Capture of mechanically interlocked molecules by rhodium-mediated terminal alkyne dimerisation

Abstract

The transition metal-mediated dimerisation of terminal alkynes is an attractive and atom-economic method for preparing conjugated 1,3-enynes. Using a phosphine-based macrocyclic pincer ligand, we demonstrate how this transformation can be extended to the synthesis of novel, hydrocarbon-based interlocked molecules: a rotaxane by ‘active’ metal template synthesis and a catenane by sequential ‘active’ and ‘passive’ metal template procedures.

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Introduction

Coordination chemistry is a prominent feature of contemporary methods for constructing mechanically interlocked molecules, such as rotaxanes and catenanes.¹ Active metal template synthesis has emerged as a particularly effective strategy, exploiting a metal to position and covalently fuse the precursor fragments together in an entangled arrangement (Fig. 1A).² Whilst this strategy has been successfully implemented using a variety of metal-mediated transformations, the overwhelming majority of examples are based on copper-mediated alkyne–azide cycloaddition reactions (I) or Glaser couplings (II) in combination with bidentate nitrogen-based macrocycles.^2,3

Fig. 1 (A) Active metal template synthesis of interlocked molecules and (B) terminal alkyne dimerisation reactions promoted by macrocyclic rhodium(I) PNP pincer complexes. Solid-state structure of complex 0 shown with thermal ellipsoids drawn at 30% probability and most H-atoms omitted.

As part of our work exploring the organometallic chemistry of terminal alkyne dimerisation reactions⁴ promoted by macrocyclic pincer complexes (Fig. 1B),^5,6 we speculated that this transformation could be adapted into an active metal template procedure for the preparation of mechanically interlocked 1,3-enynes (III). We herein describe the preparation of hydrocarbon-based rotaxane 1 and catenane 2 derived from the phosphine-based macrocyclic pincer ligand PNP-14 (Fig. 2).⁷ Despite the widespread use of phosphine ligands in organotransition metal chemistry and homogenous catalysis,⁸ this constitutes the first application in active metal template synthesis of mechanically interlocked molecules.

Fig. 2 Synthesis of rotaxane 1 and catenane 2. [B(3,5-(CF₃)₂C₆H₃)₄]⁻ counterions omitted for clarity and solid-state structure of PNP-14·2S shown with thermal ellipsoids drawn at 50% probability.

Results and discussion

Using a protocol developed previously for rhodium(I) E-enyne complex 0 (Fig. 1B),^6,9 rotaxanate 3 and pseudo-rotaxanate 4 were obtained in quantitative spectroscopic yield upon treatment of [Rh(PNP-14)(η²-COD)]⁺ (COD = cyclooctadiene; δ_31P 57.4/45.9, ²J_PP = 312 Hz, ¹J_RhP ∼135 Hz) with the novel bulky aryl stoppered terminal alkyne HC≡C(CH₂)₆C(4-tBuC₆H₄)₃ and methylene-spaced ene–yne HC≡C(CH₂)₁₃CH=CH₂, respectively, in the weakly coordinating solvent 1,2-difluorobenzene (DFB) at room temperature (Fig. 2).¹⁰ The new rhodium derivatives present ¹H NMR resonances at δ 6.95/5.94 (3) and 7.01/5.98 (4) with ³J_HH coupling constants of ∼15 Hz that are diagnostic for coordinated E-enynes, whilst the C₁ symmetry of the molecules is manifested in the observation of inequivalent ³¹P NMR resonances at δ 58.6/54.9 (3) and 56.9/53.1 (4) that are coupled to ¹⁰³Rh (¹J_RhP = 128 Hz) and display trans ²J_PP coupling constants of ∼393 Hz.¹¹ Subsequent treatment of 4 with 25 mol% Schrock's catalyst (CAS: 139220-25-0) at room temperature enabled capture of catenate 5 by ring closing olefin metathesis, with complete conversion confirmed after monitoring the reaction in situ for 5 days at room temperature using ¹H and ³¹P{¹H} NMR spectroscopy and (tandem) ESI-MS.

