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

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


Background
Since the beginning of the industrial revolution, human activity has raised the concentration of carbon dioxide (CO 2 ), amongst other important greenhouse gases, in the environment by over 50%.CO 2 gas absorbs and emits radiant energy at infrared wavelengths that causes an increase in atmospheric temperature, thus global warming and has become a main driver of climate change. 1 Society has made some progress in nding low-carbon emitting alternatives, for example in sources of energy, and now a scattered dip in CO 2 level has been observed.Minimising CO 2 emissions by at least 50% to limit the increase in the global average temperature by 2 °C by 2050 has been set as a global target. 2This will require a rapid exploitation of new energy technologies with a low-carbon or zero-carbon energy sources to restore our ecosystem.As one single technology is not expected to solve this problem, global warming alerts have drawn urgent attention to control the expansion of CO 2 concentrations in the atmosphere through the framework of carbon capture, storage, and utilisation (CCSU). 3The important challenge remains not only in carbon dioxide capture and storage but also to utilise it for the creation of value-added carbon products. 4CCSU processes add value to the conversion of CO 2 into fuels and chemicals, and can compensate the cost of capturing CO 2 .This approach has generated many new directions in various branches of science and technologies including chemical, biological and material applications. 5Extensive work has been carried out in the past few decades for CO 2 capture and utilisation (CO 2 -CU) using various chemical processes for the reduction of CO 2 into products such as formic acid and methanol, or light hydrocarbons such as methane. 6Metal and non-metal derived reagents and catalysts in homogeneous and heterogeneous systems have been explored including promising mediums such as zeolites, metal-organic frameworks (MOFs), covalent organic frameworks (COFs), nanomaterials, as well as electrochemical, photochemical and thermal processes.
The activation and chemical conversion of CO 2 requires high energy due to its high thermodynamic stability (DG f = −396 kJ mol −1 ). 7Entropy is one important factor that limits CO 2 transformations, even for some reactions where DH°< 0, DG°is positive.Conversion of CO 2 can take place at room temperature or at lower temperatures but in such transformations the carbon atom retains its oxidation state of +4 and promotes the formation of carbamates, carbonates, urea and its derivatives, polycarbonates, and polyethers, where OH − , H 2 O, amines, carbanions, olens, alkynes, and dienes have been reacted as reagents with CO 2 .Reactions that produce carboxylates from CO 2 are thermodynamically favourable.Carbon product formation such as methanol, carbon monoxide, formaldehyde, methane, and hydrocarbons from CO 2 require higher energy, and thus kinetic control to steer away from the thermodynamically favoured products.Kinetically, to obtain CO 2 reduced products with a change of oxidation state in the carbon atom requires more energy.To achieve the reduction of CO 2 , the reagent should bend CO 2 to overcome the rst energy barrier and to achieve subsequent reduction steps.Preorganised reducing agents kinetically favour the challenging reduction of CO 2 beyond carbonate or formate.
To overcome the energy barrier, external sources of energy (such as electrochemical, photochemical or thermal energy) are typically required.If value-added products are obtained at higher cost compared to the cost of CO 2 capturing and natural fuels, then the process would not be economical and could not be applied industrially. 8lthough CO 2 has been reduced to obtain chemicals and fuels, the current use of CO 2 in chemical synthesis is limited owing to the high thermodynamic stability of CO 2 that has to be overcome. 9To control the energy barrier in reduction processes of CO 2 , metals are being used that are oen rare and high cost materials, indeed transition metals are being explored making the process efficient and cost effective.The conversion of CO 2 to value-added products is highly desirable, yet the inert and highly energetically stable nature of this small molecule makes this a challenging task. 10CO 2 is a linear and an apolar molecule (dipole moment, m = 0), despite its two polar C]O bonds and is ambiphilic in nature.The length of the C]O bond in CO 2 is 116.3 pm, shorter than the approximate 140 pm bond length of a typical single C-O bond, and shorter than most other C]O double bonds, such as carbonyls. 11It offers two reaction sites, an electrophilic site at the carbon (Lewis acidic centre) due to the low-lying empty antibonding p* orbital (LUMO, lowest unoccupied molecular orbital), and two nucleophilic sites at the oxygen atoms (Lewis basic character) due to an available pair of valence electrons (HOMO, highest occupied molecular orbital) (Fig. 1).Carbon dioxide has an electron affinity (E ea ) of about −0.6 eV and a rst ionisation potential (IP) of about +13.8 eV which makes it a better electron acceptor than electron donor.Overall, a high energy of about 750 kJ mol −1 is required to break the C]O bond.Upon activation, the molecule will distort from its linear sp-hybridised geometry to a sp 2 -hybridised carbon centre with concurrent elongation of the C]O bond and a change in its molecular energy. 12lassical activation of CO 2 by nucleophilic attack at the carbon atom can be achieved using bases, 13 transition metals, 14 or by one electron reductions 15 to ultimately generate acetates, carbamates, ureas, bicarbonates, oxalates, formates, or carbon monoxide, amongst other products (Fig. 2).Further advances in research have been able to achieve value added carbon products by reducing CO 2 to products such as methanol, methane, or higher carbon chains. 16rustrated Lewis pairs (FLPs) 17 are mixtures of a Lewis acid and a Lewis base, that, because of steric hindrance, cannot combine to form a classical adduct.FLPs can perform efficient chemical transformations without losing the individual properties of the FLP system. 18This feature also enables the activation of small molecules including CO 2 and is now well-explored in the literature in several reviews. 19Herein, we cover all major developments made using p-block elements and transition metals to activate CO 2 with FLP systems with a particular emphasis on more recent reports.In this review we will cover stoichiometric as well as catalytic processes including theoretical efforts to understand the mechanism of CO 2 activation and reduction using FLPs.

Frustrated Lewis pairs (FLPs)
In the classic model of Gilbert Lewis (Fig. 3a), a Lewis base with an electron pair in the HOMO donates electron density to the LUMO of the Lewis acid by forming a dative bond. 20This process provides a HOMO of lower energy with a stabilised donor acceptor adduct and quenches the reactivity of both, the Lewis acid and base.A deviation to the classical model was observed aer the augmented work reported by Brown,21 in which no adduct formation occurred between BMe 3 and 2,6lutidine, and later Wittig, 22 Tochtermann, 23 Piers 24 and Oestreich. 25Stephan and co-workers coined the chemical term "frustrated Lewis pair" (FLP) that exists with unquenched acidity and basicity in a combination of a sterically hindered Lewis acid and Lewis base (Fig. 3b). 26This inhibition of adduct formation allows for the HOMO of the Lewis base and the LUMO of the Lewis acid to effect non-classical reactivity.The pioneering work of splitting dihydrogen heterolytically with the FLP tBu 3 P/B(C 6 F 5 ) 3 demonstrated that the unquenched reactivity of FLPs could be applied to the activation of small molecules. 27Since then, various FLP systems have been investigated to activate a variety of small molecules.Two types of FLP systems are typically considered: intermolecular or intramolecular (Scheme 1).Intermolecular FLPs are systems where the Lewis acid and Lewis base are two individual molecules that interact through secondary London dispersion interactions to bring the Lewis acid and base together where small molecules insert into the cavity of the FLP such as the combination of tBu 3 P and B(C 6 F 5 ) 3 .In intramolecular FLP systems, the Lewis acid and Lewis base are combined in one molecule by a covalent linker.An example of an intramolecular FLP system is the phoshinoborane 1 shown in Scheme 1, where the Lewis acidic boron centre and the Lewis basic phosphorus centre are separated by an aryl ring.These molecules are also able to heterolytically cleave the small molecule A-B (Scheme 1).When the FLPs heterolytically cleave the bonds in small molecules, the Lewis base donates its electron pair to the electron decient fragment of the small molecule and the Lewis acid accepts an electron pair from the HOMO of the small molecule.This results in bond formation and an ionic/zwitterionic product.It has also been found that certain combinations of Lewis acids and bases in FLPs can lead to a transfer of one electron from the Lewis basic donor to a Lewis acidic acceptor generating a reactive frustrated radical pair (FRP).This FRP can react in a homolytic way with small molecules (Scheme 2). 28

Mechanistic aspects of FLP CO 2 reduction
Amongst the small molecules activated by FLPs, CO 2 has been well-studied owing to the importance of discovering new CCSU processes.Two mechanistic pathways are proposed for the capture and activation of CO 2 by FLPs thus far (Scheme 3).One is a concerted mechanism and the other is a two-step process.Two computational models have been explored in the tBu 3 P/ B(C 6 F 5 ) 3 FLP system to study the mode of CO 2 activation.In the concerted mechanism, the reactants (FLP and free CO 2 ) and the CO 2 -FLP adduct is formed by a single transition state (TS) in which the LB-C and LA-O bonds are formed simultaneously. 29onversely, for the two-step process, when the CO 2 moves closer to the FLP system, the P-C bond is formed rst, followed by the formation of the B-O bond to give the nal CO 2 -FLP adduct.
It is well-documented that the solvent is important to stabilise the nal zwitterionic products. 30Liu and co-worker 31 led a mechanistic study in the solid-state utilising density functional theory (DFT) simulations, in which they analysed the separate roles of the Lewis acid and base without the presence of a solvent.The authors found that the reaction proceeds in a two-step process where CO 2 initially enters the cavity of the tBu 3 P/B(C 6 F 5 ) 3 FLP.The carbon atom of CO 2 then interacts with the phosphorus atom and an oxygen atom interacts with boron leading to a reduction in the O-C-O angle of CO 2 to 167.8°.This means that the CO 2 species is bent although there are no chemical bonds formed.They believe that this is due to a weak interaction between CO 2 and the FLP, where CO 2 interacts with crystal elds in the solid state created by the FLP pair.In the solution state, the crystal elds would be replaced by solvent interactions.In studying the separate roles of the Lewis acid and base, the authors suggest that the combination of a strong Lewis acid and a weak Lewis base should be selected to make the CO 2 activation thermodynamically feasible.This is due to the formation of the B-O bond being strongly exergonic while the formation of the P-C bond was deduced to be endergonic.
Other sources of energy such as light and/or electric current have been employed in other elds however, the most used source of energy for the activation of CO 2 with FLP systems is heat and pressure of CO 2 .
However, in the FLP adduct of CO 2 , the CO 2 molecule is bent, and a one-electron transfer could facilitate the reduction process.In a homogeneous system, the rst electrochemical study was performed on the FLP-CO 2 adduct for tBu 3 P-CO 2 -B(C 6 F 5 ) 3 (Scheme 4). 32Electrochemically, when an electron is added to tBu 3 P-CO 2 -B(C 6 F 5 ) 3 , a change in the bond lengths was observed.The carbon oxygen C]O bond length increased by 0.05 Å in and the C-O bond length decreased by 0.03 Å.The B-O and P-C bonds both decreased in length by 0.06 Å and 0.01 Å, respectively.Overall, it was observed that addition of electrons to the CO 2 adduct tBu 3 P-CO 2 -B(C 6 F 5 ) 3 rst generated intermediate [tBu 3 P-CO 2 -B(C 6 F 5 ) 3 ] − , then reduced CO 2 to CO also generating tBu 3 P, and [(HO)B(C 6 F 5 ) 3 ] − .In this system, the intermediate [tBu 3 P-CO 2 -B(C 6 F 5 ) 3 ] − can also react with the solvent THF causing dimerisation.
To obtain a controlled reduction of CO 2 , FLPs in the solidstate have been explored based on the phenomenon of adsorption, activation, and evolution pathways of CO 2 .For example, Yan and co-workers created a stable FLP system for the activation of carbon dioxide. 33Their system involves a composite material of zinc and tin having different electronegativities.The Lewis pairs rst capture and stabilise protons and then selectively activate CO 2 .Here the zinc oxide surface captures protons and acts as a Lewis base while the tin acts as Lewis acid.The two-electron reduction with two protons start the reaction for CO 2 activation and nally resulted in the formation of formic acid.
In this review we will cover FLP mediated activation and reduction of CO 2 that has been explored to achieve valueadded carbon products.This will include, several inter-and intramolecular FLPs systems which have been designed and developed utilising both p-and d-block elements acting as a Lewis acid in combinations with p-and d-block elements acting as a Lewis base.Each section and sub-sections will detail the different systems developed with stoichiometric and catalytic quantities of FLPs, and will be ordered by the periodic group of the Lewis acid to give structure to this review.The mechanistic and computational insights will be discussed where relevant.

