A trefoil-shaped macrocycle with [12]-imidazolium cations

A trefoil-shaped macrocycle with [12]-imidazolium cations

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Le Zhang, Li-Ying Sun, Jin-Ping Chang, Hui-Yu Xie, Ya-Wen Zhang, Yi-Fan Zhang, Ying-Feng Han^*

Chinese Chemical Letters | 2022, 33(10) : 4567 - 4571

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Chinese Chemical Letters | 2022, 33(10): 4567-4571

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A trefoil-shaped macrocycle with [12]-imidazolium cations

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Le Zhang, Li-Ying Sun, Jin-Ping Chang, Hui-Yu Xie, Ya-Wen Zhang, Yi-Fan Zhang, Ying-Feng Han^*

Affiliations

Key Laboratory of Synthetic and Natural Functional Molecule of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an 710127, China

Published: 2022-10-15 doi: 10.1016/j.cclet.2022.01.064

Outline

Abstract

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A trefoil-like two-dimensional (2D) C_3v symmetric organic [12]-imidazolium cation H₁₂–2(PF₆)₁₂ featuring three [4]-imidazolium macrocycles was synthesized in three steps. The reaction of a dodecakis H₁₂–1(PF₆)₁₂ imidazolium salt with Ag₂O resulted in the formation of a hexanuclear Ag^Ⅰ dodecacarbene assembly [Ag₆(1)](PF₆)₆. Upon UV irradiation, the photodimerization of the cinnamic ester pendants of [Ag₆(1)](PF₆)₆ led to the generation of a trefoil-like complex [Ag₆(2)](PF₆)₆ containing three closed metallacycles. Removal of metal ions allowed for the synthesis of the target molecule. All complexes were fully characterized by NMR spectroscopy (¹H, ¹³C{¹H} and 2D NMR) and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS).

Key words

Trefoil-shaped macrocycle / N-Heterocyclic carbenes / Photochemical reaction / Imidazolium

Cite this Article

Le Zhang, Li-Ying Sun, Jin-Ping Chang, Hui-Yu Xie, Ya-Wen Zhang, Yi-Fan Zhang, Ying-Feng Han. A trefoil-shaped macrocycle with [12]-imidazolium cations[J]. Chinese Chemical Letters, 2022 , 33 (10) : 4567 -4571 . DOI: 10.1016/j.cclet.2022.01.064

Full Text

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The discrete macrocyclic compounds, which contain both artificial metal-macrocycles (triangles, rhomboids, squares, hexagons, etc.) [1-12] and non-metal-macrocycles (crown ethers, cyclodextrins, cyclophanes, pillararenes, etc.) [13-18], have made significant progress in the application of host-guest chemistry, selective absorption/recognition, molecular electronic devices and supramolecular catalysis fields over the past decades [19-24]. Among these macrocyclic compounds, the polyimidazolium cyclophanes have drawn considerable attention as unique receptors due to the ionic hydrogen bond (C-H)⁺···X⁻ between the imidazolium units and guests recently [23, [25-31]. The polyimidazolium cyclophanes can also be utilized as precursors for the formation of functionalized assemblies bearing N-heterocyclic carbenes (NHCs) [32-47].

So far, attempts to make [n]-polyimidazolium-based (n > 4) cyclophanes by one-pot macrocyclization or stepwise macrocyclization processes have proved unsuccessful [48-50], probably due to the multiple incorporations of the dihaloalkanes and bis-imidazole precursors with required connectivity becomes more difficult as reactive sites increases. For example, Beer and co-workers isolated an organic polyimidazolium cation featuring 8 internal imidazolium units only as few crystals incidentally [48]. To overcome this limitation, our group developed a metal-carbene template approach and a series of three-dimensional [n]-imidazolium (n = 6, 12, 16 or 18) cages were controllably constructed [51-55].

In addition, most of the known polyimidazolium cationic compounds only have one macrocyclic unit of polyimidazolium. It should be noted that the increase in the number of macrocycles can bring potential properties [56-60]. For example, Stoddart and co-workers reported a trefoil-like rotaxane, which was used for a molecule elevator [57]. Therefore, the design and synthesis of the polyimidazolium-based compounds featuring more than one macrocyclic unit are imminent and challenging.

