP) →LUMO (H2) , HOMO (H2) →LUMO (CO2) , and from several occupied MOs of CO 2 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 CO 2 and LA activating H 2 ), 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 CO 2 reduction to HCOOH is also reported [89] by silica nanopowder supported FLP where the said system forms an FLP-CO 2 adduct on the silica surface.The activation of H 2 is followed by the conversion of the captured CO 2 to HCOOH.Again, substitution of C 6 H 5 groups on B and P centers increases the effectiveness of the whole process compared to C 6 F 5 (too electron-deficient) and C 6 H 11 (too electron-rich) substituents.
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 Figures 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 electrondeficient) 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.

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 In a recent article [90], the CO 2 capture by tBu 3 P/B(C 6 F 5 ) 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 CO 2 activation to make the reaction thermodynamically and kinetically feasible.

Activation of Other Small Molecules
Apart from H 2 activation, FLPs have proven to be useful in the activation of several other small molecules, such as CO, CO 2 , N 2 O, NO, SO 2 , 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, CO 2 reacts with FLP to produce novel carbonic acid derivatives [91].Reaction with SO 2 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 (Mes 2 PCH 2 CH 2 B(C 6 F 5 ) 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].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].(Adapted from ref. [93] with permission from American Chemical Society.Copyright © 2022, American Chemical Society).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].(Adapted from ref. [93] with permission from American Chemical Society.Copyright © 2022, American Chemical Society).A DFT-based study performed by Trujillo et al. [94] shows the effect of aromaticity in the activation power of FLPs.Replacing a BPh 2 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 fivemembered 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 H 2 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 C 4 B 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 -C 6 F 5 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 CO 2 activation.Rouf et al. [98] demonstrated an increase in the reaction feasibility of dinitrogen activation by FLPs due to an increase in aromaticity.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 fivemembered 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 antiaromatic 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 Nsulfinyltolylamine (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 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-BPh 3 ) 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 PPh 2 unit.FLP mediated amide hydrogenation [102], which uses B(2,6-F 2 -C 6 H 3 ) 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(C 6 F 5 ) 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(C 6 F 5 ) 3 instead of B(C 6 F 5 ) 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(C 6 F 5 ) 3 or Al(C 6 F 5 ) 3 acids [105].It reveals that the C-F bond activation mediated by Al(C 6 F 5 ) 3 /lutidine pair is more favorable than that mediated by B(C 6 F 5 ) 3 /lutidine pair.Hence, the Al(C 6 F 5 ) 3 acid is superior to the B(C 6 F 5 ) 3 acid for such unusual bond activation.Last year, Ison and Tubb [106], made a simple yet important calculation for M(C 6 F 5 ) 3 (M = B, Ga, Al) Lewis acids.They evaluated the ratio between orbital interaction energy (E Orb ) to the electrostatic and exchange repulsion energy (E Steric ) in an energy decomposition analysis (EDA).A correlation between this ratio and the reactivity drawn reveals that only the B(C 6 F 5 ) 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.

Summary
The high thermochemical and kinetic stability of the greenhouse gas, CO 2 , 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 CO 2 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, CO 2 , etc. Apart from the H 2 activation, FLPs may activate and react with other small molecules as well; examples include CO 2 , N 2 O, CO, NO, SO 2 , C 2 H 4 , C 2 H 2, 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 H 2 reactivity of FLP, in particular, continues to be useful in both homo-and heterogenous catalysis.Other than CO 2 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 CO 2 can occur either

Catalysts 2022 ,
12, 201    weakening of the H-H bond, which ultimately breaks to form P-H and B-H bon mechanism of H2 activation by FLP is termed as the ET model[57] (Figure2).

Figure 1 .
Figure 1.(a) Two proposed intermediates for the reactivity of B(C6F5)3 and tBu3P toward Structure of an "encounter complex" formed by a combination of B(C6F5)3 and PtBu3 pair, w distance between boron (pink) and phosphorous (yellow) centers are given in angstrom.

Figure 1 .
Figure 1.(a) Two proposed intermediates for the reactivity of B(C 6 F 5 ) 3 and tBu 3 P towards H 2. (b) Structure of an "encounter complex" formed by a combination of B(C 6 F 5 ) 3 and PtBu 3 pair, where the distance between boron (pink) and phosphorous (yellow) centers are given in angstrom.

Figure 1 .
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 2 .
Figure 2. Schematic figure of the electron transfer (ET) and electric field (EF) model-based interpretations of FLP-mediated H2 activation.

Figure 2 .
Figure 2. Schematic figure of the electron transfer (ET) and electric field (EF) model-based interpretations of FLP-mediated H 2 activation.

Catalysts 2022, 12 , 201 8 of 19 Figure 3 .
Figure 3. Partitioning scheme of the reaction of free energy to interpret the thermodynamic requirement of H2 activation by FLP.

Figure 3 .
Figure 3. Partitioning scheme of the reaction of free energy to interpret the thermodynamic requirement of H 2 activation by FLP.

Figure 4 .
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.

Figure 4 .
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.

Figure 5 .
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 .
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 6 .
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 .
Figure 6.Proposed mechanistic cycles for hydrogenation of CO 2 catalyzed by FLPs A and B, where Lewis base of FLP activates (a) H 2 and (b) CO 2. (Reprinted from ref. [87] with permission from Springer Nature.Copyright © 2022, Springer Science Business Media, LLC).

Figure 7 .
Figure 7.Some examples of small molecule activation by FLPs.

Figure 7 .
Figure 7.Some examples of small molecule activation by FLPs.

Figure 7 .
Figure 7.Some examples of small molecule activation by FLPs.

Figure 8 .
Figure 8. Schematic representation of activation of π-systems by FLPs, and optimized geometries of the transition states (TS) for the activations of CO 2 (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 9 .
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 .
Figure 9. Schematic diagrams and optimized geometries of the FLPs and the related TSs for H 2 activation.Bond distances are provided in Å unit.(Adapted from ref. [96] with permission from Springer Nature.Copyright © 2022, Springer-Verlag GmbH Germany).

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.

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.

Table 1 .
Structures of the Lewis acid and Lewis base units of some intermolecular FLPs and their roles.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

Table 2 .
Structures of some intramolecular FLPs and their role.

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.

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.

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.

Table 3 .
A comparison of energetics for H 2 activation by TM and FLPs.

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.