Group 13 Lewis acids
Borane/phosphine FLPs for CO 2 activation Boron is by far the most explored Lewis acidic element in FLPs for CO 2 activation.Different Lewis base partners such as phosphrous, nitrogen, carbon as a carbene and metals have been explored in combination with the boron Lewis acid.
Many of the rst FLP systems for CO 2 activation involved phosphorus as the Lewis basic component.Early FLPs utilised in CO 2 activation comprised of a phosphine and borane that could reversibly bind and release CO 2 including the intermolecular FLP tBu 3 P/B(C 6 F 5 ) 3 and 2 (Scheme 5).
At the time, these systems offered rare examples of metalfree CO 2 sequestration. 34Theoretical investigations show the mechanism proceeding by simultaneous formation of P-C and O-B bonds from thermochemical computed data (B97-D/ TZVPP', B2PLYP-D/TZVPP', and B2PLYP-D/QZVP(-g, -f) levels of theory).tBu 3 P reacts with B(C 6 F 5 ) 3 at room temperature and under 1 bar of CO 2 forms the desired stable product tBu 3 P-CO 2 -B(C 6 F 5 ) 3 , which upon heating at 80 °C under vacuum releases the CO 2 molecule and regenerates the starting FLP mixture.Calculations for the formation of tBu 3 P-CO 2 -B(C 6 F 5 ) 3 show that the overall reaction is exothermic.Privalov and co-workers calculated several energy pathways for CO 2 activation. 35Aer these rst reports of non-metal based inter-and intramolecular FLP-mediated reversible CO 2 activation, the scientic community explored a range of new FLPs for CO 2 activation, as shown in Fig. 4. Several other phosphine bases and boron acids have been used in the intermolecular system including iPr 3 P, XPhos and Mes 2 EtP, and B(p-C 6 F 4 H) 3 , and B(R)(C 6 F 4 H) 2 , (R = hexyl, Cl, cyclohexyl, norbornyl, Ph). 36,37More complex boranes bearing functionalised substituents with cyclic structures were also tested with tBu 3 P to trap CO 2 generating adducts 3 and 4. 38 Interestingly, when (Me 3 Si) 3 P was utilised as a Lewis base instead of tBu 3 P, with B(p-C 6 F 4 H) 3 , silyl migration was observed in the nal adducts 5 and 6 (Fig. 4 and Scheme 6).tBu 3 P and (Me 3 Si) 3 P with CO 2 in pentane at room temperature initially yields the expected adduct (Me 3 Si) 3 P-CO 2 -B(p-C 6 F 4 H) 3 .Subsequently, silyl migration from phosphorus to oxygen forms a stable compound which can be better represented as the zwitterionic compound 5.The same starting Lewis acid and base can also react with two equivalents of CO 2 in dichloromethane (CH 2 Cl 2 ) at room temperature for 24 h, providing silyl migrated product 6.Compound 6 can also be obtained from 5, when 5 is treated with CO 2 in CD 2 Cl 2 at room temperature for 24 h (Scheme 6). 39Kemp and co-workers on the other hand, investigated the phosphine base bis(di-i-propylphosphino) amine with B(C 6 F 5 ) 3 which formed the expected 1 : 1 adduct 7. The crystal structure of 7 shows that H-isomerisation took place with a migration of the proton from nitrogen to phosphorus (Scheme 7). 40tephan and co-workers have expanded the borane scope to explore the reactivity of bis-boranes to trap CO 2 with tBu 3 P.It was also found that, 1,1-bis-(C 6 F 5 ) 2 BOB(C 6 F 5 ) 2 binds with CO 2 in a monodentate manner generating 8, whilst bis-boranes of type (R 2 B) 2 C]CMe 2 , where R = Cl or C 6 F 5, provide a bidentate chelation of CO 2 to obtain a unique type of heterocyclic compounds 9. 41 The chelation of CO 2 by the two B-centres in In intramolecular systems, when the Lewis base and acid are sufficiently aligned in a geminal fashion, an increase in reactivity is observed as seen in the formation of adduct 10. 42 Another intramolecular FLP with a norbornane structure with a vicinal designed FLP was utilised to trap CO 2 to obtain adduct 11. 43 Similarly to the vicinal FLP in adduct 11, an FLP based on cyclopentane with trans-1,2-substituents was explored to form 12. 44 Other intramolecular B/P FLP systems include an active FLP borylated tetrahydrophosphole which yielded adduct 13, 45 and a cyclic six membered FLP with 1,4-phosphane/borane substituents which undergoes an addition reaction with CO 2 to form adduct 14.Erker and co-workers observed that heating 14 in n-heptane at 80 °C for 15 min under CO 2 converts 14 to its cyclotetrameric macrocyclic oligomer 15.Tetramer 15 is unstable in solution, even in a CO 2 atmosphere it slowly converts back to the monomer 14 (Fig. 5). 46zynkiewicz and co-workers reported the phosphinoboration and diphosphination of CO 2 .In 2019, they published the rst report of catalytic (with respect to the borane) diphosphination of CO 2 with a diphosphane/boron FLP.CO 2 was inserted into the relatively weak P-P bond (Scheme 8). 47Furthermore in 2019, they reported the use of diaminophosphinoboranes to Scheme 7 H-isomerisation with a migration of the proton from nitrogen to phosphorus.phosphinoborate CO 2 ; though not sterically frustrated, this compound still exhibits FLP-like reactivity (Scheme 8, bottom). 48More recently the same group built on this work, reporting the reaction of CO 2 (among several other small molecules) with a diphosphinoborane B(PtBu 2 ) 2 Ph to yield a diphospha-urea and a bicyclic diboroxane 16 (Scheme 9).The reaction proceeds by CO 2 insertion into a single B-P bond, elimination of (tBu 2 P) 2 C]O to give phenyl oxoborane PhBO.Reaction of this species with a further equivalent of the parent diphosphinoborane and 2 equivalents of CO 2 gives the product 16.While stable under N 2 , the product decomposes with loss of CO 2 and diphospha-urea to give triphenylboroxine (PhBO) 3 . 49essete and co-workers published a computational study evaluating a number of intramolecular phosphine/borane catalysts for CO 2 reduction.One nding was that, though electron-withdrawing substituents on borane like uorine stabilise the CO 2 -FLP adduct, they also destabilise the transition state (TS), increasing activation energy.Fluorination of the substituents of the phosphorus reduces its basicity and so destabilises both the transition state and the adduct formed.This highlights the importance of tuning the Lewis acidic and basic sites of FLPs to achieve stabilised transition states, but that are also Lewis acidic, or Lewis basic enough centres to bind with CO 2. 50 Jian and co-workers reported in 2017 that geminal vinylidene-bridged phosphorus/boron Lewis pairs could react with CO 2 to give a phosphinodiborated product 17, as shown in Scheme 10.Interestingly, this geminal P/B is supported with an sp 2 carbon, which is different from previous reports of geminal FLPs. 51ollowing activation of CO 2 , the subsequent transformations have been investigated initially stoichiometrically.Stephan and co-workers reported that the bis-borane, 1,2-C 6 H 4 (BCl 2 ) 2 , forms an adduct with tBu 3 P and also shows FLP reactivity with CO 2 to form the FLP-CO 2 zwitterionic compound 18 (Scheme 11).Compound 18 is remarkably more stable, with respect to the loss of CO 2 , and no decomposition was observed even on heating to 80 °C for 24 h compared to the CO 2 adducts obtained from FLPs tBu 3 P/B(C 6 F 5 ) 3 (loss of CO 2 at 80 °C), Mes 2 PCH 2 -CH 2 B(C 6 F 5 ) 2 (loss of CO 2 at −20 °C), and bis-boranes Me 2 C] C(BR 2 ) 2 where R]Cl, C 6 F 5 with tBu 3 P (loss of CO 2 at 15 °C).The chlorine atom in 18 bridges between the boron centres which enhances the Lewis acidity of the boron bound with the oxygen atom of CO 2 and results in a stronger B-O bond making the adduct more thermally stable, than other discussed examples.Hence, the strength of the bond between the Lewis acid and the  oxygen atom of CO 2 plays a critical role in establishing reversibility, this can be induced by the addition of electronwithdrawing groups, such as Cl in 18.The species 18 was reduced by Me 2 NHBH 3 followed by quenching with deuterated water (D 2 O) to obtain deuterated methanol (MeOD) as the nal product.In another way, 18 was also reduced by [C 5 H 6 Me 4 NH 2 ]/ [HB(C 6 F 5 ) 2 (C 7 H 11 )] ( 19) and quenched with D 2 O again yielding H 3 COD (Scheme 11).Here, two Lewis acidic boron sites are available, and bridging of a chlorine atom between the two stabilises the zwitterionic adduct. 52here remain two major issues with tBu 3 P/1,2-C 6 H 4 (BCl 2 ) 2 that pose limitations for a catalytic cycle.The rst issue is that H 2 cannot be activated, so H 2 surrogates such as Me 2 NHBH 3 or [C 5 H 6 Me 4 NH 2 ]/[HB(C 6 F 5 ) 2 (C 7 H 11 )] were utilised as stoichiometric reductants.Secondly, the boron centre in this FLP is more oxophilic, so the last step required quenching with D 2 O to cleave the B-O bond.
In another stoichiometric system, O'Hare and co-workers synthesised a series of FLPs based on Lewis acid {C 6 F 4 (o-C 6 F 5 )} 3 B and (C 6 Cl 5 ) 3 B with trialkylphosphines as Lewis bases (Scheme 12).The idea of synthesising these FLPs was to achieve a weaker B-O bond to facilitate the cleavage of B-O bond upon reduction and potentially generate a catalytic system.The steric congestion factor was applied as steric bulk at the ortho position alone could decrease the B-O bond strength. 53The synthesised FLPs were exposed to H 2 to form FLP-H 2 as activated salts of the type [R 3 P-H][H-BR ′ 3 ].These salts were then exposed to CO 2 (1 atm) to obtain formatoborates of type 20 in the presence of toluene at 140 °C for 24 h using Young's tap NMR tubes.The formatoborates 20 could also be prepared independently from the reaction of the FLP with formic acid in toluene at room temperature for 16 h (Scheme 12). 54The formatoborates 20 were subjected to H 2 and heated to 140 °C for 16 h but were not reduced, instead decarboxylation of the formatoborates 20 occurred and hydride salts were formed with no further reductions.
The higher stability of the formatoborates 20 and their decarboxylation at higher temperatures limited this FLP approach for a catalytic reduction of CO 2 .
Following reports on stoichiometric reactions, the rst catalytic reduction of CO 2 with an organocatalyst FLP was explored by Fontaine and co-workers in 2013.They applied hydroboranes HBR 2 [HBcat (catecholborane), HBpin (pinacolborane), 9-BBN (9-borabicyclo[3.3.1]nonane),BH 3 $SMe 2 and BH 3 $THF] to produce CH 3 OBR 2 or (CH 3 OBO) 3 following reduction of CO 2 .Upon hydrolysis, CH 3 OBR 2 or (CH 3 OBO) 3 yield methanol as the nal product in up to 99% yield (Scheme 13) with high turnover numbers (TON > 2950) and turnover frequencies (TOF = 853 h −1 ).The intramolecular phosphino-borane catalyst 21 was found to be an efficient catalyst for this reaction. 55The same authors studied the mechanism of this hydroboration of CO 2 with catalyst 21 using computational and experimental methods.It was found that an intramolecular FLP was involved in every step of the reduction and the simultaneous activation of both, the reducing agent and CO 2 , were the key to efficient catalysis in every reduction step. 56Furthermore, Fontaine and co-workers synthesised various phosphine-borane derivatives of catalyst 21 with different substituents on boron and phosphorus as shown in Scheme 13 (bottom).These were then tested for hydroboration of CO 2 using HBcat or BH 3 $SMe 2 to generate methoxyboranes.The most active species were derivatives with a catechol unit on boron.They also performed isotope labelling experiments and DFT studies and found that once the formaldehyde adduct was generated, the CH 2 O moiety remained on the catalyst system.The lowest energy barriers were found for concerted activation of catecholborane by the Lewis base and of CO 2 by the Lewis acid.The results show higher potency of "O" for the activation of hydroboranes than "P". 57Overall, FLP 21 acted as an efficient catalyst because of two important features: Firstly, 21 did not form an adduct with CO 2 , as seen previously with most FLPs that formed a stable CO 2 adduct.Exposing 21 to 1 atm of CO 2 at room temperature resulted in no spectroscopic change of the solution (by 1 H, 31 P, and 11 B NMR spectroscopy).Also, species 21 remained monomeric in solution without any P-B interaction.Secondly, the CH 2 O moiety was released upon reduction from the catalyst and made 21 available for another reaction.Hence, the higher high turnover numbers and high turnover frequencies for 21.Later, Stephan and co-workers developed another catalytic method for the reduction of CO 2 using 9-BBN as a reducing agent and phosphine as a catalyst (Scheme 14).The reaction proceeds via an FLP-type CO 2 activation intermediate 22 and the reduction products include boron-bound formate species, 23, the diolate-linked compound 24, and methoxide product 25.Intermediate 26 could be transferred to 25 in the presence of 9-BBN and to 27 in the presence of the boron-bound formate species 23.Derivatives of 27 were isolated and conrmed with single crystal X-ray diffraction analysis.With 0.02 mol% of tBu 3 P, product 25 is obtained in 98% yield at reaction temperature 60 °C.In the best scenario, the catalyst tBu 3 P provides 5556 turnovers of hydride transfers to CO 2 and a TOF of 176 h −1 . 58nstead of forming a classical adduct of tBu 3 P and 9-BBN, this system showed an FLP-type CO 2 activation and subsequent hydride transfer from boron to the carbonyl carbon in 22, releasing tBu 3 P for the next cycle and hence this system worked catalytically for the reduction of CO 2 .
Dang and co-workers reported a theoretical study on a catalytic mechanism for computationally designed bridged P/B FLPs in the activation of H 2 and CO 2 .They found that the reaction follows a one-step concerted mechanism with small reaction barriers (14.8-24.0kcal mol −1 ).Among the computationally designed bridged FLPs, some were found to successfully reduce CO 2 with molecular hydrogen in two feasible pathways.The rst pathway follows immediate hydrogenation of CO 2 aer H 2 activation (Scheme 15, top), the second follows CO 2 activation rst, then metathesis of H 2 followed by reductive elimination (Scheme 15, bottom).Both catalytic cycles provide the product HCO 2 H from the reduction of CO 2 .From all computationally designed bridged FLPs, straightforward H 2 activation takes place with those that do not have electron donating substitutions on the B's adjacent carbon site, or have a long chain between the B and P. 59 Xanthene FLPs were computationally investigated using DFT methods [level of theory: B3LYP-D3/6-311+G*(*)//M06-2X/6-31G*(*) in bromobenzene], and their reduction of CO 2 was modelled as shown in Scheme 16.Differently substituted xanthene backbones were investigated, showing that more rigid backbones have lower activation energies for CO 2 hydrogenation.
The formation of P/B-H in the rst step was shown to be exergonic and this rst intermediate is the catalyst resting stage. 60The hydride transfer from boron to the carbonyl carbon of CO 2 produces formate and subsequent protonation resulted in the formation of formic acid bringing the xanthene FLP into the next cycle for the CO 2 reduction.
The incorporation of FLPs into polymers has also seen some success in CO 2 activation.Shaver and co-workers explored the rst use of polymeric FLPs to catalyse the incorporation of CO 2 into cyclic ethers for the formation of cyclic carbonates and showed good selectivity (Scheme 17, top).Different phopshines and boranes were explored as the Lewis base and acid in the polymer (Scheme 17, bottom).These poly(FLPs) can easily be recovered and reused aer the reaction, however the efficiency of the catalyst gradually decreases due to partial phosphine oxidation and increased crosslinking. 61an and co-workers developed CO 2 -responsive dynamic gel system based on an FLP for the rst time (Scheme 18).Here, CO 2 can be regarded as a "gas glue" which crosslinks the Lewis acidic and Lewis basic sites and forms a new type of a FLP network.The trapped CO 2 FLP network undergoes reversible release of CO 2 upon heating at >60 °C.
The authors found that CO 2 -bridging crosslinks in the network are dynamic covalent linkages, which provides the gel with unique gas-tuneable viscoelastic, mechanical, and self-healing characteristics. 62The authors showed that the same (-B-CO 2 -P-) poly-FLPs are efficient catalysts in transforming amine substrates to formamide derivatives using the CO 2 poly-FLP as the starting point.
Various amines were screened and the yields for the formamide products were in the range of 41-99% with TON = 420-14 800.The highest TON of 14 800 was observed for diethylamine giving 99% yield of the corresponding diethylformamide product (Scheme 19). 63CO 2 bridges the polymer chains and a CO 2 -triggered micellisation was obtained.Addition of PhSiH 3 and R 1 R 2 NH resulted in the desired formamide products and regenerated the polymer.Aer separation of the products re-micellisation of the polymers was performed with CO 2 and a reusable catalytic system was established with a high turnover number.
Erasmus and co-workers developed efficient FLPs supported on silica nano-powder for CO 2 capture. 64A series of CO 2 adducts 28 were synthesised by reacting silica nanopowder supported Lewis acids and dissolved Lewis bases in pentane with CO 2 (2 bar) which was passed through the pentane mixture at −65 °C.At room temperature these adducts were observed to be reversible in nature (Scheme 20, top).
In a similar manner a series of silica nano-powder supported FLP-CO 2 adducts 29 were synthesised from silica nano-powder supported Lewis bases and dissolved Lewis acids.The silica nanopowder supported FLPs were also explored for the conversion of CO 2 to formic acid using hydrogen gas.Initially, the activation of H 2 was done by the supported Lewis acid/bases with FLP partners to obtain [-BH] − [HP-] + salts.Furthermore, introducing CO 2 to these salts resulted in HCO 2 H and regenerated the FLPs.HCO 2 H is a protic polar molecule and has tendency to form O/H bonds with the free -OH functionalities on the silica.The main reason for the release of HCO 2 H from the system aer reduction was the immobility of silica nanopowder bound Lewis acids (or Lewis bases) and so did not inhibit the activity of the FLPs.
In FLP systems having a P-basic centre, activation of CO 2 proceeds via the formation of a P-C bond, and depending on the type of reactive acidic site a B-O, Al-O or Ga-O bonds are generated, oen in a reversible manner.Alkyl phosphines iPr 3 P or tBu 3 P alone could not activate the CO 2 molecule.It is known that the presence of a Lewis acidic component is not necessary for capturing CO 2 when very electron rich P-nucleophiles are used. 65rane/nitrogen FLPs for CO 2 activation In addition to phosphorus as a Lewis base in FLP-CO 2 activation and reduction, there has been a wealth of FLPs described in the literature that use a nitrogen Lewis base in combination with a boron Lewis acid.A selection of the corresponding FLP-CO 2 adducts are displayed in Fig. 6. 66,67 Stephan and co-workers reported a new synthetic method for making boron amidinates.The strained ring boron amidinate derivative 30 was prepared by reacting Piers' borane, HB(C 6 F 5 ) 2 , with isopropyl carbodiimide.30 was then successfully employed to trap CO 2 incorporated into a new heterocycle 31 (Scheme 21).Compound 31 was fully characterised along with a single crystal X-ray diffraction structure. 68A theoretical study on the reaction of 30 with CO 2 found a concerted addition mechanism.
In this reaction, the C-atom and O-atom of CO 2 inserts into the B-N bond of 30 and forms the C-N and B-O bonds simultaneously.The frontier orbitals involved in the reaction mechanism were investigated as well as electric charge analysis and showed that results were consistent with charge transfer from HOMO of 30 to the LUMO of CO 2 . 69In another ndings, Chattaraj and co-workers have studied this boron amidinate 30 as a bridged B/N FLP. 70They compared 30 with a P/B bridged system shown in Scheme 15 which describes two types of cycles.In this work, a similar process shows that CO 2 hydrogenation with amidinate 30 leads to formic acid (HCO 2 H) as the nal product.In the proposed mechanisms, either H 2 is activated by the Lewis basic centre of the FLP, and CO 2 is activated by the Lewis acidic centre of the FLP, or alternatively, CO 2 can be activated by Lewis basic centre of the FLP and H 2 by Lewis acidic centre of the FLP.In both cases, simultaneous activation of CO 2 and H 2 by a single TS was conrmed by Natural Bond Orbital (NBO) analysis and this TS is the rate determining step.From energy decomposition analysis (EDA), in the TS geometry it was found that electron density was donated from the HOMO of FLP to the LUMO of H 2 and electron density from HOMO of H 2 molecule to the LUMO of CO 2 . 70Stephan and co-workers have also utilised phosphinimines and B(C 6 F 5 ) 3 to explore FLP reactivity, Ph 3 P]NR with B(C 6 F 5 ) 3 and CO 2 produced the adducts 32 (R = Ph, C 6 F 5 ) (Scheme 22). 71Mayer and co-workers applied this salt for the activation of CO 2 at 4 atm pressure and at room temperature.The air-stable formatoborate complex 36 (Scheme 25) resulted and its structure was conrmed by X-ray diffraction analysis. 76Compared to 35, Mayer and co-workers observed that 36 on heating to 80 °C resulted only in decomposition instead of transforming to other CO 2 reduced products.This restricts the method to obtain only formatoborate complex 36 in a stoichiometric way.Fontaine and co-workers explored the hydrogenation of carbon dioxide using intramolecular o-phenylene bridged B/N FLPs 37 (Scheme 26).When R = 2,4,6-Me 3 C 6 H 2 , the FLP species forms the formyl, acetal and methoxy derivatives 38, but when R = 2,4,5-Me 3 C 6 H 2 , the boron-linked product 39 formed instead. 77talytic transformations of CO 2 have also been successful using B/N FLP systems.To address the catalytic shortcomings of the reaction developed by O'Hare and his group using the FLP TMP/B(C 6 F 5 ) 3 for the reduction of CO 2 with H 2 to form methanol, Piers and co-workers developed a catalytic method by adding silane to the reaction mixture with excess B(C 6 F 5 ) 3 to form methane (Scheme 27). 78They also reported that when Et 3 SiH was not added to the reaction, then the CO 2 adduct as the salt [TMP-CO 2 -B(C 6 F 5 ) 3 ][TMPH] was formed.As seen in other systems, the formation of the CO 2 -adduct is reversible, however, when Et 3 SiH is added then it provided the [TMPH] [HB(C 6 F 5 ) 3 ] salt along with a triethylsilyl carbamate 40. 78or this reaction, Wang and co-workers carried out computational studies to look at the mechanism of CO The role of B(C 6 F 5 ) 3 was also found to be important since it promotes hydride transfer and acts as a shuttle to bring H d− from Et 3 SiH to CO 2 . 79Overall, additional B(C 6 F 5 ) 3 activates the silane reducing agent, Et 3 SiH, producing Et 3 Si + as a good oxygen acceptor and thus promotes the catalytic deoxygenation of CO 2 to CH 4 .
Cantat and co-workers explored nitrogen bases such as TBD (triazabicyclodecene), Me-TBD (MTBD), DBU (1,8-diazabicyclo [5.4.0]undec-7-ene), and others for the reduction of CO 2 in the presence of 9-BBN or CatBH.The reactions were performed at room temperature and a TON of up to 648 was achieved (Scheme 28).In this process, CO 2 is initially reduced to a borylformate which then undergoes reduction rstly to an acetal and then a methoxyborane.The stoichiometric reaction of TBD-CO 2 and 9-BBN in THF forms product 42 along with other reduced products (Scheme 28).Compound 42 was analysed by single crystal X-ray diffraction and it was found that the acidic NH proton in the TBD-CO 2 adduct was replaced with a 9-BBN unit.Compound 42 can be considered as a nitrogen/boron FLP system trapped with CO 2 .A mechanism was proposed based on rigorous control experiments.In 42, CO 2 behaves as a Lewis base and coordinates to the hydroborane R 2 BH to form adduct 43, which enables hydride transfer from the borane to carbon and forms 44.Compound 42 is regenerated when CO 2 is applied, releasing the boron formate and thus catalysed the system for CO 2 hydroboration.Finally, the boron formate is reduced to the methoxyborane.It is important to note that for MTBD it was found that the reaction proceeds with the activation of borane followed by the capture of CO 2 . 80tephan and co-workers inadvertently discovered a new class of N/B molecule 45, that consists of a strong Lewis basic phosphorus centre and weak Lewis acidic boron centre which makes it a suitable FLP system.They utilised FLP 45 in the reduction of CO 2 (5 atm) at 60 °C with BH 3 $SMe 2 as a reducing agent and obtained a boroxine product (Scheme 29). 81The reduction of CO 2 was observed catalytically in this case due to the presence of a strong basic centre and a weak Lewis acid that facilitates lability of the reduced CO 2 fragments.This shows a difference to FLPs composed of a strong Lewis acid in which only stoichiometric reduction was observed, as in the case of 18 where 1,2-C 6 H 4 (BCl 2 ) 2 is the Lewis acid (Scheme 11).
Zhang et al. found that 4 equivalents of BH 3 $NMe 3 and catalytic 6-amino-2-picoline could be used to formylate secondary amines.The proposed mechanism proceeds though dehydrocoupling of the amineborane and catalyst to form an intramolecular FLP 46, which reacts with CO 2 .The activated CO 2 is then inserted into the N-B bond which is subsequently reduced by borane with loss of H 2 BOBH 2 to give the methylated amine (Scheme 30). 82or the activation of CO 2 using FLPs, many arrangements of plausible Lewis pairs are possible.Hence, it is a challenge to nd a particular combination that is superior for catalysing CO 2 reduction.
With this in mind, Corminboeuf and co-workers proposed a map of chemical composition of FLPs for their activity towards Scheme 28 Catalytic reduction of CO 2 using N-bases and B-H reducing agents.
Scheme 29 Catalytic reduction of CO 2 using FLP 45.