Based on these results, we attempted to construct a trefoil-like 2D C_3v symmetric organic [12]-imidazolium cation featuring three [4]-imidazolium macrocycles. Unfortunately, direct synthesis of compound Ⅳ by the one-pot method using the olefin-modified dodecakisimidazolium salt Ⅰ as the precursor failed after various trials. Here, we reported an alternative method by using the precursor Ⅰ to react with Ag₂O to generate the hexanuclear Ag^Ⅰ assembly Ⅱ. Then photochemically induced [2+2] cycloaddition reaction was performed with complex Ⅱ, thereby leading to a trefoil-like complex containing three closed metallacycles. Finally, all Ag^Ⅰ ions in complex Ⅲ could be eventually removed to provide the target compound Ⅳ (Scheme 1).

The preparation of 1, 2, 3, 4, 5, 6-hexakis(4-(1H-imidazol-1-yl)phenyl)benzene (L) and N-cinnamic-ester-appended dodecakisimidazolium precursor H₁₂–1(PF₆)₁₂ is shown in Scheme 2. Firstly, the compound L was obtained from imidazole and 1, 2, 3, 4, 5, 6-hexakis(4-iodophenyl)benzene [61] by the solid-state reaction using an Ullmann coupling protocol (Fig. S1 in Supporting information) [62, 63]. Subsequently, compound L and N-p-bromomethylbenzene-N'-cinnamic acid methyl ester imidazolium bromide was mixed to yield the corresponding dodecakisimidazolium precursor H₁₂–1(Br)₁₂, followed by the anion exchanged with NH₄PF₆ in CH₃OH to form hexafluorophosphate H₁₂–1(PF₆)₁₂ in 81% yield as a white solid. The hexafluorophosphate salt of H₁₂–1(PF₆)₁₂ is excellently soluble in DMSO, DMF, and CH₃CN, whereas almost insoluble in ether and dichloromethane.

The formation of the dodecakisimidazolium precursor H₁₂–1(PF₆)₁₂ was verified by nuclear magnetic resonance (NMR) spectroscopy (¹H, ¹³C{¹H}, and 2D NMR) and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) (Figs. S2-S7 in Supporting information). The ¹H NMR spectrum of H₁₂–1(PF₆)₁₂ in CD₃CN reveals two characteristic resonances for the C2-H protons on imidazolium at δ 9.00 and 8.84 ppm, respectively (Fig. 1a). HR-ESI-MS spectrum provided further support for the formation of dodecakisimidazolium salt. The peaks of the highest intensity for H₁₂–1(PF₆)₁₂ were discovered at m/z 632.4934 (calcd. for [H₁₂–1+6PF₆]⁶⁺ 632.5114), m/z 1021.2212 (calcd. for [H₁₂–1+8PF₆]⁴⁺ 1021.2495).

The reaction of H₁₂–1(PF₆)₁₂ with an excess of Ag₂O generated the 2D C_3v symmetric hexanuclear Ag^Ⅰ dodecacarbene assembly [Ag₆(1)](PF₆)₆ in 79% yield (Scheme 3), the compound [Ag₆(1)](PF₆)₆ contains two triangular metal silver ions form an inner Ag₃ triangle inserted inside a larger outer Ag₃ arrangement. The reaction was achieved in CH₃CN under the exclusion of light, and the carbene complex was separated as a white solid after workup. The formation of the hexanuclear silver complex was unambiguously confirmed by NMR and HR-ESI-MS studies (Figs. S8-S14 in Supporting information). In the ¹H NMR spectra of complex [Ag₆(1)](PF₆)₆, the characteristic resonance of the C2-H protons of H₁₂–1(PF₆)₁₂ at δ 9.00 and 8.84 ppm were found to have disappeared (Fig. 1b). The ¹³C{¹H} NMR spectra in CD₃CN showed the two expected C_NHC signals at δ 182.1 and 180.7 ppm, accompanied by two slightly different imidazolium rings. These values are within the normal range compared with previously reported for such derivatives [64-88]. Diffusion-ordered NMR spectroscopy (DOSY) confirmed the formation of a signal configuration. All proton signals of complex [Ag₆(1)](PF₆)₆ displayed a closely related diffusion coefficient for D = 6.46 × 10⁻¹⁰ m²/s (logD = -9.19). The HR-ESI-MS spectrum further demonstrated the formation of hexanuclear silver complex. For example, the HR-ESI-MS spectrum for [Ag₆(1)](PF₆)₆ appeared two peaks at m/z 593.4065 (calcd for [Ag₆(1)]⁶⁺ 593.4365), m/z 741.0843 (calcd for [Ag₆(1)+PF₆]⁵⁺ 741.1168), these theoretical isotopic distributions are consistent with the experimental peaks (Fig. S14 in Supporting information). Unfortunately, all attempts to obtain single crystals of the hexanuclear silver complex [Ag₆(1)](PF₆)₆ used for X-ray analysis failed. However, based on the related assemblies [56-60], the 2D C_3v symmetric structure [Ag₆(1)](PF₆)₆ could be speculated in Scheme 3.

