Insertion of CO₂ and CS₂ into Bi–N bonds enables catalyzed CH-activation and light-induced bismuthinidene transfer

Abstract

The uptake and release of small molecules continue to be challenging tasks of utmost importance in synthetic chemistry. The combination of such small molecule activation with subsequent transformations to generate unusual reactivity patterns opens up new prospects for this field of research. Here, we report the reaction of CO₂ and CS₂ with cationic bismuth(III) amides. CO₂-uptake gives isolable, but metastable compounds, which upon release of CO₂ undergo CH activation. These transformations could be transferred to the catalytic regime, which formally corresponds to a CO₂-catalyzed CH activation. The CS₂-insertion products are thermally stable, but undergo a highly selective reductive elimination under photochemical conditions to give benzothiazolethiones. The low-valent inorganic product of this reaction, Bi(I)OTf, could be trapped, showcasing the first example of light-induced bismuthinidene transfer.

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Introduction

The activation of small molecules such as H₂, CO, CO₂, P₄, and ethylene used to be considered a domain of transition-metal complexes, but recent advances in main group chemistry have set the stage for s- and p-block compounds to become a unique part of this field of research.^1–8 Strategies that have successfully been applied for small molecule activation by main group compounds include the exploitation of frustrated Lewis pairs,⁶ low valent species,^1–4 (bi)radicals,^1–4,9 compounds featuring element–element multiple bonds,^1–4 ring-strain,^2,10 shuttling between unusual oxidation states,^11,12 and element-ligand cooperativity.⁸

Among the main group compounds, species based on heavy p-block elements with a main quantum number ≥5 stand out due to large covalent radii, flexible coordination environments, soft Lewis acidity, relativistic effects becoming more relevant, and low bond dissociation energies.^13–17 These properties have paved the ground for fundamentally important findings, including the reversible addition of H₂ and ethylene to distannynes,¹⁸ the reversible fixation of CO₂ and CS₂ by complexes with hyper-coordinate antimony or bismuth centers,^19,20 CO insertion into a Bi–N bond,¹⁰ and the reversible P₄ activation by a bismuth radical.²¹ The valuable insights gained from research in this field have so far mostly been focused on the activation, release, and degradation of small molecules, as well as their utilization as building blocks (for highlights on the reversible uptake and release of small molecules see: Scheme 1, top). In contrast, the concept of incorporating a small molecule into a heavy main group complex in order to grant access to new reactivity patterns in subsequent reactions is far less explored. This requires the generation of reactive sites that can be addressed by an external stimulus. Along these lines, the insertion of CO₂ and CS₂ into well-defined bismuth complexes is a promising approach, since Bi–O and Bi–S bonds can readily be cleaved via heterolytic or homolytic pathways^22–24 and even chelating bonding modes of carboxylates and dithiocarboxylates do not generate deep thermodynamic sinks due to the coordination chemical flexibility of the large central atom (Scheme 1, bottom).

Scheme 1 Activation of small molecules with compounds of heavy p-block elements. Top: reversible activation without subsequent novel reactivity (literature). Bottom: activation of CO₂ and CS₂ followed by CH activation and light-induced bismuthinidene transfer (this work).

Here we report the selective insertion of CO₂ and CS₂ into the Bi–N bonds of cationic bismuth amides and the unusual reactivity patterns of the resulting products, which cover CH activation and an unprecedented light-induced bismuthinidene transfer.

Results and discussion

Previously reported ring-strained cationic bismuth compounds 1-R were selected as starting materials since they exhibit a sufficiently high reactivity and one reactive site per bismuth atom (Scheme 2a).^25–27 Reactions of 1-R with CO₂ and CS₂ gave compounds 2-R and 3-R in 72–89% yield, resulting from the selective insertion of the heterocumulenes into the Bi–N bond (Scheme 2a). These are rare examples of compounds based on heavier p-block elements, for which well-defined insertion reactions with both, CO₂ and CS₂ could be realized.²⁸ NMR spectra of 2-R and 3-R revealed the expected signal patterns, albeit signal broadening was observed for 3-R (and for 2-R in pyridine), suggesting the accessibility of higher aggregates in equilibrium reactions. This is supported by a combination of DOSY NMR spectroscopy, high-resolution mass spectrometry, and DFT calculations. DOSY NMR spectroscopy indicated the presence of aggregated species, where the formally determined degrees of aggregation represent averaged values when involving equilibrium reactions that proceed beyond the time scale of the experiment.²⁹ For 2-H, high-resolution ESI-MS revealed a mononuclear species resulting from loss of CO₂ and the [OTf]⁻ counteranion, suggesting the possibility of de-aggregation and giving a first hint at the facile liberation of CO₂ from this molecule (vide infra). For 3-H mono- and dinuclear monocations (obtained by loss of [OTf]⁻) were detected. DFT calculations at the ZORA-BLYP-D3(BJ)/TZ2P level³⁰ with a pyridine solvent model, suggest that the aggregation to form a dinuclear species is energetically disfavored for 2-H (ΔG = +10.9 kcal mol⁻¹), but feasible for 3-H (ΔG = −8.8 kcal mol⁻¹).²⁹ Overall, these results suggest that the solution structures of compounds 2-R and 3-R involve aggregation phenomena with mononuclear species being accessible. The mononuclear representation of these compounds in Scheme 2 is chosen for simplicity.

