Crystal Structure of [1046127-79-0], 3,3′-(n-Hexane-1,6-diyl)bis(1-vinylimidazolium) Bromide

Abstract

Vinylimidazolium salts constitute a highly relevant class of polymerizable heterocycles, especially in the field of so-called polymer ionic liquids (PILs), and they are frequently used as components of polyelectrolytes. Therefore, crystallographic characterizations, in particular studies of charge distribution and conformational features, can provide fundamental insights concerning ion–ion and ion–dipole interactions. The structure of the title compound [1046127-79-0] has the space group symmetry P2₁/c. Its asymmetric unit contains one formula unit, consisting of two bromide anions and the halves of two cation moieties. The cation units possess inversion symmetry, and their C₆H₁₂ chains adopt an all-trans conformation.

1. Introduction

The title compound is a preferred crosslinker for polyelectrolytes, as its two positively charged terminal heterocyclic moieties render copolymers more swellable in polar solvents, in contrast to the similarly popular ethyleneglycol dimethacrylate, EGDMA [1,2,3], or N,N′-methylenebis(acrylamide), (MBAA) [4]. In general, charge, polarity, surface tension activity, rigidity, length of the spacer, and of course the molar fraction of crosslinking comonomers can have detrimental effects on the properties of the resulting copolymers. It is noteworthy that such bridged vinylimidazolium salts have recently been used as crosslinkers for modified polyacrylamides, enabling highly elastic and mechanically and thermally stable hydrogels [5]. Their swelling degradation resistance proved to be significantly better than that of conventional MBAA copolymer materials.

In particular, various analytical separation methods such as solid phase extraction [6] and specific modes of chromatography advantageously use porous monolithic stationary phases [7]. Effects of varying the amount of crosslinker and the porogen composition on the separation or adsorption performance of the resulting columns or resin beads are of high relevance [4]. Therefore, the use of bis-vinylimidazolium bromides with oligomethylene bridges as crosslinkers offers a powerful toolbox for the production of porous polymer networks, not least because of their high compatibility with common porogen mixtures [8]. This applies in particular to the production of molecularly imprinted polymers (MIPs), which have, apart from separation applications, become established in many fields such as biosensor technology, environmental monitoring, and drug delivery technologies [9]. Accordingly, a series of oligomethylene-bridged vinylimidazolium crosslinkers were investigated as matrix carriers for the production of molecularly imprinted polymeric GUMBOS (group of uniform materials based on organic salts) and have been shown to overcome problems in the synthesis of aqueous MIPs [10].

As for recent examples, perfluoroalkyl-tethered anion-exchange resins, which contain [1046127-79-0], have been disclosed as selective adsorbents for fluorosurfactants from water bodies [11]. In addition, sorbents containing hexamethylene-bridged vinylimidazolium salts have been developed that provide sorption-based SO₂-selective porous PILs to treat flue gas streams and minimize sulfur dioxide emissions therefrom [12]. Our interest in the present crystal structure study was originally sparked by the completely unexpected poor solubility of the analogous dichloride salt in all common solvents, including water. As a result, it has not yet been possible to obtain suitable single crystals, which coincides with a surprising lack of information in the literature about this compound. Because of its significantly cheaper synthesis based on 1,6-dichlorohexane, the dichloride salt is potentially an even more attractive crosslinker than its dibromide counterpart.

2. Results and Discussion

Crystal Structure

Crystallographic data for [1046127-79-0] and details of the crystal structure determination are collected in Table 1. The asymmetric unit contains one formula unit consisting of the halves of two cation moieties (denoted as A and B) and two bromide anions (Figure 1). Both cations possess inversion symmetry. The C₆H₁₂ chains of A and B adopt an all-trans conformation. The mean plane of the alkyl chain is twisted by 54.6 (3)° against the plane of the imidazole ring in A and by 64.1 (6)° in B. The 1-vinylimidazolium unit of cation B was found to be disordered over two sites (N1B > C8B vs. N1C > C8C) with an occupancy ratio of 0.569 (9):0.431 (9). These disorder components represent two fundamentally different orientations of vinyl group with respect to the imidazole ring (Figure 2a). The vinyl group lies almost in the plane of the imidazole ring in each component. It is oriented towards the ring atom C1 in one component [torsion angle C1B–N2B–C7B–C8B = 12.2 (14)°] and towards C2 in the other [C1C–N2C–C7C–C8C = 170.5 (9)°]. As illustrated in Figure 2a, the disorder fragments of B differ slightly in the position of the imidazole ring. For the A-type cation, the C1A–N2A–C7A–C8A torsion of 13.5 (5)° indicates an almost planar 1-vinylimidazolium geometry where the vinyl group is oriented towards the ring atom C1 (similar to the first component of B). The most significant cation⋯cation interactions in the crystal are associated with extensive van der Waals contacts. There are two sets of such interactions, one along the crystallographic c axis (A⋯A and B⋯B) and another (A⋯B) along the a axis. Therefore, cations are most tightly bound to one another within layers parallel to the ac plane; see Figure 2b (bottom). Bromide ions occupy the interlayer space so that each Br⁻ is surrounded by multiple cations and the six shortest Br⋯H separations lie between 2.7 and 3.1 Å.

Table 1. Crystal data and structure refinement.

