Metal-Free Reduction of CO2

Frédéric-Georges Fontainea and Douglas W. Stephanb

a Département de Chimie, Centre de Catalyse et Chimie Verte (C3V), Université Laval,

1045 Avenue de la Médecine, Québec (Québec), Canada, G1V 0A6

b Department of Chemistry University of Toronto, 80 St. George St. Toronto, Ontario,

Canada M5S 3H6

Email : dstephan@chem.utoronto.ca

This is the peer reviewed version of the following article: [Metal-Free Reduction of CO2, Current

Opinion in Green and Sustainable Chemistry, 2017, 3, 28- 32], which has been published in final form

at [http://dx.doi.org/10.1016/j.cogsc.2016.11.004].

Even if carbon dioxide is now considered a major pollutant and the main source of global

warming, it remains the feedstock used by Nature to generate organic compounds. There

is considerable interest to find chemical processes that could replace fossil fuels by CO2 to

generate industrially relevant carbon containing molecules. Mostly relying on frustrated

Lewis pair technology and on the nucleophilic activation of CO2 or reducing agents by

organic Lewis bases, novel catalytic opportunities have emerged to affect the metal-free

catalytic reduction of carbon dioxide.

Introduction

Efforts to mitigate the effects of climate change have prompted a surge of interest in the

functionalization of the most abundant of the green-house gases, CO2. Current industrial

use of CO2 is limited, largely a result of its thermodynamic stability. While CO2 has no

dipolar moment, the carbon centre is electrophilic and indeed it is this feature that has been

exploited to effect reactions with a range of nucleophiles. This feature is central to current

protocols for CO2 scrubbing technologies1 and current industrial applications of CO2

chemistry for the generation of urea, dyes and cyclic carbonates.2

The possibility of using CO2 as a practical C1 source is an attractive notion as this

mimics Nature and offers a renewable alternative to fossil fuels for carbon-based industrial

feedstocks and fuels. This potential has prompted a flood of studies targeting reduction

products such as methane or methanol as such conversions could in principle, be done on

a large scale, offering a strategy to reduce the CO2 output and generate value from these

emissions. To overcome the stability of CO2, catalysts are required. Indeed, various

transition metal based homogenous and heterogeneous catalysts have been explored to

effect either thermal, electrochemical or photochemical CO2 reductions.3-15 In the context

of sustainable and green chemistry, the possibility of using metal-free catalysts in lieu of

transition metal-based catalysts is another enticing approach. This avenue offers low cost,

earth-abundant, non-toxic reagents, with a significantly reduced carbon-footprint in the

overall process.16-18 This article focuses on the recent advancements in such metal-free

reduction of CO2 using main group element-based catalyst technologies.

Frustrated Lewis Pairs and Reactions of CO2

Figure 1. Frustrated Lewis complexes of CO2

CO2 Capture and Stoichiometric Reduction

Frustrated Lewis pairs (FLPs) are combinations of a Lewis acid and base that are not

mutually quenched as a result of steric or geometric constraints.19-22 These reagents are

well suited to the activation of carbon dioxide as FLPs are ambiphilic allowing nucleophilic

activation at the carbon and the subsequent electrophilic capture of the oxygen atoms in

the resulting species (Figure 1). Such a bifunctional interaction has been shown to be key

features for both the active site of carbon monoxide dehydrogenase that converts CO to

CO2 and for highly efficient transition metal CO2 reduction catalysts.23-25 In a 2009

contribution, Stephan, Erker and coworkers26 reported that FLPs derived from phosphines

and boranes can bind CO2 affording zwitterionic products in which phosphines bind to

carbon and the borane binds one of the oxygen of CO2. Analogous binding by various other

FLPs derived from donors (N-heterocyclic carbenes, phosphines, amines, phosphinimines,

pyrazoles) and acceptors (B, Al, Ti, Zr, Hf and Si electrophiles) has been reviewed.27-28

