Unveiling the potential of bismuth-based catalysts for electrochemical CO2 reduction

Negar Sabouhanian , Jacek Lipkowski * and Aicheng Chen *
Electrochemical Technology Centre, Department of Chemistry, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada. : ;

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First published on 4th December

Abstract

Electrochemical CO2 reduction has favorable industrial relevance due to its integrability with renewable energies and controllable product generation. Bismuth-based catalysts have emerged as promising candidates in this regard due to their intriguing electrochemical properties and cost-effectiveness. This review focuses on recent advances in bismuth-based catalysts for the electrochemical reduction of CO2, including synthesis methods and approaches for performance improvements. Insights into product formations using Bi-based catalysts are also presented, where in situ FTIR and Raman spectroscopic studies are highlighted to understand the structural evolution of the catalysts and to decipher the mechanisms of CO2 reduction. Further, recent progress of electrochemical CO2 reduction from an industrial perspective and strategies for further development of the bismuth-based catalysts with high activity, selectivity and stability towards practical applications are discussed.

Keywords: Electrochemical CO2 reduction; Bismuth; Nanomaterials; Electrocatalysts; In situ spectroscopy.

1. Introduction

Formate is one of the products of CO2RR having substantial market potential as a valuable industrial feedstock.12 The formic acid/formate market is anticipated to reach one megaton per year by with various applications in medical, agriculture, and textile industries.13 They are regarded as a vital intermediate for synthesizing valuable oxygen-containing compounds such as alcohols, esters, and acids in syngas catalysis. Additionally, the high density of formic acid (1.22 kg L−1) gives it a significant volumetric hydrogen capacity of 53 g H2 per liter, making it a promising candidate as a liquid hydrogen carrier.14 It may be utilized in energy production and storage systems such as hydrogen fuel cells.13 Formate/formic acid formation by electrochemical reduction of CO2 is a promising approach due to its integrability with renewable energies, relatively low cost, practical operation and being environmentally friendly.15 Among various metal-based catalysts, bismuth-based materials have exhibited significant potential for the electrochemical reduction of CO2 to formate with high selectivity.16 Bismuth is mainly found on the ores bismuthinite (bismuth sulfide) and bismite (bismuth oxide) as well as Bi crystals with an oxide layer. Bismuth is not very reactive and can sometimes be found as a native metal.17 Bi-based catalysts exhibited a lower overpotential and higher faradaic efficiency for formate formation compared with other metals. The high formate selectivity can be attributed to the low energy barrier of *COO− intermediate formation. In addition, hydrogen evolution reaction (HER) as a competitive reaction exhibits a relatively high overpotential at Bi-based catalysts.18 These factors along with the cost-effectiveness, low toxicity and high abundance in nature in contrast to other metals make Bi-based catalysts promising for CO2 reduction to formate in large-scale applications.19–24 Different nanostructured monometallic Bi catalysts (e.g., nanoparticles, nanorods, nanodendrites, and nanosheets) have been synthesized and investigated for CO2 reduction.25 However, the weaker *OCHO binding energy makes it difficult to boost formate generation.26 It has been shown that the addition of secondary elements to form bimetallic Bi-based catalysts might effectively improve their catalytic performance.27 The binding energy of oxygen in *OCHO may be adjusted by introduction of another metal (e.g., CuBi,28 BiSn,29 InBi,30 and ZnBi (ref. 31)), which may enhance the *OCHO adsorption through compressive strain and changes of surface electronic bands structure.26 Further, it is revealed that bimetallic Bi-based catalysts not only electrochemically reduce CO2 to formate, but also lead to the creation of other products such as propane and ethylene.32,33In situ characterization techniques have been developed to study the reaction mechanisms and identify the key intermediates formed during CO2 reduction. Great efforts have been made to attain industrial-compatible current densities and high stability for CO2 reduction to formate on Bi-based catalysts using flow cells or membrane electrode assemblies (MEAs). However, achieving ampere level current densities and high electrode stability remains challenging.

In this review, we discuss the recent advances in the development of bismuth-based catalysts with a focus on synthesis methods and strategies for performance enhancement. The possibility of additional product generation using Bi-based catalysts reported in the recent literature is featured. The detection of key intermediates and monitoring of product formation via in situ FTIR and Raman spectroscopy are highlighted. Finally, recent achievements from an industrial perspective are described, and future research directions are proposed.

2. Monometallic Bi-based catalysts

2.1. Synthesis methods

2.2. Strategies for improving performance

The creation of disordered metal sites has been proposed by Wang et al.19 as an efficient defect engineering strategy to activate CO2 and enhance activity for formate formation. It has been suggested that CO2 molecules tended to be adsorbed to and activated on the disorder-engineered Bi sites. It was observed that the distorted metal sites could enhance localized electron transfer to the antibonding π* orbital of adsorbed CO2 to bend the linear CO2 molecules. The richly lattice distorted Bi NSs exhibited a high value of formate FE, reaching ∼100% at a current density of 200 mA cm−2.

