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J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

39226694
10.1021/jacs.4c07044
Communication
Activation and Catalytic Degradation of SF6 and PhSF5 at a Bismuth Center
Béland Vanessa A.
https://orcid.org/0000-0001-9709-8187
Nöthling Nils
https://orcid.org/0000-0001-8171-9399
Leutzsch Markus
https://orcid.org/0000-0003-4152-7098
Cornella Josep *
Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr 45470, Germany
* cornella@mpi-muelheim.mpg.de
03 09 2024
18 09 2024
146 37 2540925415
24 05 2024
23 08 2024
23 08 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

In this work, we report the catalytic degradation of SF6 and PhSF5 using N,C,N pincer bismuthinidene complexes (1 and 5). Exposure of SF6 and PhSF5 to 1 results in the reduction of the S(VI) substrates and concomitant formation of Bi(III) and Bi(II) compounds, which were isolated and characterized. The oxidized bismuth-based products were demonstrated to undergo reduction with PMe3, recovering the starting complex 1. Having established a synthetic redox cycle, the catalytic degradation of SF6 and PhSF5 was developed through ligand optimization to 5, leading to a 528 TON for SF6 and the first reported TON for PhSF5 (3.2).

H2020 European Research Council 10.13039/100010663 850496 Max-Planck-Institut fÃ¼r Kohlenforschung NA NA Max-Planck-Gesellschaft 10.13039/501100004189 NA Deutsche Forschungsgemeinschaft 10.13039/501100001659 EXC 2033 - 390677874 - RESOLV Natural Sciences and Engineering Research Council of Canada 10.13039/501100000038 PDF-558027-2021 document-id-old-9ja4c07044
document-id-new-14ja4c07044
ccc-price
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pmcThe chemical inertness and high dielectric constant of sulfur hexafluoride (SF6) led to its industrial production starting in 1953 for use in a range of applications.1 Yet, its cherished inertness comes at a cost: SF6 is a potent greenhouse gas making it a major risk factor for the global climate. Emissions of SF6 have been steadily increasing since 1978 and were recorded to be 9,000 ± 400 tons per year in 2018.2 Aside from the negligible amount of naturally occurring SF6 (ca. 0.1 pptv), its presence in the atmosphere (ca. 10 pptv) is anthropogenic.3 Although it is currently found in low concentrations relative to other greenhouse gases, it is estimated to have 23,500 times more warming potential than CO2, due to its lifetime of 580–3,200 years.4 In response, multidisciplinary solutions to the SF6 problem have emerged,5−8 building tangible precedence from which alternative processes for its catalytic degradation can be developed.

Although SF6 succumbs to decomposition under harsh conditions,9 the key to activating the kinetically stabilized molecule under mild conditions hinges on accessing the unstable SF6•–, followed by fragmentation.1,10 Indeed, mild activation of SF6 has been reported with a plethora of transition metal compounds, including Ti, V, Cr, Fe, Ni, Zr, Rh, Ir or Pt (Figure 1A).11−16 More recently, main group complexes have also been shown to act as stoichiometric reducing agents and activate SF6, leading to diverse compounds.17−20 In particular, electron-rich Al, N, and P-based compounds as well as photoexcited complexes have proven to be promising candidates (Figure 1B).15,20,21 In a similar manner, the monodefluorinated analog PhSF5 can also be degraded with transition metal complexes based on Rh and Ni.18,19 Despite the great advances in the area, a catalytic degradation protocol of SF6 and PhSF5 with a main group catalyst still remains elusive.

Figure 1 SF6 and PhSF5 degrader toolbox. * denotes excited state reactivity.

Based on precedents in group 15 and previous work on the redox properties of low-valent N,C,N-bismuthnidenes,22−26 we envisioned that 1 would be a good candidate to activate SF6. In this work, we report a unique low-valent bismuth redox cycle capable of the degradation of SF6 and PhSF5 (Figure 1C). This uncommon main group-based protocol proceeds through the intermediacy of Bi(III) and Bi(II) complexes, which could be isolated and fully characterized. We demonstrate how these oxidation products can be successfully reduced back to 1 by the action of a simple phosphine, thus regenerating the propagating species. Further, with the use of a more electron-rich bismuthinidene supported by an asymmetric imine-amine pincer ligand (5), we show catalytic degradation of SF6 and PhSF5.

