I₅^– polymers with a layered arrangement: synthesis, spectroscopy, and structure of a new polyiodide salt in the nicotine/HI/I₂ system

2.1 Crystal structure of nicotine-1,1′-diium bis(triiodide)-diiodine (1/1) (1)

The asymmetric unit of (C₁₀H₁₆N₂)[I₃]₂·I₂ (1) consists of one dication, two triiodide anions, and one iodine molecule, all lying in general positions in the non-centrosymmetric space group P1. For the nicotine-1,1′-diium cation, its S-configuration at C6 is known from the synthesis and was independently verified by the structure determination. As a consequence of the protonation of the nitrogen atom N2 of the 1-methylpyrrolidin-2-yl moiety, an additional R-configurated stereocenter was formed (Fig. 1, upper part). Thereby, the hydrogen atoms at C6 and N2 are in an antiperiplanar conformation. Therefore, the formation of the other diastereomer should not be favorable and has never been observed in similar systems. The geometric parameters of the title dication are similar to those known from the literature (Table 1). However, it is surprising that only three doubly protonated [31–33] and a handful of singly protonated nicotine cation-containing salts [34–38] have structurally been characterised so far.

Fig. 1:

Upper part: Triiodide anions and nicotine-1,1′-diium cations form hydrogen-bonded chains along the b direction; lower part: Triiodide anions and iodine molecules are connected to chains along the [0 1 1̅] direction by halogen bonds. Displacement ellipsoids are only shown for the atoms of the asymmetric unit and are drawn at the 55 % probability level; hydrogen atoms and symmetry related atoms are drawn as spheres with arbitrary radii.

In the title structure, each nicotine-1,1′-dium cation is involved in two hydrogen bonds to two (crystallographically dependent) adjacent triiodide anions via its N–H functions. These connections lead to chains along the [010] direction (Fig. 1, upper part; Table 1) that are stacked parallel to each other in the bc plane at x = ½. According to the two N–H···I hydrogen bonds formed to only one of the terminal iodine atoms of this triiodide anion, the I–I bond lengths are significant different (Table 1). The magnitude of the resulting asymmetry of this anion introduced by the hydrogen-bonding scheme is in excellent agreement with the expectation [24].

In the bc plane at x = 0, the second crystallographically independent triiodide anion and the iodine molecule form halogen-bonded zig-zag chains along the [011̅] direction (Fig. 1, lower part; Table 1). The smallest repetition unit within these polyiodide chains is an I₅^– unit. The chains are almost flat with a maximum deviation of ~0.1 Å. The halogen-bonded zig–zag chains of the polyiodide substructure are stacked parallel to each other to form a layered motif. The primary I–I bonds within the triiodide anion as well as the bond length in the iodine molecule forming the halogen-bonded I₅^– polymer are in the typical ranges (Table 1) [1–5, 39]. The secondary halogen bonds within this polymer with values below 3.5 Å are unambiguously assigned to secondary bonding interactions. A search in the Cambridge Structural Database (CSD) resulted in more than 130 crystal structures containing the I₅^– anion, which shows that this polyiodide motif is very common [40, 41]. However, only a limited number of unbranched chains have been reported [42–47]. Even these topologically similar chains show very different geometric parameters concerning their secondary halogen bonds. In the title structure, the I₃^– and I₂ units are orientated nearly orthogonally to each other (Fig. 1, lower part; Table 1). There are at least two structure determinations reported in the literature that show comparable motifs. In the structure of a ferrocenium compound [47], the halogen-bonded zig-zag chains are stretched along the chain direction resulting in more obtuse angles of the building unit. In the structure of a selenium heterocyclic salt, the zig–zag chains, which resemble those in the title structure concerning their angles, are part of a halogen-bonded two-dimensional polymer [48]. The fit between the aforementioned two-dimensional stacking of polyiodide chains in the 0bc plane and the stacking of the hydrogen-bonded chains in the ½bc plane can easily be shown by a view perpendicular to these planes (Fig. 2). The cavities resulting from the arrangement of the polyiodide chains are occupied by the “upper part” of the 1-methylpyrrolidin-1-ium-2-yl moiety of the dication. Consequently, the layered motifs are pairwise interlocked (Fig. 2, lower part). Further significant I···I interactions can be ruled out as the corresponding iodine-to-iodine distances (>3.8 Å) are near the sum of the van der Waals’ radii [49]. In this context, it is worth mentioning that there is a growing interest in polymeric materials in which two-dimensional structures are kept together by van der Waals’ forces [50, 51].

Fig. 2:

Upper part: View along a on I₅^– polymers stacked in the 0bc plane. The gaps left by the arrangement of these chains are filled with the “upper part” of the 1-methyl-pyrrolidin-1-ium-2-yl moiety of the nicotine-1,1′-dium cation. (Radii of the iodine atoms are drawn with a radius of 1.55 Å, and the halogen bonds are indicated by dashed lines.) Lower part: View against c shows the pairwise interlocking between the polyiodide dominated layers with the hydrogen-bonded layers.