It was confirmed that the introduction of p-block atoms into bismuth could modify its electronic structure and alter the energy required for the generation of intermediates.67 The introduction of appropriate heteroatoms may modify the electronic density of Bi p-orbitals, thus enhancing the adsorption of the *OCHO intermediates and improving the intrinsic activity for CO2 reduction.68 For instance, the edge defects of Bi NSs may be coordinated by heteroatom dopants such as sulfur,37,66 titanium,69 or copper39 to enhance CO2RR selectivity, while suppressing the competing HER. Chen et al.67 showed that boron doping could induce the formation of electron-rich Bi, thus facilitating the reduction of *OCHO. A similar electron enrichment effect was also observed by Ti doping, which enhanced interactions between the active sites and *OCHO intermediates. For pure Bi, *OCHO intermediates are strongly adsorbed on the surface, making them difficult to desorb as formate.67 It was revealed that the electron enrichment of Bi could weaken the binding strengths between the active metal centers and oxygen atoms, thereby lowering the barrier for generating *OCHO intermediate.69 Furthermore, the formation of H* as a key intermediate for HER may be strongly suppressed by doping.67 A recent study conducted by Wang et al.66 demonstrated that sulfur doping not only induced charge redistribution around Bi atoms but also activated water molecules to provide sufficient for CO2RR rather than HER (Fig. 3d) to optimize the reaction pathway toward formate formation. The HR-STEM image of S-doped Bi in Fig. 3e revealed a profusion of defects due to the breaking of Bi–O bonds in the initial Bi-containing precursor following topotactic transformation. The effects of sulfur doping on the catalytic activity of Bi NSs were investigated using DFT calculations as reported by Lv et al.,37 indicating that sulfur dopants existed primarily at the edge sites of Bi NSs. This translated to the strong adsorption capacities of *OCHO intermediates while inhibiting the production of CO and H2.

Recent studies have claimed that the bismuth subcarbonate (Bi2O2CO3) is a stable Bi phase under CO2RR that may serve as the active phase for the generation of formate.70 Bi2O2CO3 can provide high carrier mobility due to its thin thickness and limits nanoparticle growth, preventing disordered aggregation and preserving the number of active sites.26 The establishment of the Bi–Bi2O2CO3 interface has been suggested as an efficient strategy to promote CO2 activation and the formation of key intermediates.63 Liu et al.25 synthesized flower-like Bi NSs and confirmed the formation of stable Bi–Bi2O2CO3 interfaces after exposure to air. It was shown that the electrode containing Bi–Bi2O2CO3 interfaces exhibited improved CO2 reduction activity in contrast to the bulk Bi electrode, and the FE of formate could be enhanced up to 89% at −1.07 V vs. RHE. Two-dimensional nanoflake Bi2O2CO3 was utilized as a substrate to load InOx nanodots for efficient CO2 reduction to formate. A good performance of the catalysts with a high FE of 90.83% at a current density of 200 mA cm−2 was attributed to the exposure of the active sites.26

3. Bimetallic Bi-based catalysts

3.1. Formate generation

In addition to the morphologies of the bimetallic CuBi catalysts, their compositions played key roles in determining their activities77 and selectivities.32 Li et al.81 prepared self-supporting Cu–Bi aerogel catalysts at different molar ratios (Fig. 4d) and showed that the selectivity could be modified by altering the molar ratio of Cu/Bi. A high formate FE (96.57%) was achieved using a Cu1Bi2 catalyst. They suggested that the 3D self-supporting structure and high surface area of Cu–Bi aerogels could facilitate electron transfer through more transport channels. As a result, it enhanced the reaction of the intermediates with protons/electrons, leading to higher CO2RR catalytic efficiency. In another study, a composite Cu1Bi1 bimetallic catalyst with a ginger root-like structure (CuO/CuBi2O4) was synthesized by Ren et al.,83 which exhibited a high FE (98.07%) for formate at −0.98 V vs. RHE. It was demonstrated that the Cu–Bi interface could provide abundant active sites for CO2RR, and the presence of bismuth–oxygen bonds stabilized the adsorbed CO2*− intermediate. According to the literature, it was demonstrated that the conversion of CO2 to formate on CuBi catalysts was more favorable through the HCOO* pathway since it was considerably downhill in energy (−0.66 eV). The corresponding free energy diagram of different pathways and the proposed mechanism of CO2 reduction are shown in Fig. 4e and f.82 CO2 was preferably converted to formate with the HCOO* intermediate.