Scheme 2 (a) Insertion of CO₂ and CS₂ into Bi–N bonds to give 2-R and 3-R. For a discussion of the CE₂-bonding mode and the aggregation behavior of 2-R and 3-R see main text. A mononuclear representation of 2-H and 3-H is chosen for simplicity. (b and c) Molecular structures of 2-H (b) and 3-Me (c) in the solid state. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity and C atoms are shown as wireframe in the right part of (b); atoms exceeding one subunit are drawn as white ellipsoids. Selected bond lengths [Å] for 2-H: Bi1–O4, 2.232(5), Bi2–O3, 2.222(5); Bi2–C14, 2.206(8). Selected bond lengths [Å] for 3-Me: Bi1–S1, 2.6260(8); Bi1–S2, 2.6354(7), Bi1–C14′, 2.305(3); S1–C1, 1.716(3); S2–C1, 1.720(3); C1–N1, 1.332(4).

The molecular structures of 2-R and 3-R in the solid state were elucidated by single-crystal X-ray diffraction analyses and evidence the coordination chemical flexibility of cationic bismuth species (the quality of the X-ray data on 2-Me and 3-H does not allow the discussion of bonding parameters, but serves as a proof of connectivity). Single-crystals of 2-H were obtained by layering a saturated solution of 2-H in acetonitrile with diethyl ether, leading to partial exchange of the solvent ligands (vide infra). The CO₂ moieties in 2-H adopt bridging μ₂-η¹,η¹-coordination modes to form a trinuclear species (one mononuclear subunit and the trimer are depicted in Scheme 2b). Two of the three triflate anions also adopt a bridging coordination mode, resulting in the interconnection of trinuclear subunits to form a double-stranded coordination polymer along the crystallographic b-axis. The bismuth atoms Bi1 and Bi2 adopt a square pyramidal coordination geometry due to contacts with one aryl, two triflate and two CO₂ units, while Bi3 shows a very unusual pentagonal pyramidal coordination geometry due to contacts with one aryl, one triflate, one CO₂ moiety and two neutral ligands (Et₂O and MeCN). The Bi–O bonds (2.22–2.30 Å) and the C–O bonds (1.27–1.30 Å) involving CO₂ moieties are similar in length, suggesting a delocalization of electron density across the CO₂ unit. The analysis of 2-Me confirmed the μ₂-η¹:η¹-coordination mode of the CO₂ group and the tendency towards aggregation in the solid state (to form a dinuclear species in this case).²⁹

The structural analysis of 3-R gives further evidence of the rich coordination chemistry of bismuth cations: 3-H forms a tetranuclear species with the CS₂ moiety in a bridging μ₂-η¹,η¹-coordination mode, while 3-Me crystallized as a dinuclear aggregate with the CS₂ unit showing a non-bridging, chelating η¹,η¹-coordination (Scheme 2c).²⁹ The bismuth atoms in 3-Me show a coordination number of seven with an irregular coordination geometry around Bi1. The Bi1–S1/2 bond lengths (2.63–2.64 Å) do not differ significantly. Together with the C1–S1/2 distances (1.72 Å), which are identical within limits of error, they indicate effective delocalization of the electron density in the CS₂ unit.

While only a few bismuth compounds have been reported to selectively react with CO₂ or CS₂, even less is known about the reactivity of the products resulting from these reactions.^19,28,31 As a prominent example, the shallow potential energy landscape related to the insertion of CO₂ into a Bi–O bond of (Ar₂Bi)₂O has been exploited to reversibly bind and release this heterocumulene.¹⁹ Inspired by these pivotal contributions, we set out to map the reactivity of 2-R and 3-R centered around the stimulus-induced release of CO₂ and CS₂.

