<figure><img src="image_319.png" /><figcaption>**Scheme 82.** The reaction of diphenylgermylene with benzylalcohol</figcaption></figure>

<figure><img src="image_320.png" /><figcaption>**Scheme 83.** The reaction of alkoxy-germylene with the second equivalent of benzylalcohol</figcaption></figure>

Overall, this work has significantly expanded the chemistry of ruthenium germanium coordinated complexes which paves new paths for applications of these new germylene complexes in future.

Finally, the sixth part explores the reactivity of cationic ruthenium half-sandwich diphosphine complexes with diphenylgermane. This reveals initial Ge–H bond activation at various coordinatively unsaturated ruthenium complexes, leading to new ruthenium germanium complexes (24, 26-29). A stoichiometric Markonikov hydrogermylation of phenylacetylene was achieved, and the involvement of the new ruthenium germanium complexes and ruthenium vinylidene complexes in the course of the hydrogermylation reaction is described.

This study has successfully investigated the reactivity of GePh₂H₂ with a number of half-sandwich ruthenium diphosphine complexes. To examine the interaction of Ge–H bonds to the activated alkyne, ruthenium vinylidene complexes were synthesised and characterized. Extensive investigation into the reactivity of these complexes with diphenylgermane has revealed new insights into the interaction between Ge–H bonds and the metal centre. Further detailed spectroscopic and structural analysis enabled the pathways between diphenyl germane and cationic complexes [CpRuP₂OTf], P₂ = (PPh₃)₂, dppe leading to the instantaneous formation of a ruthenium dihydride complex 23 or Ge–H sigma complex 26 to be established. This complex was later found to transform to a germanium-coordinated complex 24.

<figure><img src="image_321.png" /><figcaption>**Scheme 84.** Overall reaction of bis-triphenylphosphine ruthenium complex with TMSOTf and diphenyl germane.</figcaption></figure>
This work has shed light on the mechanism by which the germanium-hydrogen bond is activated at the ruthenium centre. Importantly, bidentate phosphine ligands such as dppe and dppm have shown different reactivity profile compared to *bis*-PPh₃ ligands.

<figure><img src="image_325.png" /><figcaption>**Scheme 85.** Reactivity of ruthenium bidentate diphosphine complexes with diphenyl germane.</figcaption></figure>

Finally, a detailed spectroscopic analysis of the reaction between the vinylidene complex with excess of diphenylgermane enabled the identification of a hydrogermylated product. Thus, a stoichiometric Markonikov hydrogermylation of phenylacetylene was achieved using the ruthenium vinylidene complexes **18** and **19**.

<figure><img src="image_326.png" /><figcaption>**Scheme 86.** Reactivity of ruthenium vinylidene complex with diphenyl germane.</figcaption></figure>

This stepwise investigation represents a significant advancement in understanding the mechanism of metal vinylidene complex-mediated, transition metal-catalysed hydrogermylation of alkynes. While catalytic hydrogermylation was not directly accomplished under the reaction conditions examined, the fundamental coordination chemistry principles established here provide a critical foundation for future efforts.

Even though the bonding of the ruthenium σ-germane complexes (**26**, **28**) and germanium-triflate bound complexes (**27**, **29**) were identified and characterised by NMR as discussed, further evidence were required to establish the bonding between ruthenium, germanium and triflate moieties in these complexes. Hence, DFT calculations were carried out to optimize the molecular structures of ruthenium dppe germanium complexes **26** and **27** (Figure 33). The major bond lengths provided insight into the bonding interactions.
This work has shed light on the mechanism by which the germanium-hydrogen bond is activated at the ruthenium centre. Importantly, bidentate phosphine ligands such as dppe and dppm have shown different reactivity profile compared to *bis*-PPh₃ ligands.

<figure><img src="image_322.png" /><figcaption>**Scheme 85.** Reactivity of ruthenium bidentate diphosphine complexes with diphenyl germane.</figcaption></figure>

Finally, a detailed spectroscopic analysis of the reaction between the vinylidene complex with excess of diphenylgermane enabled the identification of a hydrogermylated product. Thus, a stoichiometric Markonikov hydrogermylation of phenylacetylene was achieved using the ruthenium vinylidene complexes **18** and **19**.

<figure><img src="image_323.png" /><figcaption>**Scheme 86.** Reactivity of ruthenium vinylidene complex with diphenyl germane.</figcaption></figure>

This stepwise investigation represents a significant advancement in understanding the mechanism of metal vinylidene complex-mediated, transition metal-catalysed hydrogermylation of alkynes. While catalytic hydrogermylation was not directly accomplished under the reaction conditions examined, the fundamental coordination chemistry principles established here provide a critical foundation for future efforts.

Even though the bonding of the ruthenium σ-germane complexes (**26**, **28**) and germanium-triflate bound complexes (**27**, **29**) were identified and characterised by NMR as discussed, further evidence were required to establish the bonding between ruthenium, germanium and triflate moieties in these complexes. Hence, DFT calculations were carried out to optimize the molecular structures of ruthenium dppe germanium complexes **26** and **27** (Figure 33). The major bond lengths provided insight into the bonding interactions.
<figure><img src="image_327.png" /><figcaption>**Figure 33.** DFT calculated molecular structures of complexes [CpRu(dppe)H₂][OTf] (left), [Ru(Cp)(dppe)(η-H₂GePh₂)][(OTf) (26) (centre) and [Ru(Cp)(dppe)(GePh₂OTf)] (27) (right). Phenyl groups drawn in wireframe and CH hydrogens omitted for clarity.</figcaption></figure>

In summary, this thesis has significantly expanded the fundamental coordination chemistry of ruthenium complexes with silicon and germanium hydrides through extensive spectroscopic, structural and computational analysis. The insights gained on the activation of Si–H and Ge–H bonds using the reactive ruthenium systems provide a basis for expanding the scope of new catalytic transformations. For instance, the extension of the scope of ruthenium-catalysed hydrosilylation of nitriles and the dehydrogenative coupling is an interesting target for the synthesis of diverse organosilicon compounds at room temperature conditions.

Changing the ligands on the ruthenium centre with different steric and electronic properties can definitely lead to complexes with different properties and reactivities and such influence can be studied on the ability of the metal centre to activate Si–H and Ge–H bonds. Furthermore, new reactivity studies of this electrophilic silicon and germanium centres with other nucleophiles like ketones and ammonia would be interesting. It is important to note that both Si–H and Ge–H bonds were activated using these ruthenium complexes under ambient conditions, still their bonding interactions with similar metal centres was found different, especially in the case of [RuH₂(H₂)₂(PCy₃)₂] complex, diphenylsilane led to *bis*-σ-Si-H complex, meanwhile diphenylgermane led to germylene complex. This difference arises curiosity in bonding of heavier analogues, Sn and Pb coordination to ruthenium centre in these complexes. Also, similar reactivity studies with other Group 14 hydrides such as Sn–H could be interesting to synthesise inaccessible organotin compounds. In the case of ruthenium half-sandwich diphosphine complexes, the Si–H bond activation of hydrosilanes and the addition of the activated alkynes through ruthenium vinylidene complexes is an interesting future study.

References

[1] R. H. Crabtree, *The Organometallic Chemistry of the Transition Metals*, John Wiley & Sons, Ltd: Weinheim, **2014**, 224–258. DOI: [10.1002/9781118788301](https://doi.org/10.1002/9781118788301).

[2] C. Qin, L. Gao, and E. Wang, *Germanium: Inorganic Chemistry*, in *Encyclopedia of Inorganic and Bioinorganic Chemistry*, R.A. Scott (Ed.), **2011**. DOI: [10.1002/9781119951438.eibc0074](https://doi.org/10.1002/9781119951438.eibc0074).

[3] M. M. Banaszak Holl, D. R. Peck, *Germanium: Organometallic Chemistry*, in *Encyclopedia of Inorganic Chemistry* (R. B. King, R. H. Crabtree, C. M. Lukehart, D. A. Atwood and R. A. Scott, Editors), Wiley, **2009**. DOI: [10.1002/0470862106.ia080.pub2](https://doi.org/10.1002/0470862106.ia080.pub2).

[4] N. Auner, J. Weis, *Introduction: Organosilicon Chemistry—Recent Highlights in a Fascinating Research Area*, In *Organosilicon Chemistry VI* (N. Auner and J. Weis, Editors); John Wiley & Sons, Ltd: Weinheim, **2008**. DOI: [10.1002/9783527618224.ch](https://doi.org/10.1002/9783527618224.ch).

[5] T. Hiyama, M. Minami, A. Mori, *Transition-Metal-Catalyzed Cross-coupling of Organosilicon Compounds*, in
<figure><img src="image_324.png" /><figcaption>**Figure 33.** DFT calculated molecular structures of complexes [CpRu(dppe)H₂][OTf] (left), [Ru(Cp)(dppe)(η-H₂GePh₂)][(OTf) (26) (centre) and [Ru(Cp)(dppe)(GePh₂OTf)] (27) (right). Phenyl groups drawn in wireframe and CH hydrogens omitted for clarity.</figcaption></figure>

In summary, this thesis has significantly expanded the fundamental coordination chemistry of ruthenium complexes with silicon and germanium hydrides through extensive spectroscopic, structural and computational analysis. The insights gained on the activation of Si–H and Ge–H bonds using the reactive ruthenium systems provide a basis for expanding the scope of new catalytic transformations. For instance, the extension of the scope of ruthenium-catalysed hydrosilylation of nitriles and the dehydrogenative coupling is an interesting target for the synthesis of diverse organosilicon compounds at room temperature conditions.

