Louis Monsigny

Jean-Claude Berthet

Thibault Cantat
email: thibault.cantat@cea.fr

Depolymerization of Waste Plastics to Monomers and Chemicals Using a Hydrosilylation Strategy Facilitated by Brookhart's Iridium(III) Catalyst

Keywords: Hydrosilylation, Depolymerization, Polyesters, Polycarbonates, Plastic waste, Iridium, Catalysis

Plastic waste management is a major concern. While the societal demand for sustainability is growing, landfilling and incineration of waste plastics remain the norm and methods able to efficiently recycle these materials are desirable. Herein, we report the depolymerization, under mild conditions, of oxygenated plastics in the presence of hydrosilanes with the cationic pincer complex [Ir(PCP)H(THF)][B(C6F5)4] (PCP = 1,3-(tBu2P)2C6H3)) as catalyst. The iridium catalyst, with a low loading (0.3-1 mol%), proves selective toward the formation of silyl ethers or the corresponding alkanes depending only on the reaction temperature. It is noteworthy that the depolymerization of real household waste plastics such as PET (from plastic bottles) and polylactic acid (PLA) from 3D printer filaments is not altered by the presence of dye or other plastic's additives.

Introduction

Plastics are ubiquitous in our modern society. Thanks to their lightness, their ease to be molded and their low costs, these polymers have forced their use in various applications such as electronics, packaging, medicine, etc. It is estimated that 4-6 % of the annual production of fossil resources are used to supply the 335 million tons of plastics consumed worldwide. [START_REF]Plastics The Facts[END_REF] In consequence, millions of metric tons of waste are generated every year, which are either incinerated or deposited in landfills. This open cycle poses ecological problems, causing pollution issues and emissions of large quantities of the greenhouse gas CO2. [START_REF] Barnes | Accumulation and fragmentation of plastic debris in global environments[END_REF] The increasing awareness of the environmental issues associated with plastic wastes generates a public demand for sustainability and renewability in the plastic industries. In this context, a conglomerate of industries and associations [START_REF]The New Plastics Economy -Catalysing Action[END_REF] has called for the implementation of a circular economy of plastics lying on three main axes: drastically reducing the leakage of wastes in the environment, decoupling plastic production from fossil resources and up-taking recycling processes. [START_REF] Hong | Chemically recyclable polymers: a circular economy approach to sustainability[END_REF] The reuse of manufactured polymers remains challenging because of the high density of chemical functionalities in the polymer matrix, the presence of additives in plastics and/or the blend of different materials. Besides incineration and landfilling, physical and chemical recycling technologies are appealing approaches to improve the conservation of carbonaceous materials. [START_REF] Ignatyev | Recycling of polymers: a review[END_REF] The former processes consist of melting and remodeling the plastic, although the second generation plastics have more limited usages because of the presence of impurities. In contrast, chemical processes can afford the recovery of pure chemicals, useful for the production of recycled virgin quality plastics as well as new chemicals. [START_REF] Rahimi | Chemical recycling of waste plastics for new materials production[END_REF] In this respect, the development of new depolymerization systems recently emerged in the scientific community. [START_REF] Hong | Chemically recyclable polymers: a circular economy approach to sustainability[END_REF][START_REF] Ignatyev | Recycling of polymers: a review[END_REF][START_REF] Rahimi | Chemical recycling of waste plastics for new materials production[END_REF] Classical aminolysis, hydrolysis or transesterification reactions have been widely repurposed in the catalytic depolymerization of polyesters [START_REF] Fukushima | Organocatalytic depolymerization of poly(ethylene terephthalate)[END_REF][START_REF] Gardea | Hybrid Poly(aryl ether sulfone amide)s for Advanced Thermoplastic Composites[END_REF][START_REF] Leibfarth | Transforming polylactide into value-added materials[END_REF] and polycarbonates. [START_REF] Quaranta | Rare Earth metal triflates M(O3SCF3)3 (M = Sc, Yb, La) as Lewis acid catalysts of depolymerization of poly-(bisphenol A carbonate) via hydrolytic cleavage of carbonate moiety: Catalytic activity of La(O3SCF3)3[END_REF][START_REF] Quaranta | Depolymerization of poly(bisphenol A carbonate) under mild conditions by solvent-free alcoholysis catalyzed by 1,8-diazabicyclo[5.4.0]undec-7ene as a recyclable organocatalyst: a route to chemical recycling of waste polycarbonate[END_REF] Alternatively, the cleavage of C-O bonds can be achieved through reduction methods. [START_REF] Chauvier | A Viewpoint on Chemical Reductions of Carbon-Oxygen Bonds in Renewable Feedstocks Including CO2 and Biomass[END_REF] Robertson and co-workers tackled the recycling of waste plastics via the catalytic hydrogenation of oxygenated polymers with the soluble Ru(II) pincer complexes 1 and 2 (Scheme 1). [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] Selective deconstruction of a wide range of polycarbonates and polyesters into the corresponding valuable alcohols or carboxylic acids was achieved with excellent yields. Hydrogenation of the polymers proceeded in relatively harsh conditions (54 atm H2, 160°C). Taking advantage of the kinetically and thermodynamically favored hydrosilylation of carbonyl functions, our group recently reported the efficient catalytic reduction of a range of natural and manufactured oxygenated polymers, at room temperature, with B(C6F5)3 (3) as a catalyst. [START_REF] Feghali | Room temperature organocatalyzed reductive depolymerization of waste polyethers, polyesters, and polycarbonates[END_REF] Nonetheless, this Lewis acid catalyst is incompatible with a number of solvents and its price is close to precious metal (B(C6F5)3 ~ 110 €/mmol, [Ir(COD)Cl]2 ~ 40 €/mmol of Ir, COD = 1,5-cyclooctadiene [Sigma-Aldrich]). It is also poorly selective, and demands high loadings (2-10 mol%). These drawbacks call for more stable, efficient catalysts.

Scheme 1: Catalysts for the reductive depolymerization of plastics and polymers

The cationic Ir(III) complex 4 is a well-known hydrosilylation catalyst exhibiting similar chemical behavior as borane 3 [START_REF] Robert | Si-H bond activation: bridging Lewis acid catalysis with Brookhart's iridium(III) pincer complex and B(C6F5)3[END_REF] (Scheme 1). The electrophilic activation of hydrosilanes by Lewis acid 4 favors reduction of oxygenated functions such as carbonyl [START_REF] Park | Hydrosilation of Carbonyl-Containing Substrates Catalyzed by an Electrophilic eta-Silane Iridium(III) Complex[END_REF] or carboxyl [START_REF] Park | An efficient iridium catalyst for reduction of carbon dioxide to methane with trialkylsilanes[END_REF] groups as well as ethers, [START_REF] Yang | Scope and mechanism of the iridium-catalyzed cleavage of alkyl ethers with triethylsilane[END_REF][START_REF] Park | Hydrosilylation of epoxides catalyzed by a cationic eta1-silane iridium(III) complex[END_REF] leading to the corresponding silylated alcohols or alkanes. [START_REF] Metsanen | Insight into the mechanism of carbonyl hydrosilylation catalyzed by Brookhart's cationic iridium(III) pincer complex[END_REF] While complex 4 proved to be very efficient in hydrosilylation reactions with organic substrates, its utilization in the reduction of poly-functional materials is scarce [START_REF] Mclaughlin | Iridium-catalyzed hydrosilylative reduction of glucose to hexane(s)[END_REF][START_REF] Monsigny | Efficient reductive depolymerization of hardwood and softwood lignins with Brookhart's iridium((III)) catalyst and hydrosilanes[END_REF] and Brookhart et al. reported the unique example of depolymerization of polyethylene glycol (PEG) into silylated ethylene glycol with 4 within 4 h at 65°C. [START_REF] Yang | Scope and mechanism of the iridium-catalyzed cleavage of alkyl ethers with triethylsilane[END_REF] Here we report the efficient use of the Brookhart's complex in the catalytic depolymerization of polyesters and polycarbonates with hydrosilanes. Silyl ethers or hydrocarbons are formed selectively by controlling the temperature of the reaction, thus demonstrating the wide applicability of the iridium(III) catalyst in the depolymerization of oxygenated polymers.

Results and discussion

One of the major drawbacks in the chemical treatment of polymers is generally their insolubility in classical organic solvents which can prevent reactions or considerably slow down their kinetics. The first attempt at hydrosilylation of plastics was thus undertaken with low melting point polymers, such as the aliphatic polycaprolactone (PCL) (mp = 60°C), to form homogeneous or biphasic solutions by heating. Surprisingly, at room temperature, insoluble commercial pellets of PCL (diameter ≈ 0.4 cm) rapidly reacted in presence of the mild reductant triethylsilane (Et3SiH, 3 eq) and of the iridium catalyst 4 (0.3 mol%) in chlorobenzene. After 2 hours, no residual solid remained, [START_REF]Plastics The Facts[END_REF] H NMR and DEPT 135 [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] C NMR as well as GC-MS analyses of the clear solution revealed a complete and selective depolymerization of PCL into the silylated 1,6-hexanediol (5), obtained as the unique product in quantitative yield (Scheme 2, eq. 1). Compound 5 was isolated pure in a preparative scale (94% from 3 mmol of PCL).

Successful depolymerization of PCL under these mild conditions led us to consider a variety of aliphatic and aromatic polyesters which differ by the nature of the spacer between the carboxyl groups and with higher melting points than PCL. Though PCL, poly(dioxanone) (PDO), and poly(lactic acid) (PLA) are biodegradable polymers, their chemical recycling remain relevant to conserve the raw carbonaceous materials as well as to avoid, upon degradation, the release in the environment of CO2 (or CH4 in anaerobic conditions) [START_REF] Luckachan | Biodegradable Polymers-A Review on Recent Trends and Emerging Perspectives[END_REF][START_REF] Kubowicz | Biodegradability of Plastics: Challenges and Misconceptions[END_REF] and the additives present in the plastic matrix. PDO was found inert in reduction conditions used for PCL but further heating to 65 °C proved sufficient to successfully depolymerize PDO into a mixture of the two silylethers DEG-Si and EG-Si along with gaseous ethane and the siloxane Et3SiOSiEt3 (Scheme 2, eq. 2). While DEG-Si was the major product (82%), a complete selectivity could not be achieved suggesting that the hydrosilylation of PDO and DEG-Si are concomitant. Formation of EG-Si was reached in excellent yield (89%) under forcing conditions, when using an excess of the hydrosilane (10 equiv.) with higher temperature and prolonged reaction time (90 °C, 48 h) (Scheme 2, eq. 3). A similar behavior has been observed with PLA (Scheme 2, eq. 4). Although the depolymerization of PLA required longer reaction times than PDO (60 h vs 48 h), the reaction occurs in presence of the iridium complex 4 (0.5 mol%) and Et3SiH (3 equiv.) to yield the silylated propylene glycol PG-Si (64%) and the over-reduced product n PrOSi (31%). As for the deconstruction of PDO, the reduction of PLA and PG-Si seems to be kinetically competitive and PG-Si and n PrOSi were observed simultaneously during the progressive reduction of PLA (see SI, section II.5). Nonetheless, 1 mol% of the Ir catalyst and an excess of the hydrosilane (10 equiv.) were sufficient to yield to n PrOSi in 92% yield along with ethane and Et3OSiEt3 at 90 °C (Scheme 2, eq. 5). Interestingly, strictly similar results have been obtained from the treatment of a soluble ground powder of commercial PLA as well as large insoluble pieces of coloured PLA from 3D printer plastic (white, blue and red; ≈0.4x0.3x0.3 cm), showing the high tolerance of the Ir catalyst 4 towards the contaminants present in these PLA plastics (dyes and additives).