Recently, bimetallic bismuth-based catalysts integrated with other metals such as Sn, In, and Zn have been explored. For instance, bimetallic Bi/Sn catalysts were prepared by a two-step electrodeposition method,29 and it was found that the morphology and catalytic activity could be greatly affected by the deposition time. The needle-like structure in Fig. 5a was created through the deposition of metallic Bi for 5 min followed by the deposition of metallic Sn for 60 min (Bi5Sn60) on a copper mesh substrate. It was demonstrated that Bi5Sn60 had a high formate production rate (634.3 μmol cm−2 h−1) at −1.0 V (vs. RHE). This remarkable formate generation rate was related to the modification of the electronic structure, which enhanced the interactions between the active sites and *OCHO intermediates. A facile co-electrodeposition method was suggested by Yang et al.84 for the synthesis of bimetallic SnBi catalysts. They utilized the specifically formulated electrolyte solution with an adjusted pH of 8–10 at the temperature of 60 °C to obtain a desirable alloy with the best performance for CO2 reduction. This group reported a high formate FE of 96.1% at −1.06 V vs. RHE for a Sn0.5Bi catalyst (0.5 Sn2+/Bi3+ molar ratio), which was attributed to a large surface area with an abundance of active sites and defects. It was reported that the tin metal oxide/bismuth metal oxide interface stabilized the CO2− intermediate and suppressed HER. Also, the electronic coupling at the interfaces of Sn and Bi led to the *OCHO formation pathway, thus promoting the formate generation. Xu et al.85 prepared a Sn-doped Bi nanowire bundle (NB) through the in situ reconstruction of Sn-doped Bi2S3 precursors. By optimizing the doping concentration, a remarkable performance for CO2RR to formate could be achieved. The Sn1/24-Bi NBs showed a high FE of formate >90% over a wide potential window of −0.5 to −1.9 V vs. RHE. The excellent activity of the Sn1/24-Bi NBs catalysts originated from the electron-rich surface, as well as a lower reaction kinetic barrier. The calculation of the Fermi level of both Bi and Sn1/24-Bi catalysts exhibited the availability of more free electrons on the surface of Sn1/24-Bi catalyst, improving the CO2− adsorption.

Zhou et al.87 synthesized a defective BiIn catalyst and studied the influences of defective surfaces on the promotion of HCOOH formation, showing that the introduction of indium to bismuth is an effective approach to enhance the FE of formate via the optimization of the binding energy of *OCHO. Phosphorus-doped BiIn was used as a pre-catalyst to create defective surfaces during the self-reconstruction. It was demonstrated that the defective sites increased *OH adsorption, promoted water dissociation, and enhanced CO2RR kinetics. The Bi : In ratio in BiIn catalysts can be optimized to attain high activity and selectivity. For this purpose, Wang et al.88 prepared Bi-In2O3 nanoflower catalysts with different Bi/In ratios and investigated the relationship between the catalyst composition and CO2RR performance. It was found that the catalyst with a Bi/In ratio of 6 : 94 had a high FE of formate (88.1%) at −0.7 V vs. RHE. Likewise, indium–bismuth nanosphere catalysts with different compositions were synthesized by Tan et al.30 and tested for CO2RR to formate. The In16Bi84 had the highest FE (∼100%) of formate at −0.94 V vs. RHE as seen in Fig. 5b. It was concluded that the presence of Bi enabled electrons to flow from Bi to In and provided additional active sites for CO2RR. Improvements in the performance of bimetallic BiIn catalysts via defect engineering were proposed by Yang et al.89 The researchers showed that the oxygen vacancies originating from the lattice mismatches of Bi2O3 and In2O3 could reduce the CO2 activation energy. DFT calculations confirmed that CO2 molecules were mainly adsorbed by the oxygen vacancies, and the dominant pathway was through oxygen vacancies. MOF-derived Bi/In bimetallic oxide nanoparticles embedded in carbon networks showed excellent selectivity for formate due to the high surface area, desirable pore size distribution, and high electrical conductivity of the carbon network as well as the synergistic effect of Bi and In bimetallic components.90

As a nonprecious earth-abundant metal, Zn is a promising electrocatalyst for CO2RR. Recently, the synergistic effects of Zn and Bi have been recognized, and bimetallic ZnBi catalysts have been studied for the CO2RR to formate.91 Wang et al.31 synthesized a Zn–Bi bimetallic catalyst (Zn-Bi2O3/CN) and revealed that the surface Bi–O structure and synergistic Zn–Bi effect might enhance the CO2RR. DFT calculations indicated that the presence of Zn reduced the energy barrier of HCOO* formation, thus facilitating the production of formate. A hydrothermal process was employed by Zhang et al.91 to prepare bimetallic ZnBi catalysts with a formate FE of 94% at −0.8 V vs. RHE. A feasible and cost-effective strategy was proposed by Sabouhanian et al.86 to grow bismuth nanodendrites on the Zn surface. The ZnBi catalysts were synthesized by immersing the electrodeposited Zn in a bismuth nitrate solution. Due to differences in the reduction potentials of Zn2+ and Bi3+, galvanic replacement took place and Bi nanodendrites grew uniformly on the surface (Fig. 5c). Fig. 5d depicts the activities of Zn and ZnBi electrodes for CO2RR (solid lines) and HER (dashed lines). It was observed that the CO2RR activity increased significantly after 60 s of immersion (ZnBi-1) and was boosted further at 90 s (ZnBi-2). After 90 s, the CO2RR was only slightly improved for 120 s of the galvanic replacement time (ZnBi-3). Furthermore, the additional incorporation of Bi decreased the onset potential for CO2RR. In contrast, the HER activity remained almost the same by introducing Bi as a secondary element. In situ IR spectroscopy determined that CO2 reduction at the ZnBi catalysts proceeded through the generation of the adsorbed *COO− intermediate.