Changing the ligands on the ruthenium centre with different steric and electronic properties can definitely lead to complexes with different properties and reactivities and such influence can be studied on the ability of the metal centre to activate Si–H and Ge–H bonds. Furthermore, new reactivity studies of this electrophilic silicon and germanium centres with other nucleophiles like ketones and ammonia would be interesting. It is important to note that both Si–H and Ge–H bonds were activated using these ruthenium complexes under ambient conditions, still their bonding interactions with similar metal centres was found different, especially in the case of [RuH₂(H₂)₂(PCy₃)₂] complex, diphenylsilane led to *bis*-σ-Si-H complex, meanwhile diphenylgermane led to germylene complex. This difference arises curiosity in bonding of heavier analogues, Sn and Pb coordination to ruthenium centre in these complexes. Also, similar reactivity studies with other Group 14 hydrides such as Sn–H could be interesting to synthesise inaccessible organotin compounds. In the case of ruthenium half-sandwich diphosphine complexes, the Si–H bond activation of hydrosilanes and the addition of the activated alkynes through ruthenium vinylidene complexes is an interesting future study.

References

[1] R. H. Crabtree, *The Organometallic Chemistry of the Transition Metals*, John Wiley & Sons, Ltd: Weinheim, **2014**, 224–258. DOI: [10.1002/9781118788301](https://doi.org/10.1002/9781118788301).

[2] C. Qin, L. Gao, and E. Wang, *Germanium: Inorganic Chemistry*, in *Encyclopedia of Inorganic and Bioinorganic Chemistry*, R.A. Scott (Ed.), **2011**. DOI: [10.1002/9781119951438.eibc0074](https://doi.org/10.1002/9781119951438.eibc0074).

[3] M. M. Banaszak Holl, D. R. Peck, *Germanium: Organometallic Chemistry*, in *Encyclopedia of Inorganic Chemistry* (R. B. King, R. H. Crabtree, C. M. Lukehart, D. A. Atwood and R. A. Scott, Editors), Wiley, **2009**. DOI: [10.1002/0470862106.ia080.pub2](https://doi.org/10.1002/0470862106.ia080.pub2).

[4] N. Auner, J. Weis, *Introduction: Organosilicon Chemistry—Recent Highlights in a Fascinating Research Area*, In *Organosilicon Chemistry VI* (N. Auner and J. Weis, Editors); John Wiley & Sons, Ltd: Weinheim, **2008**. DOI: [10.1002/9783527618224.ch](https://doi.org/10.1002/9783527618224.ch).

[5] T. Hiyama, M. Minami, A. Mori, *Transition-Metal-Catalyzed Cross-coupling of Organosilicon Compounds*, in
*Organosilicon Chemistry: Novel Approaches and Reactions* (T. Hiyama, M. Oestreich, Editors), John Wiley & Sons, Ltd: Weinheim, **2020**; pp 271–332. DOI: 10.1002/9783527814787.ch9.

[6] G. A. Harvey, T. J. Lash, J. R. Rawls, *Outgassing products from orbiter TPS materials*, in *LDEF: 69 Months in Space. Third Post-Retrieval Symposium* (A.L. Levine, Editor), **1995**, pp 13-20. Document ID: 19950017376.

[7] Z. Li, X. Wang, H. Bai, M. Cao, *Polymers* **2023**, *15*, 543. DOI: 10.3390/polym15030543.

[8] L. Li, B. Li, J. Dong, J. Zhang, *J. Mater. Chem. A Mater. 2016*, **4**, 13677-13725. DOI: 10.1039/C6TA05441B.

[9] G. C. Chien, J. F Kuo, C. Y Chen, *J. Polym. Sci. A Polym. Chem.* **1993**, *31*, 2423-2436. DOI: 10.1002/pola.1993.080311002.

[10] G. A. Martin, M. C. Durupty, C. Miro datos, N. Mouaddib, V. Perrichon, *Stud. Surf. Sci. Catal.* **1991**, *63*, 269-278. DOI: 10.1016/S0167-2991(08)64591-7.

[11] E. L. Margelefsky, R. K. Zeidan, M. E. Davis, *Chem. Soc. Rev.* **2008**, *37*, 1118-1126. DOI: 10.1039/B710334B.

[12] J. M. Antonucci, S. H. Dickens, B. O. Fowler, H. H. K. Xu, W. G. McDonough, *J. Res. Natl. Inst. Stand. Technol.* **2005**, *110*, 541. DOI: 10.6028/jres.110.081.

[13] V. V. Verkholantsev, *European Coatings Journal* **2011**, 503-517.

[14] Z. F Zhang, F. Babonneau, R. M. Laine, Y. Mu, J. F. Harrod, J. A. Rahn, *J. Am. Ceram. Soc.* **1991**, *74*, 670-673. DOI: 10.1111/j.1151-2916.1991.tb04080.x.

[15] M. Lalonde, T. H. Chan, *Synthesis (Germany)* **1985**, *1985*, 817-845. DOI: 10.1055/s-1985-31361.

[16] T. Komiyama, Y. Minami, T. Hiyama, *ACS Catal.* **2017**, *7*, 631-651. DOI: 10.1021/acscatal.6b02374.

[17] Y. Yamakado, M. Ishiguro, N. Ikeda, H. Yamamoto, *J. Am. Chem. Soc.* **1981**, *103*, 5568-5570. DOI: 10.1021/ja00408a049.

[18] Y. Nakao, T. Hiyama, *Chem. Soc. Rev.* **2011**, *40*, 4893-4901. DOI: 10.1039/C1CS15122C.

[19] E. Rosenberg, *Germanium-Containing Compounds, Current Knowledge and Applications*, in *Encyclopedia of Metalloproteins* (R.H. Kretsinger, V.N. Uversky, E.A. Permyakov, Editors) **2013**, 847-855. Springer: New York, NY. DOI: 10.1007/978-1-4614-1533-6_582.

[20] K. Kojima, S. Uchida, H. Kinoshita, K. Miura, *Org. Lett.* **2021**, *23*, 4598-4602. DOI: 10.1021/acs.orglett.1c01314.

[21] T. Nakamura, Y. Shimada, K. Sato, *Bioorganic and Medicinal Organogermanium Chemistry*, in *Organogermanium Compounds: Theory, Experiment, and Applications*, John Wiley & Sons, Ltd: Weinheim, Germany, **2023**, Vol. 2, pp. 839-865. DOI: 10.1002/9781119613466.ch19.

[22] V. M. Dembitsky, *Biomedicines* **2023**, *11*, 2698. DOI: 10.3390/biomedicines11102698.

[23] R. D. Rittinghaus, J. Tremmel, A. Růžička, C. Conrads, P. Albrecht, A. Hoffmann, A. N. Ksiazkiewicz, A. Pich, R. Jambor, S. Herres-Pawlis, *Chem. Eur. J.* **2020**, *26*, 212-221. DOI: 10.1002/chem.201903949.

[24] N. Sen, S. Khan, *Chem. Asian. J.* **2021**, *16*, 705-719. DOI: 10.1002/asia.202100038.

[25] R. Dasgupta, S. Das, S. Hiwase, S. K. Pati, S. Khan, *Organometallics* **2019**, *38*, 1429-1435. DOI: 10.1021/acs.organomet.8b00673.

[26] S. Sinhababu, D. Singh, M. K. Sharma, R. K. Siwatch, P. Mahawar, S. Nagendran, *Dalton Trans.* **2019**, *48*, 4094-4100. DOI: 10.1039/C8DT05121F.

[27] T. J. Hadlington, C. E. Kefalidis, L. Maron, C. Jones, *ACS Catal.* **2017**, *7*, 1853-1859. DOI: 10.1021/acscatal.6b03306.

[28] E. Fritz-Langhals, *Reactions* **2021**, *2*, 442-456. DOI: 10.3390/reactions2040028.

[29] J. Baumgartner, C. Marschner, *Rev. Inorg. Chem.* **2014**, *34*, 119-152. DOI: 10.1515/revic-2013-0014

[30] L. D. Almeida, H. Wang, K. Junge, X. Cui, M. Beller, *Angew. Chem. Int. Ed.* **2021**, *60*, 550-565. DOI: 10.1002/anie.202008729.

[31] J. Y. Corey, *Chem. Rev.* **2011**, *111*, 863-1071. DOI: 10.1021/cr900359c.

[32] S. Lachaize, S. Sabo-Etienne, *Eur. J. Inorg. Chem.* **2006**, 2115-2127. DOI: 10.1002/ejic.200600151.

[33] G. I. Nikonov, *Adv. Organomet. Chem.*, **2005**, *53*, 217-309. DOI: 10.1016/S0065-3055(05)53006-5.

[34] S. Lachaize, S. Sabo-Etienne, *Eur. J. Inorg. Chem.* **2006**, *2006*, 4697-4699. DOI: 10.1002/ejic.200600151.

[35] G. Alcaraz, S. Sabo-Etienne, *Coord. Chem. Rev.* **2008**, *252*, 2395-2409. DOI: 10.1016/j.ccr.2008.02.006.
*Organosilicon Chemistry: Novel Approaches and Reactions* (T. Hiyama, M. Oestreich, Editors), John Wiley & Sons, Ltd: Weinheim, **2020**; pp 271–332. DOI: 10.1002/9783527814787.ch9.

[6] G. A. Harvey, T. J. Lash, J. R. Rawls, *Outgassing products from orbiter TPS materials*, in *LDEF: 69 Months in Space. Third Post-Retrieval Symposium* (A.L. Levine, Editor), **1995**, pp 13-20. Document ID: 19950017376.

[7] Z. Li, X. Wang, H. Bai, M. Cao, *Polymers* **2023**, *15*, 543. DOI: 10.3390/polym15030543.

[8] L. Li, B. Li, J. Dong, J. Zhang, *J. Mater. Chem. A Mater. 2016*, **4**, 13677-13725. DOI: 10.1039/C6TA05441B.

[9] G. C. Chien, J. F Kuo, C. Y Chen, *J. Polym. Sci. A Polym. Chem.* **1993**, *31*, 2423-2436. DOI: 10.1002/pola.1993.080311002.

