Periodic Reporting for period 4 - Let-it-Bi (Bismuth Redox Catalysis for Sustainable Organic Synthesis)
Reporting period: 2024-08-01 to 2025-07-31
Let-it-Bi questions the fundamental pillars of redox catalysis by challenging a main group element (from the p-block) to have properties of a transition metal (d-block). Traditionally, catalytic cycles where the catalyst revolves between different oxidation states have been largely restricted to transition metals, and examples based on main group elements were scattered and unstructured Let-it-Bi aimed at putting Bi at the center of a new paradigm in catalysis, where the traditional dogma of redox catalysis could be questioned. During this Action, our research group has shown that Bi is indeed an element like no other, and combines properties of both p and d block elements. This unusual situation leads to question the canonical properties that defined main group elements and separated them from transition metals. Bismuth is the last stable element and is placed in a remote area of the periodic table. By investigating the properties at the fringe, we found that a rich and vast area extended in front of us, with fertilizing a completely new field of research which we termed Bismuth Redox Catalysis.
In WP1, we discovered and implemented a high-valent Bi redox platform. In WP2, we proposed that a low-valent redox platform should also be feasible, where the Bi now revolves between Bi(I) and Bi(III). Within this framework, we were able to go beyond the Action, and discover Bi Radical Catalysis, where Bi(II) complexes can be leveraged for reactivity. In this context, also the photoactivity of Bi complexes has been discovered, which opened up new opportunities for photocatalysis (Fig. 1).
In WP1, we discovered and implemented a high-valent Bi redox platform. In WP2, we proposed that a low-valent redox platform should also be feasible, where the Bi now revolves between Bi(I) and Bi(III). Within this framework, we were able to go beyond the Action, and discover Bi Radical Catalysis, where Bi(II) complexes can be leveraged for reactivity. In this context, also the photoactivity of Bi complexes has been discovered, which opened up new opportunities for photocatalysis (Fig. 1).
During this Action, WP1 resulted in the synthesis of a family of Bi(III) complexes bearing bis-aryl sulfoximine or sulfone ligands (Fig. 2A). Mechanistic investigations on these bismuthine complexes revealed the possibility to perform canonical organometallic steps largely relegated to transition metal catalysis, namely oxidative addition, migratory insertion, transmetallation and reductive elimination. This breath of fundamental reactivity was demonstrated in the context of a C‒F (Fig. 2B), C‒OTf and C‒ONf (Fig. 2C), and C‒N (Fig. 2D). Mechanistic understanding on the latter led to the discovery to a ligand-dictated chemoselective reaction. This implies that the reactivity can be truly modulated by the stereoelectronics of the backbone on our Bi complexes.
Mechanistic studies on the 5-membered ring reductive elimination were also studied (Fig. 3).
We have also found that SO2 can insert into Bi‒C bonds forming bismuth sulfinates (Fig. 4A), both with B and Si nucleophiles (Fig. 4B).
In line with the understanding of the geometry and the bonding of Bi(V) compounds, we have achieved the synthesis of the first monomeric fluorobismuthonium and chlorobismuthonium cations (Fig. 5). In addition, we have also achieved the synthesis of the first monomeric bismuthine oxide (Ar3BiO).
WP2 focused on the two-electron Bi(I)/Bi(III) redox couple. The development of this package was extremely successful, as very early we could demonstrate that low-valent Bi(I) complexes supported by a N,C,N pincer ligand were able to catalyze a transfer hydrogenation of azo- and nitroarenes with ammonia borane and H2O (Fig. 6). In the same line, we were able to apply such two-electron redox process in the catalytic degradation of N2O, the catalytic reduction of azides to amines, the catalytic degradation of SF6, a catalytic hydrofluronation reaction and the catalytic 1,2-additon to carbonyl compounds (Fig. 6).