Despite the structural similarities between polyglycolic acid (PGA) and poly-3-hydroxybutyric acid (P3HB) with PLA, these materials were left untouched in the presence of 4 and Et3SiH regardless the quantity of reductant, the temperature and the reaction times (up to 2 weeks) (Scheme 2, eq. 6). These distinct behaviours in hydrosilylation are obviously not related to different steric congestions nor to the solubility of the polymers (soluble PLA vs insoluble

PCL).

We then investigated the depolymerization of PES (polyethylene succinate), an aliphatic biodegradable copolymer of ethylene glycol and butanedioic acid. In presence of excess Et3SiH (6 equiv.) and a 0.3 mol% 4, PES was readily depolymerized at 65 °C to the silylated butanediol 6 and ethylene glycol EG-Si (Scheme 3, eq. 7). Interestingly, the disilylether 6 was produced almost quantitatively (99%). EG-Si was obtained in lower yield (23%) due to its further reduction to ethane and siloxane Et3SiOSiEt3. PET from commercial Dacron© fibers or small chunks of Evian© bottles were finely ground and dried. The powders were then converted at 70°C, using slight excess Et3SiH (6 equiv.) and 1 mol% of the iridium complex 4, into two silylethers: silylated 1,4-benzenedimethanol (BDM-Si) and EG-Si in 63% and 48% yield (Scheme 3, eq. 8).

Scheme 3: Reductive depolymerization of PES, and PET with Et3SiH and catalyst 4

Ethane and Et3SiOSiEt3 were also present as products of the reduction of EG-Si and prolonged heating (2 weeks at 90 °C) led to para-xylene (1,4-Me-benzene) and ethane. The formation of such silyl ethers is attractive as these products can be used as sources of alkoxide groups in Ullman's coupling reactions to prepare ethers. [START_REF] Milton | Palladium-catalysed synthesis of aryl-alkyl ethers using alkoxysilanes as nucleophiles[END_REF][START_REF] Selvakumar | Copper-mediated etherification of arenes with alkoxysilanes directed by an (2-aminophenyl)pyrazole group[END_REF][START_REF] Bhadra | Decarboxylative etherification of aromatic carboxylic acids[END_REF][START_REF] Kim | DBU-Mediated Mild and Chemoselective Deprotection of Aryl Silyl Ethers and Tandem Biaryl Ether Formation[END_REF] Alternatively, the useful alcohols could be obtained through hydrolysis of the silyl ethers. In the case of PET, hydrolysis of BDM-Si affords the diol 1,4-benzenedimethanol (BDM) which is a valuable building block for the production of pesticides, perfumes or dyes (see SI, section IV.1). [START_REF] Luo | Microporous organic polymers synthesized by self-condensation of aromatic hydroxymethyl monomers[END_REF][START_REF] Chen | Using alcohols as alkylation reagents for 4-cyanopyridinium and N,N′-dialkyl-4,4′-bipyridinium and their one-dimensional iodoplumbates[END_REF] Interestingly, the reduction of PET required harsher conditions than PLA with the present hydrosilylation system, which is consistent with the chemical behavior noted by Robertson and co-workers under hydrogenation conditions. [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] However, we observed a different behavior with the organoborane B(C6F5)3 (3) and 16 h were needed to depolymerize PLA with a slight excess of Et3SiH and 5 mol% 3 at room temperature, while the same reaction was achieved within 3 h with PET. [START_REF] Feghali | Room temperature organocatalyzed reductive depolymerization of waste polyethers, polyesters, and polycarbonates[END_REF] As already observed with natural polymers, [START_REF] Monsigny | Efficient reductive depolymerization of hardwood and softwood lignins with Brookhart's iridium((III)) catalyst and hydrosilanes[END_REF] catalyst 4 proved less reactive than B(C6F5)3 requiring longer reaction times and higher temperatures for both PLA and PET.

Nevertheless, B(C6F5)3 in presence of excess silane favored rapid (24 h) cleavage of all the C-O bonds at room temperature with formation of alkanes and siloxane as the sole products. The iridium catalyst 4 exhibits a higher selectivity allowing the conservation of functionalities, such as silylethers. Moreover, when 5 mol% of the borane 3 are required to treat PLA, the iridium catalyst completed the deconstruction of this polymer with only 0.5 mol% demonstrating a higher stability.

The successful reductive depolymerization of polyesters with 4 and Et3SiH led us to consider the reduction of more oxygenated polymers. The reduction of polycarbonates is of interest as organic carbonates are classically more difficult to reduce, compared to esters, because of the delocalization of the oxygen lone pairs over the CO3 group. [START_REF] Dub | Catalytic Reductive Transformations of Carboxylic and Carbonic Acid Derivatives Using Molecular Hydrogen[END_REF] Catalytic reductive hydrosilylation of polycarbonates (and plastics in general) was only reported in 2015 by our group with the organocatalyst B(C6F5)3, [START_REF] Feghali | Room temperature organocatalyzed reductive depolymerization of waste polyethers, polyesters, and polycarbonates[END_REF] while Robertson and co-workers reported in 2014 the catalytic hydrogenation of polycarbonates (and polyesters) with Ru(II) complexes (vide supra) [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] .

We first considered polypropylene carbonate (PPC) viewed as the assemblage of the two monomers, propylene oxide and CO2. Surprisingly, reductive depolymerization of PPC using the hydrosilylation conditions of PLA (0.5 mol% of 4 and 4 equiv. Et3SiH) at 65 °C proceeded more rapidly and more selectively than what was observed with PLA (3 h versus 60 h) affording MeOSiEt3 and disilylether PG-Si in quantitative yield (Scheme 4, eq. 9). MeOSiEt3 results from the reduction of the carbonate groups and no other by-products, such as Et3SiOSiEt3, CH4 or n PrOSi, were detected. The selectivity of the reaction indicated that the hydrosilylation of the polycarbonate was faster than deoxygenation of PG-Si (see scheme 4). This difference in reactivity could be exploited for the selective depolymerization of a mixture containing pieces of PPC (insoluble) and a fine powder of PLA (soluble), suspended in chlorobenzene. In contrast to what expected from their respective solubility, PPC reacted faster than PLA. The 1 H and [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] C NMR spectra unveiled the formation of PG-Si from PPC which rapidly disappeared in the medium while soluble PLA was left untouched (see SI, section V.3). This comparative experiment highlights that the reactivity of polymers is not necessarily related to their solubility.

While the reduction of carbonyl groups usually follows the order of reactivity ketones > esters > carbonates, [START_REF] Dub | Catalytic Reductive Transformations of Carboxylic and Carbonic Acid Derivatives Using Molecular Hydrogen[END_REF] Brookhart's catalyst 4 exhibits a different reactivity. In fact, when a mixture of propylene carbonate and rac-lactide are exposed to 1 mol% 4 and an excess Et3SiH, the monofunctional carbonate is reduced before the lactide monomer (see SI, section V.2). The selective hydrosilylation of PPC over PLA hence translates a difference in reactivity between monofunctional carbonates and esters. Yet, this observation should not be taken as a general rule (vide infra). Scheme 4: Selective reduction of polycarbonates to their corresponding silylated alcohols with the 4/hydrosilane system Depolymerization of PC-BPA under conditions similar to PPC is slower (6 h vs 3 h) and afforded the disilylated phenol BPA-Si in excellent yield (88%) (Scheme 4, eq. 10). MeOSiEt3 was not detected by NMR. Yet, evolution of gaseous CH4 was observed by [START_REF]Plastics The Facts[END_REF] H NMR (in a closed J-Young NMR tube) along with Friedel-Craft-like products in the form of methylbisphenol derivatives (see SI, section II.9). Of interest, hydrolysis of the hydrosilylation products from PPC and PC-BPA afforded the corresponding alcohols, propylene glycol and BisPhenol A (BPA), respectively. They correspond to the monomers of the original polymers. For example, BPA was obtained in 88 % yield with 2.5 eq. of hydrated tetrabutylamonium fluoride (TBAF) in THF within 12 h (Scheme 4, eq. 11). An attractive alternative to TBAF involves the use of sodium hydroxide in methanol and water for the hydrolysis of silyl ethers. [START_REF] Pal | Preparation and hydrosilylation activity of a molybdenum carbonyl complex that features a pentadentate bis(imino)pyridine ligand. Inor[END_REF][START_REF] Revunova | Base-catalyzed hydrosilylation of ketones and esters and insight into the mechanism[END_REF] Yet, hydrolysis with sodium hydroxide proved inefficient when applied to triethylsilyl ethers. In fact, such transformation requires more Lewis acidic silicon derivatives such as alkoxysilane ([RO]3SiH).

This finding led us to consider the silicon industrial waste 1,1,3,3-tetramethyldisiloxane (TMDS) as a potential reductant. Treatment of 1 mmol of PC-BPA with TMDS (4 eq) and 4 (0.5 mol%) led to the formation of a gel after 12 h at 65 °C. Notably, no Friedel-Craft product could be observed in the hydrosilylation products using TMDS. Hydrolysis of the crude mixture with NaOH in water/methanol enabled the formation of the monomer BPA in 83% yield, after 2 h (Scheme 4, eq. 12).