Moiety Formula	[C16H24N4]2+·2 Br−
Empirical formula	C16H24N4Br2
Formula weight	1853.3 (4)
Temperature (K)	173 (2)
Wavelength (Å)	0.71073
Crystal system	Monoclinic
Space group	P21/c
a (Å)	11.5504 (14)
b (Å)	11.4758 (13)
c (Å)	14.7678 (15)
β (°)	108.779 (11)
Unit cell volume (Å3)	1853.3 (4)
Z/Z′	4/1
Reflections collected/Rint	12,500/0.0514
Data/restraints/parameters	3850/141/264
Goodness-of-fit on F2	1.038
R1 [I > 2 σ(I)]	0.0363
wR2 (all data)	0.0663
Largest diff. peak and hole (e · Å−3)	0.376 and −0.432

Figure 1. Molecular structure with non-H atoms depicted as ellipsoids at the 50% probability level and H atoms drawn as blue circles of random size (disorder omitted for clarity). Symmetry operations: (i) 1 − x, 1 − y, 2 − z (ii) −x, −y, −z.

Figure 2. (a) Fragment of molecule B, showing the disorder of the 1-vinylimidazolium unit (N1B > C8B vs. N1C > C8C); (b) crystal packing of cation types A and B and Br⁻ (view along [001]). The bracket at the bottom indicates a layer containing the most significant cation⋯cation interactions (ac plane).

3. Materials and Methods

The synthesis was carried out according to the literature [13] with some adjustments for a larger batch size: 1,6-dibromohexane (96% purity; 38.1 g; 150.0 mmol) and 1-vinylimidazole (99% purity; 28.3 g; 300.0 mmol) were thoroughly mixed at RT and left to stand for 3 h (Scheme 1). Due to the exothermic nature of the reaction and to avoid side reactions, the mixture was then cooled by means of an ice bath for 3 h before the reaction temperature was allowed to return to RT. After 15 h, the mixture was heated to 100 °C for 30 min in order to maximize product formation before the resulting solid block was treated with MeOH (100 mL). The suspension was ultrasonicated by means of an ultrasonic bath, until a clear solution was formed. Thereafter, Et₂O (350 mL) was added while stirring. The resulting precipitate was filtered off, washed with Et₂O (100 mL), and dried under high vacuum for 24 h, yielding 55.6 g (86%) of pure, white 3,3′-(hexane-1,6-diyl)bis(1-vinylimidazolium) bromide. M.p.: 218 °C. FT-IR (ATR, neat) ν = 3135, 3071, 3029, 2979, 2934, 2915, 2860, 1648, 1571, 1551, 1476, 1394, 1308, 1176, 1112, 956, 918, 823, 734, 622, 592 cm⁻¹. ¹H NMR (300 MHz, MeOH–d₄) δ = 9.61 (t, J = 1.9 Hz, 2H), 8.14 (t, J = 1.9 Hz, 2H), 7.94 (t, J = 1.9 Hz, 2H), 7.38 (dd, J = 15.6, 8.7 Hz, 2H), 6.04 (dd, J = 15.6, 2.7 Hz, 2H), 5.47 (dd, J = 8.7, 2.7 Hz, 2H), 4.40 (t, J = 7.4 Hz, 4H), 2.02 (m, 4H), 1.50 (m, 4H) ppm (Figure S1). ¹³C NMR (75 MHz, MeOH–d₄) δ = 136.31 (2C), 129.81 (2C), 124.49 (2C), 120.72 (2C), 109.95 (2C), 50.95 (2C), 30.50 (2C), 26.32 (2C) ppm (Figure S2).

Scheme 1. Synthesis of 3,3′-(hexane-1,6-diyl)bis(1-vinylimidazolium) bromide.

Single-Crystal Structure Determination

White needles of the title compound suitable for structure analysis were obtained through slow evaporation of a methanolic solution of [1046127-79-0]. Intensity data were collected at 173 K, using Mo radiation (λ = 0.71073 Å), on an Oxford Diffraction Gemini-R Ultra diffractometer. The data were corrected for absorption effects by means of comparison of equivalent reflections (Figure S3). The crystal structure was solved by Direct Methods with SIR2004 [14] and refined with the least-squares techniques using SHELXL [15]. H atoms were identified in difference-Fourier maps, and those bonded to carbon atoms were refined using a riding model with U_iso parameters set to 1.2 U_eq of the parent C atom. The two disorder components (i.e., N1B > C8B/N1C > C8C) were refined using restraints on chemically equivalent 1,2- and 1,3-distances. Enhanced rigid bond restraints (RIGU) were applied on the anisotropic displacement parameters of atoms in the two disorder components.

4. Conclusions and Outlook

In conclusion, the structurally characterized title compound will certainly remain an attractive starting material for many fields of materials science. Notably, it can be further converted very efficiently into a neutral, i.e., uncharged, thione system, [2127366-93-0] [16]. Such bridged heterocyclic thione derivatives [17], like other vinylimidazole-based polymers, also offer high potential for use as thermal interface materials (TIMs) with improved interface adhesion to metals [18], which is mediated by the high thiophilicity of metal surfaces. Finally, the thinoes could then be even S-alkylated by another linking reagent [16], which in turn would produce charged heterocyclic moieties bearing four polymerizable vinyl groups available in one hypercrosslinker molecule [19].