DOI: 10.1002/chem.201304376


& Organocatalysis

Transition-Metal-Free Catalytic Reduction of Carbon Dioxide Frdric-Georges Fontaine,* Marc-Andr Courtemanche, and Marc-Andr Lgar[a]

Chem. Eur. J. 2014, 20, 2990 – 2996


 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Concept workers on a pincer nickel complex that catalyses the hydroboration of CO2 into CH3OBcat using HBcat as hydrogen source (cat = catechol) with a remarkable TOF (turn-over frequency) of 495 h 1.[11] A similar iridium pincer complex exhibited exceptional activity in the catalytic hydrosilylation of CO2 to methane, reaching a TON (turn-over number) of 8293 using Me2PhSiH.[12] Although several of these processes are showing significant potential for large-scale use, this article will focus on novel approaches for the metal-free catalytic reduction of carbon dioxide.

Abstract: Metal-free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition-metal-based catalysts.

Introduction The reality of climate change and of its environmental consequences has led to several worldwide political initiatives aiming to reduce the emission of greenhouse gases, notably carbon dioxide.[1] Although several technologies exist to trap carbon dioxide from the flue gas of the major emitters, the cost and risks associated with sequestration are prohibitive for industrial requirements.[2] The price of carbon dioxide on the carbon market is too low to be an incentive for industrials to increase their efforts in recovering CO2 emissions.[3] However, the possibility of using CO2 as a C1 feedstock for the generation of industrially relevant chemicals is of high interest.[4] Indeed, not only would such initiative allow the reduction of CO2 emissions in the same way carbon capture does, but would also alleviate the cost of the sequestration by producing valuable chemicals that are in global demand. Although the use of CO2 for synthetic purpose is already known and industrially relevant, notably in the synthesis of dyes and of urea,[2] the large-scale synthesis of energy vectors has the potential to use enough CO2 to have a considerable impact on the overall global emissions.[5] Indeed, most liquid fuels currently used originate from fossil fuels, which generate “new” carbon in the troposphere. In contrast, producing liquid fuels from existing CO2 and “green” energies could significantly reduce our carbon footprint considering the very large volumes of petroleum-related energy vectors consumed daily. One of the most useful energy vectors is methanol as it could replace liquid fuels in most modern technologies. As such, it is the foundation of the methanol economy promoted notably by Nobel laureate George A. Olah.[6] Several technologies are known to reduce CO2 to methanol, notably using solid inorganic catalysts. However, most of these processes operate at high pressure and temperature.[7] In the past few years, several organometallic catalysts have shown activity in the reduction of CO2 to methanol or other high-hydrogen containing molecules under relatively mild conditions using the hydroboration,[8] hydrosilylation,[9] or hydrogenation reactions.[10] Most of these systems exploit the possibility of carbon dioxide to insert into M H bonds to generate reduced species. Notable systems include the work by Guan and co[a] Prof. F.-G. Fontaine, M.-A. Courtemanche, M.-A. Lgar Dpartement de Chimie,Universit Laval 1045 Avenue de la Mdecine, Qubec, QC, G1V 0A6 (Canada) Fax: (+ 1) 418-656-7916 E-mail: [email protected] Chem. Eur. J. 2014, 20, 2990 – 2996

Transition-Metal-Free Reduction of Carbon Dioxide Several strategies have been developed for the reduction of carbon dioxide into hydrocarbons without the use of transition-metal catalysts. One of the most efficient systems to date uses highly Lewis basic N-heterocyclic carbenes (NHC) in presence of hydrosilanes.[13] When IMes-CO2 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolium) was used as catalyst in presence of diphenylsilane as a reductant under an atmosphere of CO2, Ph2Si(OMe)2 was obtained as the main reduction product with over 90 % selectivity. The TON and TOF, calculated for this system to be 1840 and 25.5 h 1, respectively, at room temperature, exceed the performance of most catalysts. Another advantage of this system is the possibility to reduce CO2 from dry air. The authors proposed the first reduction step as either an attack of the NHC on the silane, generating a pentavalent silicon species that can deliver a highly nucleophilic hydride to CO2 (Figure 1, pathway A), or an attack of the nucleophilic carbene on the electrophilic carbon of CO2, rendering it more reactive towards silanes (Figure 1, pathway B). The former pro-

Figure 1. Suggested pathways for the NHC-catalysed hydrosilylation of CO2.