Figure 2. Stoichiometric reductive transformations of FLP CO2 adducts

Some of the products of such trapping of CO2 have been shown to undergo further

stoichiometric transformations. For example, once the FLP P(SiMe3)3/B(p-C6F5H)3 traps

CO2, it undergoes silyl migration to generate a silyl ester, which in turn traps another

equivalent of CO2 generating an unusual phosphaalkene species (Figure 2A).29 It was also

shown that the CO2 adduct of ambiphilic Me2PCH2AlMe2 undergoes methylene migration

to generate an aluminum carboxylate species (Figure 2B).30 Menard et al. described the

complex of CO2 by phosphine/aluminum FLPs such as PMes3/AlX3 leading to the

stoichiometric reduction to CO31 while subsequent reaction with ammonia borane effects

the stoichiometric reduction to methoxy-aluminium species which liberates methanol on

hydrolysis (Figure 2C).32

Catalytic Hydrosilylation of CO2

To advance metal-free strategies towards catalysis a number of studies have

exploited silane reductants. In 2009 Ying and co-workers33 described the use N-

heterocyclic carbenes (NHCs) to activate silanes leading to the catalytic reduction of CO2

to methanol and methoxysilanes with turn-over frequencies up to 25 h-1 (Figure 3, I) The

authors proposed that this reduction was a result of the increased hydridicity of the Si-H

bonds and Lewis acidity at the hypercoordinate silicon, resulting from coordination of the

NHC to silicon.

Interestingly, more recent computational studies by Zhou and Li34 demonstrated that the

likely mechanism involves the cooperative action of NHC-CO2 adduct and CO2 on the

silane. In this case the NHC-CO2 acts a Lewis base, while CO2 acts as an electrophile or

Lewis acidic hydride acceptor. This mechanism is conceptually reminiscent of what is now

referred to as the frustrated Lewis pair (FLP) mechanism35 for the hydrosilylation of

ketones, first described by Piers and coworkers.36

A more recent study37 showed that phosphazenes also mediate the reaction of CO2

with hydrosilanes, generating selectively formylsilanes or methoxysilanes, the distribution

of which is altered by reaction conditions (Figure 3, II). In addition, it was demonstrated

that DMF as the solvent impacts on the hydrosilylation of CO2, effecting reduction without

catalyst.

The stoichiometric reduction of CO2 to methane using hydrosilanes has also been

observed using Lewis acids. For example, the highly electrophilic species [AlEt2]+ (Figure

3, III),38-39 gave a mixture of reduced products when the reaction was carried out in

benzene (methane, toluene, diphenylmethane), although selectivity has been shown to be

limited at best. In a similar fashion, Piers and coworkers40 also reported that the FLP

derived from TMP/B(C6F5)3 promotes the hydrosilylation of CO2 to methane, albeit with

low yields(Figure 3, IV). In these systems, the Lewis acids are used to generate silylium

ions that promote such chemistry.

More recently, Chen and coworkers41 have optimized this catalytic reduction of

CO2 in the presence of silanes, demonstrating that high yields of methane could be obtained

when Al(C6F5)3 and B(C6F5)3 are used synergistically as catalysts (Figure 3, V). In this

system, the highly Lewis acidic Al(C6F5)3 is proposed to effect the initial reduction of CO2,

while B(C6F5)3 catalyzes the reduction of formate and methylene-diolate-intermediates in

a mechanism that is reminiscent of that described by Piers for the hydrosilylation of

carbonyls.36 Interestingly, it was elegantly demonstrated by Okuda et al.42 that weaker

Lewis acids, such as BPh3, could also promote the hydrosilylation of CO2 (Figure 3, VI).

However, in the latter case, the milder conditions allow for a selective reduction to silyl

formates.