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3.2. Generation of other products

4. In situ spectroscopic studies

The formation of bismuth subcarbonate (Bi2O2CO3) was reported and detected by in situ Raman spectroscopy during the CO2RR, contingent on the electrolyte and initial composition of the Bi precursor.95 An et al.97 reported that the formation of Bi2O2CO3 originated from the formation of a surface oxide layer (Bi3+) or oxidized metal Bi exposed to air (BiO+). In the case of Bi3+ formation, bismuth hydroxide (Bi(OH)3) was generated by the reaction of the Bi3+ and OH− from local alkalinity. However, the Bi(OH)3 was not stable and reacted with CO2 to form Bi2O2CO3 as shown in reaction (1) and (2). In the case of BiO+ formation, it further reacted with carbonate that was present in the highly alkaline CO2-purged electrolyte to form Bi2O2CO3 species as shown in reaction (3) and (4).

The existence of Bi2O2CO3 species during the CO2RR process was detected using the in situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) method. The appearance of the Bi–O stretching vibration of Bi2O2CO3 located at 182 cm−1 confirmed the formation of Bi2O2CO3 on the electrode surface.95 Wu et al.38 demonstrated that a thin layer of Bi2O2CO3 initially formed on the surface; however, it could be completely diminished and converted to metallic Bi prior to the occurrence of CO2RR. As shown in Fig. 7a, the peak at 162 cm−1 assigned to the Bi O vibration mode of the Bi2O2CO3 appeared at the open circuit potential (OCP), which verified the formation of the Bi2O2CO3. With the application of more negative potentials from OCP to −0.9 V, the peak intensity decreased and finally disappeared. Similarly, the transformation of Bi2O3 microcrystals to Bi2O2CO3 under electrochemical conditions was reported by Zeng et al.,98 it was further reduced to metallic Bi at potentials higher than −0.6 V vs. RHE. Bi2O2CO3 has also been directly employed as a catalyst for CO2RR.57,70 The abundant oxygen vacancies in Bi2O2CO3 nanosheets (VO-BOC-NS) served as durable electrocatalysts for CO2 reduction to formate with an FE of >95% at −0.62 V vs. RHE. The stability of the VO-BOC-NS catalyst under CO2 reduction conditions was characterized using in situ Raman spectroscopy in a 0.5 M KCl electrolyte. No notable change of the intensity of peak at cm−1 derived from CO32− in VO-BOC-NS was observed even at more negative potentials, showing excellent stability.99

In addition to studying the structural evolution of materials, the detection of key intermediates and the elucidation of reaction pathways are vital for gaining better insights into reaction mechanisms, and further improving performance by modifying the binding energy of intermediates.100 It was reported that the generation of formate often occurs through the oxygen-bridged *OCHO intermediate on Bi-based catalysts.57,76 The stability of the adsorbates with Bi–O bonds is systematically higher than those with C–Bi bonds. Thus, the *OCHO pathway is dominant compared with *COOH pathway, leading to higher selectivity of formate generation. Some studies have shown that the formation of the *OCHO intermediate is accompanied by the adsorption of HCO3− groups.101 Sabouhanian et al.86 studied the CO2 reduction mechanism at ZnBi catalysts using an in situ electrochemical ATR-FTIR technique. As shown in Fig. 7b, the peaks assigned to formate at cm−1 (C–H bending mode), cm−1 (C–O symmetric stretch), and cm−1 (C O asymmetric stretch) were observed. The signal that appeared at cm−1 was attributed to the asymmetric stretch of the adsorbed *COO− intermediate, revealing that the CO2 molecule was adsorbed by the carbon atom in a monodentate orientation. The time-dependent FTIR spectra of the ZnBi catalyst are presented in Fig. 7c to monitor the formation and consumption of species over time. It was observed that the peaks assigned to the symmetric and asymmetric stretches of formate followed the same trend as the peak allocated to *COO−, and they became stronger as the reaction progressed. This proves that more *COO− species resulted in the production of more formate.

