Iminophosphonamido stannylenes enable quantitative CO₂ conversion to isocyanates

Abstract

Iminophosphonamido-supported silylaminostannylenes activate CO₂ to afford siloxystannylene and isocyanates via carbamatostannylene intermediates. DFT calculations support a two-step rearrangement involving nucleophilic attack and silyl migration, offering a rare example of a main-group CO₂ valorisation through isocyanate formation.

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With the ongoing threat of global warming and the increasing atmospheric concentration of carbon dioxide (CO₂) in recent decades, reducing CO₂ emissions remains one of the most pressing global challenges. While CO₂ is well known as a major greenhouse gas, it is also recognised as a sustainable C1 feedstock for the synthesis of value-added organic compounds such as methane, formaldehyde, methanol, and isocyanates.¹ As a pioneering strategy for CO₂ transformation, transition-metal-catalysed processes have been extensively developed over the past few decades.² More recently, the utilisation of main group elements for CO₂ activation has attracted considerable attention.³ Among them, organotin compounds are promising candidates for CO₂ fixation, owing to the high oxophilicity of the tin centres and the nucleophilic nature of the Sn–X σ-bond.⁴ In particular, Sn(II) species—featuring both an electrophilic tin centre and a nucleophilic σ-bond—are generally more reactive toward CO₂ than their Sn(IV) counterparts. To date, several examples of CO₂ activation and fixation by Sn(II) compounds have been reported.^5–10 Sita and co-workers demonstrated the transformation of CO₂ into isocyanates and carbodiimides using an acyclic diaminostannylene I (Scheme 1).⁵ The well-known β-diketiminato stannylenes II, bearing alkoxy, amino, or hydride substituents at the tin centre, undergo insertion into the carbon atom of CO₂ to afford the corresponding carbonato-, carbamato-, and formato-stannylenes.⁶ Kemp reported the activation of CO₂ by an acyclic diaminostannylene III, leading to a tin-containing cluster via CO₂ reduction.⁷ Furthermore, the same group demonstrated that the reaction of a phosphine-stabilised diphosphinostannylene IV with CO₂ furnished a donor–acceptor type adduct, in which the phosphine acts as a donor and the Sn(II) centre as an acceptor.⁸ In a remarkable development, Jones achieved CO₂ reduction using HBpin, catalysed by only 1 mol% of a hydrostannylene V, yielding a methane precursor.⁹

Scheme 1 Reported stannylenes that react with CO₂.

In light of these advances, we turned our attention to the CO₂ transformation reactivity of iminophosphonamido-stannylenes. We have recently reported the synthesis of the iminophosphonamido-chlorostannylene [Ph₂P(N^tBu)₂]SnCl (1), as well as its application in the catalytic hydroboration of aldehydes, ketones, and imines.¹¹ These findings prompted us to investigate the reactivity of a series of iminophosphonamido-stannylenes towards CO₂. Herein, we report the CO₂ transformation reactions of iminophosphonamido-aminostannylenes, which afford a series of isocyanates.

First, a series of aminostannylenes [Ph₂P(N^tBu)₂]SnNRR′ (2–6) was prepared via nucleophilic substitution of 1 with the corresponding lithium amides (Scheme 2). Treatment of 1 with in situ generated LiNⁱPr₂ in THF at 0 °C afforded the aminostannylene 2 as yellow crystals in 62% yield. Similarly, bis- and monosilyl-substituted aminostannylenes 3–6 were synthesised by the reactions of 1 with the corresponding isolated lithium silylamides in toluene at ambient temperature, giving moderate yields. Compounds 4 and 6 were obtained as yellow crystals, whereas 3 and 5 were isolated as colourless crystals (Fig. 2).

Scheme 2 Synthesis of aminostannylenes 2–6 by the reactions of chlorostannylene 1 with lithium amides.

The ¹H NMR spectra of aminostannylenes 2–6 exhibited singlet signals due to the tert-butyl groups in the ligand backbone at δ 1.09, 1.09, 1.10, 1.15, and 0.99, respectively, indicating a highly symmetrical structure of the iminophosphonamide scaffold in solution. In the ³¹P{¹H} NMR spectra of 2–6, singlet resonances were observed at δ 24.8, 24.4, 23.4, 31.6, and 29.8, respectively, which are slightly shifted upfield relative to that of the precursor 1 (δ 40.4),¹¹ and comparable to the previously reported iminophosphonamido-ligated aminostannylene (δ 28.6).¹² The ¹¹⁹Sn{¹H} NMR spectra of 2–6 displayed singlet signals at δ 113.4, 88.2, 94.6, 55.8, and 32.8, respectively. Except for 6, these chemical shifts are downfield-shifted compared to those of three-coordinate aminostannylenes bearing β-diketiminate or amidinate ligands (δ −745 to 50).^6a,13