2.2 Spectrocopy of 1

Within the Raman spectrum of 1 the lines that are characteristic for the triiodide anion [52] are found at 114(vs) and 151(vs) cm^–1. For the solid phase of elemental iodine, the I–I vibration is generally observed at 180 cm^–1 [53–55]. The Raman spectrum of 1 shows a very strong line at 177 cm^–1. In the structure of 1, each iodine molecule is weakly connected to two neighbouring triiodide halogen bond donors. Consequently, a small shift of this line to lower wavenumbers compared to elemental iodine is expected. The line at 67 cm^–1 may be attributed to a lattice mode. The comparison of the spectroscopic data (Raman and IR) of 1 (see the Experimental Section) with experimentally and theoretically determined spectroscopic data on nicotine and its N-protonated cations [56] showed a good agreement.

4.1 General considerations

All chemicals were obtained from commercial sources and used as purchased. The Raman spectra were measured using a Bruker MULTIRAM spectrometer (Ettlingen, Germany) (Nd: YAG-laser at 1064 nm; InGaAs detector) with an apodised resolution of 8 cm^–1 in the region of 4000–70 cm^–1. IR spectroscopic data were collected on a Digilab FT3500 spectrometer (Krefeld, Germany) with an apodised resolution of 2 cm^–1 using a MIRacle ATR unit (Pike Technologies, Fitchburg, USA) between 4000 and 560 cm^–1. Elemental analyses (C, H, N) were performed with a HEKA-Tech (Wegberg, Germany) Euro EA3000 instrument.

4.2 Nicotine-1,1′-diium bis(triiodide)-diiodine (1/1) (1)

Nicotin-1,1′-diium bis(triiodide)-diiodine (1/1) (1) was synthesised by dissolving 0.16 mL (0.16 g, 1 mmol) of nicotine (Sigma-Aldrich) and 0.74 g (3 mmol) of diiodine (Merck Millipore) in 2 mL of 57 % aqueous hydroiodic acid (Sigma-Aldrich). Heating to ~370 K yielded a dark-coloured solution. Upon slow cooling to the room temperature, black, block-shaped crystals grew within a few hours. The yield is roughly 50 % based on nicotine. The Raman lines in the range of 50–200 cm^–1 are several orders of magnitude larger than the others. Therefore, the intensity gradations have been considered separately. – Raman (range 4000–200 cm^–1): ν = 3080(w), 3065(w), 3050(vw), 3030(w), 3010(vw), 2984(w), 2958(w, br), 2944(w), 2925(w), 2879(vw), 2840(vw), 2816(w), 1630(w), 1601(m), 1544(m), 1468(m), 1446(w, sh), 1413(w), 1390(vw), 1328(w), 1309(w), 1281(vw), 1255(s), 1236(w, sh), 1180(m), 1114(m), 1044(m), 1024(m), 978(s), 917(w), 857(m), 795(w), 772(m), 673(w), 653(w), 622(w), 548(w), 521(w), 457(w, br), 389(w), 352(m), 335(w), 302(w), 278(s), 264(s, sh), 247(vs), 231(vs), 207(vs). – Raman (range 200–50 cm^–1): ν = 177(vs), 151(s), 114(vs), 67(w). – IR (single crystal, ATR): ν (cm^–1) = 3202(s), 3156(s), 3105(s, sh), 3047(vs), 3010(vs), 2963(vs), 2954(vs), 2938(vs, sh), 2811(vs), 2765(vs), 2709(vs), 2112(w, br), 1991(w), 1952(w), 1908(w), 1831(w), 1629(m), 1597(s), 1541(s), 1448(vs), 1410(s), 1369(m, br), 1310(m), 1252(s), 1208(m), 1113(m), 1047(m), 1000(m, br), 975(m), 890(m), 854(m), 792(s), 769(s), 669(vs). – Elemental analysis: C₁₀H₁₆N₂I₈ (1179.45): calcd. C 10.8, H 1.4, N 2.4 %; found C 11.6, H 1.5, N 2.6 %.

4.3 X-ray structure determination

A single crystal of the title compound was directly selected from the mother liquor and rapidly transferred into the cold gas stream (T = 146 K) of the Xcalibur four-circle diffractometer equipped with an Eos detector [58]. The data collection generally followed the routine procedure with an increase in the exposure time, which is often needed for this class of compounds [20, 22]. An absorption correction (Gaussian method) was applied [58]. The structure solution and the refinement succeeded using the shelx program system [59, 60]. For more information on crystallographic details, see Table 2.

CCDC 1402691 contains the supplementary crystallographic data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.