Catalytic Hydroboration of CO2

Another alternative for metal-free reduction of CO2 has been the use of B-H bonds

as the reducing agent. Lewis bases including phosphines,43 guanidines,44

diazafluorenides,45 carbenes,46 and proton sponge47 (Figure 3, VII-XI) have been used in

concert with boranes or borohydrides such as 9-BBN, BH4- and Ph3BH- to effect the

reduction of CO2. It should be noted that in most of these systems, the catalysts are needed

for the first reduction of CO2 to HC(O)OBR2 since the latter species can get reduced to

methoxyboranes by several boranes without catalysis.48 The insertion of CO2 into a B-H

bond of BH4- readily generates formatoborates49 and indeed, NaBH4 can be used as a CO2

reduction catalyst in presence of excess boranes.50

In 2013, Fontaine and coworkers51 described the FLP catalyst 1-Bcat-2-PPh2-C6H4

and its use in the hydroboration of CO2 using BH3·SMe2, HBpin or HBcat to produce

methoxyboranes, with a turn-over frequencies of up to 853 h-1 (Figure 3, XII).

Interestingly, one of the criteria for the relatively high activity in this system appears to be

the very weak interaction between CO2 and the FLP precatalyst, destabilizing the adduct

and thus precluding it from being a thermodynamic sink.48 Indeed, in a subsequent

collaboration, Bourissou and Fontaine52 showed that the catalytically active species were

actually formaldehyde adducts of 1-Bcat-2-PPh2-C6H4, emphasizing that a weak Lewis

acid seems to be a key factor to effect catalytic conversions. This kind of synergistic

relation has been exploited by Cantat et al53 in ferrocene-based phosphino-boranes for the

catalytic hydroboration of CO2 to methanol, with TON up to 250 h-1 using 9-BBN as

reducing agent.

In a 2015 study, the work of Wegner et al54 showed that the diborane 1,2-(BH2)2-

C6H4 catalyzes the hydrosilylation of CO2 to give methane while reduction with HBpin

afforded methanol (Figure 3, XIII). In either reaction pathway, the intermediate is

proposed to be the aromatic heterocycle derived from by the reaction of 1,2-(BH2)2-C6H4

and CO2.

Catalytic Hydrogenation of CO2

Although academic interest in hydrosilylation and hydroboration of CO2 to

methanol and methane has garnered considerable attention, these approaches have little

potential for industrial applications as a result of the relatively high cost of boranes and

silanes. It is only catalytic reductions that are exploiting molecular hydrogen that have the

potential for commercialization. Ideally, such reduction would exploit hydrogen generated

by electrolysis driven by an alternative energy source, rather than hydrogen derived from

fossil fuels. This strategy offer a green approach to CO2 utilization. Indeed this principle

has been demonstrated at the George Olah Renewable Methanol Plant in Iceland which

uses geothermal energy to generate the reductant, H2.

In what was a seminal finding, O’Hare and Ashley55 reported that the FLP derived

from TMP/B(C6F5)3 on exposure to CO2/H2 at 160 °C for 6 days afforded the generation

of methanol albeit in 17-25% yield (Figure 3, XIV). Despite the low yield and forcing

conditions, this 2009 finding has continues to inspire much of the work discussed above as

well as continuing studies of FLPs and main group species in CO2 reduction. Despite this

early finding, the only other study that describes metal-free catalytic reduction of CO2

using H2 was recently reported by the groups of Fontaine and Stephan.56 In this work, the

FLP, 1-BMes2-2-NMe2-C6H4 was shown to mediate the conversion of CO2 and H2 to

generate formyl, diacetal and methoxyboranes (Figure 3, XV).

Figure 3. Reported catalysts for the reduction of CO2.

