Halogen-bonding-induced diverse aggregation of 4,5-diiodo-1,2,3-triazolium salts with different anions

Introduction

The halogen bond (XB) is a noncovalent interaction between electrophilic halides and Lewis bases or electron-rich regions [1,2]. Computational studies [3-7] and cyrstal architectures including XB-donors (σ-hole) such as perfluorocarbons [8-12], tetraiodoethylene [13], 1,2-diiodo-1,2-difluoroethene [14], diiodoacetylene [15] and iodo/bromoethynyl moieties [16] have revealed that the XB-donors interacting with XB-acceptors (a nucleophilic region) are in approximately linear orientation. Besides, linearity, tunability and hydrophobicity (features of the XB ) are widely applied in crystal engineering, supramolecular chemistry, anion recognition, organocatalysis, materials science and tuning of biomolecular systems [17-27]. 1,2,3-Triazole-based XB-donors, such as 5-iodo-1,2,3-triazoles A [28-33] and 5-iodo-1,2,3-triazolium B [34-37] (Figure 1), are promising candidates for XB donors, which is mainly due to the ease of preparation via a copper-catalyzed click reaction between azide and alkyne [38,39]. 1,2,3-Triazoles and 1,2,3-triazolium-based XB activators have been found applications in catalytic reactions [40,41] and anion recognition [42]. Recently, we reported neutral 4-halo-1,2,3-triazolylidenes C [43], which had a carbene character with σ-donation at the carbon and a σ-hole at the halogen atom. XB is observed by single-crystal X-ray diffraction in their coinage metal complexes. Meanwhile, 4-bromo-1,2,3-triazolylidene can catalyze H/D exchange of aldehydes [44]. Despite a variety of XB donors based on 1,2,3-triazole have been reported, no 4,5-diido-1,2,3-triazolium salts have been reported for an XB interaction. Herein, we report the synthesis and characterization of 4,5-diido-1,2,3-triazolium D with different anions. The crystal structures of these compounds show XB interactions between the triazolium moiety and anions, and different aggregations are formed. Triazolium with halide anions exists as tetramers with saddle conformation. Triazolium tetrafluoroborate exists as trimer with Chinese lantern shape conformation. Triazolium trifluoroacetate and acetate exist as dimers, respectively, while the former shows a boat conformation and the latter forms a rectangle conformation. Triazolium salts form a linear polymer with polyiodide.

Results and Discussion

Recently, we found that 4-iodotriazolylidene can be prepared by the treatment of a 4,5-unsubstituted triazolium salt with one equivalent I₂ in the presence of two equivalents of potassium tert-butoxide [43]. When 4,5-unsubstituted triazolium salt 1 was treated with two equivalents I₂ and two equivalents of potassium tert-butoxide, 4,5-diiodo-1,3-dimesityl-1,2,3-triazolium 2-I was synthesized in a good yield (Scheme 1). The product 2-I was characterized by ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry. 2-I has a poor solubility in most organic solvents such as dichloromethane, trichloromethane, tetrahydrofuran, and ethanol. A single crystal of 2-I was obtained by slow diffusion of ether into dimethylformamide solution.

Ion exchange of 2-I with AgBF₄ afforded 2-BF₄. In contrast, 2-BF₄ was soluble in dichloromethane and trichloromethane. A single crystal of 2-BF₄ was obtained by slow diffusion of ether into a dichloromethane solution. In a similar manner, 2-OAc and 2-TFA were obtained via removing the iodide anion by AgOAc and CF₃COOAg. A single crystal of 2-OAc suitable for X-ray diffraction analysis was obtained by slow diffusion of n-pentane into a dichloromethane solution. A single crystal of 2-TFA suitable for X-ray diffraction analysis was obtained by slow evaporation of dichloromethane solution (Scheme 2).