Removal of rhodium from 3 and 5 was achieved by heating with excess PMe₃ (20 equiv.) to give the corresponding rotaxane (1′, δ_31P 2.42/0.92) and catenane (2′, δ_31P 1.46/0.78), alongside [Rh(PMe₃)₄]⁺ as the rhodium-containing by-product.¹² To facilitate isolation, 1′ and 2′ were converted into the corresponding phosphine sulphides 1 and 2 by treatment with S₈ which were thereafter obtained in 32% and 38% overall yield (from PNP-14) following purification on silica. Formation of the new interlocked molecules was corroborated by analysis of isolated materials using NMR spectroscopy and ESI-MS. Threading of the enyne breaks the C₂ symmetry of the macrocycle and this desymmetrisation is evident in both the ¹H and ³¹P{¹H} NMR spectra of 1 (δ_31P 63.71/63.65) and 2 (δ_31P 63.6/63.5), alongside perturbation of the macrocycle resonances relative to independently isolated PNP-14·2S (δ_31P 64.7, NMR stack plots provided in the ESI†). The interlocked nature of 1 and 2 was also substantiated by high resolution tandem mass spectrometry experiments,¹³ whereby selective fragmentation of the [M + H]⁺ ions (1, 1584.1321, calcd 1584.1339 m/z; 2, 982.7566, calcd 982.7549 m/z) gave the [M + H]⁺ ion of PNP-14·2S (542.3154/542.3159, calcd 542.3167 m/z) as the major species in both cases.

Conclusions

These results serve as proof of principle for the application of transition mediated terminal alkyne dimerisation reactions in the synthesis of mechanically interlocked molecules, whilst also demonstrating how phosphine-based functional groups can be integrated into the structure of these intriguing molecules.

Experimental section

All manipulations were performed under an atmosphere of argon using Schlenk and glove box techniques unless otherwise stated. Glassware was oven dried at 150 °C overnight and flame-dried under vacuum prior to use. Molecular sieves were activated by heating at 300 °C in vacuo overnight. Fluorobenzene and 1,2-difluorobenzene (DFB) were pre-dried over Al₂O₃, distilled from calcium hydride and dried twice over 3 Å molecular sieves.¹⁰ CD₂Cl₂ was freeze–pump–thaw degassed and dried over 3 Å molecular sieves. THF and dioxane were distilled from sodium/benzophenone and stored over 3 Å molecular sieves. DMSO was freeze–pump–thaw degassed and dried up to five times and finally stored over 3 Å molecular sieves. SiMe₄ was distilled from liquid Na/K alloy and stored over a potassium mirror. Other anhydrous solvents were purchased from Acros Organics or Sigma-Aldrich, freeze–pump–thaw degassed and stored over 3 Å molecular sieves. PMe₃ in toluene and 1,6-dibromohexane were freeze–pump–thawed degassed and dried twice over 3 Å molecular sieves before use. Schrock's catalyst (CAS: 139220-25-0) was recrystallised from SiMe₄ at −30 °C before use. nBuLi was titrated before use.¹⁴ Br(CH₂)₆C(4-tBuC₆H₄)₃,¹⁵ 15-bromo-1-pentadecene,¹⁶ [Rh(COD)₂][BAr^F₄],¹⁷ and PNP-14 ⁷ were synthesized according to published procedures. All other reagents are commercial products and were used as received. NMR spectra were recorded on Bruker spectrometers under argon at 298 K unless otherwise stated. Chemical shifts are quoted in ppm and coupling constants in Hz. NMR spectra in DFB were recorded using an internal capillary of C₆D₆. High resolution (HR) ESI-MS and MS/MS were recorded on Bruker Maxis Plus instrument. Microanalysis was performed at the London Metropolitan University by Stephen Boyer.