The molecular structures of 2–6 were unambiguously determined by single-crystal X-ray diffraction analysis. ORTEP drawings of compounds 3–6 are shown in Fig. 1 (see the SI for 2). The conformations of 4 and 6 around the N(R)–SiMe₃ bond are similar; the bulky SiMe₃ group adopts an exo position relative to the four-membered ring in order to minimise steric repulsion with the tert-butyl groups on the ring. In sharp contrast, in 5, the Dip group in the N(Dip)SiMe₃ (Dip = 2,6-ⁱPr₂C₆H₃) moiety is oriented on the exo side, presumably because the Dip group is bulkier than the SiMe₃ group. All Sn–N bond lengths in 3–6 [2.128(2)–2.2473(15) Å] fall within the typical range reported for related three-coordinate aminostannylenes [2.038(6)–2.290(2) Å].¹³

Fig. 1 Molecular structures of 3 (a), 4 (b), 5 (c), and 6 (d). Thermal ellipsoids represent 50% probability. H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: (a) Sn1–N1 = 2.256(2), Sn1–N2 = 2.225(2), Sn1–N3 = 2.156(2), Si1–N3 = 1.734(2), Si2–N3 = 1.732(2), N1–Sn1–N2 = 66.09(7), N1–Sn1–N3 = 109.57(8), N2–Sn1–N3 = 104.48(8), (b) Sn1–N1 = 2.1411(17), Sn1–N2 = 2.2257(15), Sn1–N3 = 2.2473(15), N1–Sn1–N2 = 106.10(6), N1–Sn1–N3 = 110.27(6), N2–Sn1–N3 = 66.07(5), (c) Sn1–N1 = 2.233(3), Sn1–N2 = 2.239(2), Sn1–N3 = 2.146(2), N1–Sn1–N2 = 66.34(8), N1–Sn1–N3 = 105.72(9), N2–Sn1–N3 = 103.08(9), (d) Sn1–N1 = 2.205(2), Sn1–N2 = 2.205(2), Sn1–N3 = 2.128(2), N1–Sn1–N2 = 67.15(8), N1–Sn1–N3 = 99.91(9), N2–Sn1–N3 = 102.53(9).

Next, the reactivity of a series of aminostannylenes towards CO₂ was examined. Initially, the reaction of 2 with CO₂ (3 atm) in C₆D₆ at ambient temperature led to an immediate colour change from yellow to colourless within 1 min, along with the quantitative formation of the corresponding carbamatostannylene 7 (Scheme 3). It is well known that nitrogen–metal bonds in both main group and transition metal complexes readily insert CO₂ to form carbamate species.^6b,14 Similar reactivity yielding a carbamatostannylene has also been reported by Fulton.^6b

Scheme 3 Reaction of diisopropylaminostannylene 2 with CO₂.

The ¹H NMR spectrum of 7 displayed a sharp singlet for the tert-butyl groups and sharp doublet and septet signals for the ⁱPr groups at δ 1.09, 1.32, and 4.04, respectively. The ¹³C{¹H} NMR spectrum exhibited a characteristic resonance for the carbonyl carbon at δ 163.9. The ¹¹⁹Sn{¹H} NMR spectrum showed a doublet (J_Sn–P = 34.9 Hz) at δ −199.7, resulting from coupling with a ³¹P nucleus. This signal is markedly upfield-shifted compared with those of the related iminophosphonamido aminostannylenes (δ 32.8–137.5), and is comparable to that reported for carbamatostannylenes bearing β-diketiminate ligands.^4b

In contrast to compound 2, the reaction of bis(trimethylsilyl)aminostannylene 3 with CO₂ (3 atm) in C₆D₆ at ambient temperature for 4 h afforded the corresponding siloxystannylene [Ph₂P(N^tBu)₂]SnOSiMe₃ (8), accompanied by the quantitative formation of trimethylsilyl isocyanate 9 (Scheme 4). This transformation is reminiscent of the reaction of compound 1 with CO₂, which yields an oxostannylene along with 9 and N,N′-bis(trimethylsilyl)carbodiimide.⁵ Notably, the reaction of 3 with CO₂ produced 9 as the major product, without any detectable by-products. Reaction of 4, which features a tert-butyl-substituted amino group, with CO₂ (3 atm) at ambient temperature also led to quantitative formation of 8 and tert-butyl isocyanate 10. By contrast, the Dip-substituted aminostannylene 5 required extended heating at 70 °C for 7 days to yield 8 and Dip-substituted isocyanate 11 in quantitative conversions. The substantial difference in reactivity is ascribed to the steric bulk of the Dip group. Although several examples of main group silylamides reacting with CO₂ to give silyl isocyanate 9 have been reported,^5,15 many of these approaches suffer from drawbacks such as narrow substrate scope, harsh reaction conditions, or undesired side reactions leading to carbodiimide formation. In contrast, the transformations shown in Scheme 4 proceed smoothly under mild conditions, with broad substrate tolerance and without detectable side products.

Scheme 4 Reactions of silyl-substituted aminostannylenes 3–5 with CO₂.

Unlike 3–5, phenyl-substituted aminostannylene 6 reacted with CO₂ (3 atm) to quantitatively furnish the corresponding carbamatostannylene 12 within 3 h (Scheme 5). A characteristic resonance for the carbonyl carbon of 12 appeared at δ 165.8 in the ¹³C{¹H} NMR spectrum. The ¹¹⁹Sn{¹H} NMR spectrum of 12 exhibited a sharp doublet at δ −170.6, coupled with ³¹P nuclei (J_Sn–P = 18.2 Hz), comparable to that of 7.