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formate product by catalytic hydrogenation of CO 2 .They built the map upon linear scaling relationships, pinpointing specic FLP combinations with complementary acidity and basicity to optimally balance the energetics of the catalytic cycle.Amongst such combinations, they created a library of 60 P/N Lewis bases and 64 triaryl boranes as Lewis acids resulting in a library of 3840 FLPs.Out of these, they experimentally demonstrated the catalytic transformation of CO 2 to formate by using an inverse FLP system obtained from tris(p-bromo)tridurylborane (tbtb) as Lewis acid and DBU as the Lewis base.A turnover number of 24 ± 3 was found for this catalytic reaction (Scheme 31).This is the rst example of a metal-free CO 2 hydrogenation in which stoichiometric addition of a silylhalide was not required.This was achieved through the ne-tuning of the Lewis acid and base based on their energies of hydride and proton attachment, respectively.Here, the authors conclude that inverse FLPs, with a weaker Lewis acid and strong Lewis base or strong Lewis acid with weaker Lewis base, yet with cumulative high acid-base strength, is the ideal combination to achieve CO 2 hydrogenation.The authors highlight the importance of overcoming both activation barriers to CO 2 activation as well as H 2 activation when targeting catalytic CO 2 hydrogenation. 83n 2022, Palomero and Jones reported the preparation of bis(boramidinate)ferrocenes 47 and 48 by hydroboration of 1,1 ′dicarbodiimidoferrocenes. The resulting compounds reacted with CO 2 .The reaction of the BBN derivative 47 with CO 2 (10 atm) to form the mono-CO 2 -bound product as a yellow precipitate (90% conversion) in a process that was highly reversible (Scheme 32).Whilst this precluded isolation of the CO 2 -bound products, such reversibility may be preferable for applications in catalytic hydrogenation, facilitating release of the reduced product and catalytic turnover.Lower pressures of CO 2 were shown to reduce conversion. 84To allow the use of lower pressures, the more electron-poor bis(pentauorophenyl)borane analogue 48 was employed.At 5 atm CO 2 , it activated 2 equivalents of CO 2 , although the reaction required two weeks to go to completion.