Preparation of HC≡C(CH₂)₆C(4-tBuC₆H₄)₃

A suspension of Br(CH₂)₆C(4-tBuC₆H₄)₃ (290 mg, 504 μmol) in DMSO (5 mL) was treated dropwise with THF until homogeneous. A suspension of HC≡CLi·H₂N(CH₂)₂NH₂ (51.0 mg, 554 μmol) in THF (5 mL) was then added and the resulting suspension heated at 130 °C for 48 h. The reaction was quenched by addition of H₂O (2 mL) and the product extracted into CH₂Cl₂ (3 × 5 mL). The combined organic extracts were washed with brine (2 × 10 mL), dried over MgSO₄ and then concentrated in vacuo to give an oily white solid. Purification using a silica plug (hexane → 1 : 1 hexane : CH₂Cl₂) afforded the product as a white solid. Yield: 220 mg (422 μmol, 84%; mp. 142–143 °C).

¹H NMR (500 MHz, CDCl₃): δ 7.24 (d, ³J_HH = 8.4, 6H, m-Ar), 7.14 (d, ³J_HH = 8.4, 6H, o-Ar), 2.48–2.52 (m, 2H, Ar₃CCH̲₂), 2.13 (td, ³J_HH = 7.1, ⁴J_HH = 2.6, 2H, CH̲₂C≡CH), 1.91 (t, ⁴J_HH = 2.6, 1H, C≡CH), 1.44 (pent, ³J_HH = 7.1, 2H, CH̲₂CH₂C≡CH), 1.25–1.36 (m, 4H, 2×CH₂), 1.30 (s, 27H, tBu), 1.05–1.12 (m, 2H, CH₂).

¹³C{¹H} NMR (126 MHz, CDCl₃): δ 148.2 (s, tBuC̲), 145.0 (s, i-Ar), 129.0 (s, o-Ar), 124.5 (s, m-Ar), 84.9 (s, C̲≡CH), 68.2 (s, C≡C̲H), 55.5, (s, Ar₃C̲), 40.7 (s, Ar₃CC̲H₂), 34.4 (s, tBu{C}), 31.5 (s, tBu{CH₃}), 30.1 (s, CH₂), 28.8 (s, CH₂), 28.7 (s, C̲H₂CH₂C≡CH), 25.7 (s, CH₂), 18.5 (s, C̲H₂C≡CH).

HR ESI-MS (positive ion 4 kV): 559.3684 ([M + K]⁺, calcd 559.3701) m/z.

Preparation of HC≡C(CH₂)₁₃CH=CH₂

A suspension of 15-bromo-1-pentadecene (1.22 g, 4.24 mmol) and HC≡CLi·H₂N(CH₂)₂NH₂ (0.41 g, 4.45 mmol) in 1,4-dioxane-DMSO (10 : 5 mL) was stirred at 120 °C for 16 h. The reaction was quenched by addition of H₂O (15 mL) and product extracted into hexane (3 × 15 mL). The combined organic extracts were washed with brine (2 × 10 mL), dried over MgSO₄ and then concentrated in vacuo to give an off-white oily wax. The analytically pure compound was obtained as a colourless wax by repeated column chromatography (silica, hexane; R_f = 0.37). Yield: 244 mg (1.04 mmol, 25%; mp. 43–48 °C).

¹H NMR (500 MHz, CDCl₃): δ 5.81 (ddt, ³J_HH = 16.9, 10.2, 6.7, 1H, H₂C=CH̲), 4.99 (ddt, ³J_HH = 16.9, ²J_HH = 2.2, ⁴J_HH = 1.6, 1H, H̲₂C=CH), 4.93 (ddt, ³J_HH = 10.2, ²J_HH = 2.2, ⁴J_HH = 1.3, 1H, H̲₂C=CH), 2.18 (td, ³J_HH = 7.1, ⁴J_HH = 2.6, 2H, CH̲₂C≡CH), 2.01–2.07 (m, 2H, H₂C=CHCH̲₂), 1.94 (t, ⁴J_HH = 2.6, 1H, C≡CH̲), 1.52 (pent, ³J_HH = 7.6, 2H, CH̲₂CH₂C≡CH), 1.33–1.43 (m, 4H, 2×CH₂), 1.22–1.33 (m, 16H, 8×CH₂).