posal was shown to be more plausible by Wang et al. in a theoretical study.[14] This mechanism is supported by the observation that bulkier tertiary silanes are poor reductants in this system. After the first reduction step, generating the formoxysilane, the subsequent reductions by activated silanes can continue and lead to methoxysilane. Interestingly, although it was not observed experimentally, the final reduction step is proposed to occur through formaldehyde that would be generated from a rearrangement of the (bis)silylacetal intermediate. Cantat and co-authors have developed a TBD (TBD = 1,5,7triazabicyclo[4.4.0]dec-5-ene) catalytic reduction of CO2 to formamides using secondary amines in presence of PhSiH3 as the


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Concept reductant.[15] Optimisation of this system has shown that N-heterocyclic carbenes can also act as very efficient catalysts for this transformation, using, notably, polymethylhydrosiloxane— one of the cheapest hydride source available—as a reductant.[16] It has also been shown that CO2 can be reduced stoichiometrically by silylium ions, yielding benzoic acid, formic acid and methanol, after aqueous workup.[17] A similar strategy was developed, using a high loading of AlEt2 + (10 mol %) in presence of hydrosilanes generating methane and solvent alkylation byproducts.[18] In order to assess the role of the catalyst, the authors independently prepared the silylium ions and tested their reactivity in the same conditions. The results supported that AlEt2 + had an important catalytic role, since the activity of the silylium ions was lower in the absence of AlEt2 + . Drawbacks of this system include slow activity (50 % conversion of the silane in 14 h at 80 8C) and lack of selectivity, generating methane as well as toluene and diphenylmethane as a result of the alkylation of benzene used as solvent (Scheme 1). Using the analogous phenolate species [Al(2,6-Mes2C6H3O)2Al] + (Mes = 2,4,6,-trimethylphenyl), faster reaction rates were obtained (> 90 % conversion of Et3SiH in 25 h at 85 8C), but the reaction was still lacking selectivity.[19]

oxide has been most convincingly demonstrated using the now famous “frustrated Lewis pairs” (FLPs). These bulky main group Lewis pairs are known not to form classical adducts because of steric constraints.[23] Following pioneering reports on the splitting of molecular hydrogen using FLPs, this class of species has been able to activate several other substrates, including notably carbon dioxide.[24] In 2009, Erker and Stephan showed that a FLP system consisting of P(tBu)3 and B(C6F5)3 could rapidly bind carbon dioxide. Since then, an important number of FLP systems have been shown to activate carbon dioxide,[25] including systems containing other main group elements, such as nitrogen and aluminum. Figure 3 shows some of the CO2 adducts that have been generated using the FLP design.[26] Nevertheless, only a handful of species have proven useful for the reduction of carbon dioxide into chemicals of interest.

Scheme 1. Catalytic hydrosilylation of CO2 to CH4 and byproducts by AlEt2 + .

Putting these results together, it is clear that carbon dioxide reduction can be promoted both by main group Lewis acids and bases. However, the efficiency of the catalytic process of CO2 reduction remains to be optimised. One way of doing so is using the synergy between Lewis pairs for CO2 activation.

Ambiphilic Activation of Carbon Dioxide Carbon dioxide is known to be a non-polar molecule and is often described as thermodynamically and kinetically stable. However, in contrast to other inert molecules, such as nitrogen and alkanes, the presence of internal dipoles along the C=O bonds give the molecule an ambiphilic character: the central carbon atom acts as an electrophile whereas the terminal oxygen atoms possess a nucleophilic character. The general concept of bifunctional activation of CO2 (Figure 2) is present in the mechanism of anaerobic CO dehydrogenases, in which a nucleophilic nickel site attacks the electrophilic carbon of CO2 and in which the oxygen is stabilised by Lewis acidic FeII.[20] A similar strategy has been used in the past 30 years for the activation of CO2 by transition-metal complexes,[21] Figure 2. Ambiphilic activaand more recently for its catalytic tion of carbon dioxide. L repreduction.[22] The concept of biresents a Lewis base and Z functional activation of carbon dia Lewis acid. Chem. Eur. J. 2014, 20, 2990 – 2996

Figure 3. Some of the reported FLP CO2 adducts.[22–24]

FLP-Mediated Reduction of Carbon Dioxide A first example of reduction of CO2 using FLPs was reported in which B(C6F5)3 and TMP (TMP = 2,2,6,6-tetramethylpiperidine) were shown to bind CO2 and generate a fomatoborate product when heated at 110 8C in the presence of 1–2 atm of H2. Heating at 160 8C for several days yielded methanol as the sole C1 product after distillation (Scheme 2).[27] Although stoichiometric

Scheme 2. Stoichiometric hydrogenation of CO2 by TMP/B(C6F5)3.