Borane/carbon FLPs for CO 2 activation
Stable N-heterocyclic carbenes (NHCs) upon reaction with CO 2 form a mesomeric betaine 49 having a C-C bond between the carbene and CO 2 .On the other hand, very reactive carbenes can form oxiranones 50 (Scheme 33).The product remains in equilibrium with the starting substrates.Most attention has been focused on stable sterically hindered NHCs amongst all carbenes for the activation of small molecules, 85 and several carbenes in FLP systems have been explored and found to be efficient in the activation of CO 2 . 86he rst carbene based FLP system to activate CO 2 was reported by Tamm and co-workers in 2012 (Scheme 34). 87They showed that exposure of CO 2 to a solution of a bulky carbene (1,3-di-tert-butylimidazolin-2-ylidene) and tris[3,5bis(triuoromethyl)phenyl]borane, B(3,5-(CF 3 ) 2 C 6 H 3 ) 3 , in benzene at 60-70 °C, a white precipitate, identied as the FLP-CO 2 adduct was formed.The adduct was isolated in 66% yield.At room temperature this adduct was also obtained on exposure of CO 2 to the solution of the FLP in benzene with a 24 h reaction time and a higher yield of 86% was isolated.
Later, Tamm and his group synthesised a library of carbene based FLP CO 2 -adducts and studied their reaction prole computationally (level of theory: M05-2X/6-311G**).A 1 : 1 mixture of a bulky carbene and B(C 6 F 5 ) 3 with CO 2 provided NHC-CO 2 -B(C 6 F 5 ) 3 products 51 in 89% and 68% yield depending on the starting carbene (Scheme 35).From DFT calculations a low energy barrier was observed for the NHC-CO 2 -B(C 6 F 5 ) 3 adduct formation (10.4 (R = H) and 12.2 (R = Me) kcal mol −1 ), and it was concluded that steric changes on the NHC were more pronounced than electronic impacts. 88hu and co-workers computationally designed a boronbased carbene intramolecular FLP 52 and calculated its reactivity with various small molecules, including CO 2 .This FLP with CO 2 forms a zwitterionic species 53 and the authors discuss the important driving force of aromaticity in the nal adduct (Scheme 36). 89Baceiredo and co-workers exposed boryl(phosphine)carbene 54 to CO 2 (1 atm) and an unusual product 55 was observed.Aer analysis of the product's structure, it was found that the carbene inserted into the C]O bond of the CO 2 .Thus, incorporating carbon dioxide into the corresponding phosphoryl ketenylidene derivative (Scheme 37). 90toichiometric reduction reactions with carbene/borane FLPs have been reported. 91In 2019, Mandal and co-workers prepared an N-heterocyclic carbene-boron adduct 56 by reacting an abnormal heterocyclic carbene (aNHC) with 9-BBN.The synthesised NHC-boron adduct 56 was utilised to capture CO 2 from the atmosphere under ambient conditions in benzene overnight.Product 57 was obtained, due to moisture in the air

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leading to hydrolysis of 9-BBN and boric acid formation with the release of a cyclooctane molecule.The CO 2 was incorporated as a formate ion.Further treatment of 57 with excess 9-BBN leads to the formation of compound CH 2 (OBBN) 2 with the release of H 2 , and nally converts this to CH 3 OBBN (Scheme 38). 91This work was presented as a rst metal-free system to reduce CO 2 by capturing it from the atmosphere under ambient conditions where CO 2 remains in a concentration of ∼400 ppm.Mandal and co-workers later reported the use of the same FLP system for a catalytic reduction of CO 2 in the presence of a range of hydroboranes leading to methoxyborane (Scheme 39).Reaction of the carbene with CO 2 rstly gave the adduct whilst reaction of the carbene with 3 equivalents of 9-BBN in the presence of CO 2 , provided boron diformate 58.Zwitterionic boron diformate 58 was utilised catalytically with a loading of 0.005 mol% for the conversion of 9-BBN to the methoxide derivative CH 3 O-BBN under a CO 2 atmosphere.Catalyst 58 leads to a TON of 6000, which is the highest TON observed among all the metal-free catalysts investigated at ambient conditions.The key feature of this catalytic process is the formation two equivalents of 9-BBN formate, BBN(OCHO), from the reaction of catalyst 58 with an equivalent of 9-BBN resulting in the release of dihydrogen.This generates the carbene which further captures a CO 2 molecule regenerating 58 with 9-BBN and thus providing a catalytic process.The 9-BBN formate is nally reduced and hydroborated to CH 3 O-B in a series of steps in the presence of an excess of 9-BBN. 92

Borane/silicon FLPs for CO 2 activation
To date there is just one example of a boron/silicon FLP for CO 2 activation.Very recently Mo and co-workers reported the synthesis of a geometrically constrained bis(silylene)-stabilised borylene 59. 93 Spectroscopic and X-ray analyses reveal that structure 59 has a tricoordinate boron centre with a distorted Tshaped geometry.Computational analysis shows that the HOMO comprises a lone pair of electrons on the boron centre  93 Borane/germanium FLPs for CO 2 activation Similar to the silylene described above, germylenes are also reported as the Lewis base component of an FLP for CO 2 reduction.Kato and co-workers reported an interesting N,Pheterocyclic germylene 62 in 2016, that bears several reactive sites (including a germylene centre) and can activate two CO 2 molecules simultaneously. 94Compared to classical FLPs, 62 showed unusual behaviour of multi-reactive sites and has been with 1,3,2,5-diazadiborinine 64 featuring nucleophilic and electrophilic boron centres within the same molecule. 95his rst reported intramolecular boron-boron FLP showed high regioselectivity in the reaction with CO 2 , yielding a bicyclic product 65 (Scheme 42).Interestingly, CO 2 activation by 64 was reversible at 90 °C.To obtain insight into electronic features of 1,3,2,5-diazadiborinine 64, the authors performed DFT calculations [level of theory: B3LYP/6-311G+(d,p)]. NBO analysis showed that the compound possess both nucleophilic and electrophilic boron centres with a formal B(+I)/B(+III) mixed valence system. 95Later, Zhao and co-workers performed more detailed computational analyses of 64. 96They reported p delocalisation over the central ring which extends from the lone pair on (O) / p*(N-C), and favourable orbital overlap with CO 2 is generated from the electrophilic interaction with the Lewis acidic boron centre and nucleophilic donation to the LUMO+3 of the other boron centre.
Kinjo and co-workers synthesised another class of boron compounds in which the boron acts as a Lewis basic centre.
It was found previously that tricoordinate organoboron L 2 PhB: (L = oxazol-2-ylidene) compound 66 does not react with BEt 3 This is perhaps due to a mismatch of the soness/ hardness of the respective boron centres in 66 and BEt 3 based on HSAB (Hard So Acid Base) theory in addition to steric hindrance.As compound 66 and BEt 3 do not react, they act like an FLP.Thus 66 and BEt 3 were reacted with CO 2 in toluene at room temperature and the FLP-CO 2 adduct was isolated in 85% yield (Scheme 43). 97rane/metal FLPs for CO 2 activation In addition to p-block Lewis bases, transition metal complexes can also act as the Lewis base component of an FLP with boron as the Lewis acid.This is demonstrated using a ruthenium A solution of this FLP, when exposed to CO 2 for 12 h, provided an orange solid in 70% yield and was fully characterised using NMR and single crystal X-ray crystallography as the FLP-CO 2 adduct where the b-carbon centre had attacked the electrophilic carbon centre of CO 2 (Scheme 44). 98ass and co-workers explored reactions with a platinum(0) complex as a Lewis base in the activation of small molecules using a sterically congested boron-based Lewis acid.They found that pairing a Lewis basic platinum(0)-CO complex supported by a diphosphine ligand with B(C 6 F 5 ) 3 acts as a frustrated Lewis pair, to activate CO 2 (Scheme 45).The presence of B(C 6 F 5 ) 3 is important as no activity was observed between the platinum complex and CO 2 in the absence of the borane.In this scenario, Pt(0) acts as a donor of electron and the boron atom acts as the acceptor forming a coordinated Pt-CO 2 -B system.A substitution of CO by CO 2 on platinum was observed aer the loss of the CO molecule.
In this process 95% isotopically pure 13 CO 2 was used but the 31 P NMR analysis of the product showed a mixture of 13 C labelled and unlabelled product in a ratio of 4 : 1.The source of unlabelled product must be from the 12 CO in ligand of the starting material.This suggests that a symmetrical [C 2 O 3 ] 2− complex forms during the reaction pathway.The proposed mechanism in Scheme 45 suggests that the reaction of the platinum(0)-CO complex and B(C 6 F 5 ) 3 with CO 2 is a metalmediated oxygen transfer between CO 2 and CO rather than a simple ligand substitution. 99n 2013, Berke and co-workers showed an FLP-type activation of CO 2 using a [Re]-H/B(C 6 F 5 ) 3 system where the Re-H bond acts as a Lewis base.Catalysts 67 and 68 were prepared stepwise from a rhenium hydride precursor [ReH(PR 3 ) 2 (NO)Br] (Scheme 46).Initially, the precursor was reacted with B(C 6 F 5 ) 3 and CO 2 in benzene to form the FLP-CO 2 adduct.With a PiPr 3 ligand on the rhenium precursor, insertion of the Re-H into the FLP-bound CO 2 molecule was observed generating 67.Compound 67 could be hydrogenated with H 2 (1 bar) in toluene at 60 °C for 1 h to give 68 in 99% yield.Both 67 and 68 were screened for the hydrosilylation of CO 2 using Et 3 SiH as a reducing agent (Scheme 46).Catalyst 67 with a loading of 1 mol% provided the (Et 3 SiO) 2 CH 2 product in 87% yield (TON = 89, TOF = 5.9 h −1 ), while 68 provided the reduced product in 89% yield (TON = 95, TOF = 7.3 h −1 ).Similarly, catalysts 67 and 68 were utilised for CO 2 hydrogenation (P H 2 /CO 2 = 40/20 bar) in the presence of TMP as a base.Catalyst 68 provided the formate salt of TMP in 46% yield (TON = 92, TOF = 6.1 h −1 ). 100 In another study, Agapie and co-workers investigated the effects of the Lewis acid B(C 6 F 5 ) 3 towards the conversion of CO 2 to CO and water using a molybdenum complex (Scheme 47). 101The activation of CO 2 was found to be linearly related to the strength of the Lewis acid.When a labile Mo(0)-CO 2 adduct interacts, it will increase both the degree of activation and the kinetic stability of bound CO 2 as shown in Scheme 47.In contrast to the CO 2 displacement by a solvent that is predominantly observed in the absence of a Lewis acid, in the presence of B(C 6 The authors demonstrated that the chemistry of the labile substrate is greatly inuenced by the time the substrate resides in the metal's coordination sphere.It is shown that Lewis acid additives promote CO 2 cleavage via kinetic stabilisation rather than merely by thermodynamic activation. One nal system to note here uses the boron Lewis acid B(C 6 F 5 ) 3 with s-block metal carbonates M 2 CO 3 (M = Na, K, and Cs) for the highly efficient reduction of CO 2 to formate.Amongst the screened metal carbonates, Cs 2 CO 3 showed the highest TON of 3941. 102