2-Br and 2-Cl were synthesized by ion exchange between 2-BF₄ and the respective potassium halide in acetonitrile. A single crystal of 2-Br was obtained by slow diffusion of n-pentane into a dichloromethane solution. While the single crystal of 2-Cl was obtained by slow evaporation of dichloromethane solution. The treatment of 2-I with 2 equivalents iodine or 4 equivalents iodine afforded triazolium polyiodide 2-I^.1.5I₂ and 2-I^.3.5I₂, respectively. The single crystals were obtained by slow diffusion of ether into a dichloromethane solution.

The crystal X-ray analyses of 2-I, 2-Br and 2-Cl show tetrameric aggregation of the 4,5-diiodo-1,3-dimesityl-1,2,3-triazolium moiety with four anion halides that are bridged together to form a saddle shape through XB (Figure 2). 2-I crystallizes in the tetragonal space group . The distances of I···I are 3.306(1) Å and 3.300(1) Å which are due to the XB. The C–I···I angles are 176.9(3)° and 176.2(3)° (Table 1). The I···I distance is short: the reduction ratio R_XB [45], defined as the ratio of the actual distance over the sum of van der Waals radii, amounts to 0.81. The 2-Br crystal package has a similar package diagram. 2-Br crystallizes in the tetragonal space group . The distances of I···Br are 3.107(2) Å, 3.123(2) Å, the C–I···Br angles are 177.1(4)°, 173.4(6)°, and the R_XB values are 0.80. The crystal 2-Cl has a monoclinic crystal system and the space group is C2. The distances of I···Cl are 2.963(9) Å, 2.989(6) Å, 2.934(7) Å and 2.98(1) Å. The C–I···Cl angles are 176.8(6)°, 173.4(6)°, 176.2(6)° and 175.9(6)°. The R_XB values are 0.77. These crystal package diagrams display that a bent arrangement of the XB donors are around the central halide anion. The measured bent angles of 2-I, 2-Br and 2-Cl by mercury [46] are 146.38(3)°, 144.12(7)° and 145.6(3)°.

The crystal X-ray analyses of 2-BF₄ shows that the diiodotriazolium moiety has formed with the tetrafluoroborate anion a triangle in which only two anions and three cations are assembled and one tetrafluoroborate is independent (Figure 3). 2-BF₄ crystallizes in the triclinic space group .

The single crystal of 2-OAc crystallizes in the monoclinic space group P2₁/c. The package diagram shows a dimer which is almost a rectangle (Figure 4). As shown in Table 1, The C–I···O distances are 2.547(2) Å and 2.582(2) Å. The C–I···O angles are 174.60(7)° and 170.38(7)°. The R_XB values are 0.72 and 0.73.

The single crystal of 2-TFA crystallizes in the monoclinic space group P2₁/n, but the packing structure of 2-TFA is different. The package diagram shows that two cations and two acetates form a boat shape (Figure 5). The C–I···O distances are 2.631(8) Å, 2.739(8) Å, 2.666(6) Å and 2.68(1) Å. The C–I···O angles are 175.7(3)°, 172.7(3)°, 176.7(3)° and 176.1(3)°. The R_XB values are 0.77, 0.74 and 0.75 (Table 1).

Triazolium polyiodide 2-I·1.5I₂ was made by 2-I with iodine and crystallizes in the monoclinic space group P2₁/c. The crystal package diagram displays that a bent arrangement of XB donors are around the central iodine atom of the I₃⁻ anion (Figure 6). 2-I·3.5I₂ crystallizes in the monoclinic space group P2₁/c (Figure 7). There are two I₃⁻ and one I₂ molecule in 2-I·1.5I₂, while two I₅⁻ and three I₂ molucules can be found in 2-I·3.5I₂.

The XB interaction with neural halogen acceptors was also investigated. Diffusion of ether into the mixture of 4,4'-bipyridine (bpy) and 2-BF₄ in dichloromethane leads to the crystallization of 2-BF₄·0.5bpy (Figure 8). It crystallizes in the monoclinic space group P2₁/c. The acceptor 4,4'-bipyridine provides a complementary link for 1D chain formation. The C–I···N angles are 168.2(3)° and 173.4(4)°, close to linear, which is consistent with the high directionality of the interaction. The C–I···N distances are 2.599(9) Å and 2.580(9) Å. The R_XB value of C–I···N is 0.70. The C–I···F distances are 3.137(10) Å and 2.932(20) Å.