TMDS being a cost-efficient source of hydrides, its potential in the reductive depolymerization of plastics was further assessed. In contrast to the conditions described in eq. 1 (Scheme 2), the reduction of PCL does not occur at room temperature with the dihydrosiloxane. Nevertheless, in presence of 0.3 mol% of the iridium complex 4 and 3 equiv. of TMDS (6 equiv. of hydrides), the polymer was completely decomposed within 8 h at 65°C into numerous silyl ethers which could be further hydrolysed to 1,6 hexanediol with the NaOH/MeOH system (yield of 68 % overall, scheme 5, eq. 13). The need for a convergent depolymerization led us to consider the hydrosilylation of PCL at 110 °C to reduce the intermediate ethers to alkanes. In presence of 0.3 mol% 4 with 6 equiv. of TMDS (Scheme 5, eq.14), in situ 1 H NMR and DEPT 135 [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] C NMR spectra revealed complete reduction of PCL into hexane and siloxanes in quantitative yield. Surprisingly, after removing hexane, excess TMDS and the solvent under reduced pressure, a non-volatile viscous liquid was isolated. [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] C NMR, [START_REF] Luo | Microporous organic polymers synthesized by self-condensation of aromatic hydroxymethyl monomers[END_REF] Si NMR as well as FT-IR analyses confirmed the formation of polydimethylsiloxane PDMS. [START_REF] Kuo | Poly (dimethylsiloxane)[END_REF] SEC chromatography was performed to further characterize the resulting polysiloxane and unveiled a high molar weight and a high polydispersity (Mn = 1054 g.mol -1 , Mw = 23209 g.mol -1 and PD = 22.0). Chromatograms also revealed a complex distribution of the molecular weights. Regardless the further used of silyl ethers, [START_REF] Milton | Palladium-catalysed synthesis of aryl-alkyl ethers using alkoxysilanes as nucleophiles[END_REF][START_REF] Selvakumar | Copper-mediated etherification of arenes with alkoxysilanes directed by an (2-aminophenyl)pyrazole group[END_REF][START_REF] Bhadra | Decarboxylative etherification of aromatic carboxylic acids[END_REF][START_REF] Kim | DBU-Mediated Mild and Chemoselective Deprotection of Aryl Silyl Ethers and Tandem Biaryl Ether Formation[END_REF][START_REF] Afonso | The reactivity of silyl ethers to the Swern reagent[END_REF] stoichiometric quantities of silicon byproducts are formed, at the expense of a low atom economy for the whole process. In this respect, the use of TMDS for the complete deoxygenation of plastic wastes is more attractive than Et3SiH, as it enables the formation of a valuable by-product, PDMS, [START_REF] Brook | New Control Over Silicone Synthesis using SiH Chemistry: The Piers-Rubinsztajn Reaction[END_REF] , which is widely used as a lubricant, as a food additive, as silicone in breast implants [START_REF] Brook | Platinum in silicone breast implants[END_REF] , etc. 38 TMDS would be especially meaningful in the depolymerization of bio-sourced polymers, in order to produce a well-defined fuel by reduction of the bio-based carbon matrix and a silicon polymer. PLA was considered as the ideal candidate to test this idea. In presence of 1 mol% of the iridium catalyst 4 and 6 equiv. of TMDS, PLA was fully deconstructed at 110 °C into propane and polysiloxane PDMS (Scheme 5, eq. 15). Because of its volatility, the yield of propane could not be evaluated but the concentration of C3H8 was sufficient to detect it by 1 H and [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] C DEPT NMR (see SI, section III.2). Decreasing the amount of TMDS to 2.0 equiv. did not affect the rate of the reaction leading to propane and polysiloxane, using the same conditions of loadings catalyst, temperature, and time. After removal of the volatiles, a silicon rubber of PDMS remained in the flask (Scheme 5, eq. 16). Similarly to the PDMS silicon oil obtained from PCL depolymerization, the silicon rubber exhibits a complex mass distribution with at least two observable peaks in SEC chromatograms and a high polydispersity. (Mn = 776 g.mol -1 , Mw = 13976 g.mol -1 , and PD = 18.0).

In addition to ethers and esters, Brookhart and co-workers demonstrated that catalyst 4 is able to reduce halogeno-alkyl compounds. [START_REF] Yang | Iridium-catalyzed reduction of alkyl halides by triethylsilane[END_REF][START_REF] Yang | Reduction of Alkyl Halides by Triethylsilane Based on a Cationic Iridium Bis(phosphinite) Pincer Catalyst: Scope, Selectivity and Mechanism[END_REF] The reduction of halogenated polymers was thus attempted with the dechlorination of poly(vinyl chloride) PVC (scheme 6, eq. 17). While no reaction occurs at 65 °C and 90 °C, [START_REF] Krall | Controlled hydrogenative depolymerization of polyesters and polycarbonates catalyzed by ruthenium(II) PNN pincer complexes[END_REF] C NMR reveals that, in the presence of 0.8 mol% 4 and 2 equiv. of Et3SiH, insoluble pieces of PVC were reduced at 110 °C into soluble poly(ethylene) PE. [START_REF] Galland | 13C NMR Determination of the Composition of Linear Low-Density Polyethylene Obtained with [η3-Methallyl-nickeldiimine]PF6Complex[END_REF] The corresponding chlorosilane Et3SiCl was formed in 39% yield.

When a mixture of PVC and PLA was exposed to Et3SiH and catalyst 4, PLA was left untouched at 110 °C, although its depolymerization was observed at 65 °C in the absence of PVC (see Scheme 2, Eq. 4). In parallel, the less reactive PVC was dechlorinated to PE and Et3SiCl (see SI, section V.4). This reactivity trend is surprising considering that PVC is typically less reactive than PLA. More mechanistic investigations are certainly needed to decipher the origins of these unexpected selectivities. Importantly, a different behavior was observed when catalyst 4 was replaced with B(C6F5)3. In this case, only the polyester (PLA) was depolymerized (see SI, section V.5). Overall, we can conclude that the reactivity of isolated polymers towards a depolymerization method does not transpose directly to mixtures of plastic wastes, where reactivity trends can be inverted.

Scheme Reductive dechlorination of PVC to PE with the system 4/Et3SiH.

Conclusions

In conclusion, Brookhart's iridium(III) complex 4 has been utilized for the first time for the depolymerization of a range of oxygenated polymers, comprising household wastes. These include polyesters (PET, PCL, PLA, etc.) and polycarbonates (PC-BPA, PPC). In the presence of hydrosilanes, catalyst 4 cleaves C-O bonds in esters and carbonates to yield silylethers and alkanes. A chlorinated polymer such as PVC was dechlorinated to PE, using the same protocol.

Importantly, it was found that the reactivity of isolated esters, carbonates or well-defined polymers does not directly transpose to mixtures of polymers.

The method presents the advantage of regenerating the original monomers or valuable chemicals in a pure form. For instance, bis-phenol A could be isolated in 83 % yield from a sequence involving the depolymerization of PC-BPA with 4 and TMDS and the subsequent hydrolysis of the reaction products with NaOH in methanol. In addition, the reduction of biobased polymers such as PLA with TMDS affords bio-fuels, in the form of hydrocarbons, and PDMS (a valuable silicon polymer), in a process that valorizes both the carbon and silicon feedstocks.

Although catalyst 4 proved very efficient in the plastic depolymerization with low catalyst loadings, its price remains a major obstacle for a scale-up process development. In addition, the energy intensive production of hydrosilanes combined with the generation of silicon by-products calls for alternative methods of reduction. In this perspective, surrogates of hydrosilanes, such as sylilformates (R3SiOCHO) [START_REF] Chauvier | Silyl Formates as Surrogates of Hydrosilanes and Their Application in the Transfer Hydrosilylation of Aldehydes[END_REF][START_REF] Chauvier | Silylation of O-H bonds by catalytic dehydrogenative and decarboxylative coupling of alcohols with silyl formates[END_REF][START_REF] Cantat | Iron-Catalyzed Silylation of Alcohols by Transfer Hydrosilylation with Silyl Formates[END_REF] , may offer promising alternatives in the future by using renewable hydride sources (e.g. formic acid).

Scheme 2 :

 2 Scheme 2: Reduction of PCL, PDO and PLA into silylated alcohols with Et3SiH and 4.

Scheme 5 :

 5 Scheme 5: Reductive depolymerization of PCL and PLA with the system 4/TMDS to the

ASSOCIATED CONTENT

Supporting Information.

The following files are available free of charge. Supplementary equations, detailed descriptions of experimental methods, and results are provided in the Supporting Information.

AUTHOR INFORMATION

Corresponding Author

* E-mail: thibault.cantat@cea.fr; Fax: +33 1 6908 6640; Tel: +33 1 6908 4338 Present Addresses † NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette, France.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

For financial support of this work, we acknowledge the CEA, CNRS, CHARMMMAT Laboratory of Excellence and the European Research Council (ERC Starting Grant Agreement n.336467). T.C. thanks the Fondation Louis D. -Institut de France for its support. We thank CINES for the allowance of computer time (Project No. c2017086494).

For Table of Contents Use only

Synopsis

Several oxygenated plastics are efficiently depolymerized into valuable chemicals under mild hydrosilylation conditions with the use of molecular iridium catalyst.

TOC/Abstract Graphic


Xavier Frogneux

Enguerrand Blondiaux

Pierre Thuéry

Thibault Cantat
email: thibault.cantat@cea.fr

Bridging Amines with CO 2 : Organocatalyzed Reduction of CO 2 to Aminals

Keywords:

The 4-electron reduction of CO2 in the presence of secondary aromatic amines is described for the first time to access aminals. Under metal-free hydrosilylation conditions, the four C-O bonds of CO2 are cleaved and the organocatalysts are able to balance the reactivity of CO2 to promote the selective formation of two C-N and two C-H bonds. The methodology enables the formation of various symmetrical and unsymmetrical aminals.