Based on our previous observations on the stoichiometric activation of aromatic C–F bonds,23 we reacted 1 with SF6 in the presence of LiOTf as an in situ fluoride scavenger in MeCN at 22 °C (Scheme 1A). A gradual color change from dark teal to yellow was observed over the course of 3 days. Analysis of the reaction mixture by 1H NMR spectroscopy revealed the formation of two new bismuth species in a 2:1 ratio, attributed to 2 and 3 (see Figure S2). These compounds could be separated by selective crystallization and isolated in 67% and 55% yield, respectively. The compounds were also structurally characterized to reveal two dicationic bismuth species, which provide insight on the redox chemistry between Bi and SF6. On one hand, Bi(II) dimer (2) is the result of a 1-electron oxidation of 1 by SF6, which consumes the first four F atoms. On the other hand, Bi(III) sulfide bridged dimer 3 is the result of a formal 2-electron oxidation that consumes the S and final two F atoms. Organobismuth(II) dimers have been reported to undergo elemental chalcogen atom (O, S, Se, Te) insertion into the Bi–Bi bond.27 In order to test this possibility for the formation of 3, 2 was reacted with elemental sulfur and formation of 3 could be observed by NMR spectroscopy (72% NMR yield, Scheme 1A). Compound 2 can also be prepared directly by single electron oxidation using ferrocenium triflate (62% isolated yield, see SI),28 further confirming the 1-electron process.

Scheme 1 Activation of SF6 and PhSF5 with 1

(A) Compound 1 (6.0 equiv) was stirred with LiOTf (6.0 equiv) in acetonitrile under 0.5 bar(g) SF6 at 22 °C for 3 days; (B) Compound 1 (1.0 equiv) was heated to 60 °C with LiOTf (1.0 equiv) and PhSF5 (2.5 equiv) in acetonitrile for 3 days; (C) Solid state structures of 2, 3 and 4, visualized with 50% probability ellipsoids. For the sake of clarity, H atoms, disorder and solute molecules have been omitted. Relevant XRD distances and angles: 2: C(1)–Bi(1) 2.1879(17) Å, N(1)–Bi(1) 2.4698(14) Å, N(2)–Bi(1) 2.5438(16) Å, Bi(1)–Bi(1) 3.09439(19) Å, C(1)–Bi(1)–N(1) 71.82(6)°, C(1)–Bi(1)–N(2) 70.87(7)°; 3: C(1)–Bi(1) 2.200(3) Å, N(1)–Bi(1) 2.453(3) Å, N(2)–Bi(1) 2.572(4) Å, Bi(1)–S(1) 2.5497(8) Å, C(1)–Bi(1)–N(1) 72.04(11)°, C(1)–Bi(1)–N(2) 70.22(12)°, Bi(1)–S(1)–Bi(1) 107.54(5)°; 4: C(1)–Bi(1) 2.194(3) Å, N(1)–Bi(1) 2.491(2) Å, N(2)–Bi(1) 2.488(2) Å, Bi(1)–S(1) 2.5702(8) Å, S(1)–C(17) 1.780(3) Å, C(1)–Bi(1)–N(1) 71.37(9)°, C(1)–Bi(1)–N(2) 71.28(9)°, Bi(1)–S(1)–C(17) 98.22(10)°.

Analogous to SF6, aryl sulfur pentafluorides (ArSF5) have been shown to degrade only under strong hydrolytic conditions,29 thus finding applications as robust lipophilic groups in pharmaceuticals and materials.30 When a mixture of PhSF5, LiOTf and 1 was heated at 60 °C, quantitative conversion of 1 was observed, leading to the formation of 2 and a new bismuth(III) thiophenolate species (4), which could be crystallized and isolated from the reaction mixture in 16% yield (Scheme 1B). These findings demonstrate a rare example of PhSF5 activation,13,14 seemingly through the same 1- and 2-electron processes by which Bi activates SF6. To further confirm the nature of 4, the same compound was prepared and isolated via the reaction of benzenesulfenyl triflate with 1 (91% isolated yield, see SI).

The solid-state structures of 2, 3 and 4 were obtained by single crystal XRD (Scheme 1C). Compound 2 is dimeric with the adjacent ligand planes staggered and slightly twisted. Compound 2 is a 1,2-dication,31 which are precedented structures for heavy main group elements in low oxidation states.32 The Bi–Bi bond in 2 (3.09439(19) Å) is on the long end compared to neutral Bi(II) complexes (cf. 2.796–3.209 Å, see SI).33 Compound 3 exhibits two pincer-ligand-bearing bismuth centers bridged by a sulfur atom. The ligand planes are staggered and nearly perpendicular. Compound 4 displays a distorted square pyramidal bismuth center chelated by the pincer ligand, and coordinated by thiophenolate and triflate ligands trans to each other. The Ph group is in an anti-configuration relative to that of the pincer ligand.