5. Industrial perspective

In addition to the activity, stability is the most important issue for Bi-based catalysts due to thermodynamically driven aggregation of nanoparticles, the dissolution of reactive species, and structural reconstruction during CO2RR.39,45,106 Only a few research papers have reported a stability time of ∼100 h. As catalytic reactions take place on the catalyst's surface, surface modifications can greatly influence the catalytic reactions. The microenvironmental modulation using the molecularly modified surface layers can impact the adsorption behavior of ions and molecules on the catalyst, improving the stability during CO2 reduction.107 For example, HER on Cu nanoneedles was suppressed by a hydrophobic polytetrafluoroethylene (PTFE) coating, leading to the high C2 selectivity with 47% FE. Moreover, the Cu nanoneedle structures were well maintained during the CO2 reduction due to the PTFE coating, resulting in high stability.108 Li et al.106 recorded the most prolonged stability (120 h at 0.4 A) in a membrane electrode assembly cell. A CO2-philic defective carbon (DC) was formed over a Bi catalyst (Bi-DC), where the presence of sp3-hybrid defects in the carbon exhibited a unique sieving effect on CO2 molecules. Thus, it could effectively suppress the degradation and structural evolution of active Bi species during CO2RR. The DC species facilitated the formation of an interconnected carbon network, which enhanced the dispersion of the active Bi component. This resulted in a larger electrochemical surface area (ECSA) and, consequently, a higher current density. Schematics of the degradation of a conventional Bi catalyst and structural stabilization using a novel Bi-DC catalyst are presented in Fig. 8b and c, respectively. It has been reported that the stability of the Bi-based catalysts might be affected by the oxygenated species from the electrolyte leading to poisoning of the active sites. Zhu et al.107 introduced a molecular passivation layer of oxyphilic ascorbic acid to inhibit the poisoning of the hydroxyl. So, the free OH− preferred to bind to the outer ascorbic acid layer, and the possibility of binding to defective Bi sites was reduced. As a result, the high stability of over 120 hours was achieved at 50 mA cm−2. It has been reported that liquid metals as catalysts might have higher stability compared with solid ones due to dynamic surface properties and surface renewal ability. Liquid Bi alloy catalysts were generated by Guo et al.,109 showing 98% FE of formate over 80 h. They overcame the problem of deterioration of the traditional solid-state Bi-based catalysts during CO2 reduction. Compared with the liquid Bi alloy, the solid one exhibited a lower formate FE and less stability. To date, alkaline electrolytes have exhibited the highest activity for CO2RR to formate using Bi-based catalysts.110 For example, Peng et al.111 demonstrated that KOH solutions had improved activities over KHCO3 due to their lower solution resistance, which was confirmed by EIS. However, CO2 molecules can react with OH− ions in alkaline electrolytes and be converted to carbonate, which leads to carbonate deposition and the consumption of CO2 feedstocks.112 More importantly, the formation of carbonates can significantly threaten the stability of CO2RR, as they may obstruct the porous channels required for CO2 transport in the gas diffusion electrode, accelerate electrolyte leakage and raise cell resistance. These challenges greatly constrain the industrial potential of an alkaline or neutral CO2RR system.113 Recently, Bi-based catalysts have been utilized for CO2RR in acidic electrolytes to overcome the aforementioned issues. Formic acid is generated in acidic electrolytes rather than formate, which can reduce the industrial expenses associated with subsequent separation and purification from the electrolyte.20 However, HER typically dominates under acidic conditions. For example, in a strong acid with a pH of 1 or lower, the FE for CO2RR products is nearly zero.113 Recently, a strategy for the engineering of the local microenvironment has been proposed to enhance the CO2RR under acidic conditions. It was found that the HER in acidic electrolytes could be suppressed by creating a hydrophobic interfacial environment,112 or reducing proton coverage113 on the catalyst surface. The FE of formate up to 92.2% at a current density of −237.1 mA cm−2 was reported for CO2 reduction over Bi NSs in acidic electrolytes.113 The recently reported Bi-based catalysts with an industrially compatible current density of formate are summarized in Table 1, showing that over 90% FE was achieved.

6. Conclusions and perspectives

Continued efforts should be made to enhance the stability of Bi-based catalysts for industrial applications. It is also necessary to conduct further research into scalable synthesis techniques and cost-effective catalyst design for large-scale applications. It is vital to gain insights into the kinetics of CO2RR and identify intermediates in real-world applications. Thus, in situ characterization techniques should be developed to be compatible with high current densities. Further research should be conducted to optimize the conditions and designs of cells to generate concentrated formic acid to reduce the expenses of product purification and separation. Integrating Bi-based catalysts with renewable energy sources such as solar and wind power should be considered. The coupling of electrochemical CO2 reduction with renewable energy will reduce the reliance on fossil fuels and contribute to a greener energy landscape. Finally, CO2RR should be assessed regarding the economic perspective and environmental impact. Building a pilot-scale system that allows for real-world testing is crucial to evaluating the stability of the catalysts, the costs and the efficiency of the CO2 reduction process. It would provide valuable data on operational expenses, overall system performance, and long-term durability, offering key insights into the economic viability and potential scalability.