Because CO2 is a renewable, cost-efficient and non-toxic resource, it is a desirable carbon feedstock for the production of value-added chemicals and a lot of groups have focused their attention on designing new transformations involving CO2 over the last few years. 1 In particular, the reductive functionalization of CO2 with nitrogen reagents has known tremendous developments using various types of reductants such as hydrosilanes, hydroboranes and dihydrogen. 2 These reactions have enabled the conversion of CO2 into formamides, 2b, 3 formamidines 4 and methylamines. 5 Notably, the carbon oxidation state in these products is either +2 or -2 and the formation of C 0 organic functional groups from CO2 remains a challenge. This trend reflects the higher electrophilicity of C 0 groups compared to C +II functions in carbonyl derivatives. It is indeed well established that upon hydrogenation of CO2, formate derivatives can be accumulated while formaldehyde is an elusive species because its reduction to methanol is more rapid than the hydrogenation of formic acid. 1a As a consequence of this limitation, only a few reports have tackled the formation of C 0 species from CO2. Under hydrosilylation conditions, the selective reduction of CO2 into a bis(silyl)acetal species with triethylsilane has been revealed. 6 Using a hydroborane reductant, Bontemps, Sabo-Etienne et al. successfully trapped transient formaldehyde, obtained from CO2, with 2,6diisopropylaniline, yielding the corresponding imine. 7 To unlock new 4-electron reduction transformations of CO2, one should focus on the use of well-balanced catalysts, able to finely control the kinetics of CO2 reduction. In nature, acetogenic bacteria are able to produce over 10 9 tons of acetic acid annually, following the Wood-Ljungdahl pathway (Scheme 1). 8 In this biochemical cycle, CO2 is anchored to a diamine moiety and undergoes successive 2-electron reduction steps to yield a methylamine (C -II ) after formation of the corresponding formamide (C +II ), formamidine (C +II ) and aminal (C 0 ) intermediates. While CO2 conversion to formamides, formaminides and methylamines has been described, the synthesis of aminals 9 directly from CO2 remains unknown and was only suggested as a possible intermediate in the Ru-based methylation of amines. 5b To open up the variety of products accessible from CO2, we describe herein the first catalytic synthesis of aminals by intermolecular coupling of two amines using CO2 as a C1-bridge.

Scheme 1. Simplified mechanism of the Wood-Ljungdahl pathway for acetogenesis with CO2

Hydrosilanes are mild reductants, cheap, non-toxic with a redox potential well poised for CO2 reduction. Additionally, their slightly polar Si-H bond can be activated with metal-free catalysts, using either Lewis bases or Lewis acids. 10 Using phenylsilane as reductant, the reactivity between Nmethylaniline (1a) and CO2 has been explored using a variety of organocatalysts able to form adducts with CO2 11 or promote its reduction, such as guanidines, amidines, N-heterocyclic carbenes (NHCs) and phosphorus-bases. 3a, 3b, 5f, 5g, 12 Using 10 mol% of the NHC ItBu, CO2 undergoes hydrosilylation in the presence of N-methylaniline (1a) in CH3CN, and after 2.5 h at 80 °C, 76% of 1a was converted to the expected aminal 2a in 47% yield (Scheme 2). Nonetheless, methylamine 3a is also produced at a similar rate reaching 25% yield. Interestingly, after prolonged reaction time (24 h) methylamine 3a is formed as the main nitrogen product (>95%), indicating that the aminal is an intermediate in the formation of 3a and that the catalyst is unable to prevent over-reduction of the C 0 carbon center in 2a. Reducing the quantity of CO2 in the reaction vessel to circa 1 equiv. per amine somewhat improves the 2a/3a ratio of the reaction from 1.9 to 2.9 (Scheme 2). [a] TBD (5) PhSiH3 48 5 0 12 [b] TBD (5) PhSiH3 48 7 0 13 [c] TBD (5) PhSiH3 19 26 0

Scheme 2. Formation of aminal 2a via CO2 hydrosilylation

Reaction conditions: NMR tube (2.5 mL), catalyst, amine (0.10 mmol), hydrosilane (6 eq. "Si-H"), solvent (0.30 mL), CO2 (1 bar). Yields determined by 1 H NMR with Ph2CH2 as internal standard. [a] in toluene-d8 [b] in THF-d8. [c] at RT.

Although encouraging, these results stress the need for a catalyst having a balanced reactivity in CO2 hydrosilylation, to avoid the methylation of the amine. While IPr, Verkade's base VB Me or guanidine Me-TBD enable the formation of 2a in up to 79% yield, the best activities and selectivities were obtained with 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as catalysts for which the reaction was complete after 2.5 h, yielding 93% of the desired aminal 2a and 3a as side-product (7%) (Entries 2-6, Table 1). As expected, no reaction occurred after 24 h in the absence of CO2 or catalyst. Importantly, with a low catalyst loading of 1 mol% TBD, aminal 2a was still obtained in 92% yield after 7 h at 80 °C (Entry 8, Table 1). Whereas polymethylhydrosiloxane (PMHS) or tetramethyldisiloxane (TMDS) are unreactive in the formation of 2a, Ph2SiH2 exhibits a reactivity close to that of PhSiH3 (Entries 9-10, Table 1 and SI). The polarity of the solvent also has a positive influence on the transformation. Although 2a is formed in <10% yield from 1a in toluene or THF (ε<7.5), the corresponding yield increases to 91% in MeCN (ε=37.5) under analogous conditions (Entries 7, 11-12, Table 1). Finally, the conversion of 1a can proceed at RT, yet only 26% of the desired aminal could be obtained under these conditions and 49% of Nmethylformanilide was also formed (Entry 13, Table 1).

Reaction conditions: NMR tube (2.5 mL), TBD (0.0050 mmol), amine (0.10 mmol), PhSiH3 (0.20 mmol), CD3CN (0.30 mL), CO2 (1 bar). Yields determined by 1 H NMR with Ph2CH2 as internal standard. [a] amine (0.050 mmol).

Scheme 3. Synthesis of aminals starting from secondary aromatic amines

Having in hand a selective and efficient catalytic system for this novel reaction, the coupling of various amines to aminals was attempted with CO2 (Schemes 3). Introducing electron-donating groups (EDGs) or electron-withdrawing groups (EWGs) on the aromatic ring of N-methylaniline, the corresponding aminals 2b-2k were obtained in good yields (55-98%), after 4 h at 80 °C in CH3CN, in the presence of 2 equiv. PhSiH3 and 5 mol% TBD; and negligible over-reduction was observed within 18 h. Importantly, the reduction of CO2 to aminals is also chemoselective and oxidizing groups such as a nitrile or ketone are tolerated, as exemplified in the formation of 2j (95% yield) and 2k (98% yield). Indeed, crystals of 2k grown from the crude mixture confirmed the presence of the untouched carbonyl group and, hence, the ability of the catalyst to avoid over-reduction (Figure 1).

Figure 1. Molecular structure of 2k with displacement ellipsoids drawn at the 50% probability level

In order to widen the scope of aminal compounds available from the present methodology, the influence of the substituent on the nitrogen atom has been investigated. N-ethylaniline (1l) and N-allylaniline (1m) were converted in good 73% and 77% yields to 2l and 2m, respectively. The bulky N-benzylaniline gave the desired product 2o in 94% after 2 h at 80 °C. N,Ndiphenylaniline only furnished 24% of 2n after 21 h, presumably because of its poor nucleophilicity. Cyclic amines such as indoline (1p) and 1,2,3,4-tetrahydroquinoline (1q) were converted to their aminal analogues 2p and 2q in good 70% and 87% yields. The reaction is also viable with secondary heteroaromatic amines: the formation of 2s from 2methylaminopyridine was accomplished quantitatively and 2s was isolated in 88% yield after removal of the siloxanes byproducts. In contrast, the reaction with 4-methylaminopyridine resulted in the formation of the corresponding formamide as a major product (85%) and only 14% of the aminal 2t were observed. Naturally, the formation of the heterocycle is favored over an intermolecular reaction for diamino substrates, thus providing 2r and 2u with excellent 88 and 95% yield, respectively. However, starting from an aliphatic amine such as morpholine, only the corresponding formamide was yielded and no trace of the aminal product could be detected by 1 H NMR. This observation may be due to the stronger nucleophilicity of aliphatic amines which facilitates the production of a formamide that is unproductive in the formation of aminals.

Reaction conditions: NMR tube (2.5 mL), TBD (0.0050 mmol), amine 1 (0.050 mmol), amine 1' (0.050 mmol), PhSiH3 (0.20 mmol), CD3CN (0.30 mL), CO2 (1 bar). Average amines conversion determined by 1 H NMR with Ph2CH2 as internal standard (see SI).

Scheme 4. Synthesis of unsymmetrical aminals

The coupling of two different amines was identified as the next challenge facing this 4-component reaction, to access unsymmetrical aminals (Scheme 4). Reacting two amines with similar electronic properties (1b and 1f) with CO2, PhSiH3 and 5 mol% TBD led to a statistical distribution of all possible aminals 4a, 2b and 2f (44/44/44%). Nonetheless, when two amines of different nucleophilic character are used, the selectivity to the unsymmetrical aminals increases. For example, aminals 4b, 4c, 4d and 4e were obtained as the major products in 40, 82, 69 and 61% yield, respectively, with the symmetrical aminals being formed as side products.

Scheme 5. Proposed pathway to access aminalsThe conversion of CO2 to aminals is a rare example of a catalytic reaction leading to the complete deoxygenation of CO2. 4,13 Overall, it involves the cleavage of four C-O bonds and the formation of two C-H bonds (reduction) and two C-N bonds (functionalization). The nature of the organic intermediates involved in this reaction was investigated so as to explain the observed reactivities and facilitate future catalysts design. Possible pathways are depicted in Scheme 5. [START_REF]A reaction pathway involving the formation of transient urea species was ruled out as TBD is unable to promote the hydrosilylation of quaternary ureas to aminals (see SI). This pathway is omitted in Scheme 6 for the sake of clarity[END_REF] The reductive functionalization of CO2 to formamides is well established, TBD being a known catalyst for this transformation. 3a Nonetheless, no reaction is observed when N-methylformanilide 5a is reacted with PhSiH3 and 5 mol% TBD, at 80 °C, in the presence (step F) or absence (step E, under Ar or CO2) of N-methylaniline.