Aluminium FLPs for CO 2 activation
A number of FLPs based on aluminium Lewis acids have also been reported for CO 2 capture and reduction, although the greater oxophilicity of aluminium (potentially inhibiting product release) means that catalytic hydrogenation has yet to be reported.This oxophilicity also means that, whereas FLPs containing boron Lewis acids typically bind CO 2 in a 1 : 1 : 1 Lewis acid : Lewis base : CO 2 ratio, Al-containing FLPs oen bind it in a 2 : 1 : 1 ratio, with both oxygen atoms binding an aluminium centre. 103Studying FLPs comprised of phosphines and aluminium esters, Smythe et al., showed that the ratio of mono-to bis-bound adduct varies with Lewis acidity. 104er exposure to 1 atm of CO 2 , Al(O-C 6 H 2 Cl 3 ) 3 bound CO 2 in a predominantly mono fashion (ca.95% mono), whereas the greater Lewis acidity of Al(OC 6 Cl 5 ) 3 (ca. 1 : 1 mono : bis) and Al(OC 6 F 5 ) 3 (ca.75% bis) favoured the bis-bound adduct.
Like boron CO 2 adducts, a range of aluminium FLP-CO 2 adducts are reported (Scheme 48).Uhl and co-workers reported the synthesis of geminal ambiphilic phosphine-aluminium FLPs that can activate CO 2 to a form a cyclic adduct 69. 105Later, the same authors synthesised AlPC 2 O type heterocycle having cis/trans isomeric compounds. 106Uhl also reported a P-H functionalised Al/P FLP in 2019.The FLP reacts with CO 2 to give a ve-membered zwitterionic cycle similar to that in 69, as typical for vicinal intramolecular FLPs.However, the enhanced acidity of the phosphine means that it can be deprotonated by addition of a base (such as DABCO or nBuLi) to give a more stable precipitate. 107Similarly, Fontaine and co-workers studied the reactivity of the stable Lewis adducts [R 2 PCH 2 AlMe 2 (R = Me, Ph)] and found adducts 70 and 71. 108Harder and co-workers reported a geminal Al/P FLP, with a nitrogen rather than the In the products 74-76, the two aluminium centres each bind to one of the CO 2 's oxygen atoms.The binding strength could be tuned by varying the substituents on aluminium.The more Lewis acidic xanthene-AlCl 2 and xanthene-Al(C 6 F 5 ) 2 fragments bind CO 2 irreversibly giving 75 and 76 (Scheme 48), while xanthene-MeClAl binds CO 2 reversibly giving 74 under 2 bar CO 2 , liberating CO 2 when this excess pressure was released.For catalytic applications, this reversible binding is necessary to enable release of the product.Although most CO 2 -binding Al FLPs are Al/P rather than Al/N, one example of an Al/N FLP was described by Brewster in 2020 using the readily available 2-(methylamino)pyridine as ligand yielding 77 upon reaction with 2 equivalents of CO 2 . 112An example of an FLP-CO 2 adduct with a metal as a Lewis base was provided by Bourissou and coworkers who utilised geminal P-Al ligand [Mes 2 PC(]CHPh) AltBu 2 /Pt(PPh 3 )] in the activation of CO 2 molecule to obtain an adduct.
In this bimetallic system, platinum acts as the Lewis base and activates the CO 2 molecule by reacting at the carbon centre of the CO 2 molecule and the formed negative charge on one of the oxygen atoms is stabilised by the Lewis acidic aluminium centre (Scheme 49). 113everal of these adducts have been used in stoichiometric and catalytic transformations of CO 2 .Stephan and co-workers, reported the synthesis of CO 2 adducts 78 between AlX 3 (X = Cl, Br) and PR 3 (R = Mes) (Fig. 7).Upon treatment of these adducts with excess ammonia borane (NH 3 $BH 3 ), an Al-methoxy species was generated which aer hydrolysis resulted in the formation of MeOH at room temperature. 103To study the steps involved in the reaction, Me 3 N$BH 3 was utilised to reduce the FLP-CO 2 adduct 78 (X = C 6 F 5 , R = o-Tol).Along with the methoxy derivatives of alane, compound 79 was isolated and fully characterised. 114ater, other groups have studied the mechanism for this reaction computationally and explained the reduction of CO 2 trapped FLPs. 115In other reactions, Stephan and co-workers explored the adducts of 78 (X = Cl, Br, I, C 6 F 5 , OC(CF 3 ) 3 and R = Mes, o-tolyl) for the stoichiometric transformation of CO 2 to CO. 116 While uorination of substituents is a common way to increase Lewis acidity in FLP design, an alternative is the use of a cationic Lewis acid.Harder reported an FLP comprised of Lewis basic PPh An unusual report of an oxygen-bridged geminal Al/P FLP 82 was made by Wickemeyer et al. (Scheme 51).It was prepared by reaction of the parent alane and phosphine oxide, giving an initial zwitterionic compound 83 which slowly eliminates H 2 to give the FLP 82.82 was found to bind CO 2 to give the heterocyclic CO 2 adduct 84.The hydrogenated zwitterion 83 can also be generated in small quantities by exposure of the FLP to H 2 .This species bound CO 2 irreversibly, and exposure of the hydrogen adduct to CO 2 gave stoichiometric CO 2 reduction to the aluminium bound formate 85.
However, the instability of the hydrogen adduct and strong Al-O bond make the system not well suited for catalytic applications (Scheme 51). 118uang et al. reported a variety of group 12 and group 13 formamidinate FLPs.While formamidinates are able to coordinate as bidentate ligands, the incorporation of strongly electron-withdrawing C 6 F 5 substituents on nitrogen increases the preference of the monodentate species with a vacant coordination site on the metal.The free "N" and unsaturated metal in proximity are able to act as an FLP, 119 and the compounds' potential for catalytic CO 2 hydrosilylation.While the formamidinates investigated (B, Al, Ga, In and Zn) showed poor activity for this reaction on their own, signicantly improved performance was seen when combined with B(C 6 F 5 ) 3 or Al(C 6 F 5 ) 3 .The highest activity for complete conversion of triethylsilane under 1 bar CO 2 aer 10 h at 80 °C was observed with the aluminium formamidinate/B(C 6 F 5 ) 3 , yielding almost exclusively CH 4 .Replacing Et 3 SiH with Ph 2 SiH 2 gave selective formation of the bis(silyl ether).
However, mechanistic studies involving the aluminium formamidinate suggest that the catalyst decomposes under the reaction conditions to generate other aluminium species, which were the catalytically active species, and were not identied.
Surawatanawong and co-workers compared the reactivity of geminal P/Al and B/P FLPs with CO 2 and H 2 based on systems previously published by Lammertsma et al. 42 The compounds investigated consisted of an sp 2 -carbon bridged FLP (Mes 2 P-C(]CHPh)-EtBu 2 ) and an sp 3 -carbon bridged FLP (tBu 2 P-CH 2 -EPh 2 ) (E = B, Al).In their comparative study between the geminal B/P and Al/P FLP activation of CO 2 and H 2 (Scheme 52), the main conclusions the authors drew are that the FLPs are more reactive towards CO 2 than H 2 , and that the geminal B/P FLPs involve stronger orbital interactions with CO 2 than their Al/P counterparts.Distortion-interaction decomposition showed that the distortion energy in the H 2 fragment is higher than that in the CO 2 transition state leading to a higher energy barrier for H 2 activation than CO 2 activation.This again highlights the importance of considering energy barriers to the activation of both CO 2 and H 2 , similarly highlighted by the work of Corminboeuf above (Scheme 31).The type of geminal linker, sp 2 or sp 3 , was found not to affect the reactivity. 120se-free CO 2 reduction with group 13 Lewis acids In the nal example using only Group 13 Lewis acids, without a base, Chen and co-workers reported the rst example of a mixed Lewis acid system consisting of Al(C 6 F 5 ) 3 and B(C 6 F 5 ) 3 for the highly selective reduction of CO 2 into CH 4 via a tandem hydrosilylation (Scheme 53).The reaction proceeds in a catalytic manner.In the rst step, Al(C 6 F 5 ) 3 effectively mediates the overall hydrosilylation cycle xing CO 2 into HCO 2 SiEt 3 by activating the carbonyl group.For this initial transformation B(C 6 F 5 ) 3 was found to be inefficient but for the subsequent reduction steps to CH 4 (Scheme 53, steps 2-4) B(C 6 F 5 ) 3 was found to be crucial to give CH 4 in up to 94% yield through a frustrated Lewis pair (FLP)-type Si-H activation.The higher Lewis acidity of Al(C 6 F 5 ) 3 relative to the corresponding borane led to the formation of stable intermediates ([Al]-substrate adducts and [Al]-intermediates).In this reaction for the overall reduction of CO 2 to CH 4 , the role observed for both Lewis acids are not only complementary but also synergic where the rst reduction step is initiated by the aluminium catalyst and later by the boron catalyst.

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systems are known for indium (see later).Uhl and co-workers reported a dimeric gallium hydrazide displaying FLP-like reactivity able to insert CO 2 into the Ga-N bond, yielding a sevenmembered C 2 O 4 Ga 2 cycle 86 (Scheme 54, top). 122An atypical example of FLP-like reactivity with CO 2 was also reported by Goicoechea using a phosphanyl phosphagallene.
The compound adds to CO 2 with oxidation of the phosphanyl phosphorus, with gallium bound to the phosphanyl phosphorus and one of the oxygen atoms bound to gallium 87 (Scheme 54, middle). 123Kemp and co-workers prepared a P,Pchelated heteroleptic complex bis[bis-(diisopropylphosphino) amido]indium chloride [(iPr 2 P) 2 N] 2 InCl.In both the solid-state and solution, it was found that CO 2 inserted into two of the four M-P bonds to produce [O 2 CP(iPr 2 )NP(iPr 2 )] 2 InCl 88 (Scheme 54, bottom).Experimental analysis showed that the time taken for the insertion of CO 2 at room temperature in solution condition was less than 1 minute and less than 2 h in the solid-gas reaction.The complex was stable up to 60 °C under vacuum but released CO 2 when heated above 75 °C. 124

Group 14 Lewis acids
Compared to group 13, group 14 elements have been less studied as Lewis acid components of FLPs for CO 2 activation and conversion.Although carbenium ions such as trityl are isoelectronic with boron, the activation of CO 2 with carbon Lewis acids within an FLP are not known to the best of our knowledge.Examples with heavier Group 14 Lewis acids are known, however, and are described below.

Silicon FLPs for CO 2 activation
Silicon cations are highly electrophilic and are therefore good candidates as the Lewis acid component of FLPs for small molecules activation.In addition, CO 2 transformation into products such as benzoic acid, formic acid, and methanol using silicon cations formed from a [Ph 3 C][B(C 6 F 5 ) 4 ]/R 3 SiH system in different solvents has already been shown to be effective. 125ith sterically hindered phosphines, triarylsilylium borates [Ar 3 Si + ][B(C 6 F 5 ) 4 ] form FLPs. Müller and co-workers studied a series of silylium ion/phosphane Lewis pairs [Ar 3 Si + /PR 3 ].When Ar = Me 5 C 6 and R = tBu or Cy these FLPs were able to activate CO 2 (1 atm, 30 min) in benzene at room temperature to obtain the FLP-CO 2 adducts [R 3 P-CO 2 -SiAr 3 ] (Fig. 8). 126In 2015, Mitzel and coworkers reported the rst synthesis of a neutral Si/P FLP (C 2 F 5 ) 3 -SiCH 2 PtBu 2 and this was utilised in trapping CO 2 at room temperature as a cyclic adduct 89 in quantitative yields. 127he stability of the adduct of a trimethylsilylium and a congested N-heterocyclic carbene (90) was found to be strongly dependent on the nature of the counterion used in the reaction.The stability was found to increase with decreasing nucleophilicity of the ion X, or increasing Lewis acidity of the silylating agent Me 3 SiX (X = I, OTf, NTf 2 ; Tf = SO 2 CF 3 ). 128Tamm and coworkers explored the N-heterocyclic carbene-silylium ion frustrated FLP for the synthesis of adduct 90 (Fig. 8). 129ike the N/B intramolecular FLP described earlier, Cantat and co-workers synthesised a series of TBDR 2 SiX [R = Me, iPr, Ph; X = Cl, B(C 6 F 5 ) 4 , I; TBD = triazabicyclodecene] compounds and utilised them for CO 2 capture to obtain N/Si + FLP-CO 2 adducts 91 (Fig. 8).The formation and stability of the adducts are dependent on the steric and electronic environment at the silicon centre.
Among the synthesised series, R = Me and X = Cl was found to be a good FLP adduct in reducing CO 2 to methoxyboranes (R 2 BOMe) using 9-BBN as a reducing agent both in a stoichiometric and catalytic way.The authors carried out DFT calculations in support of their experimental results to explore the role of N/Si + FLP-CO 2 adducts in the catalytic reduction of CO 2 with different boranes. 130They synthesised a series of o-phenylenebridged phosphorus-silicon Lewis pairs and investigated their reactivity towards CO 2 but no reaction was observed. 131Stephan and co-workers applied silyl triates of the form R 4−n Si(OTf) n (R = C 6 F 5 , Ph, Me; n = 1, 2; OTf = OSO 2 CF 3 ) to activate CO 2 for Scheme 54 FLP type reactivity of homogenous indium and gallium systems with CO 2 .Germanium and tin FLPs for CO 2 activation It has been proven that the cleavage of FLP-CO 2 adducts are quite difficult to great extent due to the strong hard-hard interaction of oxygen and the typical hard Lewis acids used in the FLP system according to the HSAB principle. 133Ge and Sn are soer elements and are less oxophilic, thus their use could provide benecial to enable catalytic CO 2 reduction.Although FLPs with a germanium Lewis acidic centre are known and have been shown to activate small molecules, their use in CO 2 activation and conversion is not yet reported. 134itzel and co-workers reported in 2019 the synthesis of a geminal Sn/P FLP (F 5 C 2 ) 3 SnCH 2 PtBu 2 by reacting LiCH 2 PtBu 2 with (F 5 C 2 ) 3 SnCl.When the FLP (F 5 C 2 ) 3 SnCH 2 PtBu 2 was exposed to CO 2 at −70 °C, it formed an adduct that was found to be reversible at 25 °C. 135Fernandez performed a theoretical analysis of the FLP systems (F 5 C 2 ) 3 E-CH 2 -PtBu 2 (E = Si, Ge, Sn) to understand the effect of the nature of these group 14 elements on their reactivity.Moving down the group, the reactivity of these species is kinetically enhanced (Si < Ge < Sn).Quantitatively, this trend of reactivity was analysed by the "activation strain model" of reactivity in combination with the energy decomposition analysis method.A ve-membered TS with CO 2 lead to the experimentally observed zwitterionic products.The model identies the interaction energy between the deformed reactants as the main factor controlling the reactivity of these geminal FLPs containing Si/Ge/Sn, where the lone pair of phosphorus donates into the p* orbital of C]O and a stronger electrostatic and orbital interaction is observed for Sn over Si. 136 Similarly, Pati and co-workers computationally explored the ability of the FLP system (F 5 C 2 ) 3 E-CH 2 -D(tBu) 2 where E = Si, Ge, Sn and D = P, N to act as hydrogenation catalysts using CO 2 as a substrate. 137For the FLPs where D = N, simultaneous proton and hydride migration take place, whereas for D = P FLPs, proton transfer is followed by hydride transfer.NBO analysis shows that LP(O) / s* (D-H) and s(E-H) / p* (C]O) dominate along the energy prole.From their studies, they predict that FLP (C 2 F 5 ) 3 Sn-CH 2 -N(tBu) 2 would be able to perform CO 2 hydrogenation particularly well.
Hulla reported an application of tin-based FLPs in the form R 3 SnX/N-base (R = alkyl and X = OTf − or NTf 2 − ] which can catalyse the formation of azoles from ortho-substituted anilines via complete deoxygenation of CO 2 in the presence of H 2 . 138mputational insights into group 13 and 14 Lewis acids in FLP catalysed CO 2 activation Grimme reported mechanistic insights, based on extensive DFT calculations, on all steps of the FLP catalysed reduction of CO 2 to boryl formate, H 2 CO, bis(boryl) acetal, and methoxyl borane products in 2020. 139The work addressed three FLP catalysts that had been previously reported; (i) Fontaine's B/P intramolecular FLP reported in 2013, 55,56 (ii) Stephan's intermolecular FLP consisting of tBu 3 P and 9-BBN from 2014, 58 and (iii) Cantat's 2016 Si/N FLP with 9-BBN (Fig. 9, top). 130The report unveils the importance of the Lewis-basic CH 2 O "oxide" site in promoting a hydride transfer, from calculations (PW6B95-D3+COSMO-RS// TPSS-D3+COSMO level of theory in THF).Initial formation of a zwitterionic FLP-H 2 CO adduct had been proposed previously and was veried in this report, in the intramolecular FLP reported by Fontaine, this is generated through the Lewis-basic Bcat oxygen atoms 95 (Fig. 9).
Subsequent hydride transfer from the FLP-H 2 CO adduct to CO 2 then forms boryl formate HCOOBcat through a series of steps, identifying the Lewis acidic Bcat group as the 'base shuttle'.For Stephan's intermolecular tBu 3 P/9-BBN FLP, a hydride transfer from tBu 3 P-CH 2 O-9-BBN to CO 2 via 96 (Fig. 9) is exergonic by −16.1 kcal mol −1 with a barrier of 7.2 kcal mol −1 , which is feasible at room temperature.The nal reduction step from H 2 C(O-9-BBN) 2 into H 3 CO-9-BBN is the slowest reduction step, with a barrier of 23.3 kcal mol −1 .Lastly, when investigating the mechanism for Cantat's Si/N FLP, Scheme 55 Si-based CO 2 adducts using silyl triflates as Lewis acids.