¹³C{¹H} NMR (126 MHz, CDCl₃): δ 139.4 (s, H₂C=C̲H), 114.2 (s, H₂C̲=CH), 85.0 (s, C̲CH), 68.2 (s, C≡C̲H), 34.0 (s, CH₂=CHC̲H₂), 29.80 (s, 2×CH₂), 29.76 (s, 2×CH₂), 29.7 (s, 2×CH₂), 29.31 (s, CH₂), 29.27 (s, CH₂), 29.1 (s, CH₂), 28.9 (s, CH₂), 28.7 (s, CH₂), 18.6 (s, C̲H₂C≡CH).

Anal. calcd for C₁₇H₃₀ (234.43 g mol⁻¹): C, 87.10; H, 12.90; N, 0.00. Found: C, 86.99; H, 13.02; N, 0.00.

Preparation of rotaxane 1

A solution of [Rh(PNP-14)(η²-COD)][BAr^F₄] (8.3 μmol, generated in situ from [Rh(COD)₂][BAr^F₄] and PNP-14 as previously described⁹) in DFB (0.5 mL) was treated with HC≡C(CH₂)₆C(4-tBuC₆H₄)₃ (9.5 mg, 18.2 μmol) and stirred at RT for 5 min. Volatiles were removed in vacuo to afford 3 as an orange foam upon removal of volatiles. Crude 3 was then dissolved in DFB (330 μL) and treated with a solution of PMe₃ in toluene (170 μL, 1 M, 170 μmol) and the resulting solution heated at 80 °C for 5 days. Volatiles were removed in vacuo and the residue extracted with hexane. The resulting orange oil was treated with S₈ (12.6 mg, 49.1 μmol) in DFB (0.5 mL) and stirred at RT for 16 h. Finally, removal of the volatiles in vacuo and purification by preparative TLC (silica; 9 : 1 CH₂Cl₂ : MeOH; R_f = 0.4–0.5) afforded the product as a white solid. Yield: 4.2 mg (2.7 μmol, 32%; mp 112 °C).

Data for 3

¹H NMR (500 MHz, CD₂Cl₂, selected data): δ 7.76 (t, ³J_HH = 7.9, 1H, p-py), 7.33 (d, ³J_HH = 7.9, 1H, m-py), 7.31 (d, ³J_HH = 7.9, 1H, m-py), 6.95 (dt, ³J_HH = 14.6, ³J_HH = 6.9, 1H, C≡CCH=CH̲), 5.94 (d, ³J_HH = 15.3, 1H, C≡CC̲H=CH), 1.31 (s, 12H, tBuC), 1.30 (s, 38H, tBuC), 0.93 (d, ³J_PH = 12.3, 9H, PtBu), 0.89 (d, ³J_PH = 12.3, 9H, PtBu).

³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 58.6 (dd, ²J_PP = 394, ¹J_RhP = 129, 1P), 54.9 (dd, ²J_PP = 394, ¹J_RhP = 127, 1P).

¹H NMR (400 MHz, DFB, selected data): δ 7.55 (t, ³J_HH = 8.0, 1H, p-py), 5.99 (d, ³J_HH = 15.0, 1H, C≡CCH̲=CH), 1.12–1.17 (m, 54H, tBuC), 0.82–0.89 (m, 18H, PtBu).