Indeed, compounds 2-R eliminate CO₂ at ambient temperature in pyridine or at 60 °C in THF, irrespective of the presence or absence of ambient light. The decay of 2-R and the formation of CO₂ was monitored by ¹H and ¹³C NMR spectroscopy, respectively. Remarkably, the CO₂ release goes along with selective formation of the previously reported^25–27ortho CH-activation products 4-R in quantitative spectroscopic yield (Scheme 3a). The formation of 4-Me was also confirmed by single-crystal X-ray analysis.²⁹ Access to the ortho CH-activated species 4-R has previously been achieved by heating 1-R in pyridine at 80 °C (Scheme 3a, bottom).^25,27 When THF is used as the solvent, 1-H in particular can be kept at 60 °C over an extended period of time without any signs of decomposition or transformation.³² In the presence of 10 mol% of 2-H, however, compound 1-H was readily transformed into 4-H with >90% spectroscopic yield over 4 days in THF (Scheme 3a, bottom). Lowering the amount of 2-H in the reaction mixture to 1 mol% still resulted in the selective formation of 4-H (84% spectroscopic yield), albeit longer reaction times of 17 d were necessary. These observations formally correspond to a CO₂-catalyzed CH-activation reaction.³³ In contrast to the CO₂-insertion products 2-R, the sulfur analogs 3-R proved to be stable in THF at 60 °C over days, when ambient light was excluded.

Scheme 3 (a) Reactivity of compounds 2-R in the context of CH activation of 1-R to give 4-R ((i) THF, 60 °C, (ii) pyridine, r.t.). (b) Proposed mechanism for the extrusion of CE₂ from 2-H (black) and 3-H (dark grey) to give 4-H. Gibbs energies are given in kcal mol⁻¹.²⁹

Mechanistic aspects of the CE₂ extrusion from 2-H and 3-H were investigated by DFT calculations (Scheme 3b). Mononuclear model systems were chosen based on the investigations of their aggregation behavior (vide supra), to keep the computational approach feasible at a sufficiently high level of theory, and because a higher reactivity for mononuclear species has been proposed in related systems.³⁴ We suggest a heterolytic cleavage of the Bi–E bond as the initiating step of the reaction to give the charge-separated intermediate Int-1-E. This intermediate is energetically more favorable for E = O than for E = S due to stabilization of the negative charge by the electronegative oxygen atoms in the (R₂NCO₂)⁻ moiety. Release of CE₂ with concomitant addition of the cationic bismuth center to the phenyl group via transition state TS-1-E gives Int-2-E. Again, the oxygen-based species is lower in energy, which is due to the energy gain resulting from the formation of the C=E double bond in CE₂ being higher for E = O than for E = S.³⁵ Re-aromatization of Int-2-E gives the final product 4-H – a sequence of reactions that has previously been investigated in detail during the CH activation of 1-H and is thus suggested to proceed in an analogous manner here.²⁵ Overall, the results of this mechanistic study indicate release of CO₂ at early stages of the reaction, rationalize the feasibility vs. reluctance of CE₂ release for 2-R and 3-R, and demonstrate the accessibility of previously identified reaction pathways for bismuth-mediated CH activation upon CE₂ elimination. The sensitivity of the CH activation 1-H → 4-H towards the polarity of the reaction medium³² motivated us to investigate the effect of the CO₂ insertion on the polarity of the bismuth species by DFT methods. Indeed, the CO₂-insertion-product 2-H shows a high dipole moment of 6.1 D, exceeding those of pyridine (2.3 D) or 1,2-difluorobnezene (2.6 D), which also facilitate the CH activation of 1-H when used as solvents.²⁹ These findings indicate that an increase of the polarity of the solvent medium as a result of CO₂ insertion is an important factor facilitating the CO₂-catalyzed CH activation 1-H–(cat. 2-H) → 4-H.