Formamides thus appear as competition products in CO2 conversion to aminals. Classically, aminals can be prepared by condensation of amines onto paraformaldehyde and an alternative pathway for the formation of aminals from CO2 could thus rely on catalytic hydrosilylation of CO2 to a C 0 bis(silyl)acetal species and subsequent nucleophilic addition of the two amine reagents. Using NHCs as catalysts, Zhang and Ying et al. have indeed detected successfully bis(silyl)acetal derivatives upon hydrosilylation of CO2, prior to the formation of silylmethoxides end-products. 12b These considerations suggest that the conversion of CO2 to aminals proceeds via reduction of CO2 to a silylacetal species (steps A and B) which undergoes two successive nucleophilic attacks (steps C and D). Although steps C and D are similar processes, they involve different electrophiles, namely a bis(silyl)acetal and an aminosilylacetal derivative. Because the bis(silyl)acetal is a stronger electrophile, the most nucleophilic amine should be rapidly consumed in step C, so that unsymmetrical aminals can be selectively formed. According to this mechanism, the reduction and functionalization steps are consecutive but their relative rates are important to ensure CO2 conversion to the desired aminal while avoiding formamide and silylmethoxide competition products. Experimentally, highly nucleophilic amines (e.g. morpholine) indeed prevent the formation of aminals because they are readily converted to their corresponding formamides (vide supra) and catalysts able to promote the rapid conversion of CO2 to silylacetals will be necessary to access aminals from aliphatic amines. Conversely, electron poor amines such as diphenylamine are not nucleophilic enough to trap the bis(silyl)acetal intermediate and the latter is reduced to a silylmethoxide product prior to the formation of C-N bonds (steps, A, B and G). Experimentally, silylmethoxide species (CH3OSi(O)Ph) were indeed observed as the major product in the conversion of diphenylamine with CO2, PhSiH3 and 5 mol% TBD.

Capitalizing on this mechanism, we envisioned that other nucleophiles, such as malonates, could efficiently replace the amine reagents to promote the challenging formation of C-C bonds from CO2. [START_REF] Li | [END_REF] In fact, addition of 2 equiv. PhSiH3 to diethylmalonate, under an atmosphere of CO2, resulted in the formation of 58% 6 after 5 h at 80 °C, in the presence of 5 mol% TBD (Eq. 1). 6 formally results from the methylenation of two malonate moieties with CO2 and, to the best of our knowledge, it represents the first example of a homogeneous catalytic reaction leading to the formation of 2 C-C bonds at the CO2 carbon atom. This reaction is under further investigation in our laboratories. conclusion, we have described herein a novel catalytic transformation to promote the conversion of CO2 to aminal derivatives via a 4-component reaction. The organocatalysts are able to balance the reactivity of CO2 reduction and selectively stabilize carbon(0) products for the formation of both symmetric and unsymmetric aminals.

ASSOCIATED CONTENT Supporting Information

Details of experimental procedures, physical properties of new compounds, tables of crystal data, atomic positions and displacement parameters, anisotropic displacement parameters, and bond lengths and bond angles in CIF format. This information is available free of charge via the Internet at http://pubs.acs.org.

Table 1 . Catalyst screening for the coupling of 1a to 2a

 1

		Catalyst (mol%) Reductant Time (h) Yield 2a (%)	Yield 3a (%)
	1	ItBu (10)	PhSiH3	2.5	52	18
	2	IPr (10)	PhSiH3	4.5	79	3
	3	VB Me (10) PhSiH3	2.5	71	0
	4 5	Me-TBD (10) DBU (10) PhSiH3 PhSiH3	4.5 2.5	78 93	0 4
	6	TBD (10) PhSiH3	2.5	93	7
	7	TBD (5)	PhSiH3	3.0	91	6
	8	TBD (1)	PhSiH3	7.0	92	6
	9	TBD (5)	PMHS	96	5	0
	10	TBD (5)	TMDS	96	0	0
	11

ACKNOWLEDGMENT

For financial support of this work, we acknowledge CEA, CNRS, ADEME (Fellowship to E.B.), the CHARMMMAT Laboratory of Excellence, the University Paris-Saclay (Fellowship to X.F.) and the European Research Council (ERC Starting Grant Agreement n.336467). T.C. thanks the Foundation Louis D. -Institut de France for its support.

AUTHOR INFORMATION

Corresponding Author *thibault.cantat@cea.fr Author Contributions ‡These authors contributed equally.


Von Wolff

N ; Lefèvre

G ; Berthet

J.-C ; Thuéry

P ; Cantat

The Implications of CO 2 Activation by Frustrated Lewis Pairs in the Catalytic Hydroboration of CO 2 : A View Using N/Si + Frustrated Lewis Pairs

Keywords: Organocatalysis, CO2, hydroboration, mechanisms, DFT calculations

A series of base-stabilized silylium species were synthesized and their reactivity towards CO2 explored, yielding in the characterization of a novel N/Si+ FLP-CO2 adduct. These silicon species are active catalysts in the hydroboration of CO2 to the methoxide level with 9-BBN, catecholborane (catBH) and pinacolborane (pinBH). Both experiments and DFT calculations highlight the role of the FLP-CO2 adduct in the catalysis. Depending on the nature of the hydroborane reductant, two distinct mechanisms have been unveiled. While 9-BBN and catBH are able to reduce an intermediate FLP-CO2 adduct, the hydroboration of CO2 with pinBH follows a different and novel path where the B-H bond is activated by the silicon-based Lewis acid catalyst. In these mechanisms, the formation of a highly stabilized FLP-CO2 adduct is found detrimental to the kinetics of the reaction.

Introduction

Carbon dioxide is an abundant, inexpensive, low toxic and renewable carbon feedstock and therefore an appealing C1-building block in organic chemistry. [1][2][3] The efficient reduction of CO2 to important raw materials for the chemical industry or to value-added chemicals is however hampered by its high thermodynamic stability and kinetic inertness towards reduction. This drawback calls for efficient reduction catalysts able to synchronize C-O bond cleavage with C-H and C-C bond formation, at a high kinetic rate and with a low energy demand. While CO2 activation and reduction at a metal center has been thoroughly investigated over the last four decades, 2,[4][5][6][7][8] organic systems able to perform this task, based on main group elements (e.g. B, N, O, P, Si), remain sporadic and underexplored. Many different coordination modes have been unveiled for the interaction of CO2 with a transition metal or an f-element, based on the nature of the metal ion and the redox state of the CO2 ligand. 9,10 In organic chemistry, CO2 activation is quite monotonous and relies on the formation of stable adducts with organic Lewis bases, such as guanidines or N-heterocyclic carbenes (NHCs). [11][12][13] Nevertheless, most progress has been gained via the synergistic action of Lewis pairs, able to coordinate both the C and O centers of CO2.

Within the concept of Frustrated Lewis Pairs (FLPs), introduced by Stephan and Erker, [14][15][16][17] several CO2 adducts have been successfully isolated over the last 6 years, in which the C atom is attached to a C, N or P donor while one or two O atoms coordinate a group XIII Lewis acid (B or Al). [18][19][20][21][22][23][24][25][26][27][28] Although FLP-CO2 adducts are commonly proposed as catalytic intermediates in the reduction of CO2 with hydrosilanes or hydroboranes, their possible involvement and role in the catalysis remain yet unclear. [29][30][31][32][33][34] Experimentally, FLPs that catalyze the hydroelementation of CO2 do not form observable adducts with CO2 and, conversely, only a single N/B FLP-CO2 adduct has been reported so far to promote the catalytic hydroboration of CO2, by our group. 27,35 Mechanistically, CO2 adducts may have a decreased reactivity compared to free CO2 in the presence of a reductant and reactive CO2 adducts that are not thermodynamically favored under catalytic conditions may form without being detected. Scheme 1. Silylium-based FLP systems for the activation of small molecules (CO2 and H2).

In order, to better address the role and influence of CO2 adducts in the metal-free catalytic reduction of CO2, we have thus sought stable CO2 adducts able to perform as catalysts in the hydroboration of CO2. Whereas FLP-CO2 adducts mostly feature borane or alane Lewis acids, we reasoned that an isolobal and isoelectronic silylium cation (R3Si + ) would exhibit a stronger Lewis acidity and a greater affinity for the O-center in CO2. In addition, the versatile synthesis of organosilicon compounds may facilitate the access to structures exhibiting different affinities for CO2. In fact, a few FLP systems involving highly electrophilic silyl cations in combination with phosphines as Lewis base were reported to activate small molecules. In 2011, Müller and coworkers demonstrated that triarylsilylium cations in combination with bulky phosphines could activate H2 and the same system was shown recently to activate CO2 in the formation of an adduct (I) with the intermolecular P/Si + FLP (Scheme 1). 36,37 Recently, Stephan and co-workers characterized P/Si + FLP-CO2 adducts (II) starting from basic phosphines (namely PEt3 and Pt-Bu3) and silyl triflates. 38 Further Ashley et al. reported on a phospine stabilized silylium species able to cleave H2. 39 We report herein the formation of novel intramolecular FLP-CO2 adducts by reaction of CO2 with nitrogen stabilized silyl cations. These systems exhibit excellent catalytic activity in the hydroboration of CO2 and the catalytic involvement of FLP-CO2 adducts is addressed, based on mechanistic experimental and DFT investigations (Scheme 1).

Results and discussion

Synthesis of novel FLP-CO2 adducts. We recently reported that both 1,5,7triazabicyclo[4.4.0]dec-5-ene (TBD) and guanidine substituted borane III are able to bind CO2 to afford adducts IV and V, respectively (Scheme 2). 27 Importantly, these species are highly active hydroboration catalysts and they promote the reduction of CO2 to the methanol level with 9borabicyclo[3.3.1]nonane 9-BBN, at room temperature. In order to increase the activation of the C-O bond in CO2, the replacement of the organoborane Lewis acid with a more oxophilic silyl cation was envisioned. Unstabilized silyl cations are strong and  acceptors 40 and can undergo unwanted side reactions with the solvent. 41,42 Scheme 2. TBD and TBD-substituted borane II form CO2-adducts III and IV.

The formation of base stabilized silyl species with modulated reactivity was thus attempted by reacting the guanidinate [K + ,TBD -] with various dichlorosilanes (Scheme 3a).

Scheme 3. Synthesis of hypervalent silanes and their reaction with CO 2

Following the procedure developed by Andell and coworkers, 43 the hypervalent chlorosilane 1 was isolated from [K + ,TBD -] and Me2SiCl2. Replacing Me2SiCl2 with i-Pr2SiCl2 and Ph2SiCl2 led to the isolation of the analogous compounds iPr 1 and Ph 1, respectively. Crystals of 1, grown from a pentane solution, revealed a monomeric structure with a hypervalent silicon center bound to two methyl groups, the two nitrogen donors of TBD and one chloride (Figure 1). [START_REF]Andell and coworkers solved a dimeric structure of 1 by cooling down a refluxing pentane solution to -30 °C. This might indicate a higher thermodynamic stability of the dimeric structure. (45) Pounds[END_REF] The silicon center adopts a distorted trigonal bipyramidal geometry, the Cl1 and N2 atoms occupying the axial positions with a Cl-Si-N2 bond angle of 159.20(4)°. The Si-N2 bond length is significantly longer than the Si-N1 bond length (1.9843(12) vs 1.7914(12) Å) and the Si-Cl bond is elongated in comparison to Si-Cl bonds in tetravalent chlorosilanes (2.2783( 5) vs e.g. 2.01(2) Å in SiCl4).