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Concept amounts of the FLP are needed for the reduction to occur, this finding demonstrated that frustrated Lewis pairs can be used for the reduction of carbon dioxide to methanol. Pushing the latter concept developed by Ashley and O’Hare further, Piers et al. reinvestigated the system using hydrosilanes as reductants in presence of additional equivalents of the Lewis acid.[28] The general strategy Figure 4. Simplified (it was shown computationally that the TMPH + moiety had an important role to play behind this modification was to throughout the catalysic process) mechanism of the first hydrosilylation step of CO by TMP/B(C F ) . 2 6 5 3 promote the B(C6F5)3-catalysed hydrosilylation of the formatoborate species generated by the TMP/B(C6F5)3 FLP tandem. Under those conditions, the system Developing an Efficient Catalytic System for effectively catalysed the reduction of CO2 to methane. In addithe Reduction of CO2 to Methanol tion, it allowed the spectroscopic monitoring of several reacAs their appellation entails, frustrated Lewis pairs require the tion intermediates, thus allowing the authors to propose acid and base fragments to be free of Lewis interactions to aca stepwise hydrosilylation process that was later supported tivate substrates. In a previous study, our group has demoncomputationally (Figure 4).[29] This contribution convincingly affirmed the potential of Lewis pairs to act as catalysts for the restrated that ambiphilic species (R2PCH2AlMe2)2 (R=Me, Ph), duction of carbon dioxide. Unfortunately, the calculated turnforming classical Lewis adducts, can activate carbon dioxide to over numbers were still far from competing with transitiongenerate CO2 adducts similar to those expected with FLP systems.[34] This report demonstrated that carbon dioxide can be metal-based systems. Mnard and Stephan reported a FLP system consisting of activated by Lewis pairs in solution or at the solid state even if PMes3/AlX3 (X = Cl or Br) in which CO2 was coordinated by two they form stable Lewis adducts in both media. Unfortunately, the CO2 adducts generated in this manner were not stable. Al O and one P C interactions. The CO2 fragment was reduced in presence of NH3BH3 to generate methoxyaluminum Indeed, they rapidly rearranged to generate thermodynamically stable carboxylate species (Scheme 4). species.[30] Hydrolysis of this species yielded methanol, destroyInterestingly, changing the phosphine moiety from PMe2 to ing the FLP system in the process and preventing catalysis (Scheme 3). Although this reaction is limited by its stoichiometPPh2 showed very little difference in reactivity. This led us to believe that highly Lewis basic groups would not be essential for CO2 activation. This aspect is of particular interest, since although the presence of the Lewis base in FLP systems favours the formation of CO2 adducts, a strong Lewis base could reduce the rate of reduction by making the carbon atom less electrophilic, thus slowing down the hydride transfer. This inhibiting effect of strong Lewis bases was recently proposed by Musgrave et al. and supported by in silico studies.[35] Scheme 3. Stoichiometric reduction of a FLP-CO2 adduct by BH3NH3. Since the reactivity of the methylene bridge of (R2PCH2AlMe2)2 was shown to be a problem in the design of a CO2 reduction catalyst, we investigated a more robust ric nature, it had an important impact in expanding the scope system inspired by the aryl-bridged phosphine–boranes develof reducing agents to hydroboranes. Further experimentation oped by Bourissou and used for small molecule activation.[36] [31] using a large array of Lewis acids and amine–boranes and Our attempt to generate the monophosphine analogue orthotheoretical studies, later revealed possible mechanistic pathPh2PC6H4AlMe2 was dampened by the observation that such ways for this reaction.[32, 35] The activation of carbon dioxide by the FLP system was deemed essential to allow the CO2 to react with the reducing agent. A similar system was also recently shown to perform the stoichiometric reduction of CO2 to CO.[33] Scheme 4. Fixation of carbon dioxide by Al–P ambiphilic molecule and subsequent carboxylation. Chem. Eur. J. 2014, 20, 2990 – 2996


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Concept species undergo rearrangement under synthetic conditions to generate the triphosphine species Al((C6H4)PPh2)3. Interestingly, this species interacts with carbon dioxide to generate classical CO2 adducts, albeit with a weaker interaction than in the case of previously reported Al-based species (Scheme 5). Indeed, it was shown that the latter interaction was reversible under ambient conditions, without any signs of degradation of the complex.[37]

Scheme 5. Reversible fixation of carbon dioxide by aryl-bridged Al–P ambiphilic molecule.