To better understand the 1,2,3-triazole based XB donors, model 1,2,3-triazole A, 1,2,3-triazolium B, 1,2,3-triazolylidene complex C-CuI and diiodotriazolium D were calculated by DFT calculations (Figure 9). The calculation results show that σ holes in diiodotriazolium D are mainly located in the elongation of two C–I bonds. The DFT calculation also shows that σ hole of in diiodotriazolium D and 1,2,3-triazolium B are comparable, and much larger than the 1,2,3-triazole A and 1,2,3-triazolylidene complex C-CuI due to positive charge effect.

Conclusion

In summary, we synthesized 4,5-diiodo-1,3-dimesityl-1,2,3-triazolium salts with different anions. When the anion is chloride, bromide or iodide, the crystal is a tetramer. Strong XB was observed in these forms. When the anion is changed to tetrafluoroborate, it takes Chinese lantern shape as a trimer. Triazolium trifluoroacetate and acetate exist as a dimer, while the former shows a boat conformation and the latter forms a rectangle conformation. Triazolium salts form a linear polymer with polyiodide. 2-BF₄ forms co-crystals with 4,4'-bipyridine via halogen bonding. DFT calculation results show that the σ holes of 4,5-diiodo-1,2,3-triazolium is similar to the σ hole of 5-iodo-1,2,3-triazoliums salts.

Experimental

General

Unless otherwise noted, all reagents were obtained from commercial sources and used without further purification. All solvents were dried then stored over 4 Å molecular sieves prior to use. All syntheses were carried out under an atmosphere of dry nitrogen or in a glovebox. At the same time, the syntheses were performed under a standard Schlenk vacuum line. ¹H NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. High-resolution mass spectra (HRMS) were acquired with a Thermo Scientific (Q-Exactive) instrument using electrospray ionization mode (ESI). Elemental analyses (C, H, N) were performed on Flash EA 1112 Analyzer.

Synthesis

2-I: 4,5-Unsubstituted triazolium salt 1 (450 mg, 1 mmol), potassium tert-butoxide (250 mg, 2.2 mmol) and I₂ (510 mg, 2 mmol) were added in a Schlenk tube under nitrogen, then THF (20 mL) was added at −78 °C. The mixture was stirred for 12 hours. After the evaporation of THF, dichloromethane (100 mL) was added and inorganic salts were removed by filtration. The pure product (521 mg) was obtained by evaporation of dichloromethane and washed with ether. The yield was 76%. A single crystal of 2-I was obtained by slow diffusion of ether into a dimethylformamide solution due to the poor solubility. ¹H NMR (400 MHz, CDCl₃) δ 7.07 (s, 4H), 2.41 (s, 6H), 2.00 (s, 12H); ¹³C NMR (100 MHz, DMSO-d₆) 143.2, 134.9, 131.9, 130.2, 111.8, 21.2, 17.1; HRMS (m/z): [M – I⁻]⁺ calcd for C₂₀H₂₂I₂N₃⁺, 557.9898; found, 557.9891; anal. calcd. for C₂₀H₂₂I₃N₃ (685.13): C, 35.06, H, 3.24, N, 6.13%; found: C, 35.41, H, 3.32, N, 6.06%.

2-BF₄: AgBF₄ (60 mg, 0.30 mmol) and 2-I (205 mg, 0.30 mmol) were added in a Schlenk tube, then dichloromethane (5 mL) was added and the solution stirred under nitrogen in the dark for 6 h. After AgI was removed by filtration, the solution was washed with water. The pure product was obtained by evaporation of the dichloromethane phase (178 mg, 92% yield). A single crystal of 2-BF₄ was obtained by slow diffusion of ether into a dichloromethane solution. ¹H NMR (400 MHz, CDCl₃) δ 7.12 (s, 4H), 2.40 (s, 6H), 2.02 (s, 12H); ¹³C NMR (100 MHz, CDCl₃) δ 134.8, 134.2, 131.5, 130.3, 104.4, 12.4, 17.3; anal. calcd. for C₂₀H₂₂BF₄I₂N₃ (645.03): C, 37.24, H, 3.44, N, 6.51%; found: C, 37.51, H, 3.52, N, 6.42%.