The photochemical reaction of the N-cinnamic-ester-appended hexanuclear Ag^Ⅰ dodecacarbene assembly [Ag₆(1)](PF₆)₆ was subsequently investigated (Scheme 3). The complex [Ag₆(1)](PF₆)₆ was treated with a Philips high-pressure mercury lamp (λ = 365 nm) in CD₃CN solvent at ambient temperature converted exclusively into the organometallic complex [Ag₆(2)](PF₆)₆. The photochemical [2+2] cyclization reaction was finished within 24 h and the conversion was easily demonstrated by ¹H, ¹³C{¹H} NMR spectroscopy.

The formation of the hexanuclear complex [Ag₆(2)](PF₆)₆ featuring three cyclobutanes bridged was monitored by NMR spectroscopy (Figs. S15-S20 in Supporting information). Before irradiation, the ¹H NMR spectrum from the olefin protons of [Ag₆(1)](PF₆)₆ features characteristic two doublet resonances of the cinnamic ester groups at δ 7.56 (³J = 16.1 Hz, partly overlapped by other resonances) and 6.46 (³J = 16.1 Hz) ppm (in CD₃CN, Fig. 1b). The ¹H NMR spectrum revealed that olefin protons resonances of H₁₂–1(PF₆)₁₂ (Fig. 1a) slightly shifted upfield after producing to [Ag₆(1)](PF₆)₆. The ¹³C{¹H} NMR spectrum of [Ag₆(1)](PF₆)₆ features signals for the C=C carbon atoms were observed at δ 143.4 and 120.2 ppm (Fig. S9 in Supporting information). After UV-irradiation (λ = 365 nm) for 24 h, the intensity of the olefin doublet resonances completely vanished and two new multiplets for the cyclobutane protons of [Ag₆(2)](PF₆)₆ generated at δ 4.43 and 4.03 (both d, ³J = 5.51 Hz) ppm (in CD₃CN, Fig. 1c), while two characteristic peaks for cyclobutane carbons were also found at δ 52.7 and 46.1 ppm (Fig. S16 in Supporting information). Additionally, the formation of [Ag₆(2)](PF₆)₆ featuring three cyclobutanes have demonstrated by the HR-ESI-MS spectrum, showed peaks of highest intensity at m/z 593.4272 (calcd for [Ag₆(2)]⁶⁺ 593.4365), m/z 741.1054 (calcd. for [Ag₆(2)+PF₆]⁵⁺ 741.1168) (Fig. S21 in Supporting information). Subsequently, UV–vis spectra also clearly revealed the disappearance of the characteristic absorption peaks at around 280 nm, attributed to the C=C double bonds (Fig. S29 in Supporting information). The photochemical [2+2] cycloaddition reaction of precursor H₁₂–1(PF₆)₁₂ was performed under the same experimental conditions. However, no target photoproduct was found except the trans-cis photoisomerization as indicated by ¹H NMR spectroscopy (Fig. S22 in Supporting information).

In our previous work, it has been confirmed that the newly generated terminal cyclobutanes can connect two poly-NHC ligands after removing the silver ions [51-55]. Inspired by these results, the next generation of trefoil-like dodecakisimidazolium cation H₁₂–2(PF₆)₁₂ was prepared. The addition of excess NH₄Cl in CH₃OH successfully bring the liberation of the six Ag⁺ ions from hexanuclear complex [Ag₆(2)](PF₆)₆, followed by anion exchange with NH₄PF₆ provided hexafluorophosphate H₁₂–2(PF₆)₁₂ in 82% yield as a white solid (Scheme 3). The trefoil-like dodecakisimidazolium cation H₁₂–2(PF₆)₁₂ was fully characterized by NMR spectroscopy and HR-ESI mass spectrometry (Figs. S23-S28 in Supporting information). In the ¹H NMR spectrum of H₁₂–2(PF₆)₁₂, two resonances were observed at δ 9.10 and 9.03 ppm (Fig. 1d) because of imidazolium groups of dodecakisimidazolium salt H₁₂–2(PF₆)₁₂ have two different chemical environments. This is consistent with the dodecakisimidazolium precursor starting material. The intensity ratio of the cyclobutane protons and the imidazolium protons was checked to be 1:1:1:1, indicating the fully de-metalation of complex [Ag₆(2)](PF₆)₆. All proton signals of H₁₂–2(PF₆)₁₂ showed a relatively narrow interval with a diffusion coefficient of D = 5.62 × 10⁻¹⁰ m²/s (logD = −9.25), verifying that it is a single assembly (Fig. 1e). Moreover, the HR-ESI-MS spectrum of compound H₁₂–2(PF₆)₁₂ (positive-ion mode) revealed peaks of highest intensity at m/z = 438.1339 (calcd for [H₁₂–2+4PF₆]⁸⁺ 438.1424), m/z = 521.4350 (calcd for [H₁₂–2+5PF₆]⁷⁺ 521.4434) (Fig. 2). Furthermore, the addition of 3 equiv. Ag₂O to H₁₂–2(PF₆)₁₂ resulted in regeneration of the hexanuclear cycloaddition product [Ag₆(2)](PF₆)₆. This reaction was achieved after 24 h at 60 ℃ under the exclusion of light (Scheme 3). The HR-ESI-MS spectrum data were similar to those of [Ag₆(2)](PF₆)₆ obtained from the photochemical cycloaddition of [Ag₆(1)](PF₆)₆ (Fig. S21 in Supporting information).