Al((C6H4)PPh2)3 acted as a precatalyst for the reduction of CO2 in presence HBcat, but close monitoring of this system showed that the aluminum species rearranged to an intractable solid and that the active catalytic species proved to be the phosphine–borane analogue 1-catecholboryl-2-diphenylphosphinobenzene 1 (vide infra). Although the characterisation of the refractive material was not possible, carrying out the reaction with MeOBcat rather than HBcat allowed the structural characterisation of Al(k2-O,O-(MeO)2Bcat)3, which suggest once more that the aluminum species rearranges to generate stable inorganic aluminum alkoxide derivatives.[37] Together, these results suggest that the kinetic instability of the Al R bonds and the oxophilicity of aluminum are detrimental to the generation of ambiphilic catalysts for CO2 reduction.

Ambiphilic Boron-Containing Species as Catalyst for CO2 Reduction Following the previous observations, we investigated the possibility for ambiphilic and FLP systems to activate CO2 in presence of weak Lewis acids and bases. Indeed, the stabilities of the CO2 adducts and of the reduced products with the FLPs seem to be one of the important limiting factors in obtaining important catalytic activity. Results in that direction have already been reported by Lammertsma et al. in 2012 as they synthesised the ambiphilic phosphinoborane (tBu)2PCH2BPh2.[38] In this molecule, the borane moiety is significantly less electrophilic than the perfluorinated groups usually used in FLP systems and was shown to bind CO2. However, in this case, the contribution of a very strongly basic phosphine moiety cannot be overlooked, as the formation of the CO2 adduct is irreversible, even under high vacuum. No reduction chemistry was reported using this adduct. Another aspect that we considered important in the design of a new CO2 reduction catalyst was to use an ambiphilic molecule as an FLP. Indeed, it is logical to think that the entropic cost for the activation CO2 will be lowChem. Eur. J. 2014, 20, 2990 – 2996

ered with an ambiphilic molecule compared to a system in which three components (Lewis base, Lewis acid, and CO2) have to come together. As explained above, the phenylene linker was of interest and demonstrated great stability.[36] The preparation of the so far unreported 1-catecholboryl-2-diphenylphosphinobenzene (1) was carried out using classical synthetic pathways.[39] As previously reported by Bourissou for this family of species, no intra- or intermolecular Lewis adducts were observed. Initial numerical DFT modelling of this molecule at the B3PW91/631G** level of theory predicted that an interaction with CO2 could generate an adduct that would be disfavoured by 9.9 kcal mol 1 (DH) in comparison to the free fragments (Scheme 6). According to this value, the concentration of the CO2 adduct expected in solution would be undetectable by most spectroscopic techniques. NMR spectroscopy corroborated these results, since no adduct was observed in solution under a CO2 atmosphere.

Scheme 6. Reversible fixation of carbon dioxide by aryl-bridged phosphine– borane ambiphilic molecule.

Nonetheless, it was possible to observe that carbon dioxide could undergo rapid hydroboration in the presence of 1 to afford catBOBcat and MeOBcat, which could be hydrolysed to methanol. Under mild conditions (1 atm, 70 8C), a maximum TOF of 166 h 1 was calculated and the reaction was followed for over 650 turn-overs (Scheme 7). It was shown that for the first time a metal-free system could catalyse the hydroboration of carbon dioxide at similar rates as transition-metal catalysts. Interestingly, organocatalyst 1 could reduce CO2 in presence of BH3·SMe2. As can be expected, when 1 was added to the borane source, the formation of the BH3-protected phosphine was observed. It is still unclear if BH3 needs to dissociate from 1 to be able to reduce carbon dioxide, but the dissociation is expected to be possible at 70 8C. When 1 was reacted with 100 equivalents of BH3·SMe2 under about 2 atm of CO2 at

Scheme 7. General reaction for the hydroboration of CO2 to methanol by catalyst 1.