In the case of 3 and 4, sulfur S(1) is bent, attributed to the lone-pair-bearing sulfide and thiophenolate ligands in the complexes: a structural beacon of the multiple electron reduction that occurred from the S(VI) starting materials.

To explore the possibility of reducing the oxidative addition products, we selected trimethyl phosphine (PMe3), due to its ability to scavenge both S and F and for its inertness toward the ligand imines.12,16,18−20

The reduction studies were performed in the presence of excess wet [NMe4][F] to mirror the fluoride activity in an envisioned catalytic manifold. It was found that in the presence of stoichiometric PMe3 and excess [NMe4][F], 2, 3 and 4 could be reduced to 1 in >95%, 67% and 85% NMR yield, respectively (Scheme 2A). In all cases, phosphine oxide (OPMe3) (δP = 36.9 ppm) is observed as a byproduct instead of (OTf)2PMe3, due to in situ hydrolysis. In the case of 3, SPMe3 is also observed as a byproduct (δP = 30.9 ppm) and exclusively accounts for the fate of the sulfur atom. The NMR yields were determined to be 61% for OPMe3 and >95% for SPMe3. These yields manifest that the reduction of the triflate portion could account for the low recovery of 1. In the case of 4, OPMe3 and ammonium thiophenolate ([NMe4][SPh]) are observed as byproducts (see Figures S6–S8), indicating that F– likely displaces PhS– from bismuth.

Scheme 2 (A) Reduction of 2, 3 and 4 Using PMe3 as Electron Donor and Atom Acceptor; (B) Disproportionation of 2

Interestingly, we found that the presence of [NMe4][F] results in the rapid and quantitative disproportionation of 2 into 1 and 1·[F]2. Moreover, PMe3 can also reduce 1·[F]2 to 1 in 73% NMR yield, thus highlighting that the reduction protocol is agnostic to such disproportionation events (Scheme 2B).

Having validated the stoichiometric redox cycling, the degradation of SF6 was attempted using catalytic amounts of 1. With PMe3 as a reducing agent, the formation of F2PMe3 and phosphine sulfide (SPMe3) could be observed with 2.0 mol % 1. The reaction was slow, with only 32% yield of SPMe3 and 4.0 TON SF6 formed after 1 month at 25 °C. However, 1 was observed to be the resting state and the concentration remained constant throughout the entire NMR monitoring time (1 month), indicating the robustness of the catalyst to the reaction conditions (Figures S16–S17). Switching to a bismuthinidene supported by an asymmetric pincer ligand (5)—where one of the supporting imine arms is replaced with a stronger σ-donating amine—results in a more electron-rich Bi(I) complex (E1/2: 5 = −1.01 V, cf. 1 = −0.85 V).24,25 With 2.1 mol % 5 and using PMe3 as reducing agent, 70% SPMe3 and 7.9 TON could be achieved in 11 days under 1 bar(g) of SF6 (Scheme 3A). All SF6 was consumed from solution and PMe3 remained in excess by the end of the NMR monitoring time. When the reaction mixture was heated to 60 °C, a 97% NMR yield of SPMe3 was obtained after 3 days, corresponding to SF6 TON of 528. This TON is almost an order of magnitude higher than the one reported by Zámostná and Braun with a rhodium catalyst (86 TON).16

Scheme 3 Catalytic Degradation of SF6 and PhSF5

Whereas the catalytic degradation of SF6 has a good driving force with the formation of F2PMe3 and SPMe3, removal of thiophenolate from Bi requires the use of an electrolyte. Analogous to the stoichiometric reduction of 4, the use of wet [NMe4][F] was found to be necessary for PhSF5 to be catalytically degraded using 10.5 mol % 5 and PMe3 as reducing agent, achieving 3.2 TON at 80 °C (Scheme 3B). To the best of our knowledge, this represents the first example of catalytic degradation of PhSF5 and provides a blue print for further developments.