Catalytic Amination of CO2

An alternative avenue for the utilization of CO2 is its reduction via amination. This

provides a strategy of the production of compounds that might have applications as

precursors for pharmaceutical and agrochemical processes. In a break though finding

Cantat et al57 reported a three-component reaction between secondary or primary amines,

hydrosilanes and CO2 could be catalyzed by Lewis base 1,5,7-triazabicyclo[4.4.0]dec-5-

ene (TBD) to generate the corresponding formamides under mild reaction conditions. This

metal-free catalyst was shown to function by the hydrosilylation of the carbamate

generated by the reaction of CO2 and the amine. Interestingly, it was subsequently shown

that N-heterocyclic carbenes could catalyze the related reaction using the industrial silane

polymethylhydrosiloxane (PMHS).58 Since these studies, other catalysts including

thiozolium carbenes,59 diazaphospholene60 and imidazolium-based ionic liquids61 have

been shown to promote the catalytic synthesis of formamides from CO2 and amines. In an

interesting perturbation, using superbases such as N-heterocyclic carbenes and Verkade’s

base, Cantat and coworkers62 observed reduction of CO2 in the presence of 9-BBN to

methylamines. Further variations of this theme afforded routes to aminals,

benzothiazoles,63 benzimidazole, formamidine and quinazolinone.64

Conclusion

The use of metal-free catalysts, including most notably FLPs, for the reduction of

CO2 has been the subject of study for less than a decade old. Some of these systems have

been shown to compete in activity and selectivity with the best transition metal catalysts.

It is clear that such metal-free catalyst systems will continue to be the subject of study and

that the correlations of the structure-reactivity relationship will facilitate the careful design

of ambiphilic FLP catalyst systems by judicious modification of the acidity and basicity

accommodating the intrinsic polarity of carbon dioxide. The low financial and

environmental cost together with high catalytic efficiencies of the envisioned metal-free

catalysts offer significant industrial potential and a green alternative to transition metal

catalysts.

Acknowledgements

The authors would like to thank NSERC for funding.

References and recommended reading

1. D. M. D'Alessandro, B. Smit and J. R. Long, Angew. Chem. Int . Ed., 2010, 49, 6058-6082. 2. M. Ricci, Recovery and Utilization of Carbon Dioxide, Kluwer Academic Publishers, The

Netherlands, 2003. 3. J. L. White, M. F. Baruch, J. E. P. III, Y. Hu, I. C. Fortmeyer, J. E. Park, T. Zhang, K. Liao, J.

Gu, Y. Yan, T. W. Shaw, E. Abelev and A. B. Bocarsly, Chem. Rev., 2015, 115, 12888-12935. 4. C. Federsel, A. Boddien, R. Jackstell, R. Jennerjahn, P. J. Dyson, R. Scopelliti, G. Laurenczy

and M. Beller, Angew. Chem. Int . Ed., 2010, 49, 9777-9780. 5. R. Langer, Y. Diskin-Posner, G. Leitus, L. J. W. Shimon, Y. Ben-David and D. Milstein,

Angew. Chem. Int . Ed., 2011, 50, 9948-9952. 6. E. Balaraman, C. Gunanathan, J. Zhang, L. J. W. Shimon and D. Milstein, Nat Chem, 2011,

3, 609-614. 7. C. A. Huff and M. S. Sanford, J. Am. Chem. Soc., 2011, 133, 18122-18125. 8. S. Park, D. Bézier and M. Brookhart, J. Am. Chem. Soc. , 2012, 134, 11404-11407. 9. S. Wesselbaum, T. V. Stein, J. Klankermayer and W. Leitner, Angew. Chem. Int. Ed. , 2012,

51, 7499-7502. 10. S. Bontemps, L. Vendier and S. Sabo-Etienne, Angew. Chem. Int . Ed., 2012, 51, 1671-1674. 11. S. Bontemps and S. Sabo-Etienne, Angew. Chem. Int. Ed., 2013, 52, 10253−10255. 12. S. Bontemps, L. Vendier and S. Sabo-Etienne, J. Am. Chem. Soc., 2014, 136, 4419−4425. 13. C. Federsel, R. Jackstell and M. Beller, Angew. Chem., Int. Ed., 2010, 49, 6254−6257. 14. T. Schaub and R. A. Paciello, Angew. Chem., Int. Ed., 2011, 50, 7278−7282. 15. M. S. Jeletic, M. T. Mock, A. M. Appel and J. C. Linehan, J. Am. Chem. Soc., 3013, 135,