³¹P{¹H} NMR (162 MHz, DFB): δ 56.1 (dd, ²J_PP = 393, ¹J_RhP = 129, 1P), 52.5 (dd, ²J_PP = 393, ¹J_RhP = 127, 1P).

Data for 1′

³¹P{¹H} NMR (162 MHz, PhF, selected data): δ 2.42 (s, 1P), 0.92 (s, 1P).

Data for 1

¹H NMR (500 MHz, CDCl₃): δ 7.45 (d, ³J_HH = 7.6, 1H, m-py), 7.41 (t, ³J_HH = 7.6, 1H, p-py), 7.24 (d, ³J_HH = 8.6, 6H, m-Ar), 7.23 (d, ³J_HH = 8.6, 6H, m-Ar), 7.13 (d, ³J_HH = 8.3, 12H, 2×o-Ar), 7.12 (obscured, 1H, m-py), 6.17 (dt, ²J_HH = 16.0, ³J_HH = 6.9, 1H, C≡CCH=CH̲), 5.95 (dt, ²J_HH = 16.0, ⁴J_HH = 2.0, 1H, C≡CCH̲=CH), 3.84 (app t, ²J_PH = ²J_HH = 13.9, 1H, pyCH̲₂), 3.60 (dd, ²J_HH = 14.1, ²J_PH = 11.1, 1H, pyCH̲₂), 3.37 (app t, ²J_PH = ²J_HH = 13.7, 1H, pyCH̲₂), 3.35 (dd, ²J_HH = 13.8, ²J_PH = 11.3, 1H, pyCH̲₂), 2.45–2.53 (m, 4H, 2×Ar₃CCH̲₂), 2.18–2.28 (m, 1H, CH̲₂C≡CCH=CH), 2.02–2.16 (m, 4H, C≡CCH=CHCH̲₂ [δ 2.11, 2H] + PCH₂ [δ 2.08, 1H] + CH̲₂C≡CCH=CH [δ 2.07, 1H]), 1.80–1.94 (m, 5H, CH₂), 1.11–1.68 (m, 34H, CH₂), 1.292 (s, 27H, tBuC), 1.290 (s, 27H, tBuC), 1.24 (d, ³J_PH = 15.6, 18H, 2×PtBu), 0.98–1.11 (m, 4H, 2×Ar₃CCH₂CH̲₂).

¹³C{¹H} NMR (126 MHz, CDCl₃): δ 153.6 (dd, ²J_PC = 6, ⁴J_PC = 1, o-py), 153.5 (d, ²J_PC = 7, o-py), 148.2, (s, tBuC̲), 148.1 (s, tBuC̲), 145.03 (s, i-Ar), 144.97 (s, i-Ar), 143.4 (s, C≡CCH=C̲H), 135.6 (s, p-py), 128.96 (s, o-Ar), 128.95 (s, o-Ar), 124.53 (s, m-Ar), 124.50 (s, m-Ar), 123.4 (br, m-py), 123.3 (br, m-py), 113.3 (s, C≡CC̲H=CH), 92.3 (s, C̲≡CCH=CH), 82.1 (s, C≡C̲CH=CH), 55.43 (s, Ar₃C̲), 55.39 (s, Ar₃C̲), 40.83 (s, Ar₃CC̲H₂), 40.80 (s, Ar₃CC̲H₂), 37.1 (d, ¹J_PC = 42, pyC̲H₂), 36.3 (d, ¹J_PC = 41, pyC̲H₂), 35.3 (d, ¹J_PC = 47, PtBu{C}), 35.2 (d, ¹J_PC = 47, PtBu{C}), 34.4 (s, 2×tBuC̲{C}), 33.3 (s, C≡CCH=CHC̲H₂), 31.6 (s, 2×tBuC̲{CH₃}), 31.2 (d, ²J_PC = 15, 2×CH₂), 30.8 (s, CH₂), 30.6 (s, CH₂), 28.8–30.0 (m, 12×CH₂), 28.2 (d, ¹J_PC = 48, PCH₂), 27.7 (d, ¹J_PC = 47, PCH₂), 26.1 (s, 2×Ar₃CCH₂C̲H₂), 25.8 (s, 2×PtBu{CH₃}), 23.8 (d, ³J_PC = 4, CH₂), 23.2 (d, ³J_PC = 4, CH₂), 21.0 (s, C̲H₂C≡CCH=CH).