[10] G. A. Martin, M. C. Durupty, C. Miro datos, N. Mouaddib, V. Perrichon, *Stud. Surf. Sci. Catal.* **1991**, *63*, 269-278. DOI: 10.1016/S0167-2991(08)64591-7.

[11] E. L. Margelefsky, R. K. Zeidan, M. E. Davis, *Chem. Soc. Rev.* **2008**, *37*, 1118-1126. DOI: 10.1039/B710334B.

[12] J. M. Antonucci, S. H. Dickens, B. O. Fowler, H. H. K. Xu, W. G. McDonough, *J. Res. Natl. Inst. Stand. Technol.* **2005**, *110*, 541. DOI: 10.6028/jres.110.081.

[13] V. V. Verkholantsev, *European Coatings Journal* **2011**, 503-517.

[14] Z. F Zhang, F. Babonneau, R. M. Laine, Y. Mu, J. F. Harrod, J. A. Rahn, *J. Am. Ceram. Soc.* **1991**, *74*, 670-673. DOI: 10.1111/j.1151-2916.1991.tb04080.x.

[15] M. Lalonde, T. H. Chan, *Synthesis (Germany)* **1985**, *1985*, 817-845. DOI: 10.1055/s-1985-31361.

[16] T. Komiyama, Y. Minami, T. Hiyama, *ACS Catal.* **2017**, *7*, 631-651. DOI: 10.1021/acscatal.6b02374.

[17] Y. Yamakado, M. Ishiguro, N. Ikeda, H. Yamamoto, *J. Am. Chem. Soc.* **1981**, *103*, 5568-5570. DOI: 10.1021/ja00408a049.

[18] Y. Nakao, T. Hiyama, *Chem. Soc. Rev.* **2011**, *40*, 4893-4901. DOI: 10.1039/C1CS15122C.

[19] E. Rosenberg, *Germanium-Containing Compounds, Current Knowledge and Applications*, in *Encyclopedia of Metalloproteins* (R.H. Kretsinger, V.N. Uversky, E.A. Permyakov, Editors) **2013**, 847-855. Springer: New York, NY. DOI: 10.1007/978-1-4614-1533-6_582.

[20] K. Kojima, S. Uchida, H. Kinoshita, K. Miura, *Org. Lett.* **2021**, *23*, 4598-4602. DOI: 10.1021/acs.orglett.1c01314.

[21] T. Nakamura, Y. Shimada, K. Sato, *Bioorganic and Medicinal Organogermanium Chemistry*, in *Organogermanium Compounds: Theory, Experiment, and Applications*, John Wiley & Sons, Ltd: Weinheim, Germany, **2023**, Vol. 2, pp. 839-865. DOI: 10.1002/9781119613466.ch19.

[22] V. M. Dembitsky, *Biomedicines* **2023**, *11*, 2698. DOI: 10.3390/biomedicines11102698.

[23] R. D. Rittinghaus, J. Tremmel, A. Růžička, C. Conrads, P. Albrecht, A. Hoffmann, A. N. Ksiazkiewicz, A. Pich, R. Jambor, S. Herres-Pawlis, *Chem. Eur. J.* **2020**, *26*, 212-221. DOI: 10.1002/chem.201903949.

[24] N. Sen, S. Khan, *Chem. Asian. J.* **2021**, *16*, 705-719. DOI: 10.1002/asia.202100038.

[25] R. Dasgupta, S. Das, S. Hiwase, S. K. Pati, S. Khan, *Organometallics* **2019**, *38*, 1429-1435. DOI: 10.1021/acs.organomet.8b00673.

[26] S. Sinhababu, D. Singh, M. K. Sharma, R. K. Siwatch, P. Mahawar, S. Nagendran, *Dalton Trans.* **2019**, *48*, 4094-4100. DOI: 10.1039/C8DT05121F.

[27] T. J. Hadlington, C. E. Kefalidis, L. Maron, C. Jones, *ACS Catal.* **2017**, *7*, 1853-1859. DOI: 10.1021/acscatal.6b03306.

[28] E. Fritz-Langhals, *Reactions* **2021**, *2*, 442-456. DOI: 10.3390/reactions2040028.

[29] J. Baumgartner, C. Marschner, *Rev. Inorg. Chem.* **2014**, *34*, 119-152. DOI: 10.1515/revic-2013-0014

[30] L. D. Almeida, H. Wang, K. Junge, X. Cui, M. Beller, *Angew. Chem. Int. Ed.* **2021**, *60*, 550-565. DOI: 10.1002/anie.202008729.

[31] J. Y. Corey, *Chem. Rev.* **2011**, *111*, 863-1071. DOI: 10.1021/cr900359c.

[32] S. Lachaize, S. Sabo-Etienne, *Eur. J. Inorg. Chem.* **2006**, 2115-2127. DOI: 10.1002/ejic.200600151.

[33] G. I. Nikonov, *Adv. Organomet. Chem.*, **2005**, *53*, 217-309. DOI: 10.1016/S0065-3055(05)53006-5.

[34] S. Lachaize, S. Sabo-Etienne, *Eur. J. Inorg. Chem.* **2006**, *2006*, 4697-4699. DOI: 10.1002/ejic.200600151.

[35] G. Alcaraz, S. Sabo-Etienne, *Coord. Chem. Rev.* **2008**, *252*, 2395-2409. DOI: 10.1016/j.ccr.2008.02.006.
Agustin MORALES

ESR15

**Thesis co-directors**

Dr. Sébastian Bontemps
Dr. Olivier Baslé

Laboratoire de Chimie de Coordination, Toulouse, France

Prof. Agustí Lledós

Universitat Autònoma de Barcelona, Bellaterra, Spain

**Thesis defense**

Laboratoire de Chimie de Coordination, 15 December 2023
# Agustin MORALES

## ESR15

### Thesis co-directors

Dr. Sébastian Bontemps
Dr. Olivier Baslé

Laboratoire de Chimie de Coordination, Toulouse, France

Prof. Agustí Lledós

Universitat Autònoma de Barcelona, Bellaterra, Spain

### Thesis defense

Laboratoire de Chimie de Coordination, 15 December 2023
<figure><img src="image_328.png" /></figure>
<figure><img src="image_325.png" /></figure>
# Single Electron Reduction of NHC-CO₂ and NHC-CO₂-BR₃ Adducts

## Introduction

The carbon dioxide radical anion [CO₂]⁻ is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO₂ to generate [CO₂]⁻ occurs at very negative reduction potentials, which is often a limiting factor for applications. My doctoral work concerns the synthesis of NHC-CO₂ and NHC-CO₂-BR₃ adducts (BR₃: borane) and the evaluation of their one-electron reduction chemistry through a combined experimental and theoretical study. Firstly, a brief literature review is given followed by a theoretical study on the one-electron reduction of NHC-CO₂ and NHC-CO₂-BR₃ adducts with the aim of understanding how these species behave after the addition of one electron. This study would guide us towards the choice of adduct/s that may exhibit most accessible reduction potential for its further synthesis. We show that NHC-CO₂-BR₃ species undergo single electron reduction at a less negative potential than free CO₂. A net gain of more than one volt was notably measured with a C_yCAAC-CO₂-B(C₆F₅)₃ adduct, which was chemically reduced to afford [CAAC-CO₂-B(C₆F₅)₃]⁻. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. A theoretical analysis is presented, where the computational method is initially validated based on previous experimental results, and then various properties related to the one-electron reduction of these adducts are rationalized using quantum chemistry-based tools. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO₂ moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO₂]⁻ toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO₂]⁻ like species. For more information about this work, see [1].

## Bibliographic section

In this first section, the subject of this work will be contextualised with relevant examples reported in the literature. The two pathways for generating the radical anion of CO₂, [CO₂]⁻, will be briefly described. Then, we will provide two key examples of its reactivity towards open-shell and unsaturated species as alkenes. The second part will focus on the one-electron reduction of NHC-CO₂ adducts through electrochemical and chemical means. Finally, the synthesis of NHC-CO₂-BR₃ adducts reported in the literature will be also summarised.

### Generation of CO₂ radical anion and formation of C-C bonds

As shown in the diagram below, the generation of the CO₂ radical anion can originate from two main sources: either from CO₂ gas or from a formate salt through electrochemical and photochemical means (on the left, Scheme 1). It has been demonstrated that it can then react either with an open-shell species via radical-radical coupling or with alkenes via radical addition (on the right, Scheme 1).

The direct one-electron reduction of carbon dioxide is the most straightforward process to generate the CO₂ radical anion, [CO₂]⁻. However, this approach is challenging as it requires particularly high negative potentials, E<sub>red</sub> = -2.2 V in DMF and -3 V in CH₃CN in respect to Fc⁺/₀ redox couple [2-5]. In the literature, the use of organic dye species such as *p*-terphenyl as a photocatalyst to achieve this
# Single Electron Reduction of NHC-CO₂ and NHC-CO₂-BR₃ Adducts

## Introduction

The carbon dioxide radical anion [CO₂]⁻ is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO₂ to generate [CO₂]⁻ occurs at very negative reduction potentials, which is often a limiting factor for applications. My doctoral work concerns the synthesis of NHC-CO₂ and NHC-CO₂-BR₃ adducts (BR₃: borane) and the evaluation of their one-electron reduction chemistry through a combined experimental and theoretical study. Firstly, a brief literature review is given followed by a theoretical study on the one-electron reduction of NHC-CO₂ and NHC-CO₂-BR₃ adducts with the aim of understanding how these species behave after the addition of one electron. This study would guide us towards the choice of adduct/s that may exhibit most accessible reduction potential for its further synthesis. We show that NHC-CO₂-BR₃ species undergo single electron reduction at a less negative potential than free CO₂. A net gain of more than one volt was notably measured with a C_yCAAC-CO₂-B(C₆F₅)₃ adduct, which was chemically reduced to afford [CAAC-CO₂-B(C₆F₅)₃]⁻. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. A theoretical analysis is presented, where the computational method is initially validated based on previous experimental results, and then various properties related to the one-electron reduction of these adducts are rationalized using quantum chemistry-based tools. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO₂ moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO₂]⁻ toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO₂]⁻ like species. For more information about this work, see [1].