When solutions of compounds 2-R were irradiated with a low-pressure Hg-lamp, only mixtures of products were obtained, which could not be identified to date.²⁹ While compounds 3-R are stable in the dark, even at elevated temperature (vide supra), it quickly became evident that they are light-sensitive: at 23 °C, exposure of THF solutions of 3-R to ambient light induced a quantitative and highly selective transformation over five days with concomitant precipitation of a dark solid. Irradiation of THF solutions of 3-R with a low-pressure Hg-vapor lamp dramatically accelerated these reactions without diminishing the excellent selectivities (full conversion after 45 min).²⁹ The THF-soluble products of these reactions were identified as benzothiazolethiones 5-R (Scheme 4, top). This corresponds to a C–S bond formation, formally resulting from the reductive elimination of Bi^I(OTf), which we suggest as a logical intermediate. DFT calculations confirm that a concerted mechanism only involving closed-shell species is too high in energy to be viable.²⁹ In contrast, DFT calculations suggest that the light-induced Bi–S homolysis in 3-R is feasible and gives a short-lived intermediate (Int-5-S; ESI†), from which the elimination of Bi(thf)₂(OTf) proceeds with a free activation energy of only 9.0 kcal mol⁻¹.²⁹ It should be noted in this context that Bi^I(OTf) has recently been isolated as a cyclic alkyl(amino) carbene adduct and proved to be unstable in solution.³⁶ In general, non-stabilized bismuthinidenes BiX (X = monoanionic, monodentate ligand) are unstable in solution,^16,37 with disproportionations such as 3 BiX → 2 Bi⁰ + BiX₃ being a typical degradation pathway, which accounts for the commonly observed dark precipitate (“bismuth black”).³⁸ In order to trap the potential intermediate Bi^I(OTf) in photochemical reactions of cationic bismuth species, 3,5-di-tert-butyl-o-benzoquinone (6) was added to a THF solution of 3-H. ¹H NMR spectroscopy revealed that the quinone coordinated 3-H, but no reaction occurred at ambient temperature in the dark. Upon irradiation of the reaction mixture containing the quinone (Scheme 4, bottom), the precipitation of a black solid was not observed and quantitative conversion to 5-H and a new compound (later identified as 7) was detected by NMR spectroscopy. Despite similar solubilities of these two products, compound 7 could be isolated in moderate yield and was fully characterized.²⁹

Scheme 4 Photochemically-induced elimination of BiOTf from 3-R to give heterocycle 5-R in the absence (top) and presence (bottom) of 3,5-di-tert-butyl-o-benzoquinone (6), which is introduced as bismuthinidene trapping reagent.

While the chemistry of mononuclear bismuth compounds used to be centered around Bi^III and Bi^V species, recent advances have begun to uncover an unexpectedly rich redox chemistry for this class of compounds.^7,11,14,39 This includes examples of reductive elimination reactions for selective bond formation in stoichiometric or catalytic regimes, which operate under thermal conditions.^15,40 While the light-sensitivity of molecular bismuth compounds has been phenomenologically described,⁴¹ the exploitation of their photochemistry for selective bond formations is still in its infancy.^23,24 Similarly, the field of cationic bismuth(I) compounds is virtually unexplored, with a single example of an isolable species³⁶ and the reactivity of these species being entirely unknown.^17,42 Along these lines, reactions of compounds 3-R demonstrate for the first time that cationic bismuth compounds may be exploited both, for the light-driven construction of organic heterocycles and for photochemically-induced transfer of low-valent Bi(I) building blocks.

Conclusions

In summary we have shown, that ring-strained bismuth amide cations selectively insert CO₂ and CS₂. The insertion products show striking differences in their reactivity: the CO₂ activation products undergo thermochemical CO₂ elimination, which goes along with a CH activation to furnish previously reported cationic bisma-cycles. These reactions were transferred to the catalytic regime, which formally corresponds to an unprecedented CO₂-catalyzed CH activation reaction. In the case of the CS₂ activation product, the highly selective, photochemical elimination of low-valent BiOTf (a so-called bismuthinidene) is observed, while benzothiazolethiones are generated through selective C–S bond formation. Chemical trapping of the photochemically released BiOTf was realized for the first time, opening up the field of light-driven bismuthinidene transfer reactions, which are subject of future investigations in our laboratories.

Data availability

Experimental data is provided in the ESI.†

Author contributions

Synthesis: K. O. (exclusively); X-ray diffraction analyses: K. O., A. H. (equal); DFT calculations: J. P. (lead); F. M. B., C. L. (supporting); DOSY studies: X. X. (lead); K. O., C. L. (supporting) composition of the manuscript: K. O., C. L. (lead); J. P. (supporting); discussion of results and correction of manuscript: all authors; project administration: C. L. (lead); acquisition of funding: K. O., J. P., F. M. B., C. L.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Benedikt Ritschel for his contributions. Financial support by the FCI, the DFG (LI2860/3-1, LI2860/5-1), the LOEWE Program, the Cusanuswerk, the Netherlands Organisation of Scientific Research (NWO), and the Spanish Government (PID-2019-106830GB-I00, CEX2021-001202-M), and the Catalan Government (2021SGR442) is gratefully acknowledged. This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 946184).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, crystallographic information; computational details; cartesian coordinates of calculated compounds. CCDC 2223276–2223278. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc01635h

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