2-Cl: Potassium chloride (740 mg, 10 mmol) and 2-BF₄ (120 mg, 0.19 mmol) were mixed in acetonitrile (30 mL) in a round bottom flask, then the mixture was stirred for 24 h under air. Then the acetonitrile was removed under reduced pressure. Dichloromethane (50 mL) was added and the excess potassium chloride was removed by filtration. Then the dichloromethane was removed by evaporation to give the final white product (2-Cl) (99 mg, 87% yield). ¹H NMR (400 MHz, CDCl₃) δ 7.05 (s, 4H), 2.39 (s, 6H), 1.98 (s, 12H); ¹³C NMR (100 MHz, CDCl₃) δ 142.9, 134.2, 132.1, 130.1, 113.2, 21.4, 17.4; anal. calcd. for C₂₀H₂₂ClI₂N₃ (593.68): C, 40.46, H, 3.74, N, 7.08%; found: C, 40.81, H, 3.79, N, 6.96%. A single crystal of 2-Cl was obtained by slow evaporation of a dichloromethane solution.

2-Br: Potassium bromide (1190 mg, 10 mmol) and 2-BF₄ (128 mg, 0.2 mmol) were mixed in acetonitrile (30 mL) in a round bottom flask, then the mixture was stirred under air for 24 h. Then the acetonitrile was removed under reduced pressure, dichloromethane (70 mL) was added and the excess potassium bromide was removed by filtration. Then the dichloromethane was removed by evaporation to give the white product (2-Br) (112 mg, 88% yield). ¹H NMR (400 MHz, CDCl₃) δ 7.05 (s, 4H), 2.39 (s, 6H), 1.99 (s, 12H); ¹³C NMR (100 MHz, CDCl₃) δ 143.0, 134.2, 132.1, 130.1, 113.8, 21.4, 17.4; anal. calcd. for C₂₀H₂₂BrI₂N₃ (638.13): C, 37.64, H, 3.48, N, 6.59%; found: C, 37.91, H, 3.65, N, 6.41%.

2-OAc: AgOAc (52 mg, 0.30 mmol) and 2-I (205 mg, 0.30 mmol) were added in a Schlenk tube, then dichloromethane (6 mL) was added and the solution stirred in a glove box in the dark for 6 h. Then AgI was removed by filtration and the solution was washed with water (10 mL) to remove the excess of silver. The pure product was obtained by evaporation of dichloromethane solution (176 mg, 92%). ¹H NMR (400 MHz, CDCl₃) δ 7.00 (s, 4H), 2.32 (s, 6H), 1.92 (s, 12H), 1.75 (s, 3H); ¹³C NMR (100 MHz, CDCl₃) δ 176.6, 142.6, 133.9, 131.8, 129.8, 110.8, 23.6, 21.2, 17.2; anal. calcd. for C₂₂H₂₅I₂N₃O₂ (617.27): C, 42.81, H, 4.08, N, 6.81%; found: C, 42.96, H, 4.26, N, 6.72%. The single crystal of 2-OAc was obtained by slow diffusion of n-pentane into a dichloromethane solution.

2-TFA: CF₃COOAg (77 mg, 0.35 mmol) and 2-I (205 mg, 0.3 mmol) were added in a Schlenk tube, then dichloromethane (6 mL) was added and the solution stirred in the glovebox for 6 h in the dark. Then AgI was removed by filtration and the solution was washed with water (5 mL) to remove the excess of silver. The pure product was obtained by evaporation of the dichloromethane solution (125 mg, 53%). ¹H NMR (400 MHz, CDCl₃) δ 7.08 (s, 4H), 2.40 (s, 6H), 2.00 (s, 12H); ¹³C NMR (100 MHz, CDCl₃) δ 161.0 (q, J = 35.0 Hz), 143.3, 134.2, 131.9, 130.2, 116.5 (q, J = 297.2 Hz), 110.0, 21.4, 17.4; anal. calcd. for C₂₂H₂₂F₃I₂N₃O₂ (671.24): C, 39.37, H, 3.30, N, 6.26%; found: C, 39.51, H, 3.49, N, 6.18%.