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Concept 70 8C, the formation of (MeOBO)n type products was found to be extremely rapid. These products were then hydrolysed to generate methanol and an unprecedented TOF of 853 h 1 was calculated for this reaction and no decrease in reaction rate was observed after 2950 turn-overs. Based on the reactivity and mechanistic investigations of previous systems, we hypothesised that the reduction would occur through a stepwise process. First generating a formatoborate, which would then be reduced to formaldehyde that could finally generate the methoxide derivative. The formation of these possible intermediates was explored using DFT computational modelling and energy calculations. We were thus able to optimise the structure of intermediate products and their possible interaction with the ambiphilic catalyst. The first optimised structure represents a formatocatecholborate (2) fragment coordinated to the ambiphilic centre and features a six-membered ring in which the oxygen atom of the catechol group plays a role in stabilizing the intermediate. Interestingly, this adduct is only 1.4 kcal mol 1 (DH) more stable than the free fragments. Another minimum was located on the potential energy surface at which an adduct between the catalyst and formaldehyde is generated. This time the bonding is much stronger, with an enthalpy of formation of 14.7 kcal mol 1. This rather strong binding energy explains why the formaldehyde adduct of 1 is the only experimentally observable species during catalysis by using HBcat as a reducing agent. This observation is important as it was previously reported that for the reduction of CO2 to methanol, a formaldehyde intermediate had to be formed at some point. However, it was never observed experimentally until our initial report of the catalytic activity of 1 and soon after by Sabo-Etienne and Bontemps in a related study.[40] (Figure 5) Although the full mechanism is still under investigation and many possible rearrangements are possible in the reaction mixture, especially at the boron centre, these preliminary calculations with HBcat provide clues regarding the possible reactive intermediates.

Summary and Outlook Several inorganic and organometallic systems are known to be efficient catalysts for the transformation of CO2 in chemicals of industrial interest. However, the possibility of using metal-free systems for the low-temperature reduction of CO2 was until recently unprecedented. In the past five years, it was shown that several FLP systems could interact with carbon dioxide, in some instances initiating the reduction in presence of hydrogen-containing reductants, such as hydroboranes, hydrosilanes and molecular hydrogen. However, most of the systems reported exhibit very low catalytic activity. Interestingly, the high catalytic activity of 1 surpasses most of the organometallic systems for the reduction of CO2 to methanol, radically in contrast with traditional FLP systems. Although full investigation of the reactivity is underway, one can expect that the key feature of this system involves: 1) The availability of both Lewis acids and bases for interaction with CO2, as in classical FLPs; 2) an ambiphilic system to entropically favour the binding of CO2, especially when compared to FLPs with multiple components; but 3) without strong acidity or basicity to initiate reduction. Indeed, it can be expected that a strong interaction between the Lewis base and CO2 will reduce the rate of the hydride transfer to the carbon atom. Alternately, the use of a strong Lewis acid will prevent dissociation of the reduced species and limit catalytic turn-overs by deactivating the catalyst. A fine tuning of the Lewis acidity/basicity of the active centres is thus of prime importance. Current efforts are focused on understanding the important mechanistic parameters as well as varying the functional groups on phosphorus and boron in order to achieve maximal catalytic efficiency. A desirable, but challenging avenue would be to implement inexpensive molecular hydrogen as primary reductant and move away from hydroboranes and hydrosilanes. However, such avenue will be limited by the formation of formic acid and water that can easily poison most of the FLP derivatives.

Acknowledgements This work was supported by the National Sciences and Engineering Research Council of Canada (NSERC, Canada) and the Centre de Catalyse et Chimie Verte (Quebec). M.-A.C. and M.A.L. would like to thank NSERC and FQRNT for scholarships. We would like to thank R. Nadeau for the frontispiece picture. Keywords: carbon dioxide · frustrated Lewis pairs · main group elements · organocatalysis · reduction [1] D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. 2010, 122, 6194 – 6219; Angew. Chem. Int. Ed. 2010, 49, 6058 – 6082. [2] M. Aresta, A. Dibenedetto, Dalton Trans. 2007, 2975 – 2992. [3] M. Ricci in Recovery and Utilization of Carbon Dioxide (Eds.: M. Aresta), Kluwer Academic Publishers, The Netherlands, 2003, pp. 395 – 402. [4] H. Arakawa, M. Aresta et al., Chem. Rev. 2001, 101, 953 – 996. [5] Z. Jiang, T. Xiao, V. L. Kuznetsov, P. P. Edwards, Philos. Trans. R. Soc. A 2010, 368, 3343 – 3364. [6] G. A. Olah, A. Goeppert, G. K. Surya Prakash, J. Org. Chem. 2009, 74, 487 – 498.

Figure 5. Possible intermediates in the reduction of CO2 by phosphine– borane 1 and their interaction with the catalyst. Chem. Eur. J. 2014, 20, 2990 – 2996


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Received: December 5, 2013 Published online on February 12, 2014


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Transition-metal-free catalytic reduction of carbon dioxide.

Metal-free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic ...
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