Taken together, these data indicate a 3c/4e -interaction for the axial bonding. 45,[START_REF] Weinhold; Landis | Valency and Bonding A Natural Bond Orbital Donor-Acceptor Perspective[END_REF] A Natural Bond Orbital (NBO) analysis indeed confirms a notable acidity at the Si atom with a positive charge of +1.89. A similar structure was found for iPr 1 (see SI). Treating 1 with a Lewis acid (i.e. AgNTf2, [Ph3C][B(C6F5)4] or Me3SiI) readily led to the formation of dimeric dicationic species in agreement with the reported ease of halogen-silicon bond ionization of base-stabilized halosilanes(Scheme 3b). [START_REF] Kummer | [END_REF] Crystals suitable for X-ray analysis were obtained for both [1 + ,B(C6F5)4 -]2 and [1 + ,NTf2 -]2 (Fig. 1 and SI). A dicationic dimeric structure is also proposed for [1 + ,I -]2 due to its very poor solubility in common polar solvents, such as THF, DMSO, acetonitrile and pyridine. 48 16), N3-C5 1.3598 (17).

Having in hand a series of base-stabilized chlorosilanes and silyl cations, the activation of CO2 was explored. Exposing a d8-THF solution of 1 to an atmosphere of CO2 readily produces a novel species, namely [2 + ,Cl -], in 30 % conversion, according to the 1 H and 13 C NMR spectra of the crude solution (see SI, Figure S20). Crystals suitable for X-ray diffraction were successfully obtained by slow diffusion of pentane into a concentrated pyridine solution of 1 under a CO2 atmosphere. As depicted in Figure 2, [2 + ,Cl -] is a novel CO2 adduct where the carbon atom coordinates to a N-donor on the TBD backbone while one oxygen atom binds to the silylium center.

The formation of 2 + also reveals that 1 can act as an intramolecular FLP. The addition of CO2 is reversible and 1 is recovered after evaporating a THF solution of [2 + ,Cl -] under high vacuum (10 -2 mbar) overnight (Scheme 3c). Interestingly, the formation of [2 + ,Cl -] is more favored in CH2Cl2 compared to THF and it reaches a conversion of 80 % (see SI, Figure S22). This is in agreement with the observed ease of ionization of Si-Cl bonds by polar solvents reported by Kummer and co-workers [START_REF] Kummer | [END_REF] and the more polar CH2Cl2 solvent might thus shift the equilibrium towards the ionized CO2-adduct. Starting from [1 + ,B(C6F5)4 -]2, CO2-adduct [2 + ,B(C6F5)4 -] is obtained quantitatively under a CO2 atmosphere (see SI, Figure S24). This adduct shows a much greater stability as 7 days under reduced pressure (10 -2 Values in parentheses correspond to shorter reaction times with corresponding yields. a Reaction conditions: catalyst (2.5 mol%) in 400 L of THF, (0.48 mmol of B-H), 1 atm CO2. Yields determined by 1 H-NMR vs internal standard (mesitylene). b TOFs were calculated under steadystate regime as the derivative of TON (B-H bond used) vs reaction time. c in CH3CN. d in pyridine. e in DCM. f in toluene. g in benzene.

It is notable that the formation of CO2 adducts with the intramolecular N/Si + FLPs seems sensitive to the steric crowding around the silylium center. In fact, no adduct is observed when the isopropyl or phenyl derivatives iPr 1 or Ph 1 are exposed to an atmosphere of CO2 (Scheme 3e). This behavior provides FLP systems for which activation of CO2 is disfavored and hence a platform to study the occurrence and catalytic importance of CO2 activation by FLPs (vide infra).

Catalytic hydroboration of CO2

The catalytic hydroboration of CO2 is an efficient method to reduce CO2 to the methanol level under mild reaction conditions, to produce methoxyboranes or methylamines (in the presence of an amine source). This catalytic reaction, first unveiled by the Guan group in 2010 with nickel(II) catalysts, 49,50 has attracted much attention and a variety of organometallic catalysts have been proposed. [51][52][53] Interestingly, in 2013, our group 27 has shown in parallel with the work of Fontaine and coworkers 31 that simple organic catalysts such as TBD, Me-TBD or N/B and P/B FLPs (e.g.

V) could also serve as efficient catalysts in this reductive process. As shown with the synthesis and chemical behaviour of 1, 1 + , iPr 1, Ph 1 and 2 + , the introduction of a silyl cation on the guanidine backbone of TBD enables a fine tuning in the activation of CO2. Whereas a strong coordination of CO2 is observed in 2 + , no reaction with CO2 is observed in the case of iPr 1 and Ph 1 showing that the corresponding adducts are not thermodynamically favored (although they could exist as metastable intermediates). These compounds therefore constitute a suitable platform to investigate the effect of CO2-adduct formation in the catalytic reduction of CO2.

In the presence of 9-BBN and 1 atm of CO2 in THF, 2.5 mol% of 1 catalyzes the hydroboration of CO2 to the corresponding methoxyborane MeOBBN in >99 % yield, after 20 h at 20 °C (Table 1, entry 1, TOF = 1.3 h -1 ). 54 THF is the best solvent under these conditions, as both more polar (CH3CN, pyridine) and less polar solvents (toluene, CH2Cl2) gave lower TOFs (Table 1, entries 2-5, TOFs up to 0.3 h -1 ). In fact, as shown in the work of Kummer and co-workers, [START_REF] Kummer | [END_REF] the solvent (Gutmann donor and acceptor number) can have an important influence on the ionization of the Si-Cl bond and hence on the Lewis acidity of the silicon center. The observed solvent effects could however not be correlated to the respective solvent acceptor (nor donor) numbers and might therefore be due to differences in CO2 and borane solubility. Increasing the steric bulk around silicon leads to a lower reactivity for the hydroboration of CO2 using 9-BBN. After 24 h at 20 °C, MeOBBN was obtained in 60 % yield using isopropyl compound iPr 1 (TOF = 0.7 h -1 ) and in 56 % yield with the phenyl analogue Ph 1 (TOF = 0.8 h -1 ) (Table 1 entries 6 and7). The nature of the anion X -also has a drastic influence on the catalytic performances and replacing the chloride ligand in 1 with the non-coordinating B(C6F5)4 -anion inhibits the catalytic hydroboration of CO2 (Entries 8, Table 1, TOFs up to 0.1 h -1 ). 55 Poorly soluble in THF, [1 + ,I -]2 promotes the reduction of CO2 to MeOBBN, after an induction period of about 3 h, in 100 % yield, after 24 h (TOF=1.3 h -1 after induction period). [1 + ,NTf2 -]2 does not react with CO2 and shows no catalytic activity, in the presence of 9-BBN, presumably because of its very poor solubility in THF.

When 9-BBN is replaced with catechol borane (catBH), the selective reduction of CO2 to methoxycatecholborane (MeOBcat) proved less efficient requiring 26 h at 70 °C in 74 % yield (TOF = 0.5 h -1 , Entry 10, Table 1). Although the catalytic performances are modest, these results are interesting as neither the parent guanidine base TBD, nor the dimeric boron analogue III, nor the N/B FLP-CO2 adduct V (Scheme 2) show any catalytic activity with catBH and pinBH. 27 The same trends in reactivity are observed for catBH and 9-BBN when catalyst 1 is replaced with iPr 1, Ph 1, [1 + ,I -]2 or [1 + ,B(C6F5)4 -]2 and the catalytic activity plummets in the presence of poorlycoordinating anions or with bulky silyl cations (Entries 11-14 in Table 1). All together, these results suggest that the formation of a CO2-adduct (e.g. 2) facilitates the catalytic hydroboration of CO2 with 9-BBN and catBH. Nevertheless, catalysts binding strongly to CO2 such as [1 + ,B(C6F5)4 - ]2) are sluggish.

Although pinacolborane (pinBH) presents a much lower hydridic character and a weaker Lewis acidity at boron than 9-BBN or catBH, 56 it was found to be an efficient hydride source in the hydroboration of CO2 with 1. 75 % MeOBpin was obtained selectively after 28 h at 90 °C under 1 atm of CO2, from pinBH and 2.5 mol% 1 (Entry 15 in Table 1, TOF = 0.8 h -1 ). Whereas metalbased catalysts have been shown to effectively catalyse the hydroboration of CO2 using a variety of borane sources (e.g., (9-BBN)2, BH3SMe2 and catBH), few catalysts exist that utilize pinacolborane. For example, Sabo-Etienne, Bontemps et al. have reported a ruthenium hydride complex able to afford MeOBpin in a low 40 % yield after 5 h at 70 °C (TOF = 5 h -1 ). 52 Only three metal-free systems have been described so far as catalysts in the hydroboration of CO2 with pinBH:

Stephan et al. designed a ring expanded-carbene providing a mixture of two-four-and six-electron reduction products, 30 whereas Fontaine et al. prepared a phosphino-borane FLP producing 60 % pinBOMe at 70 °C (TOF = 21 h -1 ). 29,31 Recently the group of Wegner reported a bidentate borane catalyst able to reduce CO2 to the methanol level in 89 % with pinBH at 70° C with high reactivity (TOF = 93 h -1 ). 33 As the catalyst itself possesses six active B-H bonds, this TOF-value must be considered with caution and a TOF of around 16 h -1 is calculated per active B-H bond, facilitating its comparison with other catalysts bearing only one active site.

Strikingly, catalysts iPr 1 and Ph 1 perform better than 1 in the hydroboration of CO2 with pinBH, while the opposite effect was noted with 9-BBN and catBH. MeOBpin is formed in 82 and 100 % yield within 28 h at 90 °C with Ph 1 and iPr 1 respectively, corresponding to TOFs of 0.9 and 2 h -1 (Entries 16 an 17 in Table 1). Although the amino-borane catalyst of Fontaine and co-workers showed higher catalytic activity (TOF = 21 h -1 , at 70 °C), 31 iPr 1 is able to promote the formation of MeOBpin quantitatively without the formation of other reduction side-products within only 16 h. In contrast to reactions with 9-BBN and catBH, the influence of the anion is less marked in the case of pinBH. Both compounds [1 + ,B(C6F5)4 -]2 and [1 + ,I -]2 exhibit a catalytic activity comparable to that of 1, affording pinBOMe in 40 and 50 % yield respectively after 24 h (TOFs of 0.5 and 0.6 h -1 respectively, Table 1 entries 18 and19). It is remarkable that pinBH imposes different demands on the nature of the catalyst and, unlike 9-BBN and catBH, catalysts showing no CO2 activation (e.g. iPr 1 and Ph 1) are more efficient for the hydroboration of CO2 with pinBH.