11533−11536. 16. G. Fiorani, W. Guo and A. W. Kleij, Green Chem., 2015, 17, 1375-1389. 17. F.-G. Fontaine, M.-A. Courtemanche, M.-A. Légaré and É. Rochette, Coord. Chem. Rev.,

2016, DOI: 10.1016/j.ccr.2016.1005.1005. 18. S. Bontemps, Coord. Chem. Rev. , 2016, 308, 117-130. 19. D. W. Stephan and G. Erker, Angew. Chem. Int . Ed., 2015, 54, 6400-6441. 20. D. W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2010, 49, 46-76. 21. D. W. Stephan, J. Am. Chem. Soc., 2015, 137, 10018-10032. 22. D. W. Stephan, Accts Chem. Res., 2015, 48, 306-316. 23. H. Dobbek, V. Svetlitchnyi, L. Gremer and O. Meyer, Science, 2001, 293, 1281-1285. ●●

This manuscript describes how ambiphilic systems could be used to activate carbon dioxide based on the mechanism of carbon monoxide dehydrogenase.

24. T. J. Schmeier, G. E. Dobereiner, R. H. Crabtree and N. Hazari, J. Am. Chem. Soc. , 2011, 133, 9274-9277.

25. M. Beley, J.-P. Collin, R. Ruppert and J.-P. Sauvage, J. Am. Chem. Soc., 1986, 108, 7461-7467.

26. C. M. Mömming, E. Otten, G. Kehr, R. Fröhlich, S. Grimme, D. W. Stephan and G. Erker, Angew. Chem. Int. Ed., 2009, 48, 6643-6646. ●● This seminal discovery is the first report of a FLP CO2 adduct.

27. D. W. Stephan and G. Erker, Chem. Sci., 2014, 5, 2625-2641. 28. F.-G. Fontaine, M.-A. Courtemanche and M.-A. Légaré, Chem. Eur. J., 2014, 20, 2990-2996. 29. K. Takeuchi and D. W. Stephan, Chem. Commun., 2012, 48, 11304-11306. 30. J. Boudreau, M. A. Courtemanche and F. G. Fontaine, Chem. Commun., 2011, 47, 11131-

11133.

31. G. Ménard and D. W. Stephan, Angew. Chem. Int. Ed., 2011, 50, 8396-8399. 32. G. Ménard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796-1797. 33. S. N. Riduan, Y. G. Zhang and J. Y. Ying, Angew. Chem. Int . Ed., 2009, 48, 3322-3325. ●

Although the reactivity is limited, it represents the first example of a metal free catalyst for the reduction of CO2.

34. Q. Zhou and Y. Li, J. Am. Chem. Soc., 2015, 137, 10182-10189. 35. S. Rendler and M. Oestreich, Angew. Chem., Int. Ed., 2008, 47, 5997-6000. 36. D. J. Parks and W. E. Piers, J. Am. Chem. Soc., 1996, 118, 9440-9441. ● This transformation

is recognized as one of the seminal example of catalysis using what is now know as a FLP type transformation.

37. M. A. Courtemanche, M. A. Legare, E. Rochette and F. G. Fontaine, Chem. Commun., 2015, 51, 6858-6861.

38. M. Khandelwal and R. J. Wehmschulte, Angew. Chem. Int . Ed., 2012, 51, 7323-7326. 39. R. J. Wehmschulte, M. Saleh and D. R. Powell, Organometallics, 2013, 32, 6812-6819. 40. A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132, 10660-10661. 41. J. Chen, L. Falivene, L. Caporaso, L. Cavallo and E. Y. Chen, J. Am. Chem. Soc., 2016, 138,