## Bibliographic section

In this first section, the subject of this work will be contextualised with relevant examples reported in the literature. The two pathways for generating the radical anion of CO₂, [CO₂]⁻, will be briefly described. Then, we will provide two key examples of its reactivity towards open-shell and unsaturated species as alkenes. The second part will focus on the one-electron reduction of NHC-CO₂ adducts through electrochemical and chemical means. Finally, the synthesis of NHC-CO₂-BR₃ adducts reported in the literature will be also summarised.

### Generation of CO₂ radical anion and formation of C-C bonds

As shown in the diagram below, the generation of the CO₂ radical anion can originate from two main sources: either from CO₂ gas or from a formate salt through electrochemical and photochemical means (on the left, Scheme 1). It has been demonstrated that it can then react either with an open-shell species via radical-radical coupling or with alkenes via radical addition (on the right, Scheme 1).

The direct one-electron reduction of carbon dioxide is the most straightforward process to generate the CO₂ radical anion, [CO₂]⁻. However, this approach is challenging as it requires particularly high negative potentials, E<sub>red</sub> = -2.2 V in DMF and -3 V in CH₃CN in respect to Fc⁺/₀ redox couple [2-5]. In the literature, the use of organic dye species such as *p*-terphenyl as a photocatalyst to achieve this
transformation using ultraviolet (UV) light has been reported. As a key example, Jamison and collaborators [6] disclosed a photoredox strategy for the synthesis of α-amino acids in continuous flow under UV light through the carboxylation of the C(sp³)-H bond of tertiary benzylamine derivatives (N-benzylpiperidine derivatives) in the presence of a base, KCO₂CF₃. Open-shell species are generated as follows: upon irradiation of the photocatalyst, its singlet excited state undergoes a reducing quenching with the amine, resulting in the oxidised form of the amine and the reduced photocatalyst. Then, the oxidised amine is deprotonated by a base, producing a carbon-centred radical. After the formation of the carbon dioxide radical anion via one-electron transfer with the reduced form of the photocatalyst, a new C-C bond is formed through radical-radical coupling, ultimately resulting in α-amino acids with yields of up to 99% and a regioselectivity of 52:1 in just 10 minutes (Scheme 2).

<figure><img src="image_329.png" /><figcaption>**Scheme 1.** Generation of CO₂ radical anion [CO₂]⁻ and its use in C-C bond formation.</figcaption></figure>

<figure><img src="image_330.png" /></figure>
transformation using ultraviolet (UV) light has been reported. As a key example, Jamison and collaborators [6] disclosed a photoredox strategy for the synthesis of α-amino acids in continuous flow under UV light through the carboxylation of the C(sp³)-H bond of tertiary benzylamine derivatives (N-benzylpiperidine derivatives) in the presence of a base, KCO₂CF₃. Open-shell species are generated as follows: upon irradiation of the photocatalyst, its singlet excited state undergoes a reducing quenching with the amine, resulting in the oxidised form of the amine and the reduced photocatalyst. Then, the oxidised amine is deprotonated by a base, producing a carbon-centred radical. After the formation of the carbon dioxide radical anion via one-electron transfer with the reduced form of the photocatalyst, a new C-C bond is formed through radical-radical coupling, ultimately resulting in α-amino acids with yields of up to 99% and a regioselectivity of 52:1 in just 10 minutes (Scheme 2).

<figure><img src="image_326.png" /><figcaption>**Scheme 1.** Generation of CO₂ radical anion [CO₂]⁻ and its use in C-C bond formation.</figcaption></figure>

<figure><img src="image_327.png" /></figure>
<figure><img src="image_331.png" /><figcaption>**Scheme 3.** Hydrocarboxylation of alkenes derivatives with formate salts catalysed by either an organic or an Ir-based photocatalyst.</figcaption></figure>

Furthermore, formate salts have been used instead of CO₂ in order to avoid highly reducing potentials required for the generation of the CO₂ radical anion. The radical anion of carbon dioxide can be generated through hydrogen atom transfer (HAT) from formate using a suitable HAT agent or by single-electron oxidation. As a key example, Li and colleagues [7] described the β-selective hydrocarboxylation of substituted alkenes using either Eosin B or an iridium-based photocatalyst (Scheme 3). The authors suggested two plausible mechanisms. In strategy A, the excited state of the photocatalyst oxidises the formate salt to form a formyloxyl radical capable of trapping one hydrogen atom from the formate salt and producing [CO₂]⁻ and formic acid. Subsequently, [CO₂]⁻ undergoes a radical addition to the alkene, resulting in a Giese-type species that it is subsequently reduced. This
<figure><img src="image_328.png" /><figcaption>**Scheme 3.** Hydrocarboxylation of alkenes derivatives with formate salts catalysed by either an organic or an Ir-based photocatalyst.</figcaption></figure>

Furthermore, formate salts have been used instead of CO₂ in order to avoid highly reducing potentials required for the generation of the CO₂ radical anion. The radical anion of carbon dioxide can be generated through hydrogen atom transfer (HAT) from formate using a suitable HAT agent or by single-electron oxidation. As a key example, Li and colleagues [7] described the β-selective hydrocarboxylation of substituted alkenes using either Eosin B or an iridium-based photocatalyst (Scheme 3). The authors suggested two plausible mechanisms. In strategy A, the excited state of the photocatalyst oxidises the formate salt to form a formyloxyl radical capable of trapping one hydrogen atom from the formate salt and producing [CO₂]⁻ and formic acid. Subsequently, [CO₂]⁻ undergoes a radical addition to the alkene, resulting in a Giese-type species that it is subsequently reduced. This
latter species is protonated to yield the desired product. In strategy B, the reduced form of the catalyst can be excited again and reduce the alkenes to form an alkene radical anion, which could react with the CO₂ generated *in situ* from the formate salt. Finally, the resulting functionalized alkene radical anion is reduced and protonated as in strategy A to yield the desired carboxylate product (Scheme 3).

### Reduction of NHC-CO₂ adducts

Examples of one-electron reduction in NHC-CO₂ adducts is rather limited in the literature (Scheme 4). During the reduction of these adducts, it can lead to the formation of formate (Scheme 4A) and methane (Scheme 4B) in the presence of a proton source. In the absence of any proton source, its radical anion (product of one-electron reduction) is generated either through electrochemical (Scheme 4C) or chemical means (Scheme 4D).

<figure><img src="image_332.png" /><figcaption>**Scheme 4.** Reduction of NHC-CO₂-based adducts. A) Generation of the formate ion. B) Generation of methane. C) Electrochemical reduction of CAAC-CO₂ adduct. D) Chemical reduction of CAAC-CO₂ adduct.</figcaption></figure>

### Synthesis of NHC-CO₂-BR₃ adducts.
latter species is protonated to yield the desired product. In strategy B, the reduced form of the catalyst can be excited again and reduce the alkenes to form an alkene radical anion, which could react with the CO₂ generated *in situ* from the formate salt. Finally, the resulting functionalized alkene radical anion is reduced and protonated as in strategy A to yield the desired carboxylate product (Scheme 3).

### Reduction of NHC-CO₂ adducts

Examples of one-electron reduction in NHC-CO₂ adducts is rather limited in the literature (Scheme 4). During the reduction of these adducts, it can lead to the formation of formate (Scheme 4A) and methane (Scheme 4B) in the presence of a proton source. In the absence of any proton source, its radical anion (product of one-electron reduction) is generated either through electrochemical (Scheme 4C) or chemical means (Scheme 4D).

<figure><img src="image_329.png" /><figcaption>**Scheme 4.** Reduction of NHC-CO₂-based adducts. A) Generation of the formate ion. B) Generation of methane. C) Electrochemical reduction of CAAC-CO₂ adduct. D) Chemical reduction of CAAC-CO₂ adduct.</figcaption></figure>

### Synthesis of NHC-CO₂-BR₃ adducts.
Over the past decade, Tamm and colleagues [8,9] have described three examples of NHC-CO₂-BR₃ adducts in the context of Frustrated Lewis Pair (FLP) chemistry. These systems consist of two components: a classic bulky diamino-carbene with *tert*-butyl groups as *N*-substituents and a bulky boron-based Lewis acid, tris[3,5-bis(trifluoromethyl)phenyl]borane (abbreviated as B(*m*-XyF₆)₃) and tris(pentafluorophenyl)borane, (B(C₆F₅)₃).

<figure><img src="image_333.png" /><figcaption>**Scheme 5.** Synthesis of NHC-CO₂-BR₃ adducts.</figcaption></figure>

**Computational study on reduction potentials of NHC-CO₂ and NHC-CO₂-BR₃ adducts.**

Firstly, we investigated the influence of the *N*-Heterocyclic Carbenes (NHC) in the one-electron reduction of NHC-CO₂ adducts. In this work, we aimed to estimate the reduction potential of a collection of NHC-CO₂ adducts bearing different *N*-substituents, as well as other types of NHC-based compounds (1-8) (Scheme 6).
Over the past decade, Tamm and colleagues [8,9] have described three examples of NHC-CO₂-BR₃ adducts in the context of Frustrated Lewis Pair (FLP) chemistry. These systems consist of two components: a classic bulky diamino-carbene with *tert*-butyl groups as *N*-substituents and a bulky boron-based Lewis acid, tris[3,5-bis(trifluoromethyl)phenyl]borane (abbreviated as B(*m*-XyF₆)₃) and tris(pentafluorophenyl)borane, (B(C₆F₅)₃).