³¹P{¹H} NMR (162 MHz, CDCl₃): δ 63.71 (s, 1P), 63.65 (s, 1P).

HR ESI-MS (positive ion, 4 kV): 1584.1321 ([M + H]⁺, calcd 1584.1339) m/z.

HR ESI-MS/MS (positive ion; 120 eV @ +1584): 542.3154 ([PNP-14·2S + H]⁺, calcd 542.3167) m/z.

Preparation of catenane 2

A solution of [Rh(PNP-14)(η²-COD)][BAr^F₄] (10.7 μmol, generated in situ from [Rh(COD)₂][BAr^F₄] and PNP-14 as previously described⁹) in DFB (0.5 mL) was treated with HC≡C(CH₂)₁₃CH=CH₂ (194 μL, 116 mM, 22.5 μmol) and stirred at RT for 5 min. Volatiles were removed in vacuo to afford 4 as an orange oil. Crude 4 was dissolved in fluorobenzene (10 mL) and treated with 25 mol% Schrock's catalyst in 5 mol% portions (0.4 mg, 0.52 μmol) daily over 5 days and periodically assayed by ESI-MS. Volatiles were removed in vacuo to afford 5 as an orange oil. Crude 5 was then dissolved in DFB (300 μL) and treated with a solution of PMe₃ in toluene (200 μL, 1 M, 200 μmol) and the resulting solution heated at 80 °C for 5 days. Volatiles were removed in vacuo and the residue extracted with hexane. The resulting orange oil was treated with S₈ (12.6 mg, 49.1 μmol) in DFB (0.5 mL) and stirred at RT for 16 h. Finally, removal of the volatiles in vacuo and purification by preparative TLC (silica; 9 : 1 CH₂Cl₂ : MeOH; R_f = 0.4–0.5) afforded the product as a colourless oil. Yield: 3.7 mg (3.8 μmol, 38%).

Data for 4

¹H NMR (500 MHz, CD₂Cl₂, selected data): δ 7.77 (t, ³J_HH = 7.8, 1H, p-py), 7.36 (overlapping d, ³J_HH = 7.9, 2H, m-py), 7.01 (dt, ³J_HH = 14.6, ³J_HH = 6.9, 1H, C≡CCH=CH̲), 5.98 (d, ³J_HH = 15.3, 1H, C≡CCH̲=CH), 5.82 (ddt, ³J_HH = 16.8, ³J_HH = 9.8, ³J_HH = 6.7, 2H, CH̲=CH₂), 4.98 (d, J_HH = 17, 2H, CH=CH̲₂), 4.91 (d, J_HH = 10, 2H, CH=CH̲₂), 3.37–3.51 (m, 4H, pyCH̲₂), 0.97 (d, ³J_PH = 12.3, 9H, tBu), 0.91 (d, ³J_PH = 12.2, 9H, tBu).

³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 56.9 (dd, ²J_PP = 394, ¹J_RhP = 129, 1P), 53.1 (dd, ²J_PP = 394, ¹J_RhP = 127, 1P).

¹H NMR (400 MHz, DFB, selected data): δ 8.09–8.15 (m, 8H, Ar^F), 7.53 (obscured t, ³J_HH = 8.2, p-py), 7.49 (br, 4H, Ar^F), 6.04 (d, ³J_HH = 15.4, 1H, C≡CCH̲=CH), 5.82 (dt, ³J_HH = 15.4, ³J_HH = 8.3, CH̲=CH₂), 4.80–4.98 (m, 2H, CH=CH̲₂), 0.87 (app t, J_PH = 12.8, 18H, tBu).