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Grimme and co-workers found the neutral adduct between the Lewis-basic N and 9-BBN to be the most energetically favourable starting point.Here, the B-H is partially activated by the Si/N centres.Hydride transfer to CO 2 is then exergonic by −7.5 kcal mol −1 via 97 (Fig. 9).In summary, zwitterionic FLP-H 2 CO adducts were found to be the active catalysts, strong oxygen and nitrogen Lewis bases were found to stabilise the hydride transfer steps to CO 2 , and nally, Lewis-acidic groups such as Bcat were found to act as a base shuttle.

Group 15 Lewis acids
Generally, in FLP chemistry, group 15 elements are employed as the Lewis base component due to the presence of a lone pair when in the +3 oxidation state.Although nitrogen Lewis acids are known in FLPs, and have been used for small molecule activation, their application for CO 2 activation has not been explored. 140On the other hand, there are a few examples using phosphorus as the Lewis acid.An example was reported by Stephan who prepared a CO 2 -adduct 98 based on intramolecular amidophosphoranes where the phosphorus acts as a Lewis acidic centre and a nitrogen centre in the parent FLP acts as a nucleophilic centre to capture CO 2 (1 atm) at ambient temperatures.Similarly, the bis-CO 2 -adduct 99 (Fig. 10) was also prepared under the same reaction conditions. 141A detailed computational mechanism was studied for the adduct 98 by Zhu and co-workers.They investigated that ring strain, and the trans-inuence are the key factors in amidophosphoranes to capture CO 2 . 142

Transition metal Lewis acids
Earlier we have discussed examples of how low valent transition metals can act as the Lewis base of an FLP when combined with boron Lewis acids.In this section we will discuss selected reports where the transition metal behaves as the Lewis acid to activate CO 2 in an FLP fashion.Several early examples by Piers reported the use of Lewis acidic scandium complexes in combination with B(C 6 F 5 ) 3 and a silane to be operative under an FLP type mechanism to reduce CO 2 . 143,144Examples by Wass and co-workers in 2011, however, were the rst to extend the concept of FLPs to transition metals through the use of cationic zirconocene-phosphinoaryloxide complexes. 145Wass reported the synthesis of zirconocene-phosphinoaryloxide complexes 100 and their applications in the FLP activation of H 2 to generate 101 and activation of CO 2 to give the FLP-CO 2 adduct 102.102 showed no further reaction with H 2 , however 101 could insert into CO 2 under mild conditions to generate 103 (Scheme 56, top).A similar system also reported by Wass focuses on intermolecular zirconium/phosphorus FLPs where a zirconium(IV) cation 104 is combined with a tertiary phosphine.Activation of CO 2 occurred under mild conditions to yield the adduct 105 (Scheme 56, bottom). 146ystematic modication of the phosphine Lewis base showed that FLPs with modest Tolman steric parameters are highly reactive and have the maximum selectivity for the intended product.The base was found to affect the selectivity, and PEt 3 gave the cleanest results.These later ndings demonstrate that transition metal FLPs do not require intramolecular systems and allow for the construction of intermolecular transition metal frustrated or cooperative Lewis pairs.Another zirconium based FLP has been reported by Erker in the form of an intramolecular cationic geminal Zr + /P pair 106 which could react with CO 2 to form a ve-membered metallaheterocyclic adduct 107 (Scheme 57). 147Systems based on 106 have been the subject of theoretical studies for the reactivity of the Zr + /P pair system in the activation of CO 2 . 148Whereas, computational investigations reveal that the activation reaction The heavier group 4 metal hafnium has also been shown to undergo FLP-type CO 2 activation between the metal centre and a pendant Lewis basic centre on the ligand.
The hafninum complex 108 was found to react with one or two equivalents of CO 2 to give a series of monometallic and bimetallic CO 2 activated products (109-111) depending upon the substituents on the phosphine ligand (Scheme 58). 149In these complexes, the phosphinoamines binds to hafnium via the nitrogen atom, and binds weakly through the soer phosphorus atom.Reaction of metallocene cation complexes [Cp* 2 HfMe][B(C 6 F 5 ) 4 ] with trimethylsilyl-(diarylphosphino) acetylenes yielded internal phosphane stabilised hafnium cations [Cp* 2 Hf-C(Me)-C(SiMe 3 )PPh 2 ][B(C 6 F 5 ) 4 ].As with other vicinal compounds, Hf + /P 112 exhibits FLP-like reactivity and generates the adduct 113 when reacted with CO 2 (Scheme 59). 150n alternative approach is to use a transition metal to assist CO 2 activation as demonstrated by Streubel and co-workers.The 3-imino-azaphosphiridine complex 114 was prepared and reacted with CO 2 to obtain a heterocyclic compound 115 (Scheme 60). 151hile numerous creative strategies are being developed for CO 2 capture, there is an increasing interest in using carbon dioxide as a C1 carbon source.The hydrogenation of CO 2 to formic acid and its derivatives is one such well-developed strategy based on Ru. 152 In 2012, Stephan and co-workers developed an elegant catalytic system based on ruthenium hydride 116 (Scheme 61).This salt was explored in the reduction of CO 2 catalytically using HBpin as a reducing agent.One equivalent of 116 and 18 equivalents of HBpin under an atmosphere of CO 2 gave the MeOBPin product catalytically aer 96 h at 50 °C.Increasing the ratio of 116 : HBpin to 1 : 100 resulted in a small increase in the TON. 153In this system the RuNP ring in the catalyst is similar to FLP systems, and the binding of CO 2 depends upon the cooperative action of the Lewis acidic metal centre with one of the Lewis basic phosphine centres in the ligand.Cleavage of the C-P bond upon reduction with HBpin and a transfer of oxygen from Ru to Bpin allows the system for a catalytic hydroboration of CO 2 .A similar cooperativity between metal and ligand was observed by Crispin and coworkers 154  Scheme 62 Cooperative activation of CO 2 using an iridium catalyst.
Scheme 61 Catalytic reduction of CO 2 using a Ru-H salt and pinBH.

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These species can activate a range of small molecules including CO 2 (R]H) in an FLP fashion between a Lewis acidic iridium centre and a Lewis basic nitrogen atom on the pyridyl ligand (Scheme 62).
Similar to the tungsten system described earlier, other metals have been employed to assist classical FLPs in CO 2 reduction.A copper-hydride system has been developed by Bertrand and co-workers for the activation and reduction of CO 2 in synergy with a N/B FLP.Amongst several screened reactions, both stoichiometric as well as catalytic, the authors found that a catalytic system consisting of a (CAAC)CuH (CAAC = cyclic (alkyl)(amino)carbene) 118 with B(C 6 F 5 ) 3 in a 1 : 2 ratio and DBU (10 mmol) forms the formate salts of DBU from CO 2 (15 bar) and H 2 (45 bar) when heating the reaction mixture at 100 °C for 24 h in THF (Scheme 63).The key step in this reaction is the insertion of CO 2 into the copper hydride and regeneration of copper hydride with H 2 .While the Cu-H bond readily inserts CO 2 , it is difficult for copper to activate H 2 .Thus, the FLP assists by activating H 2 to allow regeneration of the copper hydride.The TON for this reaction is observed as 1881. 155inc metal has been explored for the reduction of CO 2 to CO. Stephan reported the in situ formation of the catalytically active species Et 3 P]C]PEt 3 through the reduction of CO 2 to CO employing CH 2 I 2 .This (bis)ylide was found to interact with CO 2 , eliminating the phosphine oxide Et 3 PO as a by-product and forming an interim phosphaketene.The addition of catalytic ZnBr 2 was found to facilitate the process through an FLP-type activation mode and was important for the regeneration of the (bis)ylide with simultaneous removal of CO (Scheme 64). 156The same authors described Zn-based FLP chemistry for functionalising CO 2 , using tBu 3 P/ZnEt 2 FLPs. 157he transition metal based Lewis acids in the activation of CO 2 benet from a higher coordination number when compared to the lighter main-group Lewis acids that were discussed previously.The ability of the TM species, such as in 110 and 116, to bind CO 2 as well as ligand systems in more intricate intermediary species offers a higher degree of netuning of the stability of adducts formed.Hence, higher turnover numbers may be observed for transition metal systems, as regeneration of the active catalyst species is kinetically more favourable.This may also explain for the more accessible use of H 2 as the reducing source compared to silanes or hydrogen surrogates oen used for the main group systems.This is seen for 118 and 100, whereas main group based Lewis acids frequently require pre-organised reducing sources, limiting the reaction to hydroboration or hydrosilylation of CO 2 rather than direct hydrogenation.Nonetheless, concerns of toxicity and environmental impact drive an increased interest in transition metal free reagents.The work on main-group CO 2 activation has focused on mimicking transition metals in synergistically accepting and donating electrons in the activation of CO 2 with species that combine lled and vacant orbitals.