<figure><img src="image_330.png" /><figcaption>**Scheme 5.** Synthesis of NHC-CO₂-BR₃ adducts.</figcaption></figure>

**Computational study on reduction potentials of NHC-CO₂ and NHC-CO₂-BR₃ adducts.**

Firstly, we investigated the influence of the *N*-Heterocyclic Carbenes (NHC) in the one-electron reduction of NHC-CO₂ adducts. In this work, we aimed to estimate the reduction potential of a collection of NHC-CO₂ adducts bearing different *N*-substituents, as well as other types of NHC-based compounds (1-8) (Scheme 6).
Using the "direct approach," the Gibbs energy associated with the one-electron reduction of NHC-CO₂ adducts allowed us to estimate an absolute reduction potential for each adduct. The next step is to refer it to a reference electrode or redox couple. The most commonly used reference electrodes are the Standard Hydrogen Electrode (SHE) and the Saturated Calomel Electrode (SCE). In the literature, the absolute reduction potential of SHE, E<sub>red</sub><sup>SHE</sup>, has been theoretically determined in various ways. However, it can vary from 4.05 V to 4.44 V depending on parameters used, such as the Gibbs energy of H⁺ and the surface potential, among others [10-13]. The theoretical reduction potential of SCE depends on SHE, which can complicate the assessment of precise reduction potentials. Therefore, we used the ferrocenium/ferrocene (Fc<sup>+/-</sup>) redox couple as a reference as it does not depend on the SHE. The absolute reduction potential used for this redox couple was 4.780 V [14].

In NHC-CO₂ systems where N-substituents are alkyl groups such as methyl (1), tert-butyl (2), and adamantyl (3), their reduction potential is more negative than the reduction of a free CO₂ molecule (approximately -3 V vs. Fc⁺/⁻). In the case of adducts based on diaminocarbene with aryl derivatives as N-substituents (4-6, Scheme 6), the reduction potential of these species is more favourable than the reduction potential of the previous NHC-CO₂ adducts (1-3, Scheme 6). Only adduct 6 has a reduction potential less negative than in the case of free CO₂. The use of NHCs with N-substituents as aryl groups allowed for more accessible reduction potentials than in the case of adducts with alkyl groups. In other specific cases such as Enders' and CyCAAC carbene adducts, their reduction is more accessible than in the case of free carbon dioxide (Scheme 6).

<figure><img src="image_334.png" /><figcaption>**Scheme 6.** The studied NHC-CO₂ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/⁻.</figcaption></figure>

Next, the influence of adding a borane was studied. We wondered if a borane (BR₃) would facilitate the reduction of NHC-CO₂ adducts to less negative reduction potentials. We considered the use of three different boranes with different Lewis acidities: the simplest borane BH₃ and two more acidic boranes, BCl₃ and B(C₆F₅)₃.
Using the "direct approach," the Gibbs energy associated with the one-electron reduction of NHC-CO₂ adducts allowed us to estimate an absolute reduction potential for each adduct. The next step is to refer it to a reference electrode or redox couple. The most commonly used reference electrodes are the Standard Hydrogen Electrode (SHE) and the Saturated Calomel Electrode (SCE). In the literature, the absolute reduction potential of SHE, E<sub>red</sub><sup>SHE</sup>, has been theoretically determined in various ways. However, it can vary from 4.05 V to 4.44 V depending on parameters used, such as the Gibbs energy of H⁺ and the surface potential, among others [10-13]. The theoretical reduction potential of SCE depends on SHE, which can complicate the assessment of precise reduction potentials. Therefore, we used the ferrocenium/ferrocene (Fc<sup>+/-</sup>) redox couple as a reference as it does not depend on the SHE. The absolute reduction potential used for this redox couple was 4.780 V [14].

In NHC-CO₂ systems where N-substituents are alkyl groups such as methyl (1), tert-butyl (2), and adamantyl (3), their reduction potential is more negative than the reduction of a free CO₂ molecule (approximately -3 V vs. Fc<sup>+/-</sup>). In the case of adducts based on diaminocarbene with aryl derivatives as N-substituents (4-6, Scheme 6), the reduction potential of these species is more favourable than the reduction potential of the previous NHC-CO₂ adducts (1-3, Scheme 6). Only adduct 6 has a reduction potential less negative than in the case of free CO₂. The use of NHCs with N-substituents as aryl groups allowed for more accessible reduction potentials than in the case of adducts with alkyl groups. In other specific cases such as Enders' and CyCAAC carbene adducts, their reduction is more accessible than in the case of free carbon dioxide (Scheme 6).

<figure><img src="image_331.png" /><figcaption>**Scheme 6.** The studied NHC-CO₂ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc<sup>+/-</sup>.</figcaption></figure>

Next, the influence of adding a borane was studied. We wondered if a borane (BR₃) would facilitate the reduction of NHC-CO₂ adducts to less negative reduction potentials. We considered the use of three different boranes with different Lewis acidities: the simplest borane BH₃ and two more acidic boranes, BCl₃ and B(C₆F₅)₃.
As shown in Scheme 7, the trend of the reduction potential remains unchanged compared to the adducts without borane (1-8, Scheme 6). The addition of a borane has a beneficial impact on facilitating the one-electron reduction of NHC-CO₂ adducts (1-8, Scheme 6). With the addition of a borane, these adducts ((1-8)-BH₃, Scheme 7) exhibit less negative reduction potentials than NHC-CO₂ adducts (1-8, Scheme 6). At this stage, the reduction potential of the adducts (1-8)-BH₃ is less negative than in the case of the free CO₂ molecule.

After observing that there is an impact on the reduction potentials of NHC-CO₂-BR₃ adducts, we also wondered about the impact of the Lewis acidity of the borane on the reduction potentials. The results are shown in Scheme 8. By increasing the Lewis acidity of the borane, we can access even less negative reduction potentials (Scheme 8) than in the case of free CO₂, NHC-CO₂ adducts, and NHC-CO₂-BH₃ adducts (Scheme 8).

After evaluating NHC-CO₂-BR₃ adducts by increasing the Lewis acidity of the borane ((1-8)-BCl₃), we were interested in using highly acidic boranes to experimentally synthesise adducts with these properties. According to the literature, there is experimental evidence that NHC-CO₂-B(C₆F₅)₃ adducts can be synthesised with moderate yields. We questioned if these adducts (1-8)-B(C₆F₅)₃ exhibited similar behaviour to their analogues, (1-8)-BCl₃. The reduction potential of NHC-CO₂-B(C₆F₅)₃ adducts (1-8)-B(C₆F₅)₃ is slightly lower than the reduction potential of NHC-CO₂-BCl₃ adducts. Despite this fact, the reduction potential of (1-8)-B(C₆F₅)₃ is more accessible than in the case of NHC-CO₂ adducts (1-8) and the free CO₂ molecule (Scheme 9).

<figure><img src="image_335.png" /><figcaption>**Scheme 7.** The studied NHC-CO₂-BH₃ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/₀.</figcaption></figure>
As shown in Scheme 7, the trend of the reduction potential remains unchanged compared to the adducts without borane (1-8, Scheme 6). The addition of a borane has a beneficial impact on facilitating the one-electron reduction of NHC-CO₂ adducts (1-8, Scheme 6). With the addition of a borane, these adducts ((1-8)-BH₃, Scheme 7) exhibit less negative reduction potentials than NHC-CO₂ adducts (1-8, Scheme 6). At this stage, the reduction potential of the adducts (1-8)-BH₃ is less negative than in the case of the free CO₂ molecule.

After observing that there is an impact on the reduction potentials of NHC-CO₂-BR₃ adducts, we also wondered about the impact of the Lewis acidity of the borane on the reduction potentials. The results are shown in Scheme 8. By increasing the Lewis acidity of the borane, we can access even less negative reduction potentials (Scheme 8) than in the case of free CO₂, NHC-CO₂ adducts, and NHC-CO₂-BH₃ adducts (Scheme 8).

After evaluating NHC-CO₂-BR₃ adducts by increasing the Lewis acidity of the borane ((1-8)-BCl₃), we were interested in using highly acidic boranes to experimentally synthesise adducts with these properties. According to the literature, there is experimental evidence that NHC-CO₂-B(C₆F₅)₃ adducts can be synthesised with moderate yields. We questioned if these adducts (1-8)-B(C₆F₅)₃ exhibited similar behaviour to their analogues, (1-8)-BCl₃. The reduction potential of NHC-CO₂-B(C₆F₅)₃ adducts (1-8)-B(C₆F₅)₃ is slightly lower than the reduction potential of NHC-CO₂-BCl₃ adducts. Despite this fact, the reduction potential of (1-8)-B(C₆F₅)₃ is more accessible than in the case of NHC-CO₂ adducts (1-8) and the free CO₂ molecule (Scheme 9).

<figure><img src="image_332.png" /><figcaption>**Scheme 7.** The studied NHC-CO₂-BH₃ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/₀.</figcaption></figure>
<figure><img src="image_336.png" /><figcaption>**Scheme 8.** The studied NHC-CO₂-BCl₃ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/₀.</figcaption></figure>

<figure><img src="image_337.png" /><figcaption>**Scheme 9.** The studied NHC-CO₂-B(C₆F₅)₃ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/₀.</figcaption></figure>
<figure><img src="image_333.png" /><figcaption>**Scheme 8.** The studied NHC-CO₂-BCl₃ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/₀.</figcaption></figure>

<figure><img src="image_334.png" /><figcaption>**Scheme 9.** The studied NHC-CO₂-B(C₆F₅)₃ adducts with their calculated theoretical reduction potential in dichloromethane using the M06-2X-D3 functional (in blue). Potentials are expressed in volts (V) in respect to Fc⁺/₀.</figcaption></figure>
**Experimental approach to one-electron reduction in NHC-CO₂-BR₃ adducts.**

We conducted an exploratory study with the ultimate goal of understanding the behaviour of NHC-CO₂-based species upon the addition of a single electron to avoid the synthesis of a large collection of NHC-CO₂ and NHC-CO₂-BR₃ adducts. Nevertheless, to validate whether the computational methodology accurately describes reduction in NHC-CO₂ and NHC-CO₂-BR₃ adducts, a significantly smaller collection of NHC-based adducts was experimentally tested. According to theoretical results, DFT analysis suggests that NHC-CO₂-BR₃ adducts would result in less negative reduction potentials compared to NHC-CO₂ adducts. According to DFT analysis, the addition of boranes with higher Lewis acidity (such as BCl₃ and B(C₆F₅)₃) should more easily promote one-electron reduction compared to cases where the borane is less acidic (like BH₃) or absent (NHC-CO₂ adducts).