Reaction mechanism and DFT calculations

The catalytic hydroboration of CO2 with 1, 1 + , iPr 1 and Ph 1 constitutes an appropriate system to explore the role and impact of FLP-CO2 adducts in the reduction of CO2, because these four active catalysts interact differently with CO2. Whereas 1 and 1 + form stable adducts with CO2, replacing the methyl groups on the silicon atom in 1 with i-Pr or Ph substituents provides iPr 1 and Ph 1, which do not react with CO2. In addition, different behaviours and trends were noticed for 9-BBN and pinBH suggesting that different mechanisms might take place in this important transformation.

Regardless of the nature of the hydroborane R2BH, the formoxyborane HCOOBR2 and the acetal H2C(OBR2)2 are intermediates in the formation of MeOBR2. In addition, the distribution of the products over time reveals that the reduction of CO2 to methoxyborane is kinetically controlled by the first hydroboration step, yielding HCOOBR2 (see ESI Page 6). The mechanism of the catalytic hydroboration of CO2 to formoxyboranes has thus been investigated using both DFT calculations and stoichiometric reactions. Mechanism involving the activation of CO2. CO2-adducts are often described as keyintermediates in the catalytic cycle of CO2 reduction processes 25,57 and both adducts IV and V were shown to react with an electrophilic borane (9-BBN) to yield formate species. 27 In light of these data and the reaction chemistry of 1 described above, activation of CO2 by 1 is a reasonable starting point for the hydroboration of CO2 with 1 (Scheme 4). DFT calculations, performed at the M05-2X/PCM=THF/6-31+G* (6-311++G** for the active hydride) level of theory, confirm that the formation of a CO2 adduct [2 + ,Cl -] from 1 is indeed exergonic by -6.1 kcal/mol with a low activation barrier of only 10.6 kcal/mol (Scheme 4). Interestingly, the coordination of the chloride anion to the silylium center is also possible and the corresponding CO2 adduct 2 lies at -8.6 kcal/mol. These data are consistent with the experimental results showing a reversible CO2 activation with 1 and the possible formation of hypervalent guanidinate chlorosilanes. Reduction of the activated CO2 molecule in 2 with 9-BBN was found to involve the addition of the B-H bond to the unbound C=O functionality with a low lying transition state (TS2-3) at 13.9 kcal/mol. The resulting intermediate 3 features a tetrahedral carbon. Elimination of the product HCOOBBN (4) from 3 is essentially barrier-less and favoured by the thermodynamics of the overall process (ΔG= -13.9 kcal/mol). The hydroboration of CO2 with 9-BBN catalyzed by 1 thus involves three successive steps, namely the activation of CO2 by the intramolecular FLP, the addition of the hydroborane and, finally, the elimination of HCOOBBN to regenerate catalyst 1 (Scheme 4).

Reduction of 3 before release by a second equivalent of hydride might therefore also be likely, although experimentally the first hydride transfer is rate determining (see SI). As depicted in Scheme 4, CO2 adduct 2 and TS2-3 are the rate determining states and they control the kinetics of the catalysis. [58][59][60][61] The computed energy span of 22.5 kcal/mol is much lower than the one required for the uncatalyzed addition of 9-BBN to CO2 (30.8 kcal/mol) 27 and is consistent with a fast reaction at room temperature (computed TOF of 0.7 h -1 ). 57 Notably, as CO2 adduct 2 constitutes a TOF determining intermediate (TDI), increasing its stability is detrimental to the catalytic performances. Computationally, removing the chloride anion in the energy surface depicted in Scheme 4, an energy span of 24.7 kcal/mol (1 + as 0.0 kcal/mol reference, see Figure S37) is obtained, that is 2.2 kcal/mol than with catalyst 1. This result is excellent agreement the experimental findings pointing to a significant diminution of the catalytic performance when The formation of CO2 adducts from iPr 1 and Ph 1 is around 4 kcal/mol higher in energy compared to 2 (see SI Table S1). Although this destabilization could have a positive effect on the catalysis, it also results in a lower degree of activation of the CO2 molecule and, hence, in a greater activation barrier of 25.2 and 22.6 kcal/mol for the rate determining C-H bond formation, for the iPr and Ph substituted sytems, respectively (see ESI Page 8 for detailed span values). Again, these results are in good qualitative agreement with the catalytic data showing a lower catalytic activity for iPr 1 and Ph 1 compared to 1 and they underline the necessity for a fine balance in the activation of CO2. Scheme 5. Stoichiometric and catalytic reactions between 1, 2 + , CO2 and hydroboranes.

1 is replaced with [1 + ,B(C6F5)4 -]2.
Obviously, changing 9-BBN with catBH or pinBH does not modify the energy level of the CO2 adduct 2. Nevertheless, it affects the position of the corresponding TS for the hydride transfer to CO2 (TS2-3). Both catBH and pinBH possess a boron atom that is less Lewis acidic than 9-BBN, because the π lone pairs on the oxygen substituents are delocalized into the boron vacant p orbital.

As such, coordination of the hydroborane to the C=O group of the activated CO2 molecule is disfavoured and the TOF determining TS2-3 cat and TS2-3 pin are higher in energy than 2 (spans of 34.0 and 37.9 kcal/mol for catBH and pinBH, respectively). The computed order of reactivity 9-BBN > catBH > pinBH is thus in agreement with the experimental observations. Nonetheless, a span of 39.7 kcal/mol is relatively high for a catalytic reaction proceeding at 70 °C, thereby questioning the validity of a mechanism relying on CO2 activation for pinBH. In addition, while iPr 1 presents a better catalytic performance than 1 with pinBH, its computed energy span is 1.8 kcal/mol higher in energy (37.9 kcal/mol). Experimentally, no reaction is observed upon mixing [2 + ,B(C6F5)4 -]2 with pinBH for >30 h at 90 °C, although 2.5 mol% [1 + ,B(C6F5)4 -]2 efficiently catalyze the reduction of CO2 to MeOBpin at 90 °C (Scheme 5 and Table 1, entry 18). These facts definitively rule out a mechanism relying on CO2 activation for the hydroboration of CO2 with pinBH and an alternative pathway was sought. Scheme 6. Computed pathway for the electrophilic activation of hydroboranes in the reduction of CO2 to formoxyboranes (M05-2X/PCM THF). Values given are Gibbs Free Energies with respect to 1 (0.0).

Mechanism involving the electrophilic activation of the hydroborane. In order to account

for the catalytic activity of 1 and [1 + ,B(C6F5)4 -]2 in the reduction of CO2 with pinBH, an alternative mechanism was explored based on the potential electrophilic activation of the B-H bond with the catalyst (Scheme 6). Indeed, the ionization of the silicon-chloride bond is readily accessible in hypervalent chlorosilanes. [START_REF] Kummer | [END_REF] Starting from 1, such dissociation would be also favoured by the presence of an electrophile such as a hydroborane, to yield masked silylium ion 1 + . The hydride transfer from R2BHCl -to 1 + was computed for the three hydroborane reagents, to form hydrosilane 5 (Scheme 6). The formation of 5 and chloroborane R2BCl (6) by-product is slightly endergonic for the three hydroborane sources (+7.0 kcal/mol for 9-BBN, +10.9 kcal/mol for catBH and +7.8 kcal/mol for pinBH, Scheme 6). Nevertheless, the respective transition states energies are all accessible, suggesting that it is a viable intermediate on the hypersurface (17.0 kcal/mol for 9-BBN, 21.8 kcal/mol for catBH and 24.8 kcal/mol for pinBH, Scheme 6). Species 5 appears to be a potent reductant and outersphere hydride transfer from the Si-H group in 5 to an incoming CO2 molecule yields silylium 1 + and the formate anion with a low energy barrier of 18.9 kcal/mol, in agreement with the reported reactivity of other hypervalent hydrosilanes towards CO2. 62 Subsequent salt metathesis between HCOO -, R2BCl and 1 + finally regenerates catalyst 1 and releases formoxyborane HCOOBR2, without any significant energy barrier (<6 kcal/mol).

TS5-4 associated with the reduction of CO2 with 5 constitutes the highest energy rate determining state in this pathway. [START_REF]The formation of hypervalent hydrosilanes has also been proposed as a competent catalytic pathway in the hydrosilylation of CO2, based on DFT calculation[END_REF] Interestingly, the structure of TS5-4 does not depend on the nature of the hydroborane although its energy level is indirectly dictated by the formation of the chloroborane co-product R2BCl (6). Although catalyst 1 is the lowest energy state in this pathway, CO2 adduct 2 easily forms under the applied conditions, as revealed by 1 This mechanistic route hence highlights two important experimental observations. First, an electrophilic pathway significantly lowers the energetic span (35.3 kcal/mol) by about 3 kcal/mol for the hydroboration of CO2 with pinBH compared to a route where CO2 is activated via the formation of an adduct (37.9 kcal/mol). Second, a stable CO2-adduct hampers reaction kinetics in CO2 hydroboration under electrophilic activation, as witnessed by the lower reactivity of [1 + ,B(C6F5)4 -]2 compared to 1. In fact, such mechanism also explains the differences in reactivity between 1, iPr 1 and Ph 1, but care has to be taken to avoid quantitative conclusions, as the calculated spans are close in energy. Computationally, the energy span for the catalytic hydroboration of CO2 with pinBH decreases from 35.3 kcal/mol with 1 to 34.2 and 32.4 kcal/mol with Ph 1 and iPr 1, respectively. The DFT model is thus consistent with the experimental data providing the following order of reactivity: iPr 1 > Ph 1 > 1. The DFT model is thus consistent with the experimental data providing the following order of reactivity: iPr 1 > Ph 1 > 1. Although the order of reactivity is reproduced correctly, these results have to be taken as a qualitative measure, given the accuracy of the DFT models. It is notable that, although the TOF-limiting transition state with iPr 1 corresponds to the hydride abstraction from pinBH (TS5-4), the main factor governing its catalytic activity is the destabilization of CO2-adduct iPr 2 (+3.7 kcal/mol with respect to 2) (see SI, Table S1).