When Bi(I) is oxidized with bulky oxyaryl one-electron oxidants, it leads to a Radical Equilibrium Complex (Fig. 7A). It was found that upon coordination to Bi(II), the BDFE of N‒H or O‒H bonds are severally weakened, permitting the O radical to undergo rapid HAA. This coordination bond-weakening properties are mostly reported for transition metals or lanthanides. Yet, the fact that Bi is able to perform in this manner opens the door to consider this main group element to weaken more X–Y bonds.
The discovery of this reactivity was truly profound. The generation of radical Bi(II) meant that one-electron redox processes could also be feasible. This was a key moment in this Action, as we felt compelled to devote our efforts to these new possibilities, as this would likely open up completely new perspectives in catalysis.
We found that Bi(I) complexes are able to engage in one-electron oxidative addition (Fig. 8). This new property is unconventional for main group elements, which usually engage lone pairs and therefore, two electron chemistry. However, the possibility of revolving through one electron in a redox process is a revolutionary new reactivity paradigm for bismuth, that can be exploited in a myriad of contexts.
We applied this concept in the catalytic decarboxylative amination reaction (Fig. 9A). We devoted efforts in elucidating the intermediacy of Bi(II) radicals. We successfully synthesized and fully characterized organobismuth(II) pincer complexes by EPR, SQUID and Xray (Fig. 9B).
We noticed that such low-valent Bi(I) complexes absorb red-light (ca 660 nm) via an MLCT (Fig. 10). This new reactivity mode was exploited in the oxidative addition of aryl (pseudo)halides. Due to the novelty of the Bi photoredox process, we elucidated the nature of the excited state dynamics.
We have found that under blue light irradiation, processes such as LMCT or LLCT are also operative (Fig. 11). This concept was applied in the trifluoromethylation of arenes, the cyclopropanation of olefins, the desaturation of amides or the arylation of C–H bonds.
We also made the first mono-coordinated Bi(I) compound (Fig. 12A). In collaboration with Profs. Neese, De Beer, Schnegg and others, it was found that the triplet ground state dominates, but due to the incredibly large contribution of the Spin-Orbit coupling for Bi, the Zero Field Splitting is gigantic (5422 cm–1, the largest ever measured) and thermally isolates the Ms=0 state from the other magnetic sublevels (Fig. 12B). This was the reason for the compound to be “magnetically silent”. In order to study the effect of SO to the electronic structure of the Bi, we synthesized the Sb analogue. However, in this case, it is not a monomer, but a dimer. We have shown that such a dimer is capable of reacting with H2 and ethylene akin TM activate hydrogen and olefins, leading to stable Ar*–SbH2 and Ar*–Sb(C2H4) (Fig. 13A).
We synthesized Ar*–BiH2 (Fig. 13B). Ar*–BiH2 is very unstable and releases H2 at 0 °C.
We also achieved the heaviest analogue of a π-allyl cation (Fig. 13C). This structure was the first of its kind and highlights the possibility of delocalizing electron density in carbon-like structure with heavy elements.
In a nutshell, this Action contains many distinct discoveries that made the concept of Bi Redox Catalysis a reality. We believe we opened up a new field in catalysis, with this Action containing the pioneering results.
Mechanistic studies on the 5-membered ring reductive elimination were also studied (Fig. 3).
We have also found that SO2 can insert into Bi‒C bonds forming bismuth sulfinates (Fig. 4A), both with B and Si nucleophiles (Fig. 4B).
In line with the understanding of the geometry and the bonding of Bi(V) compounds, we have achieved the synthesis of the first monomeric fluorobismuthonium and chlorobismuthonium cations (Fig. 5). In addition, we have also achieved the synthesis of the first monomeric bismuthine oxide (Ar3BiO).