In the literature, reports have confirmed the experimental isolation of NHC-CO₂-BR₃ adducts where the NHC and highly acidic borane are sterically hindered, inspired by a Frustrated Lewis Pair (FLP) approach in the presence of CO₂. Following the work of Tamm [8,9], we considered the synthesis of NHC-CO₂-B(C₆F₅)₃ adducts. To confirm if our theoretical methodology is suitable for predicting reduction potentials in NHC-CO₂ and NHC-CO₂-BR₃ adducts, regardless of the electronic properties of the NHC, we tested several types of NHC. We synthesised I'Bu-CO₂-B(C₆F₅)₃ (2-B(C₆F₅)₃, previously reported by Tamm [9]) and IAd-CO₂-B(C₆F₅)₃ (3-B(C₆F₅)₃), with adamantyl groups as N-substituents, IPr-CO₂-B(C₆F₅)₃ (5-B(C₆F₅)₃) with substituted aromatic groups as N-substituents, and finally, CyCAAC-CO₂-B(C₆F₅)₃ (8-B(C₆F₅)₃) inspired by the previous works of Machan and Gilliard (CyCAAC-CO₂, 8) [15,16]. This allowed us to test our theoretical methodology in a diverse group of systems.

After the synthesis and characterisation of NHC-CO₂-B(C₆F₅)₃ adducts (2-, 3-, 5-, 8-B(C₆F₅)₃), we measured their reduction potential. Measurements were performed by cyclic voltammetry. Due to the stability of the compounds in dichloromethane (NMR spectra were recorded in CH₂Cl₂), this solvent was used for measuring the reduction potential of the synthesised adducts.

After defining the experimental conditions, we tested the reproducibility of our system for determining the reduction potential of NHC-CO₂ and NHC-CO₂-BR₃ adducts. To our knowledge, there is only one example where the reduction potential of NHC-CO₂ adducts has been successfully determined experimentally. Machan, Gilliard, and their colleagues jointly reported the reduction potential of CyCAAC-CO₂ adduct (8) as -2.08 V (in CH₃CN) and -2.15 V (in THF) vs. Fc⁺/⁻ [15]. We measured a reduction potential of CyCAAC-CO₂ at -2.13 V vs. Fc⁺/⁻ in CH₂Cl₂. Next, we measured the reduction potentials of all studied adducts (2-, 3-, 5-, 8-B(C₆F₅)₃) (Figure 1).

In the case of adducts with alkyl N-substituents, tert-butyl (2-B(C₆F₅)₃) and adamantyl (3-B(C₆F₅)₃), an irreversible behaviour with a reduction potential at -2.30 V (2-B(C₆F₅)₃) and -2.44 V (3-B(C₆F₅)₃) vs. Fc⁺/⁻, respectively, was observed. This may indicate a potential instability of the monoreduced NHC-CO₂-B(C₆F₅)₃ generated. For the compound 5-B(C₆F₅)₃, which has aryl derivatives as N-substituents, its reduction potential is -2.08 V vs. Fc⁺/⁻ (Figure 1, brown curve). It is slightly less negative than in the previous cases (2- and 3-B(C₆F₅)₃). Finally, the compound 8-B(C₆F₅)₃, with the NHC moiety based on CAAC exhibits a "duck-shaped" curve associated with a perfectly reversible process. The reduction potential of this adduct was determined to be -1.34 V vs. Fc⁺/⁻. Among all synthesised adducts, 8-B(C₆F₅)₃ presented the most accessible reduction potential. In this example, one-electron reduction is more favourable than in other synthesised adducts (8, 2-, 3-, 5-B(C₆F₅)₃). As predicted by our preliminary theoretical studies, the presence of B(C₆F₅)₃ in a NHC-CO₂-B(C₆F₅)₃ adduct enable the
Experimental approach to one-electron reduction in NHC-CO₂-BR₃ adducts.

We conducted an exploratory study with the ultimate goal of understanding the behaviour of NHC-CO₂-based species upon the addition of a single electron to avoid the synthesis of a large collection of NHC-CO₂ and NHC-CO₂-BR₃ adducts. Nevertheless, to validate whether the computational methodology accurately describes reduction in NHC-CO₂ and NHC-CO₂-BR₃ adducts, a significantly smaller collection of NHC-based adducts was experimentally tested. According to theoretical results, DFT analysis suggests that NHC-CO₂-BR₃ adducts would result in less negative reduction potentials compared to NHC-CO₂ adducts. According to DFT analysis, the addition of boranes with higher Lewis acidity (such as BCl₃ and B(C₆F₅)₃) should more easily promote one-electron reduction compared to cases where the borane is less acidic (like BH₃) or absent (NHC-CO₂ adducts).

In the literature, reports have confirmed the experimental isolation of NHC-CO₂-BR₃ adducts where the NHC and highly acidic borane are sterically hindered, inspired by a Frustrated Lewis Pair (FLP) approach in the presence of CO₂. Following the work of Tamm [8,9], we considered the synthesis of NHC-CO₂-B(C₆F₅)₃ adducts. To confirm if our theoretical methodology is suitable for predicting reduction potentials in NHC-CO₂ and NHC-CO₂-BR₃ adducts, regardless of the electronic properties of the NHC, we tested several types of NHC. We synthesised I'Bu-CO₂-B(C₆F₅)₃ (2-B(C₆F₅)₃, previously reported by Tamm [9]) and IAd-CO₂-B(C₆F₅)₃ (3-B(C₆F₅)₃), with adamantyl groups as N-substituents, IPr-CO₂-B(C₆F₅)₃ (5-B(C₆F₅)₃) with substituted aromatic groups as N-substituents, and finally, CyCAAC-CO₂-B(C₆F₅)₃ (8-B(C₆F₅)₃) inspired by the previous works of Machan and Gilliard (CyCAAC-CO₂, 8) [15,16]. This allowed us to test our theoretical methodology in a diverse group of systems.

After the synthesis and characterisation of NHC-CO₂-B(C₆F₅)₃ adducts (2-, 3-, 5-, 8-B(C₆F₅)₃), we measured their reduction potential. Measurements were performed by cyclic voltammetry. Due to the stability of the compounds in dichloromethane (NMR spectra were recorded in CH₂Cl₂), this solvent was used for measuring the reduction potential of the synthesised adducts.

After defining the experimental conditions, we tested the reproducibility of our system for determining the reduction potential of NHC-CO₂ and NHC-CO₂-BR₃ adducts. To our knowledge, there is only one example where the reduction potential of NHC-CO₂ adducts has been successfully determined experimentally. Machan, Gilliard, and their colleagues jointly reported the reduction potential of CyCAAC-CO₂ adduct (8) as -2.08 V (in CH₃CN) and -2.15 V (in THF) vs. Fc⁺/° [15]. We measured a reduction potential of CyCAAC-CO₂ at -2.13 V vs. Fc⁺/° in CH₂Cl₂. Next, we measured the reduction potentials of all studied adducts (2-, 3-, 5-, 8-B(C₆F₅)₃) (Figure 1).

In the case of adducts with alkyl N-substituents, tert-butyl (2-B(C₆F₅)₃) and adamantyl (3-B(C₆F₅)₃), an irreversible behaviour with a reduction potential at -2.30 V (2-B(C₆F₅)₃) and -2.44 V (3-B(C₆F₅)₃) vs. Fc⁺/°, respectively, was observed. This may indicate a potential instability of the monoreduced NHC-CO₂-B(C₆F₅)₃ generated. For the compound 5-B(C₆F₅)₃, which has aryl derivatives as N-substituents, its reduction potential is -2.08 V vs. Fc⁺/° (Figure 1, brown curve). It is slightly less negative than in the previous cases (2- and 3-B(C₆F₅)₃). Finally, the compound 8-B(C₆F₅)₃, with the NHC moiety based on CAAC exhibits a "duck-shaped" curve associated with a perfectly reversible process. The reduction potential of this adduct was determined to be -1.34 V vs. Fc⁺/°. Among all synthesised adducts, 8-B(C₆F₅)₃ presented the most accessible reduction potential. In this example, one-electron reduction is more favourable than in other synthesised adducts (8, 2-, 3-, 5-B(C₆F₅)₃). As predicted by our preliminary theoretical studies, the presence of B(C₆F₅)₃ in a NHC-CO₂-B(C₆F₅)₃ adduct enable the
assessment of less negative reduction potentials. For example, the addition of B(C₆F₅)₃ to the CₚCAAC-CO₂ adduct (8) results in a difference of 0.79 V compared to adduct 8.

<figure><img src="image_338.png" /><figcaption>**Figure 1.** Determination by voltammetry of the reduction potentials of the adducts **2-**, **3-**, **5-**, **8-B(C₆F₅)₃** in CH₂Cl₂ in respect to Fc⁺/₀.</figcaption></figure>

We also considered the formation of the corresponding monoreduced species by chemical means. To our knowledge, only the groups of Machan and Gilliard have jointly reported the one-electron reduction of NHC-CO₂ adducts by chemical means.

Until now, chemical reduction of NHC-CO₂-B(C₆F₅)₃ adducts has not been reported in the literature. In our study, chemical reactions were conducted at room temperature in THF using a stoichiometric amount of KC₈, the reducing agent to avoid the possibility of forming dianionic species. Potassium graphite (KC₈) is a powerful reducing agent capable of reducing systems with low reduction potentials to -2.73 V vs. SCE in THF (-3.29 V vs. Fc⁺/₀) [17,18]. Therefore, the reduction of the studied adducts (**2-**, **3-**, **5-**, **8-B(C₆F₅)₃**) is thermodynamically favourable using KC₈.