³¹P{¹H} NMR (121 MHz, DFB): δ 56.5 (dd, ²J_PP = 394, ¹J_RhP = 128, 1P), 52.1 (dd, ²J_PP = 394, ¹J_RhP = 127, 1P).

HR ESI-MS (positive ion, 4 kV): 1048.7587, ([M]⁺, calcd 1048.7398) m/z.

Data for 5

¹H NMR (400 MHz, CD₂Cl₂, selected data): δ 7.79 (t, ³J_HH = 8.3, 1H, p-py), 7.36 (br d, ³J_HH = 7.3, 2H, m-py), 7.03 (br, 1H, C≡CCH=CH̲), 6.00 (d, ³J_HH = 16, 1H, C≡CCH̲=CH), 5.39 (br, 2H, CH=CH), 3.43 (br, 4H, pyCH̲₂), 0.98 (d, ³J_PH = 11, 9H, PtBu), 0.92 (d, ³J_PH = 12, 9H, PtBu).

³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 56.7 (d, ²J_PP = 394, ¹J_RhP = 129, 1P), 53.3 (d, ²J_PP = 393, ¹J_RhP = 127, 1P).

¹H NMR (400 MHz, DFB, selected data): δ 8.09–8.15 (m, 8H, Ar^F), 7.49 (br, 4H, Ar^F), 6.01 (br d, ³J_HH = 14.4, 1H, C≡CCH̲=CH), 5.35 (br, 2H, CH=CH), 3.30 (br, 4H, pyCH̲₂), 0.88 (br d, ³J_PH = 12, 9H, tBu), 0.83 (br d, ³J_PH = 10, 9H, tBu).

³¹P{¹H} NMR (121 MHz, DFB): δ 56.6 (d, ²J_PP = 394, ¹J_RhP = 129, 1P), 52.5 (d, ²J_PP = 393, ¹J_RhP = 127, 1P).

HR ESI-MS (positive ion, 4 kV): 1020.7092, ([M]⁺, calcd 1020.7085) m/z.

HR ESI-MS2 (positive ion, 70 eV @ +1020): 578.2543 ([{Rh(PNP-14)}-H₂]⁺, calcd 578.2546) m/z.

Data for 2′

³¹P{¹H} NMR (162 MHz, DFB, selected data): δ 1.46 (s, 1P), 0.78 (s, 1P).

Data for 2

¹H NMR (500 MHz, CDCl₃): δ 7.45–7.54 (m, 2H, py), 7.20 (br d, ³J_HH = 5.2, 1H, py), 6.22 (dt, ³J_HH = 16.0, ³J_HH = 7.0, 1H, C≡CCH=CH̲), 5.98 (dt, ³J_HH = 16.1, ⁴J_HH = 2.0, 1H, C≡CCH̲=CH), 5.36–5.39 (m, 2H, CH=CH), 3.88 (app t, J_PH = J_HH = 14, 1H, pyCH̲₂), 3.59–3.69 (m, 1H, CH₂), 3.41 (app t, J_PH = J_HH = 13, 2H, pyCH̲₂), 2.04–2.41 (m, 6H, CH₂), 1.89–1.99 (m, 9H, CH₂), 1.38–1.47 (obscured m, ∼16H, CH₂), 1.25–1.35 (m, ∼65H, CH₂ + PtBu).