Rare earth metals
Rare earth metals are oen employed as Lewis acids for a variety of reactions, and have also been employed as Lewis acids in FLPs.Dihydrogen was readily activated by combination of homoleptic rare-earth metal aryloxides, RE(OAr) 3 (RE = La, Sm, and Y) with N-heterocyclic carbenes (NHCs) under mild conditions.In addition, the La/NHC pair exhibited FLP-like reactivity towards carbon dioxide, affording 1,2-addition products 119, as shown in Scheme 65. 158

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Chemical Science centres in the solid structure activate CO 2 .Here, the understanding the chemistry of reactants, intermediates, and products on surfaces is crucial for designing catalytic nanostructures that transform carbon dioxide into carbon-based fuels.Several systems have been reported using indium as a Lewis acid.For example, indium oxide nanocrystals, In 2 O 3x (OH) y , can catalyse the reverse water gas shi reaction, reducing carbon dioxide to carbon monoxide and water. 159Surface hydroxide groups and oxygen vacancies facilitate this reaction as shown in Scheme 66.
The enhancement of activity in the gas-phase reverse water gas shi process has also been investigated, as well as the distinct photoactive behaviour of pristine and defective indium oxide surfaces. 160Based on TD-DFT calculations, this study discovered that surface FLP in In 2 O 3x (OH) y has Lewis acidic indium sites close to a Lewis basic surface hydroxide.These acquired more acidity/basicity making them more active in the excited state relative to the ground state.In the photochemical reaction this reduces the activation energy relative to the thermal reaction, and could provide a mechanism to design improved photocatalytic systems for solar fuel production.In 2018, Ozin and co-workers, reported a similar system based on a rod-like nanocrystal superstructure of In 2 O 3−x (OH) y , that could effectively catalyse the hydrogenation of CO 2 to methanol under light at atmospheric pressure.The rate of conversion was found to be 0.06 mmol g −1 h −1 with a 50% selectivity and a longterm working stability. 161nother heterogenous system based on indium has been developed by Wang and co-workers (Fig. 11).The authors developed a photocatalytic material based on a ZnIn 2 S 4 /In(OH) 3 − x heterojunction that works in a cooperative fashion to reduce CO 2 into CO driven by light.The ZnIn 2 S 4 functions to harvest the light and transport an electron to the FLP-activated CO 2 on the In(OH) 3−x surface.In In(OH) 3−x , the hydroxyl-decient vacancies (OH Vs ) acts as a Lewis acid, and the adjacent hydroxyl groups act as a Lewis base generating the FLP which activates CO 2 .This composite showed a CO formation rate of 1945.5 mmol g −1 h −1 . 162bove we have seen that heterogenous FLPs can capture and react with H 2 and CO 2 , boosting photocatalytic CO 2 reduction.Isomorphous substitution of In 3+ with Bi 3+ has been found to increase catalytically active surface FLPs.Isomorphous substitution optimises surface catalytic active sites and affects optoelectronic characteristics, improving our understanding of photocatalytic CO 2 reduction.Such isomorphous substitution will help to develop CO 2 reduction materials with higher catalytic performance by tuning surface FLP site strength. 163nother bismuth containing heterogenous system has been reported by Wang who synthesised Sn-doped BiOBr with oxygen vacancies.The synthesised material possesses surface frustrated Lewis acidic (bismuth) and Lewis basic (lattice oxygen) pairs in BiOBr through the substitution of Bi 3+ with Sn 4+ .4Sn-BiOBr showed the best performance for photocatalytic CO 2 reduction into CO with a yield of 165.6 mmol g −1 h −1 . 164Another main group heterogenous system also reported is B 3 P 3 doped hexa-cata-hexabenzocoronene, a model of nanographene (B 3 P 3 $NG) which reacted with carbon dioxide.This multi FLP device binds three CO 2 molecules sequentially or simultaneously on the B 3 P 3 $NG surface. 165For the CO 2 reduction via dissociative chemisorption of H 2 , nanocarbon-based FLP bifunctional catalysts are becoming promising due to their unquenched electron transport capability.One study proposes a nanocarbon-based FLP catalyst for the CO 2 reduction via the dissociative chemisorption of H 2 . 166he catalyst consists of nitrogen/phosphorus doped graphene and M(C 6 F 5 ) 3 (M = B, Al, Ga, In) as Lewis acids.The study demonstrates the potential of doped carbon-based FLPs as innovative nanostructure catalysts for CO 2 reduction via molecular hydrogen.N-doped FLP catalysts with activation barriers between 0.01 and 0.11 eV are promising for CO 2 reduction, potentially enabling CO 2 reduction catalytic material design.
Converting and storing solar energy through light-driven CO 2 reduction is a promising area of research.A particular approach for converting CO 2 to methane gas uses hydroxyls inherent on an oxyhydroxide photocatalyst, such as in CoGeO 2 (OH) 2 , as a proton source.Irradiation of CoGeO 2 (OH) 2 causes the lattice hydroxyls to be oxidised by photogenerated holes, leading to the formation of oxygen vacancies (OVs) and protons.These OVs and Lewis acid-base pairs bind CO 2 and protons to activate it before reducing it to CH 4 .In the presence

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of water molecules, the surface lattice hydroxyls regenerate, allowing for continuous CO 2 conversion as shown in Scheme 67.This strategy has the potential to pave the way for a novel use of photocatalysis in the eld of energy conversion. 167inc based heterogenous systems have also been reported.Neumann and co-workers studied the coordination of CO 2 to a Zn(II) Lewis acid site in Wells-Dawson type polyoxometalates a 2 -(P 2 W 17 O 61 Zn) 8− which bound CO 2 in an FLP fashion (Fig. 12).This system reveals two distinct binding modes: stronger "side-on" binding at higher temperatures and weaker "end-on" architectures at lower temperatures. 168This interaction with 2,4,6-collidine is possible through the development of a frustrated Lewis pair at lower temperatures.
Like the indium oxide systems described above, efficient photocatalysts for CO 2 reduction have also been reported using titanium.Anatase TiO 2−x hierarchical hollow boxes with FLPs can be synthesised through in situ topological modication of perovskite as shown in Fig. 13.These structures possess strong adsorption and activation properties, converting CO 2 to CO without auxiliary substances.This innovative approach converts solar energy into chemical energy. 169he above examples of heterogeneous FLPs are important for the activation of CO 2 compared to synthesising FLP-CO 2 adducts.
There are some challenges to select suitable methods for achieving light absorption, electron-hole separation, energy gap matching for the reduction of CO 2 to different products (product selectivity) in a photochemical way.The examples In 2 O 3x (OH) y , ZnIn 2 S 4 /In(OH) 3−x , 4Sn-BiOBr, and CoGeO 2 (OH) 2 are promising systems for the photochemical reduction of CO 2 .The use of clean sources of the reducing agent H 2 in these systems will suppress the chemical waste which is generated when activated reducing agents for the reduction of CO 2 adducts are used.
Several reports of cerium as a Lewis acid in heterogenous FLPs are reported.Qu synthesised a defect-enriched cerium oxides (CeO 2 ) with constructed interfacial FLPs (Ce 3+ $$$O 2− ) that activate CO 2 efficiently via the interactions between the carbon atom of the CO 2 molecule with the Lewis basic lattice O 2− in CeO 2 , and the two oxygen atoms of CO 2 with two adjacent Lewis acidic Ce 3+ centres in CeO 2 .This CeO 2 solid material showed FLP-inspired tandem activation of CO 2 and reactions with alkenes to catalytically form selective cyclic carbonates. 170avide et al. reported that CO 2 activation is shown to occur via a bidentate carbonate bridging the FLP through a Ce 3+ -to-CO 2 charge transfer (Scheme 68). 171The authors performed a detailed study of the system in which an FLP was formed over a highly defective sample of CeO 2 .
The reaction of CO 2 with MeOH formed monomethylcarbonate through an FLP mechanism involving Ce 3+ and oxygen vacancies.
Recently, other cerium based FLP systems have also been explored for the reduction of CO 2 into products such as CH 4 and carbonates. 172[175] Stoichiometric and catalytic reduction of CO 2 : scope and limitations So far, we have witnessed a wealth of FLP systems for CO 2 activation and reduction to different products.In this nal section we summarise the key ndings of the different FLP systems described in terms of CO 2 activation and the stability of the CO 2 adducts, as well as the CO 2 reduction strategies using hydrogen, silane or borane reducing agents.

An insight into the stability of CO 2 adducts
In this review many different FLP adducts with CO 2 are discussed.Stability of the FLP-CO 2 adducts is dependent on different factors, such as the state of the system (solid or solution phase), temperature and conditions (e.g.such as applying vacuum), the strength of the Lewis base-C (CO 2 ) and Lewis acid-O (CO 2 ) bonds, steric effects around the ligand attached to the acidic or basic reactive centre, and the geometry of the FLP system either as intra-or intermolecular systems.Several systems undergo reversible CO 2 activation which is highly  dependent upon the geometry of the FLP (intra-or intermolecular) as well as the electronic and steric effects at the Lewis acid and basic sites.Among the described examples of the FLP systems for CO 2 activation, the rst was described for the B/P intra-and intermolecular systems.The FLP tBu 3 P/B(C 6 F 5 ) 3 with CO 2 was observed to form a stable adduct tBu 3 P-CO 2 -B(C 6 F 5 ) 3 at room temperature but, upon heating under vacuum releases CO 2 and regenerates the FLP.The same outcome was observed for intramolecular system 2, which produces a cyclic adduct with CO 2 with lower stability which decomposes even at −20 °C. 34When the Lewis base and acid are aligned in a geminal fashion, an increase in reactivity is observed as seen in the formation of adduct 10 with a non-uorinated FLP. 42A unique binding mode of CO 2 in the FLP system bis-borane was observed that resulted in compound 9 as a six-membered stable adduct in which two boron Lewis acidic centres in the FLP bind to the two oxygen atoms in the CO 2 molecule.This, however, was not the case for the FLP tBu 3 P/O(B(C 6 F 5 ) 2 ) 2 , where chelation of CO 2 by two B-centres was not observed due to steric effects and as well as a signicant p-character in the B-O bonds supported by crystal structure information. 41With the more Lewis acidic and oxophilic aluminium Lewis acids, more stable CO 2 adducts were typically observed and in several cases both oxygen atoms of CO 2 bound to a Lewis acidic site, through coordination to two aluminium centres for example in compounds 74-76 and 78. 103,111The binding strength could be tuned by varying the substituents on aluminium.The more Lewis acidic centres -AlCl 2 and -Al(C 6 F 5 ) 2 bind CO 2 irreversibly, while less Lewis acidic -AlMeCl binds CO 2 reversibly under 2 bar CO 2 , liberating CO 2 when the excess pressure is released.For catalytic applications, this reversible binding is necessary to enable release of the product.In several cases, not only CO 2 activation is observed but also further reactivity of the adduct with the FLP to generate more stable CO 2 activated products.These reactions, however, would be irreversible and therefore stoichiometric.Examples of this have been observed in the B/N FLP 34 in which a cyclohexyl group migrates from boron to an adjacent carbon centre. 72Overall, in the activation of CO 2 molecule specially in the homogeneous FLP-CO 2 adduct systems, a species that can reversibly form weak adducts of CO 2 with almost no energy barrier in either direction would be an incredible valuable tool to enable catalytic transformations.Alternatively, combinations of LA and LB are promising that show reversible CO 2 binding.Homogeneous and heterogenous metal based FLP systems have been shown to be very promising in the activation of CO 2 through an FLP mechanism and offer much promise for catalytic turnover (see later).

CO 2 reductions in FLP systems
As we have seen in this review, the binding and activation modes of small molecules (in this case CO 2 and H 2 ) are different in homogeneous (metals and non-metal in inter-and intramolecular) and heterogeneous FLP systems.In homogeneous systems, the combination of Lewis  One strategy to overcome this could be to use inverse frustrated Lewis pair systems.This was observed for the use of excess Lewis base DBU in combination with the Lewis acid tbtb (tris(pbromo)tridurylborane), an inverse FLP as shown in Scheme 31. 83Another example of catalytic hydrogenation of CO 2 to formate was explored by using K 2 CO 3 /B(C 6 F 5 ) 3 with H 2 . 102etal systems as a Lewis basic centre in combination with a Lewis acid show a different way of activating CO 2 and coordinate to the CO 2 molecule through the C]O bond (Fig. 14B) as

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seen with Lewis basic Pt, Re and Mo systems.The Lewis acidic component of the FLP then activates the oxygen atom.CO 2 can be found in a reduced state with a Re system where reduction to formate is observed through CO 2 hydrogenation using H 2 .
When the transition metal is incorporated as the Lewis acid component of an FLP, then the mode of CO 2 activation is similar to that observed for the main group Lewis acids with TM-O bond formation (Fig. 14C).Here, it is interesting to note that the system before reaction with CO 2 can cleave H 2 heterolytically, similar to other FLPs and the TM-H bond then inserts into the CO 2 molecule.An example for this type of reaction is shown for cationic zirconocene-phosphinoaryloxide complexes in Scheme 56.Heterogeneous FLPs systems activate CO 2 using a similar concept but via a different mechanism (Fig. 14D) and are generally not limited by some of the challenges that main group systems face.In these systems the surface has Lewis acidic metal sites such as indium or titanium.However, the Lewis basic sites are typically an oxygen atom (oen as a hydroxy group).This tolerance to hydroxy functional groups is a signicant advantage in the heterogenous systems and provides easier routes for CO 2 reduction.In many of the main group FLP systems described herein the strong binding to oxygen centres, while benecial for CO 2 activation, is detrimental to catalytic turnover.
CO 2 reduction by FLPs: using H 2 , activated B-H and Si-H Several catalytic reduction methods have been developed utilising different FLP systems using different reducing agents.For CO 2 reduction, direct hydrogenation provides the best approach as it is clean and generates no waste.However, many FLP systems, especially those using the main group elements, have used other reducing agents such as silanes or boranes due to the inability of the system to activate H 2 in conjunction with CO 2 .Thus, although some FLPs showed promising results for the CO 2 reduction with a direct use of H 2 gas as a reducing agent, heterogeneous systems have shown to be more promising with the direct use of H 2 .As described by several computational studies considering hydrogenation of CO 2 with only H 2 as the reducing agent, the energy barrier to the activation of H 2 by the FLP system is generally higher than the activation barrier for that of CO 2 .Fine-tuning of both the Lewis acid's ability to accept a hydride from H 2 and the Lewis base's ability to accept a proton from H 2 is necessary to consequently reduce activated CO 2 .The computational work herein have highlighted the importance for a cumulative high Lewis basicity and acidity, whilst avoiding the combination of a very strong Lewis acid with a very strong Lewis base as this negatively impacts both the activation barriers to H 2 and CO 2 .
For many systems seen in this review, boron reducing agents have commonly been employed in homogeneous systems, for example R 2 BH/HBpin.Here either the Lewis base or acid activates the reducing agent towards reduction, as shown in Fig. 15.The rst mode of CO 2 reduction with R 2 BH and the Lewis base is the nucleophilic activation of R 2 BH.Strong nucleophiles favor hydride transfer to CO 2 by increasing the hydridicity of the B-H bond (Fig. 15A).Alternatively, the Lewis acids may abstract the hydride of the B-H bond to yield a boron electrophile and convert the Lewis acid catalyst to a strong hydride donor (Fig. 15B). 176In these two mechanisms, the Lewis acid component is then trapped by the generated oxygen anion.In these cases, the formation of stable CO 2 adducts oen hampers catalysis by stabilising the catalyst's resting state.As we have seen above, the FLP catalyst can also activate the CO 2 molecule directly.As CO 2 is a weak Lewis base, this mode of action usually involves a bifunctional activation of CO 2 with the cooperative effect of a Lewis base and a Lewis acid (Fig. 15C).The borohydride reducing agent then can directly hydrogenate this species.In many cases, "activation" of CO 2 in the form of an adduct is both deleterious and necessary to the catalytic activity, as it stabilises the lowest intermediate in the potential energy surface yet prepares CO 2 for the subsequent reduction steps by removing electron density from the carbon by coordination to the Lewis acid.It is noteworthy that future catalytic systems based on this approach should be target compounds that show a lower affinity for CO 2 , based on thermodynamics, yet increase the electrophilicity of the carbon centre.Instead of borane reducing agents, silanes such as Et 3 SiH have also been used for catalytic CO 2 reduction.For example, using R 3 SiH with a Lewis acid, an electrophilic activation is observed similar to the way shown Fig. 15B.Although several examples use H 2 as the reducing agent, this remains a challenge with main group systems and more success has been observed in this regard when using either homogenous or heterogenous transition metal systems.