In the case of **2-** and **3-B(C₆F₅)₃**, no colour change was observed 10 minutes after the addition of KC₈. The transparent solution turned into a black heterogeneous mixture after 12 hours. In contrast, marked colour changes were observed for initially colourless solutions of **5-** or **8-B(C₆F₅)₃** after the addition of the reducing agent. The adduct [**8-B(C₆F₅)₃(K)**]⁺ was isolated by recrystallisation from a saturated solution of THF-pentane at -37 °C, yielding orange crystals suitable for X-ray diffraction analysis.

In the case of [**8-B(C₆F₅)₃(K)**]⁺, the CₚCAAC part is completely planar (0.1°) with respect to the CO₂ moiety, unlike its neutral form (**8-B(C₆F₅)₃**) (79.2°). After reduction, the carbon atom of CO₂ exhibits a closer sp² hybridisation than in its neutral form. Additionally, the C-O bonds are elongated, while the C-C bond between the NHC part and the CO₂ moiety is shortened. The potassium cation coordinates to the terminal oxygen of the CO₂ moiety and interacts with the aromatic π system and two THF molecules. However, despite several attempts, the isolation of the adduct [**5-B(C₆F₅)₃(K)**]⁺ in its crystalline form was unsuccessful.

To prove the formation and electronic structure of the resulting monoreduced NHC-based systems, Continuous-Wave X-band Electron Paramagnetic Resonance (CW-EPR) spectroscopy was
assessment of less negative reduction potentials. For example, the addition of B(C₆F₅)₃ to the CₚCAAC-CO₂ adduct (8) results in a difference of 0.79 V compared to adduct 8.

<figure><img src="image_335.png" /><figcaption>**Figure 1.** Determination by voltammetry of the reduction potentials of the adducts 2-, 3-, 5-, 8-B(C₆F₅)₃ in CH₂Cl₂ in respect to Fc⁺/₀.</figcaption></figure>

We also considered the formation of the corresponding monoreduced species by chemical means. To our knowledge, only the groups of Machan and Gilliard have jointly reported the one-electron reduction of NHC-CO₂ adducts by chemical means.

Until now, chemical reduction of NHC-CO₂-B(C₆F₅)₃ adducts has not been reported in the literature. In our study, chemical reactions were conducted at room temperature in THF using a stoichiometric amount of KC₈, the reducing agent to avoid the possibility of forming dianionic species. Potassium graphite (KC₈) is a powerful reducing agent capable of reducing systems with low reduction potentials to -2.73 V vs. SCE in THF (-3.29 V vs. Fc⁺/₀) [17,18]. Therefore, the reduction of the studied adducts (2-, 3-, 5-, 8-B(C₆F₅)₃) is thermodynamically favourable using KC₈.

In the case of 2- and 3-B(C₆F₅)₃, no colour change was observed 10 minutes after the addition of KC₈. The transparent solution turned into a black heterogeneous mixture after 12 hours. In contrast, marked colour changes were observed for initially colourless solutions of 5- or 8-B(C₆F₅)₃ after the addition of the reducing agent. The adduct [8-B(C₆F₅)₃(K)]* was isolated by recrystallisation from a saturated solution of THF-pentane at -37 °C, yielding orange crystals suitable for X-ray diffraction analysis.

In the case of [8-B(C₆F₅)₃(K)]*, the CₚCAAC part is completely planar (0.1°) with respect to the CO₂ moiety, unlike its neutral form (8-B(C₆F₅)₃) (79.2°). After reduction, the carbon atom of CO₂ exhibits a closer sp² hybridisation than in its neutral form. Additionally, the C-O bonds are elongated, while the C-C bond between the NHC part and the CO₂ moiety is shortened. The potassium cation coordinates to the terminal oxygen of the CO₂ moiety and interacts with the aromatic π system and two THF molecules. However, despite several attempts, the isolation of the adduct [5-B(C₆F₅)₃(K)]* in its crystalline form was unsuccessful.

To prove the formation and electronic structure of the resulting monoreduced NHC-based systems, Continuous-Wave X-band Electron Paramagnetic Resonance (CW-EPR) spectroscopy was
performed in diluted THF solutions at room temperature. As expected, no signal was observed following the reduction of 2- and 3-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adducts (Figure 2), as previously indicated; no signal was observed. The irreversibility in the CV curves is related to the instability of monoreduced forms during reduction. Despite several attempts, this might explain the difficulty in measuring an EPR signal for the monoreduced adducts, [2-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* and [3-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]*, respectively. Regarding the other adducts ([5-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]*), an EPR signal was obtained. In the case of monoreduced adducts based on CAAC, [8(K)]* (Figure 2, violet curve) and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* (Figure 2, green curve), the EPR signal consists of a triplet corresponding to the coupling interaction between the unpaired electron and the nitrogen atom of the CAAC part. Regarding the EPR coupling constant (a<sub>N</sub>), the addition of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> has a minor effect on this constant.

We also considered the use of a less powerful reducing agent, decamethylcobaltocene (CoCp*₂), which has a less negative reduction potential (-1.94 V vs. Fc⁺/₀ in CH₂Cl₂) [18]. Nevertheless, it is still thermodynamically capable of transferring an electron to the adduct 8-B(C₆F₅)₃ (-1.34 V vs. Fc⁺/₀ in CH₂Cl₂) and generating the corresponding monoanionic radical, [8-B(C₆F₅)₃(Co)]*. The EPR experiment showed a very similar signal to the previous measurement using KC₈ as a reducing agent (Figure 2, red curve). The addition of B(C₆F₅)₃ allows the use of less strong reducing agents to obtain monoanionic radicals.

<figure><img src="image_339.png" /><figcaption>**Figure 2.** Continuous-wave X-band EPR spectra of [8(K)]* and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* taken in a THF solution, as well as [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(Co)]* in a CH<sub>2</sub>Cl<sub>2</sub> solution at room temperature.</figcaption></figure>

<figure><img src="image_340.png" /><figcaption>**Figure 3.** Continuous-wave X-band EPR spectrum of [5-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* taken in a THF solution at room temperature.</figcaption></figure>

In the case of the adduct based on NHC IPr, [5-B(C₆F₅)₃(K)]*, the EPR signal exhibited a quintuplet corresponding to the interaction between the unpaired electron and the two equivalent nitrogen
performed in diluted THF solutions at room temperature. As expected, no signal was observed following the reduction of 2- and 3-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adducts (Figure 2), as previously indicated; no signal was observed. The irreversibility in the CV curves is related to the instability of monoreduced forms during reduction. Despite several attempts, this might explain the difficulty in measuring an EPR signal for the monoreduced adducts, [2-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* and [3-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]*, respectively. Regarding the other adducts ([5-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]*), an EPR signal was obtained. In the case of monoreduced adducts based on CAAC, [8(K)]* (Figure 2, violet curve) and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* (Figure 2, green curve), the EPR signal consists of a triplet corresponding to the coupling interaction between the unpaired electron and the nitrogen atom of the CAAC part. Regarding the EPR coupling constant (a<sub>N</sub>), the addition of B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> has a minor effect on this constant.

We also considered the use of a less powerful reducing agent, decamethylcobaltocene (CoCp*₂), which has a less negative reduction potential (-1.94 V vs. Fc⁺/₀ in CH₂Cl₂) [18]. Nevertheless, it is still thermodynamically capable of transferring an electron to the adduct 8-B(C₆F₅)₃ (-1.34 V vs. Fc⁺/₀ in CH₂Cl₂) and generating the corresponding monoanionic radical, [8-B(C₆F₅)₃(Co)]*. The EPR experiment showed a very similar signal to the previous measurement using KC₈ as a reducing agent (Figure 2, red curve). The addition of B(C₆F₅)₃ allows the use of less strong reducing agents to obtain monoanionic radicals.

<figure><img src="image_336.png" /><figcaption>**Figure 2.** Continuous-wave X-band EPR spectra of [8(K)]* and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* taken in a THF solution, as well as [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(Co)]* in a CH<sub>2</sub>Cl<sub>2</sub> solution at room temperature.</figcaption></figure>

<figure><img src="image_337.png" /><figcaption>**Figure 3.** Continuous-wave X-band EPR spectrum of [5-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]* taken in a THF solution at room temperature.</figcaption></figure>

In the case of the adduct based on NHC IPr, [5-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]*, the EPR signal exhibited a quintuplet corresponding to the interaction between the unpaired electron and the two equivalent nitrogen
atoms of the NHC part (Figure 3). Regarding the coupling constant, it has the lowest value among all the studied adducts (4.45 G). Following the previous explanation for adducts [8(K)]\* and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]\*, one can extrapolate that the spin density at the carbene carbon decreases with the addition of borane and the change in the NHC part. This fact could allow the localisation of spin density at the carbon of the CO<sub>2</sub> moiety.

**Theoretical analysis of the one-electron reduction of NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub> adducts.**

Finally, we evaluated other parameters of interest using DFT-based tools that would ultimately allow us to rationalise the behaviour of these adducts after their one-electron reduction after the validation of the computational methodology. Among these parameters, we can highlight the distribution of spin density in the one-electron-reduced NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub> adducts, particularly at the carbon of the CO<sub>2</sub> moiety and the orbital description of these adducts in their neutral and one-electron-reduced forms, among others. In this summary, we will only address the spin density distribution.

In addition to determining the reduction potentials of NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub> adducts, we investigated whether their one-electron-reduced species mimic the behaviour of the CO<sub>2</sub> radical anion. For this reason, we studied how the spin density (additional electron) is localised in their one-electron-reduced species. During the reduction of NHC-CO<sub>2</sub> adducts, the additional electron will likely be localised in the vacant *p*-orbital of the carbene carbon atom (I, Scheme 10). Through resonance, some of the spin density could also reside at the carbon of the CO<sub>2</sub> group (II). We wondered if the addition of a Lewis acid enhances the spin density at the carbon of the CO<sub>2</sub> group (II-BR<sub>3</sub>, Scheme 10).