Data availability

Conflicts of interest

Acknowledgements

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Metal-catalysed radical cross-coupling reactions represent a conceptual paradigm shift from the historical two-electron polar disconnections1, resulting in a new approach for the synthesis of organic molecules2. Particularly, disconnections based on the coupling of alkyl-radical fragments have been shown to hold tremendous potential in making C(sp3)–C and C(sp3)–heteroatom bonds3,4. The evolution and application of such a synthetic strategy is linked to advances in the fields of photoredox catalysis5,6,7 and electrochemical synthesis8,9 and, especially, their combination with first-row transition-metal catalysis10,11,12,13. Indeed, elements such as Fe, Co, Ni or Cu hold a preferential place when one-electron processes are required in cross-coupling cycles, resulting in redox events occurring via (n)/(n + 1)/(n + 2) oxidation states (Fig. 1a, right). This particular chemical behaviour leads to the facile generation of alkyl-radical fragments through single-electron transfer (SET) oxidative addition from precursors such as redox-active esters (RAEs) or Katritzky salts (KSs).

Recent years have witnessed significant efforts towards mimicking the redox behaviour of transition metals by main-group elements14. For instance, pnictogens can take part in SN2-type polar oxidative additions resulting in two-electron manoeuvering throughout (n)/(n + 2) redox catalytic cycles, emulating those of late transition metals (Fig. 1a, left)15. However, radical oxidative additions of redox-active electrophiles have generally been restricted to first-row transition metals and well-defined examples of this process with a main-group complex remain elusive6,16. Very recently, bismuth redox catalysis has been established as an emerging platform for organic synthesis17 and our group has shown how Bi(III/V) or Bi(I/III) catalytic cycles can lead to the development of C–F (ref. 18), C–O (ref. 19) or C–H (ref. 20) bond-forming reactions, among others21. Nevertheless, despite the fact that persistent and stable radicals of heavier main-group elements are known22, bismuth radical catalysis has been significantly underexplored23. Bi(II/III) catalytic cycles have been postulated for the living radical polymerization of alkenes24 or the cycloisomerization of 4-iodoalkenes25. This, together with further reports probing the existence of bismuth(II)-centred radicals26, prompted us to explore the behaviour of the Bi(I/II) pair in SET-based oxidative additions of redox-active alkyl electrophiles. In this Article, we show how a well-defined bismuthinidene (1) reacts with alkyl phthalimide esters and alkyl KSs to give alkyl-bismuth(III) adducts, which were found to behave as Bi–C radical-equilibrium complexes (Fig. 1b, bottom). Additionally, we discovered that α-amino alkyl-radical fragments resulting from this process can be easily oxidized by Bi(II), giving rise to iminium ions27,28 that can be trapped by N-nucleophiles. This observation led to the development of a Bi-catalysed radical C–N cross-coupling reaction with a wide scope of both coupling partners (Fig. 1b, top). In spite of the vast number of alkyl-radical couplings developed during the past decade, only a few examples of C(sp3)–N bond formation from redox-active radical precursors have been reported29, mainly relying on photoredox set-ups30,31,32,33, electrochemical synthesis34,35 or the use of an excess of chemical oxidant36. In this Article, we demonstrate that catalytic amounts of a Bi(I) complex can promote this type of transformation in an autonomous manner, without the need for a photoredox system, a chemical oxidant, an external base or an electrochemical set-up.