5321-5333. 42. D. Mukherjee, DanielF.Sauer, A. Zanardi and J. Okuda, Chem. Eur.J., 2016, 22, 7730 -7733. 43. T. Wang and D. W. Stephan, Chem. Commun., 2014, 50, 7007-7010. 44. C. Das Neves Gomes, E. Blondiaux, P. Thuery and T. Cantat, Chem. Eur. J., 2014, 20, 7098-

7106. 45. Y. Yang, M. Xu and D. Song, Chem. Commun., 2015, 51, 11293-11296. 46. S. Y. F. Ho, C. W. So, N. Saffon-Merceron and N. Mezailles, Chem. Commun., 2015, 51,

2107-2110. 47. M.-A. Legare, M.-A. Courtemanche and F.-G. Fontaine, Chem. Commun., 2014, 50, 11362-

11365. 48. M.-A. Courtemanche, M.-A. Légaré, L. Maron and F.-G. Fontaine, J. Am. Chem. Soc., 2014,

136, 10708-10717. 49. I. Knopf and C. C. Cummins, Organometallics, 2015, 34, 1601-1603. 50. K. Fujiwara, S. Yasuda and T. Mizuta, Organometallics, 2014, 33, 6692-6695. 51. M. A. Courtemanche, M. A. Legare, L. Maron and F. G. Fontaine, J. Am. Chem. Soc., 2013,

135, 9326-9329. ●● This is the first highly active FLP system for CO2 reduction, surpassing the activity of many transition metal catalysts, demonstrating that milder Lewis acids and bases could be used for increased activity.

52. R. Declercq, G. Bouhadir, D. Bourissou, M.-A. Légaré, M.-A. Courtemanche, K. S. Nahi, N. Bouchard, F.-G. Fontaine and L. Maron, ACS Catalysis, 2015, 5, 2513-2520.

53. A. Tlili, A. Voituriez, A. Marinetti, P. Thuery and T. Cantat, Chem. Commun., 2016, 52, 7553-7555.

54. Z. Lu, H. Hausmann, S. Becker and H. A. Wegner, J. Am. Chem. Soc., 2015, 137, 5332-5335. 55. A. E. Ashley, A. L. Thompson and D. O'Hare, Angew. Chem. Int. Ed., 2009, 48, 9839-9843.

● This is still the only metal-free catalysts that can generate methanol directly from CO2 and H2.

56. M. A. Courtemanche, A. P. Pulis, É. Rochette, M. A. Légaré, D. W. Stephan and F. G. Fontaine, Chem. Commun., 2015, 51, 9797-9800.

57. C. D. N. Gomes, O. Jacquet, C. Villiers, P. Thuery, M. Ephritikhine and T. Cantat, Angew. Chem. Int. Ed., 2012, 51, 187-190. ●● The first example of a metal-free reductive amination of CO2 exhibiting an enchanced scope compared to transition metal catalysts.

58. O. Jacquet, C. D. Gomes, M. Ephritikhine and T. Cantat, J. Am. Chem. Soc., 2012, 134, 2934-2937.

59. S. Das, F. D. Bobbink, S. Bulut, M. Soudani and P. J. Dyson, Chem. Commun., 2016, 52, 2497-2500.

60. C. C. Chong, H. Hirao and R. Kinjo, Angew. Chem. Int. Ed., 2014. 61. L. Hao, Y. Zhao, B. Yu, Z. Yang, H. Zhang, B. Han, X. Gao and Z. Liu, ACS Catalysis, 2015, 5,

4989-4993. 62. X. Frogneux, E. Blondiaux, P. Thuéry and T. Cantat, ACS Catalysis, 2015, 5, 3983-3987. 63. X. Gao, B. Yu, Z. Yang, Y. Zhao, H. Zhang, L. Hao, B. Han and Z. Liu, ACS Catalysis, 2015, 5,

6648-6652. 64. O. Jacquet, C. Das Neves Gomes, M. Ephritikhine and T. Cantat, ChemCatChem, 2013, 5,

117-120.