<figure><img src="image_341.png" /><figcaption>**Scheme 10.** Resonance structures in the monoreduced form of NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub>.</figcaption></figure>

According to the computational analysis, their reduced forms in the absence of a highly acidic borane exhibit more than half of the total spin density localized in the carbene carbon, between 0.56 and 0.66. The common feature for all NHC-CO<sub>2</sub> adducts is that none of them displays spin density at the carbon of the CO<sub>2</sub> group. In comparison with the free CO<sub>2</sub> radical anion, the one-electron-reduced
atoms of the NHC part (Figure 3). Regarding the coupling constant, it has the lowest value among all the studied adducts (4.45 G). Following the previous explanation for adducts [8(K)]\* and [8-B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(K)]\*, one can extrapolate that the spin density at the carbene carbon decreases with the addition of borane and the change in the NHC part. This fact could allow the localisation of spin density at the carbon of the CO<sub>2</sub> moiety.

**Theoretical analysis of the one-electron reduction of NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub> adducts.**

Finally, we evaluated other parameters of interest using DFT-based tools that would ultimately allow us to rationalise the behaviour of these adducts after their one-electron reduction after the validation of the computational methodology. Among these parameters, we can highlight the distribution of spin density in the one-electron-reduced NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub> adducts, particularly at the carbon of the CO<sub>2</sub> moiety and the orbital description of these adducts in their neutral and one-electron-reduced forms, among others. In this summary, we will only address the spin density distribution.

In addition to determining the reduction potentials of NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub> adducts, we investigated whether their one-electron-reduced species mimic the behaviour of the CO<sub>2</sub> radical anion. For this reason, we studied how the spin density (additional electron) is localised in their one-electron-reduced species. During the reduction of NHC-CO<sub>2</sub> adducts, the additional electron will likely be localised in the vacant *p*-orbital of the carbene carbon atom (I, Scheme 10). Through resonance, some of the spin density could also reside at the carbon of the CO<sub>2</sub> group (II). We wondered if the addition of a Lewis acid enhances the spin density at the carbon of the CO<sub>2</sub> group (II-BR<sub>3</sub>, Scheme 10).

<figure><img src="image_338.png" /><figcaption>**Scheme 10.** Resonance structures in the monoreduced form of NHC-CO<sub>2</sub> and NHC-CO<sub>2</sub>-BR<sub>3</sub>.</figcaption></figure>

According to the computational analysis, their reduced forms in the absence of a highly acidic borane exhibit more than half of the total spin density localized in the carbene carbon, between 0.56 and 0.66. The common feature for all NHC-CO<sub>2</sub> adducts is that none of them displays spin density at the carbon of the CO<sub>2</sub> group. In comparison with the free CO<sub>2</sub> radical anion, the one-electron-reduced
NHC-CO₂ adducts seem to be far from reproducing the spin density distribution of the free CO₂ radical anion.

In the case of one-electron-reduced NHC-CO₂-BH₃ adducts, the addition of BH₃ does not significantly alter the spin density at the carbon of the CO₂ moiety. Using more acidic Lewis acids such as BCl₃ or B(C₆F₅)₃, we observed that spin density can also reside at the carbon of the CO₂ group but with a greater effect. The use of more acidic boranes, such as BCl₃ and B(C₆F₅)₃, has a beneficial effect on the spin density at the CO₂ group. Among all the studied one-electron-reduced NHC-CO₂-BR₃ compounds, [5-B(C₆F₅)₃]*- has the highest spin density at the carbon of the CO₂ moiety (0.42).

The accessible reduction potentials found for NHC-CO₂-BR₃ adducts allow the use of milder conditions to generate their reduced form than in the case of CO₂. Furthermore, the presence of spin density on the carbon atom of the CO₂ moiety in their reduced form is also one of the beneficial properties of these molecules that could make them good candidates as C1 synths to construct new C-C chemical bonds.

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NHC-CO₂ adducts seem to be far from reproducing the spin density distribution of the free CO₂ radical anion.

In the case of one-electron-reduced NHC-CO₂-BH₃ adducts, the addition of BH₃ does not significantly alter the spin density at the carbon of the CO₂ moiety. Using more acidic Lewis acids such as BCl₃ or B(C₆F₅)₃, we observed that spin density can also reside at the carbon of the CO₂ group but with a greater effect. The use of more acidic boranes, such as BCl₃ and B(C₆F₅)₃, has a beneficial effect on the spin density at the CO₂ group. Among all the studied one-electron-reduced NHC-CO₂-BR₃ compounds, [5-B(C₆F₅)₃]*- has the highest spin density at the carbon of the CO₂ moiety (0.42).

The accessible reduction potentials found for NHC-CO₂-BR₃ adducts allow the use of milder conditions to generate their reduced form than in the case of CO₂. Furthermore, the presence of spin density on the carbon atom of the CO₂ moiety in their reduced form is also one of the beneficial properties of these molecules that could make them good candidates as C1 synths to construct new C-C chemical bonds.

## References

[1] A. Morales, C. Gonçalves, A. Sournia-Saquet, L. Vendier, A. Lledós, O. Baslé, S. Bontemps, *Chem. Sci.* **2024**, DOI: 10.1039.D3SC06325A.

[2] W. H. Koppenol, J. D. Rush, *J. Phys. Chem.* **1987**, 4429–4430. DOI: 10.1021/j100300a045.

[3] E. Lamy, L. Nadjo, J. M. Saveant, *Electroanal. Chem. Interfacial Electrochem.* **1977**, *78*, 403–407. DOI: 10.1016/S0022-0728(77)80143-5.

[4] D. C. Grills, Y. Matsubara, Y. Kuwahara, S. R. Golisz, D. A. Kurtz, B. A. Mello, *J. Phys. Chem. Lett.* **2014**, *5*, 2033–2038. DOI: 10.1021/jz500759x.

[5] J. Wu, Y. Huang, W. Ye, Y. Li, *Adv. Sci.* **2017**, *4*, 1700194. DOI: 10.1002/advs.201700194.

[6] H. Seo, M. H. Katcher, T. F. Jamison, *Nat. Chem.* **2017**, *9*, 453–456. DOI: 10.1038/nchem.2690.

[7] Y. Huang, J. Hou, L. W. Zhan, Q. Zhang, W. Y. Tang, B. D. Li, *ACS Catal.* **2021**, *11*, 15004–15012. DOI: 10.1021/acscatal.1c04684.

[8] E. L. Kolychev, T. Bannenberg, M. Freytag, C. G. Daniliuc, P. G. Jones, M. Tamm, *Chem. Eur. J.* **2012**, *18*, 16938–16946. DOI: 10.1002/chem.201202840.

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<figure><img src="image_342.png" /></figure>
<figure><img src="image_339.png" /></figure>
<figure><img src="image_343.png" /></figure>

UAB
Universitat Autònoma de Barcelona

<figure><img src="image_344.png" /></figure>

UNIVERSITY
of York

<figure><img src="image_345.png" /></figure>

UNIVERSITATEA DIN BUCUREŞTI
VIRTUTE ET SANITATE

<figure><img src="image_346.png" /></figure>

UNIVERSITAT
LEIPZIG

<figure><img src="image_347.png" /></figure>

Danmarks
Tekniske
Universitet

<figure><img src="image_348.png" /></figure>

<figure><img src="image_349.png" /></figure>

UNIVERSITAT
ROVIRA I VIRGILI

<figure><img src="image_350.png" /></figure>

<figure><img src="image_351.png" /></figure>

UNIVERSITÉ
TOULOUSE III
PAUL SABATIER

<figure><img src="image_352.png" /></figure>

TOULOUSE
INP

<figure><img src="image_353.png" /></figure>

Elkem
A Bluestar Company

tecnal:a

MEMBER OF BASQUE RESEARCH & TECHNOLOGY ALLIANCE

<figure><img src="image_354.png" /></figure>

Energies
nouvelles

BASF

We create chemistry

Henkel

<figure><img src="image_355.png" /></figure>

Italmatch Chemicals

<figure><img src="image_356.png" /></figure>

Johnson Matthey

<figure><img src="image_357.png" /></figure>

The CCIMC beneficiaries and partners
<figure><img src="image_340.png" /></figure>

UAB
Universitat Autònoma de Barcelona

<figure><img src="image_341.png" /></figure>

UNIVERSITY
of York

<figure><img src="image_342.png" /></figure>

UNIVERSITATEA DIN BUCUREŞTI
VIRTUTIS ET SANITATIS

<figure><img src="image_343.png" /></figure>

UNIVERSITAT
LEIPZIG

<figure><img src="image_344.png" /></figure>

Danmarks
Tekniske
Universitet

<figure><img src="image_345.png" /></figure>

<figure><img src="image_346.png" /></figure>

UNIVERSITAT
ROVIRA I VIRGILI

<figure><img src="image_347.png" /></figure>

<figure><img src="image_348.png" /></figure>

UNIVERSITÉ
TOULOUSE III
PAUL SABATIER

<figure><img src="image_349.png" /></figure>

TOULOUSE
INP

<figure><img src="image_350.png" /></figure>

Elkem
A Bluestar Company

tecnal:a

MEMBER OF BASQUE RESEARCH & TECHNOLOGY ALLIANCE

<figure><img src="image_351.png" /></figure>

Energies
nouvelles

BASF

We create chemistry

Henkel

<figure><img src="image_352.png" /></figure>

Italmatch Chemicals

<figure><img src="image_353.png" /></figure>

Johnson Matthey

<figure><img src="image_354.png" /></figure>

The CCIMC beneficiaries and partners