WP2 focused on the two-electron Bi(I)/Bi(III) redox couple. The development of this package was extremely successful, as very early we could demonstrate that low-valent Bi(I) complexes supported by a N,C,N pincer ligand were able to catalyze a transfer hydrogenation of azo- and nitroarenes with ammonia borane and H2O (Fig. 6). In the same line, we were able to apply such two-electron redox process in the catalytic degradation of N2O, the catalytic reduction of azides to amines, the catalytic degradation of SF6, a catalytic hydrofluronation reaction and the catalytic 1,2-additon to carbonyl compounds (Fig. 6).
When Bi(I) is oxidized with bulky oxyaryl one-electron oxidants, it leads to a Radical Equilibrium Complex (Fig. 7A). It was found that upon coordination to Bi(II), the BDFE of N‒H or O‒H bonds are severally weakened, permitting the O radical to undergo rapid HAA. This coordination bond-weakening properties are mostly reported for transition metals or lanthanides. Yet, the fact that Bi is able to perform in this manner opens the door to consider this main group element to weaken more X–Y bonds.
The discovery of this reactivity was truly profound. The generation of radical Bi(II) meant that one-electron redox processes could also be feasible. This was a key moment in this Action, as we felt compelled to devote our efforts to these new possibilities, as this would likely open up completely new perspectives in catalysis.
We found that Bi(I) complexes are able to engage in one-electron oxidative addition (Fig. 8). This new property is unconventional for main group elements, which usually engage lone pairs and therefore, two electron chemistry. However, the possibility of revolving through one electron in a redox process is a revolutionary new reactivity paradigm for bismuth, that can be exploited in a myriad of contexts.
We applied this concept in the catalytic decarboxylative amination reaction (Fig. 9A). We devoted efforts in elucidating the intermediacy of Bi(II) radicals. We successfully synthesized and fully characterized organobismuth(II) pincer complexes by EPR, SQUID and Xray (Fig. 9B).
We noticed that such low-valent Bi(I) complexes absorb red-light (ca 660 nm) via an MLCT (Fig. 10). This new reactivity mode was exploited in the oxidative addition of aryl (pseudo)halides. Due to the novelty of the Bi photoredox process, we elucidated the nature of the excited state dynamics.
We have found that under blue light irradiation, processes such as LMCT or LLCT are also operative (Fig. 11). This concept was applied in the trifluoromethylation of arenes, the cyclopropanation of olefins, the desaturation of amides or the arylation of C–H bonds.
We also made the first mono-coordinated Bi(I) compound (Fig. 12A). In collaboration with Profs. Neese, De Beer, Schnegg and others, it was found that the triplet ground state dominates, but due to the incredibly large contribution of the Spin-Orbit coupling for Bi, the Zero Field Splitting is gigantic (5422 cm–1, the largest ever measured) and thermally isolates the Ms=0 state from the other magnetic sublevels (Fig. 12B). This was the reason for the compound to be “magnetically silent”. In order to study the effect of SO to the electronic structure of the Bi, we synthesized the Sb analogue. However, in this case, it is not a monomer, but a dimer. We have shown that such a dimer is capable of reacting with H2 and ethylene akin TM activate hydrogen and olefins, leading to stable Ar*–SbH2 and Ar*–Sb(C2H4) (Fig. 13A).
We synthesized Ar*–BiH2 (Fig. 13B). Ar*–BiH2 is very unstable and releases H2 at 0 °C.
We also achieved the heaviest analogue of a π-allyl cation (Fig. 13C). This structure was the first of its kind and highlights the possibility of delocalizing electron density in carbon-like structure with heavy elements.
In a nutshell, this Action contains many distinct discoveries that made the concept of Bi Redox Catalysis a reality. We believe we opened up a new field in catalysis, with this Action containing the pioneering results.
The specific progress on let-it-Bi has been summarized in the point above. It is fair to state that we have achieved the overarching goal of the Action, albeit several deviations were required throughout. Indeed, the deviations did not change the main goal of this project: the development and establishment of Bismuth Redox Catalysis as new field of chemistry, with our group becoming the pioneer.