2-I^.1.5I₂: 2-I (35 mg, 0.05 mol) and I₂ (25 mg, 0.1 mmol) were mixed in dichloromethane (6 mL) in a round bottom flask. The single crystals were obtained by slow diffusion of ether into a dichloromethane solution. Brown solid, 21 mg, 39% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.20 (s, 4H), 2.47 (s, 6H), 2.09 (s, 12H); anal. calcd. for C₂₂H₂₂I₆N₃ (1065.84): C, 22.54, H, 2.08, N, 3.94%; found: 22.63, H, 2.19, N, 3.82%.

2-I^.3.5I₂: 2-I (35 mg, 0.05 mol) and I₂ (50 mg, 0.2 mmol) were mixed in dichloromethane (10 mL) in a round bottom flask. The single crystals were obtained by slow diffusion of ether into a dichloromethane solution, Brown solid, 63 mg, 81% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.21 (s, 4H),2.48 (s, 6H), 2.10 (s, 12H); anal. calcd. for C₂₂H₂₂I₁₀N₃ (1073.46): C, 15.72, H, 1.41, N, 2.67%; found: C, 15.93, H, 1.66, N, 2.41%.

2-BF₄^.0.5bpy: 2-BF₄ (65 mg, 0.1 mmol) and 4,4'-bipyridine (16 mg, 0.1 mmol) was mixed in dichloromethane (4 mL) in a round bottom flask. The single crystals were obtained by slow diffusion of ether into a dichloromethane solution. Colourless solid, 28 mg, 31% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.60 (d, J = 6.1 Hz, 2H), 7.57 (d, J = 6.2 Hz, 2H), 7.11 (s, 4H), 2.39 (s, 6H), 2.02 (s, 12H); ¹³C NMR (100 MHz, CDCl₃) δ 150.0, 145.7, 143.5, 134.0, 131.6, 130.2, 121.9, 107.6, 21.4, 17.2.

X-ray diffraction measurements

Single crystal diffraction data for 2-I, 2-Br and 2-Cl were collected at 200 K using an IμS micro-focus sealed X-ray tube with Mo Kα radiation (λ = 0.71073 Å) on a Bruker D8 venture Kappa Duo diffractometer equipped with a PHOTON 100 detector. Low-temperature holding was achieved by a Cryostream Cooler (Oxford Cryosystems). Single crystal diffraction data for 2-OAc, 2-TFA, 2-BF₄^.0.5bpy, 2-I^.1.5I₂ and 2-I^.3.5I₂ were collected at 150 K while 2-BF₄ was collected at 298 K. All the data were collected 0.5 degree per step and using the ω scan mode. Frames were integrated using the Bruker SAINT [47] software. Semi-empirical absorption correction was applied with the SADABS program [48].

All the structures were solved by SHELXT [49] and refined by SHELXL [50] programs against |F|² using all data following established refinement strategies [51] through olex2 [52]. Their packing diagrams were prepared by using Mercury [46].

Computational details

All calculations were performed with the Gaussian 16 (G16) program package [53]. The DFT method using the M06-2X functional [54] relying on relativistic pseudo-potentials was used, namely the small core ECP46MWB [55] for I atoms. The C, H and N atoms were treated with a basis set of 6-311G** [56]. The Cu atom was treated with a basis set of LanL2DZ [57]. Geometry optimizations were performed without any constraints, and the frequency analysis confirmed that there were no image frequencies for these structures. Visualization of the electrostatic potential was performed using the Gauss View 6.0 package [58].