Stoichiometric and catalytic reduction of CO 2 in FLPs
Various homogeneous and heterogeneous FLPs systems have been investigated for the reduction of CO 2 .It depends on certain properties of the FLP system to guide the CO 2 reduction either for a stoichiometric or a catalytic reaction pathway.For CO 2 adducts, a strong Lewis base and a weak Lewis acid adduct of CO 2 appear suitable for catalytic CO 2 reduction whereas stronger Lewis acids form stable CO 2 adducts.To carry out a catalytic reduction of CO 2 in a zwitterionic system, a rst condition is that the bond between O atom of CO 2 and the Lewis

Review
Chemical Science acid should be weak, i.e. the Lewis acid should not be too acidic or oxophilic (see Fig. 16).Secondly, the cation of the activated reducing agents (R 2 B-H or R 3 Si-H) aer supplying hydride ion should be able to trap the oxygen atom, cleaving the O-LA bond.
Here, both the Lewis acid and base are not coordinated, and the system will be able to reversibly activate and reduce CO 2 leading to a catalytic pathway.Several CO 2 adduct systems have been discussed for catalytic CO 2 reductions.6][57] In the tBu 3 P/9-BBN FLP system, hydride transfer from boron to the carbonyl carbon releases tBu 3 P for the next cycle, and hence this system also works catalytically. 58Another interesting catalytic example is for the TMP/B(C 6 F 5 ) 3 FLP system, where Et 3 SiH is employed as a silane reducing agent producing Et 3 Si + following B(C 6 F 5 ) 3 Si-H activation. 78Et 3 Si + is a good oxygen acceptor and thus promotes the catalytic deoxygenation of CO 2 to CH 4 and (Et 3 Si) 2 O.Although catalytic, the stoichiometric use of silane is not desirable in the longer term and routes that employ H 2 as a hydrogen source should be sought.Making use of FLPs in direct catalytic hydrogenations of CO 2 remain difficult.
Although hydride transfer from [LA-H] to CO 2 have been described in this review, the proton transfer from [LB-H] does not occur readily due to the formation of a strong O-LA bond (Fig. 16).Hence, most FLP systems are seen to terminate at adduct formation as O-LA, and although the H 2 activation barrier may have been overcome, full hydrogenation of CO 2 is prohibited.Some success has been achieved with inverse FLP systems (e.g.tris(p-bromo)tridurylborane (tbtb)/DBU). 83To obtain a suitable FLP for the direct catalytic hydrogenation of CO 2 , the intrinsic reactivity of each of the components is very important i.e., free energy of proton attachment to the Lewis base and free energy of hydride attachment to the Lewis acid.
For high turnover numbers and turnover frequencies, a catalyst should be stable enough and should regenerate in the catalytic cycle.In some of the examples, the highest in carbene system 58 using 9-BBN as the reducing agent, FLPs have shown a good TON and TOF for the reduction of CO 2 .As shown in the latter part of this review, metals and heterogeneous FLPs are generally efficient to activate and use H 2 as a direct reducing source for CO 2 .For example, indium oxide has been found to be a particularly good example of a heterogeneous FLP system where H 2 is directly utilised for the reduction of CO 2 . 159The potential of transition metals is that they provide reactive sites for the activation of H 2 as well as CO 2 , but in several cases, they are not cost efficient.

Conclusions
Tremendous progress in CO 2 activation and reduction to value added products in both homogeneous and heterogeneous systems on bench scale has been seen.Both homogenous and heterogenous transition metal systems, as Lewis acids and bases have been efficient for the reduction of CO 2 .However, main group elements have emerged as alternatives and remarkable progress has been made here.Group 13 elements, boron and aluminium as Lewis acids have been heavily explored combined with Lewis bases such as phosphines, amines, or carbenes.In many cases, activation of CO 2 has been achieved up to full conversion under mild conditions, and in several examples reduced products can be obtained in the form of methanol, formates, acetates and CH 4 upon addition of a silane or borane, or in some cases H 2 .Most commonly, the formation of a zwitterionic product is the initial key activation step in the reduction of CO 2 .However, subsequent product liberation has been seen to be the most limiting step in these reactions, thus limiting the scope of catalytically viable reactions.Currently H 2 is utilised as a reducing agent on industrial scale as it is cheap and widely available, but its use in CO 2 reduction with FLPs is limited and other reducing agents such as hydrosilanes, hydroboranes or ammonia boranes are oen used.However, a drawback of these reducing agents in CO 2 reduction is that they form strong Si-O and B-O bonds and form oxidised products such as siloxanes or boroxanes, making the process less atom economic.Thus, there still need to be signicant development of FLP CO 2 reduction using H 2 as the reducing agent.Compared to main group FLP-CO 2 adduct systems, transition metals possessing empty orbitals that offer site selective coordination have been applied as more suitable systems to activate the non-polar covalent bond in H 2 and utilise this as a direct reducing source.Conversely, heterogeneous systems provide the opportunity of recycling and have also been seen to achieve higher TONs and TOFs.Present chemical methods of recycling siloxanes or boroxanes to the corresponding hydrosilanes or hydroboranes are energy intense.Electrochemical methods are efficient and have made material recycling possible.Therefore, electrochemical methods may be an attractive and efficient approach over chemical routes for recycling of the oxides to their corresponding hydrides.The key to obtain high TON for hydrosilylation, hydroboration, or hydrogenation relies on the stability of the catalyst, the release of the reduced products and then catalyst regeneration.Thermodynamic control is the main limitation in accessing CO 2 reduced products beyond carboxylates, whilst kinetic control is the main limitation in achieving high output catalytic cycles that regenerate the catalyst.Although some progress has been made to achieve CO 2 reduction in a catalytic manner, the challenge is still to achieve a robust and effective FLP system that can catalyse CO 2 reduction at ambient temperature and pressure utilising H 2 as a reducing agent.In addition, there must be more focus on selective reduction reactions to give valuable products that are of interest to industry.Further investigations should focus on developing highly active and selective FLP systems for the catalytic conversion to C 1 or C 2 products, and could include various methods for conversion such as thermal, photochemical, or electrochemical methods.Chemical Science Review

Fig. 3
Fig. 3 Frontier molecular orbital presentation of (a) a classic Lewis acid-base adduct and (b) a frustrated Lewis pair.LA = Lewis acid; LB = Lewis base.

8
was restrained due to steric crowding as well as a signicant pcharacter in the B-O bonds, which was evident from the relatively large B-O-B bond angle of 139.5(2)°.Whereas, in 9 B-C-B angles of 117.3(2)°(R = Cl) and 121.2(2)°(R = C 6 F 5 ) show a six membered planar structure.

Scheme 10
Scheme 10 Reaction of geminal vinylidene-bridged P/B Lewis pair with CO 2 .
2 reduction to methane with Et 3 SiH catalysed by the ion pair [TMPH] [HB(C 6 F 5 ) 3 ] in combination with B(C 6 F 5 ) 3 in detail.The mechanism proposed by Piers was conrmed to be energetically feasible in this study.The reduction proceeds via CO 2 insertion into [TMPH][HB(C 6 F 5 ) 3 ], followed by three successive hydride transfers from Et 3 SiH to the CO 2 centre.It was conrmed that the insertion of CO 2 into the H-B bond of [TMPH][HB(C 6 F 5 ) 3 ] proceeds in a stepwise manner with H d+ and H d− in the salt rst transferring to CO 2 to form 41 (Scheme 27, insert).

Scheme 33
Scheme 33 Reaction of carbenes with CO 2 .
and is delocalised over the Si-B-Si unit.Compound 59 shows single electron transfer reactivity towards B(C 6 F 5 ) 3 forming a frustrated radical pair [(SiNSi)B]c + [B(C 6 F 5 ) 3 ]c − .The reaction of 59 with CO 2 (1 atm) in C 6 D 6 at room temperature forms a new product 60 by cleaving CO 2 which, upon hydroboration with two equivalent of 9-BBN, forms compound 61 in quantitative yields.The structure of 61 showed that boron and silicon atoms are bridged by boryloxymethylene (CHOBR 2 ) formed by the hydroboration of the C]O group.The Si-O-B bridge in 60 was cleaved along with the formation of BH and SiOBR 2 units.Compound 61 can also be obtained directly by treating 59 with CO 2 and 2 equivalents of 9-BBN in C 6 D 6 at room temperature with an isolated yield of 45% (Scheme 40).The catalytic performance of 59 (5 mol%) and 60 (5 mol%) shows an efficient transformation of N-methylaniline into the corresponding formamide (92% yields in each case) by capturing and hydroborating CO 2 with HBpin (Scheme 40).

Scheme 39
Scheme 39 Preparation of zwitterionic boron diformate 58 and its use as a catalyst for the reduction of CO 2 .

Fig. 9
Fig. 9 Substrates and intermediates involved in the FLP activation of CO 2 .

Fig. 10
Fig. 10 Phosphorus as a Lewis acid in CO 2 capture.
Scheme 58 CO 2 adduct formation using an intramolecular hafnium FLP.

Fig. 11
Fig. 11 An FLP ZnIn 2 S 4 /In(OH) 3 − x (ZIOS) heterojunction system for the reduction of CO 2 to CO. Scheme 67 Heterogenous Ge FLP in the reduction of CO 2 to CH 4 .M = Metallic element.
base and acid in frustrated pairs typically cleave the H-H bond heterolytically and form ion pairs [LB-H] + [LA-H] − .[LA-H] − acts as a hydride source and can reduce CO 2 by transferring H − to the carbon atom.The oxygen anion generated is then trapped by the Lewis acid generating formates (Fig. 14A).The formation of formate salts of CO 2 with H 2 surrogates [LB-H] + [LA-H] − are shown as examples in Schemes 12, 24 and 25.To utilise this strategy for CO 2 reduction in a catalytic way has been studied computationally showing a possible reduction of CO 2 to HCO 2 H in Schemes 15 and 16 but practically has not been demonstrated.For a practical feasibility, the ion pair [LB-H] + should be able to supply H + to the formed formate ion [HCOO-LA] − .If this occurs, then the FLP would catalyse hydrogenation of CO 2 , but the limiting factor is the release of the formate from [HCOO-LA] − due to the strength of the O-LA bond.In other words, for a catalytic hydrogenation of CO 2 using FLPs, the ion pair [LB-H] + [LA-H] − should regenerate the free Lewis acid and base following CO 2 reduction.This remains a key challenge in main group FLP-CO 2 reduction and the use of a strong Lewis acid oen precludes product release.
Scheme 47 Mo based FLP for CO 2 activation.BAr F 24 = tetrakis(3,5bis(trifluoromethyl)phenyl)borate.Chemical Science Review more common carbon linker.Like the carbon-linked Al/P FLP reported earlier by Fontaine, this reacts with CO 2 to give a 2 : 2 eight-membered ring product 72 by insertion of CO 2 into the Allinker bond with cis-and trans-isomers. 109Limberg et al. utilised a biphenylene backbone to prepare a strained intramolecular P/ Al-based FLP which was reacted with CO 2 (2 bar) at room temperature for 5 minutes in deuterated dichloromethane to obtain adduct 73. 110They also reported xanthene-linked intramolecular Al/P FLPs containing two Al centres which are able to activate CO 2 . 111