As a result of the high nucleophilicity of the 6p2 lone pair on the Bi(I) centre, bismuthinidene 1 (refs. 37,38) has recently been shown to engage in polar SN2-type reactions with alkyl halides and triflates39. Similarly, 1 reacted quantitatively with a range of benzyl (pseudo)halides (Cl, Br, I, mesylate) to give benzyl bismuth(III) complexes 5–8 (Fig. 2b). Cyclic-voltammetry analysis of 1 (E1/2 = −0.85 versus Fc0/+, the ferrocene/ferrocenium couple) provides evidence that C‒X (X = halide) cleavage should proceed through a classical SN2 pathway (Ep/2 < −2.0 V versus Fc0/+). On the other hand, the electrochemical behaviour suggested that 1 could potentially engage in SET oxidative-addition processes with alkyl redox-active electrophiles (Fig. 2a). Accordingly, reaction of 1 with 1 equiv. of tetrachlorophthalimide (TCPhth) ester 2 (Ep/2 = −1.2 V versus Fc0/+) cleanly afforded benzyl bismuth(III) complex 9 after SET, fragmentation, release of CO2 and radical recombination (given that the potential difference between 1 and 2 is approximately 0.35 V, SET between 1 and 2 can be estimated to be approimately 8 kcal mol−1 uphill, but subsequent release of CO2 can drive the oxidative-addition process)40. The resulting alkyl-bismuth(III) adduct could be fully characterized by NMR, high-resolution mass spectrometry (HRMS) and single-crystal X-ray diffraction. Furthermore, KS 4 (Ep/2 = −1.3 V versus Fc0/+) also underwent radical oxidative addition with 1 to give 10. As expected, non-chlorinated phthalimide ester 3 (Ep/2 = −2.0 V versus Fc0/+) remained unreacted when mixed with 1. Besides benzyl groups, the same process occurs with primary (12) or secondary (13) RAEs, leading to stable alkyl-bismuth(III) complexes. Tertiary RAEs such as the one derived from 1-adamantanecarboxylic acid did also react with 1, but the resulting adducts were found to be unstable and could not be characterized under standard conditions. Interestingly, the process is orthogonal to classical polar transition-metal oxidative additions, as it could be performed in the presence of an aryl bromide, giving 11 as the sole product in 93% yield (Fig. 2b). This reactivity is a rare example where bismuth, besides emulating the redox behaviour of first-row transition metals during oxidative addition, allows the isolation and characterization of the corresponding alkyl‒metal species resulting from radical recombination. We also found that, whereas classical SN2 reactivity is sensitive to steric effects (>24 h for 14), single-electron oxidative addition of the corresponding RAE led to quantitative formation of complex 12 in <5 min. Furthermore, we found complexes 9, 12 and 13 to be active by electron paramagnetic resonance (EPR) spectroscopy, especially upon light irradiation. Low-temperature EPR analysis of 12 suggests the formation of two radical species that decay at different rates. This is consistent with the homolysis of the C–Bi bond (see Supplementary Information for details)41,42. To investigate this behaviour further, the reaction of bismuthinidene 1 with cyclopropylmethyl iodide was monitored by NMR at low temperature in the dark (Fig. 2c). Complete conversion into cyclopropylmethyl adduct 16 was observed within 1 h at −20 °C. When the mixture was warmed to 50 °C, a slow but steady conversion to ring-opening compound 18 was observed (35% after 12 h).

This indicates that homolysis of the Bi‒C bond in 16 takes place, leading to an in-cage radical pair (int-1). Furthermore, subjecting a solution of 16 to blue light-emitting diode (LED) irradiation resulted in complete conversion into open product 18 within 5 min, showing that light can accelerate the radical ring-opening process25. Conversely, when cyclopropylmethyl RAE 17 was reacted with bismuthinidene 1, a complex analogous to 16 was not observed; instead, radical ring-opening product 19 was immediately obtained, even in the dark. This is consistent with the two distinct mechanistic scenarios for the oxidative addition. On one hand, polar SN2-type reaction of cyclopropylmethyl iodide with 1 initially leads to 16, which eventually ring-opens via alkyl-radical formation. On the other hand, SET and fragmentation of RAE 17 lead to an in-cage bismuth(II)/alkyl radical pair (int-1), for which cyclopropane ring-opening is faster than radical recombination, resulting in the formation of 19 (Fig. 2b). This alkyl radical-type reactivity is consistent with the behaviour displayed by these complexes: the secondary alkyl radical derived from 13 reacts with Michael acceptors such as phenylvinylsulfone giving Giese addition product 21, either in the dark (57%) or under blue-light irradiation (85%) (Fig. 3a, left). Additionally, the alkyl fragment of several complexes reacted with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical (TEMPO) leading to C(sp3)‒TEMPO adducts. Moreover, we observed that catalytic amounts of 1 can promote Giese-type reactions, among others, upon blue-light irradiation (Supplementary Information)43. When investigating the stability of the Bi(III)‒alkyl compounds in solution, it was found that benzyl bismuth(III) complex 9 was especially sensitive to light irradiation, resulting in decomposition mainly to benzyl–benzyl dimers and unselective benzylation of the N,C,N ligand. On the other hand, complex 13 was stable in solution, even after 3 days at 60 °C (Fig. 2b)44. However, under blue-LED irradiation, 13 underwent slow but clean conversion into elimination product 15 and Bi(I), in a radical-type elimination reminiscent of that of alkylcobaloximes45. Furthermore, scrambling experiments confirm that the exchange of alkyl fragments between two different Bi(III) adducts is also possible (Supplementary Information). Interestingly, when attempting the isolation of α-amino alkyl-bismuth(III) adduct 23 derived from proline, we observed the exclusive formation of the product of decarboxylative amination 24, with recovery of bismuthinidene 1 (Fig. 3a, right)30,33. It was speculated that product 24 would arise from the oxidation of the corresponding α-amino alkyl radical by a highly reactive bismuth(II) species. This would lead to the formation of an electrophilic iminium ion28, which ultimately reacts with the TCPhth anion to make the C‒N bond (Fig. 4 and Supplementary Information).