¹³C{¹H} NMR (126 MHz, CDCl₃): δ 153.5–153.8 (m, py), 143.3 (s, C≡CCH=C̲H), 135.7 (s, py), 130.54 (s, CH=CH), 130.46 (s, CH=CH), 123.4 (s, py), 123.3 (s, py), 113.6 (s, C≡CC̲H=CH), 92.5 (s, C̲≡CCH=CH), 82.1 (s, C≡C̲CH=CH), 37.2 (d, ¹J_PC = 42, pyC̲H₂), 36.5 (d, ¹J_PC = 41, pyC̲H₂), 35.5 (d, ¹J_PC = 23, PtBu{C}), 35.1 (d, ¹J_PC = 24, PtBu{C}), 32.8 (s, CH₂), 32.1 (s, CH₂), 32.4 (s, CH₂), 31.2 (s, CH₂), 29.9 (s, CH₂), 29.2–29.8 (m, multiple CH₂), 29.2 (s, CH₂), 29.12 (s, CH₂), 29.09 (s, CH₂), 29.07 (s, CH₂), 28.88 (s, CH₂), 28.86 (s, CH₂), 28.7 (s, CH₂), 28.3 (s, CH₂), 27.9 (s, CH₂), 27.6 (s, CH₂), 25.77 (s, PtBu{CH₃}), 25.75 (s, PtBu{CH₃}), 23.8 (d, ³J_PC = 4, CH₂), 23.3 (d, ³J_PC = 4, CH₂), 22.9 (s, CH₂), 20.8 (s, CH₂).

³¹P{¹H} NMR (121 MHz, CDCl₃): δ 63.6 (s, 1P), 63.5 (s, 1P).

HR ESI-MS (positive ion, 4 kV): 982.7566, ([M + H]⁺, calcd 982.7549) m/z.

HR ESI-MS2 (positive ion, 60 eV @ +982): 542.3159 ([PNP-14·2S + H]⁺, calcd 542.3167) m/z.

Preparation of PNP-14·2S

A solution of PNP-14 (8.5 mg, 17.8 μmol) in DFB (0.5 mL) was treated with S₈ (1.2 mg, 4.68 μmol) and stirred at RT for 16 h. Volatiles were removed, and the resulting residue washed with methanol (2 × 0.5 mL) and then dried in vacuo to afford the product as a white microcrystalline solid. Yield: 9.4 mg (17.3 μmol, 97%; mp. 139–140 °C).

¹H NMR (500 MHz, CDCl₃): δ 7.59 (t, ³J_HH = 7.8, 1H, p-py), 7.35 (d app t, ³J_HH = 7.8, J_PH = 2, 2H, m-py), 3.49 (app t, ²J_PH = ²J_HH = 13, 2H, pyCH̲₂), 3.43 (app t, ²J_PH = ²J_HH = 14, 2H, pyCH̲₂), 1.99–2.10 (m, 2H, PCH₂), 1.68–1.86 (m, 4H, PCH₂ [δ 1.80, 2H] + CH₂), 1.24–1.58 (m, 22H, CH₂), 1.17 (d, ³J_PH = 15.9, 18H, tBu).

¹³C{¹H} NMR (126 MHz, CDCl₃): δ 153.7 (dd, ²J_PC = 8, ⁴J_PC = 2, o-py), 136.6 (t, ⁴J_PC = 2, p-py), 123.4 (app t, J_PC = 3, m-py), 38.9 (d, ¹J_PC = 39, pyC̲H₂), 34.8 (d, ¹J_PC = 47, tBu{C}), 30.2 (d, ²J_PC = 15, CH₂), 27.9 (s, CH₂), 27.80 (s, CH₂), 27.79 (s, CH₂), 27.7 (s, CH₂), 26.3 (d, ¹J_PC = 47, PCH₂), 25.3 (s, tBu{CH₃}), 22.4 (d, ³J_PC = 4, CH₂).

³¹P{¹H} NMR (162 MHz, CDCl₃): δ 64.7 (s).

HR ESI-MS (positive ion, 4 kV): 542.3160 ([M + H]⁺, calcd 542.3167) m/z.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the European Research Council (ERC, grant agreement 637313) and Royal Society (UF100592, UF150675, A. B. C.) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Characterisation data and NMR stack plots. CCDC 2063081. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ra00566j

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