To further underline the feasibility of CO2 reduction by a based-substituted silane 5 (Scheme 7), the synthesis of such a species was attempted. Guanidine substituted hydrosilane Ph 5 is easily accessible through dehydrogenative coupling of TBD with diphenylsilane and shows a characteristic hydride peak at 5.03 ppm in the 1 H NMR (Scheme 7, Figure S26). Crystals suitable for X-ray analysis were obtained by slowly cooling down a concentrated acetonitrile solution of Ph 5 from 90 °C to room temperature (Figure 3). With bond angles of 97.4, 114.7(2)° and 116.8 (2) for N1-Si-H1, N1-Si-C8a and N1-Si-C14a, respectively, the silicon geometry is slightly distorted from a tetrahedron, although a -bond as in 1 or iPr 1 is less likely due to a small bond angle of 153.6 ° for N2-Si-H1 and a long N2-Si distance of 2.648(5) Å. Under one atmosphere of CO2, a THF solution of Ph 5 evolves within minutes at room temperature to yield a formate species (Scheme 7, Figure S28). In the absence of an electrophilic scavenger, such as a chloroborane, this species then converts to formylated TBD by deoxygenation of the formate anion (Figure S28). This chemical behaviour demonstrates the strong reductive capability of Ph 5 in the presence of CO2 64 , in agreement with the DFT model depicted in Scheme 6. As an example, catalyst Me-TBD or pro-azaphosphatranes operate through this process. 27,74 In a recent study by Bourissou and Fontaine 32 the importance of an intermediate formaldehyde adduct was highlighted, which can also serve as a Lewis base. In contrast, Lewis acids may abstract the hydride of the B-H bond to yield a boron electrophile and convert the catalyst to a strong hydride donor, as observed for iPr 1 with pinBH (Scheme 8B). In these two mechanisms, the formation of stable CO2 adducts hampers the catalysis by stabilizing the catalyst's resting state. Finally, the organocatalyst can also activate the CO2 molecule, directly. As CO2 is a weak Lewis base, this mode of action usually involves a bifunctional activation of CO2 with the cooperative effect of a Lewis base and a Lewis acid (Scheme 8C). Catalysts 1, 2 + and IV were shown to follow this path, with 9-BBN. Importantly, the degree of activation of the CO2 molecule must be finely tuned to open this reduction path. Indeed, the "activation" of CO2 in the form of an adduct is both deleterious and necessary to the catalytic activity, as it stabilizes the lowest intermediate in the potential energy surface yet prepares CO2 for the subsequent reduction step 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 thus target compounds that show a low affinity for CO2, based on thermodynamics, yet increase the electrophilicity of the carbon center with strong Lewis acids.

Conclusion

novel class of intramolecular nitrogen-silylium FLPs, e.g. TBDSiR2Cl (R = Me, i Pr, Ph) and their cationic derivatives, have been reported and their reactivity with CO2 has been investigated. Formation and stability of the CO2-adducts can be modulated with the steric and electronic environment at the silicon center and only the methylated CO2-FLP have been observed and characterized. All of these novel silicon species were shown to promote the catalytic hydroboration of CO2 to the methoxide level, in high yield under mild reaction conditions. DFT calculations were carried out, in support to experimental results, to better explore the role of FLP-CO2 adducts in the catalytic reduction of CO2 with 3 different boranes (9-BBN, catBH, pinBH). To date, CO2-adducts 2 + and V, where CO2 is activated by a N/Si + or N/B intramolecular FLP, remain the only two examples of stable FLP-CO2 adducts exhibiting a catalytic activity in the hydro-elementation of CO2.

From a mechanistic point of view, the reduction of CO2 by 9-BBN with 1 relies on a hydride transfer from boron to carbon through a [2+2] mechanism, where the B-H bond adds to the C=O

Figure 1 .

 1 Figure 1. ORTEP view of 1 (left) and cation (1 + )2 in [1 + ,B(C6F5)4 -]2C4H8O2 (right).The solvent molecules and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) in 1: Si1-Cl1 2.2783(5), Si1-N1 1.7914(12), Si1-N2 1.9843(12), N1-C1 1.3578(17), N2-C1 1.3087(18), N1-Si1-N2 69.09(5); in (1 + )2: Si1-N1 1.7714(13), Si1-N3 1.7796(13), N1-C1 1.3628(16), N3-C5 1.3598(17).

  mbar) are needed to promote the decarboxylation of [2 + ,B(C6F5)4 -]. While adducts I and II, reported by Müller et al. and Stephan et al., respectively, were found unstable and could not be characterized structurally, 38 [2 + ,Cl -] and [2 + ,B(C6F5)4 -] (Scheme 3d) represent the only examples of N/Si + FLP-CO2 adducts, which are characterized structurally in the solid state. The bonding of CO2 in adduct 2 + resembles that of V and IV, with a more pronounced activation of the C-O bonds. Indeed, the activation of CO2 with FLPs bends the linear CO2 molecule and the O-C-O bond angle is smaller in 2 + (121.4(3) °) compared to V (128.6(2) °) and IV (129.9(2) °). In parallel the C-O bond activated by the Lewis acid is somewhat longer in 2 + (1.322(4) Å), compared to V (1.299(3) Å) and IV (1.257(3) Å), while the C=O bond is shorter (1.206(4) vs 1.222(2) in V and 1.229(2) Å in IV). These data confirm the stronger Lewis acidity of the silyl cation in the activation of CO2, compared to the N/B system V. Spectroscopically, the 13 C resonance for CO2 is shifted upfield in 2 + (145.3 ppm, see SI Figures S25 or S23), compared to V and IV (153.1 and 154.4 ppm, respectively).

Figure 2 .

 2 Figure 2. ORTEP view of cation 2 + in [2 + ,Cl -] 0.5C 5 H 5 N. The solvent molecules and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Si1-O1 1.671(2), Si1-N1

Scheme 4 .

 4 Scheme 4. Computed pathway for the catalytic hydroboration of CO2 via CO2 activation (M05-

  Moreover, the addition of one equivalent of 9-BBN (monomer) to a 70:30 equilibrium mixture of 1/[2 + ,Cl -] results in the fast formation of the acetal CH2(OBBN)2 after only 2.5 h at RT while a reaction time of 17 h is required for [1 + ,B(C6F5)4 -]2 (Scheme 5a,b).

  H NMR spectroscopy, and it should therefore be included on the energy surface. 2 is thus an off-cycle intermediate that further enlarges the surface landscape yielding energetic spans 34.5 kcal/mol 35.3 kcal/mol (pinBH) and 38.3 kcal/mol (catBH) for the different hydroboranes.

Scheme 7 .

 7 Scheme 7. Reduction of CO 2 withPh 5 to a formate species.

Figure 3 . 8 .

 38 Figure 3. ORTEP view of Ph 5. Only one position of the disordered groups is represented (see SI for details). Selected bond lengths (Å): Si1-N1 1.722(3), Si1-H1 1.427, N1-C1 1.341(4), N2-C1 1.323(4).Overall, the mechanistic route depicted in Scheme 6 relies on the abstraction of a hydride from

Table 1 .

 1 Catalyst screening for the hydroboration of CO 2 to methoxyboranes.

	Entry	Catalyst	Borane	T[°C]	t[h]	Yield[%] a	TOF(h -1 ) b
	1	1	9-BBN	20	23(6.5)	100(77)	1.3 (3.7)
	2 c	1	9-BBN	20	78	17.5	0.1
	3 d	1	9-BBN	20	78	0	0.0
	4 e	1	9-BBN	20	78	83	0.3
	5 f	1	9-BBN	20	78	43	0.2
	6	iPr 1	9-BBN	20	24	60	0.7
	7	Ph 1	9-BBN	20	24	56	0.8
	8	0.5[1 + ,B(C6F5)4 -]2	9-BBN	20	72(3)	30(6)	0.1(0.9)
	9	0.5[1 + ,I -]2	9-BBN	20	24	100	1.3
	10	1	catBH	70	120	74	0.5
	11	iPr 1	catBH	70	67	31	0.3
	12	Ph 1	catBH	70	96	44	0.3
	13	0.5[1 + ,B(C6F5)4 -]2	catBH	70	42	18	0.2
	14	[1 + ,I -]2	catBH	70	96	49	0.3
	15	1	pinBH	90(70)	28(28)	75(21)	0.8(0.2)
	16	iPr 1	pinBH	90(70)	16(28)	100(78)	2(0.8)
	17	Ph 1	pinBH	90	28	82	0.9
	18	0.5[1 + ,B(C6F5)4 -]2	pinBH	90	90	50	0.6
	19	0.5[1 + ,I -]2	pinBH	90	96	40	0.5
	20	Ph 5	catBH	70	96	40	0.3
	21 g	7	catBH	90	24	10	0.1

groups on the hydrosilylation of C=O and C-O bonds, where the electrophilic borane (B(C6F5)3) abstracts the hydride from the hydrosilane. [64][65][66][67][68][69][70] Here DFT calculations suggest that the inverse reaction should also be possible. With the work of Oestreich and co-workers 71,72 on the transient formation of such Si-H-B species during the hydrosilylation of carbonyl functional groups and the recent isolation and characterization of such a species by Piers and co-workers, 73 the availability of a transition state (TS1-5) for the hydride transfer from a chloroborate (e.g. [pinBH-Cl] -) to the masked silylium 1 + is further accredited. Interestingly, such mechanism relying on an electrophilic activation of a hydroborane reductant is unprecedented in hydroboration chemistry, although it resembles the mode of action of Lewis acid catalysts in hydrosilylation chemistry. It is found to be favoured for electron rich boranes such as pinBH (and possibly catBH, although DFT calculation don not allow to draw a final conclusion), with a boron center stabilized by electron donors.

bond of the activated CO2 molecule in the FLP-CO2 adduct. On the other hand, using pinBH with iPr 1 as catalyst a novel electrophilic pathway is at play. For catBH and other catalyst a clear distinction between both pathways cannot be made. Nevertheless, a strong stabilization of CO2 in the form of an adduct is deleterious to the catalytic activity in all cases.

ASSOCIATED CONTENT Supporting Information. Experimental procedures, characterization of compounds, kinetic profile of catalytic CO2 hydroboration, crystallographic data, Cartesian coordinates all optimized stationary points and transition states with the corresponding energies and lowest frequencies, overview of the energetic spans of the different proposed reaction mechanisms including different catalysts and hydroboranes sources, as well as an alternative reaction pathway. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*Thibault Cantat, NIMBE, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette, France Fax: (+33) 1.6908.6640 E-mail: thibault.cantat@cea.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.