In summary, based on the rational design of a hexagonal poly-NHC precursor, we described the synthesis of two different discrete 2D C_3v symmetric hexanuclear Ag^Ⅰ dodecacarbene metallacycles and corresponding trefoil-like 2D C_3v symmetric organic [12]-imidazolium cation featuring three [4]-imidazolium macrocycles. To the best of our knowledge, this is the first example of 2D polyimidazolium derivative containing three [4]-imidazolium macrocycles. In addition, this approach potentially prepares newly functional two-dimensional polyimidazolium materials under study in our group.

Declaration of competing interest

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

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The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 22025107, 21722105, 22101225), the National Youth Top-notch Talent Support Program of China, the Key Science and Technology Innovation Team of Shaanxi Province (Nos. 2019TD-007, 2019JLZ-02), and the FM & EM International Joint Laboratory of Northwest University.

Supplementary materials

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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.01.064.

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doi: 10.1016/j.cclet.2022.01.064

Receive Date：2021-11-27
Online Date：2025-12-24
Published：2022-10-15

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Received：2021-11-27
Revised：2022-01-19
Accepted：2022-01-23

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Key Laboratory of Synthetic and Natural Functional Molecule of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi'an 710127, China

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https://castjournals.cast.org.cn/joweb/ccl/EN/10.1016/j.cclet.2022.01.064

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表12种不同金属材料的力学参数

科 Family	属数 Number of genus	种数 Number of species	占总种数比例 Percentage of total species (%)	属 Genus	种数 Number of species	占总种数比例 Percentage of total species (%)
鹅膏菌科Amanitaceae	2	11	5.26	鹅膏菌属 Amanita	10	4.78
小菇科 Mycenaceae	2	12	5.74	丝盖伞属 Inocybe	5	2.39
多孔菌科 Polyporaceae	8	14	6.70	蜡蘑属 Laccaria	5	2.39
红菇科 Russulaceae	3	23	11.00	小皮伞属 Marasmius	6	2.87
				小菇属 Mycena	11	5.26
				光柄菇属 Pluteus	5	2.39
				红菇属 Russula	17	8.13
				栓菌属 Trametes	5	2.39

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Scheme 1 Cartoon presentation of the synthetic strategy for the construction of a trefoil-like dodecakisimidazolium cation.

Scheme 2 Synthesis of compounds L and H₁₂–1(PF₆)₁₂.

Fig. 1 Partial ¹H NMR spectra (400 MHz, 298 K) in CD₃CN of (a) dodecakisimidazolium salt H₁₂–1(PF₆)₁₂; (b) complex [Ag₆(1)](PF₆)₆ before irradiation; (c) complex [Ag₆(2)](PF₆)₆ obtained after irradiation; (d) dodecakisimidazolium salt H₁₂–2(PF₆)₁₂; (e) ¹H DOSY spectrum of H₁₂–2(PF₆)₁₂.

Scheme 3 Preparation of the hexanuclear Ag^Ⅰ dodecacarbene assembly [Ag₆(1)](PF₆)₆, of the organometallic complex [Ag₆(2)](PF₆)₆ and of the dodecakisimidazolium cation H₁₂–2(PF₆)₁₂.

Fig. 2 HR-ESI-MS spectrum (position ion mode) of H₁₂–2(PF₆)₁₂, inset experimental (blue) and calculated (red) isotope distributions for peaks corresponding to [H₁₂–2+4PF₆]⁸⁺ and [H₁₂–2+5PF₆]⁷⁺.

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