At this point, it was envisaged that this reactivity could exploit the resulting iminium intermediates with external N-nucleophiles, leading to a formal C–N cross-coupling reaction. After optimization of the conditions, we found that the reaction of RAE 22 with 3 equiv. of benzimidazole (25) in the presence of 10 mol% of 1 in dimethylacetamide (DMA) at 25 °C afforded the product of C–N cross coupling (26) in 88% yield within 2 h (Fig. 3b). Under these conditions, the only observed side products were 24 (nucleophilic competition by TCPhth) and the expected amide bond-formation product 27, which could be minimized by controlling the stoichiometry and selecting the appropriate solvent (Fig. 3b, entry 1 versus entry 3) (see Supplementary Information for optimization details). Control experiments without the Bi catalyst led exclusively to an acyl-transfer product (Fig, 3b, entry 2). The high efficiency of the optimized reaction relies on the faster kinetics of the Bi-catalysed radical reaction compared to the background amide formation. For example, the reaction could be carried out at −30 °C in DMF (Fig. 3b, entry 4) or at room temperature in DMA in only 10 min (Fig. 3b, entry 5), giving the desired product in 56 and 85% yield, respectively. To exclude completely the requirement of photoexcitation for any of the steps of the transformation to proceed, the reaction was carried out under exclusion of ambient light, giving comparable results (Fig. 3b, entry 6). As expected, the addition of TEMPO completely inhibited the reaction (Fig. 3b, entry 7)41. Other bismuth(I) complexes and different redox couples tested led to decreased yields or inactivity, thus highlighting the importance of the finely tuned redox properties of 1 (Supplementary Information).

This methodology led us to the assembly of a wide variety of products containing an aminal- or a hemiaminal-ether structural motif, which would be challenging to construct from the parent halide (Table 1). RAEs of readily available natural and non-natural α-amino acids were investigated (either fully protected or with free N–H bonds) as electrophilic partners. C–N coupling products derived from proline (26), phenylalanine (29), valine (30 and 34), leucine (35), glutamic acid (36) or pipecolic acid (32 and 33) were successfully obtained in good to excellent yields. Synthetically relevant N-heterocycles bearing free N‒H bonds were evaluated. Using proline-derived RAE 22, the corresponding C–N products of benzimidazoles (26, 28 and 45), triazole (37), imidazoles (38, 44 and 46) and pyrazoles (32, 33 and 40–43) were obtained.

Non-symmetrical heterocycles such as benzotriazole (to give 39) could also be accommodated, providing the product in a 5:1 N1/N2 ratio of regioisomers. A range of functional groups with different electronic properties were also tolerated (to give 43, 46 and 50). Since the radical process is orthogonal to classical transition-metal-catalysed cross-coupling reactions, different heteroaryl halides (to give 40–42) and heteroaryl boronic esters (to give 32 and 33) could be well tolerated. The strategy was successfully applied in the modification of bioactive molecules, such as theophylline (to give 47, 57%, single regioisomer). The successful coupling using thiabendazole (to give 48, 61%) provides another illustrative example of the orthogonal reactivity to transition metals, as the Lewis-basic sites on both starting material and product could inhibit catalysis by binding to a metal centre. Interestingly, carbamate-like N‒H bonds could also be accommodated as demonstrated by the preparation of 49, a hydroxylated analogue of riluzole. In the absence of external nucleophiles, the product of decarboxylative amination via formal CO2 extrusion was obtained. For this process, both α-amino RAEs and α-oxo RAEs reacted, giving hemiaminal-ether structures such as 50 and 51 in good yields. Overall, this strategy is complementary to the photochemical protocol reported by Fu and co-workers30, allowing the use of α-heteroatom RAEs instead of unbiased alkyl substrates.

To shed light on the mechanism, we monitored the catalytic reaction of α-amino RAE 22 by NMR with 10 mol% of 1 at −40 °C, using DMF-d7 as the solvent. In this scenario, the pair of rotamers of α-amino alkyl-bismuth(III) intermediate 23 accumulated upon consumption of the RAE, coexisting with bismuthinidene 1. It is important to mention that complex 23 was characterized by reaction of 1 with 22 in a separate stoichiometric experiment (see Supplementary Information for details). The accumulated 23 decays into 1 after 1 h at −20 °C (Fig. 4, bottom, Bi(I/II/III) pathway). However, we observed that the consumption of RAE 22 to give decarboxylative-amination product 24 occurs at a higher rate than that of the former process (see Supplementary Information for details of kinetic analysis). Thus, an alternative pathway should be considered in which the corresponding in-cage radical pair reacts directly through SET, leading to the iminium cation upon regeneration of Bi(I) (Fig. 4, bottom, Bi(I/II) pathway)46. Alternatively, radical recombination of the aforementioned radical pair leads to some accumulation of 23, which eventually collapses into the reaction product (24) and 1. Overall, the radical oxidative addition appears to be the rate-limiting step of the dominant pathway, as suggested by the continuous presence of 1 throughout the entire course of the reaction. Importantly, low-temperature EPR-spectroscopy analysis allowed us to detect an intense single-line signal, in agreement with the presence of the corresponding α-amino alkyl-radical fragment. This strong EPR signal was observed even in the dark. This is consistent with the fact that the Bi(I/II) pair can promote this reactivity in the absence of external light irradiation.