Our proposed catalytic cycle is initiated by single electron transfer (SET) from bismuthinidene I to SF6, to generate a Bi(II) fluoride species (II) and sulfur(IV) tetrafluoride (SF4) (Scheme 4). Species II rapidly disproportionates to I and difluoro Bi(III) species IV. Compound IV is then reduced by PMe3 to I via reductive defluorination. The control experiment (Table S4, entry 3) as well as precedence from Dielmann et al.20 demonstrate that PMe3 cannot activate SF6 on its own, solidifying the necessity of 5 in this initial step. Although SF4 and SF2 have not been directly observed in the reaction mixture, we speculate their short-lived presence based on the stoichiometric studies. These highly reactive fluorinating agents have been shown to directly scavenge phosphines,34 and this is likely competitive with subsequent reactivity with I. Nonetheless, our stoichiometric studies have shown the possibility of propagating through a bridging sulfide Bi(III) species (III). The latter can undergo reductive atom transfer by PMe3, affording SPMe3. In the case of PhSF5, the sulfur turnover is dependent on anion metathesis of fluoride for thiophenolate at bismuth. This would result in IV, which undergoes reductive dehalogenation by PMe3, followed by the hydrolysis of both PhS– and F2PMe3 to give thiophenol and OPMe3 (Scheme S1).

Scheme 4 Postulated Mechanism for the Catalytic Degradation of SF6

Red: reduction; AT: Atom transfer; Ox: oxidation; Disprop: disproportionation.

In this work, we have reported the stoichiometric degradation of SF6 and PhSF5 using a Bi(I) complex supported by a N,C,N pincer ligand (1). We demonstrate that Bi(I) is capable of reducing SF6 via SET, leading to mixtures of Bi(II) and Bi(III) intermediates, which could be isolated and fully characterized. The oxidation products were reduced back to catalytically active 1 by using PMe3. Finally, with all organometallic steps validated in a stoichiometric fashion, the catalytic degradation of SF6 and PhSF5 was developed with low-valent Bi(I) complex 5. The catalyst scores a remarkable 528 TON for SF6 and provides the first reported catalytic destruction of PhSF5 (3.2 TON).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c07044.Experimental and characterization details (PDF)

Supplementary Material

ja4c07044_si_001.pdf

Financial support for this work was provided by Max-Planck-Gesellschaft, Max-Planck-Institut für Kohlenforschung and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy - EXC 2033-390677874 - RESOLV. V.A.B. is grateful for fellowship funding provided by the Natural Sciences and Engineering Research Council of Canada (NSERC, fellowship number: PDF-558027-2021). This project has received funding from European Union’s Horizon 2020 research and innovation program under Agreement No. 850496 (ERC Starting Grant, J.C.). Open access funded by Max Planck Society.

The authors declare no competing financial interest.

Acknowledgments

We thank the MS, GC, NMR, X-ray and glassblowing departments as well as the Weidenthaler group of Max-Planck-Institut für Kohlenforschung for analytic and technical support. We thank N. Pfänder for TEM-EDX analysis. We thank Prof. Dr. A. Fürstner for generous support.

Abbreviations

AT atom transfer

bar(a) bar absolute pressure

bar(g) bar gauge pressure

Disprop disproportionation

Equiv. equivalents

NMR nuclear magnetic resonance

NOESY Nuclear Overhauser Effect Spectroscopy

Ox oxidation

Red reduction

SET single electron transfer

tBu tertiary butyl

TON turnover number

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction.
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CCDC deposition numbers: 102011, 164446, 164447, 195482, 242846, 242846, 242847, 242847, 242849, 247981, 284976, 284977, 284977, 606899, 675732, 689595, 704047, 740995, 740996, 740997, 742476, 789992, 808072, 879222, 879222, 978281, 981809, 981810, 999347, 1114845, 1114846, 1114847, 1114848, 1149034, 1166085, 1190325, 1292751, 1416634, 1416635, 1416635, 1416636, 1446889, 1500596, 1500597, 1500598, 1500599, 1500600, 1559486, 1559488, 1559488, 1559488, 1585346, 1937537, 1937538, 1962843, 1962843, 1983873, 1985754, 1985754, 1985754, 1985754, 1985755, 1990333, 2012044, 2077927, 2077927, 2077928, 2077928, 2077929, 2077929, 2080779, 2080779, 2080853, 2125092, 2125092, 2125093, 2125093, 2125603, 2203730, 2203730, 2243049, 2243049, 2243050, 2243054, 2243054, 2278294.

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