Scheme 5 Reaction of phenyl- and silyl-substituted aminostannylene 6 with CO₂.

Interestingly, 12 was gradually converted in solution into the corresponding 8 and phenyl isocyanate 13, suggesting that 12 serves as a key intermediate in the CO₂ activation process leading to isocyanate formation. In contrast to compounds 3–5, this transformation proceeded much more slowly at room temperature. After 7 days, the ³¹P{¹H} NMR spectrum of the reaction mixture revealed 8 and 12 in relative NMR yields of 27% and 53%, respectively, along with a new signal at δ 40.2 (19%), assigned to an unidentified by-product. In addition, insoluble precipitates were observed in the NMR tube (see SI). These findings suggest that the conversion between 8 and 12, accompanied by the generation of 13, is reversible. Aryl isocyanates generally exhibit higher reactivity than their alkyl analogues, and the high reactivity of 13 likely facilitates the reverse reaction from 8 back to 12. This behaviour is consistent with the previous report by Fluton on the reversible reaction between tri-coordinated alkoxyplumbylene and 13.¹⁶

To gain deeper insight into the reaction mechanism of CO₂ activation by the aminostannylenes leading to 8 and isocyanates formation, we performed DFT calculations using compound 3 (Fig. 2). All calculations were conducted at the B3PW91-D3(BJ)/6-31+G(d,p) level of theory for C, H, N, O, P, and Si atoms, and using the SDD basis set for Sn (see SI for details). The first step involves aminostannylation of CO₂ by 3, resulting in the formation of the corresponding carbamatostannylene intermediate (INT1) via nucleophilic attack of the amino group on the carbon atom of CO₂ (TS1). This step is exergonic (ΔG = −18.1 kcal mol⁻¹) and proceeds with a relatively low activation barrier (ΔG^‡ = 14.6 kcal mol⁻¹). In TS1, the Sn–O distance (2.803 Å) is shorter than the sum of the van der Waals radii of tin and oxygen atoms (3.69 Å), suggesting a significant interaction between these atoms. The Sn–N(SiMe₃) bond lengthens from 2.160 Å in 3 to 2.300 Å in TS1, accompanied by a decrease in Wiberg bond index (WBI) from 0.44 to 0.32, indicating partial cleavage of the Sn–N(SiMe₃) bond and initiation of Sn–O bond formation. INT1 features a carbamate ligand coordinated to the tin centre. This intermediate undergoes isomerisation via a 1,2-silyl shift from nitrogen to the oxygen atom of the carbonyl group through TS2, yielding stannylene INT2, an energetically less favourable species (ΔG_INT1→INT2 = +9.2 kcal mol⁻¹). Subsequently, INT2 undergoes a concerted Sn–O bond formation and cleavage via a four-membered transition state (TS3), furnishing isocyanate 9 and 8 as final products. The fragmentation into two molecules results in a significantly positive entropy change (ΔS_INT2→8+9 = 12.8 kcal mol⁻¹ K⁻¹), which contributes substantially to the thermodynamic driving force of the reaction. The rate-determining step of the overall process is the second migration step (TS3), which affords the 8 and 9. This process has a relatively low energy barrier (ΔG^‡ = 23.8 kcal mol⁻¹), which is consistent with the fact that no intermediates were detected during the reaction of 3 with CO₂.

Fig. 2 Energy profiles of the formations of 8 and 9 by the reaction of 3 with CO₂ involving a silyl shift calculated at the B3PW91-D3(BJ)/6-31+G(d,p) for C, H, N, O, P, and Si, sand SDD for Sn level of theory.

In conclusion, we have demonstrated that iminophosphonamido-supported (silylamino)stannylenes react with CO₂ to afford siloxystannylene and isocyanates via carbamatostannylene intermediates. DFT calculations revealed a two-step migration mechanism involving the formation and rearrangement of the carbamatostannylene species, in good agreement with the experimental observations. Given the quantitative nature of the transformation and the broad substrate compatibility, this reaction represents a promising synthetic approach for preparing isocyanates from CO₂ as a C1 feedstock.

This work was partially supported by JSPS KAKENHI (grant number: JP22K05138 to N. N.) and research grants from the Iwatani Naoji Foundation (N. N.) and the International Polyurethane Technology Foundation (N. N.).

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information: NMR spectra, X-ray crystallographic analysis, DFT calculations. See DOI: https://doi.org/10.1039/d5cc04149j.

CCDC 2473573 (2), 2473574 (3), 2473575 (4), 2473576 (5) and 2473577 (6) contain the supplementary crystallographic data for this paper.^17a–e

Notes and references

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17. (a) CCDC 2473573: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p0ypc; (b) CCDC 2473574: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p0yqd; (c) CCDC 2473575: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p0yrf; (d) CCDC 2473576: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p0ysg; (e) CCDC 2473577: Experimental Crystal Structure Determination, 2025 DOI:10.5517/ccdc.csd.cc2p0yth.

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Footnote

† These authors contributed equally.

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