Crystal structures of five 1-alkyl-4-aryl-1,2,4-triazol-1-ium halide salts

To investigate the predominant intermolecular interactions in 1-alkyl-4-aryl-1,2,4-triazol-1-ium halide salts, five salts were prepared and crystallographically characterized. The halide ions generally interact with the H atoms of the triazolium cation forming extended sheets. When the aryl ring lies on the plane of the triazolium cation, the cationic core formed two-dimensional networks that lead into layer-like assembled structures. The triazolium core exhibits π–π interactions with the iodide and/or the aryl ring of another layer. The melting-point temperatures of each salt were also determined.

Most recently, Strassner has introduced a new group of ionic liquids called 'TAAILs' (tunable aryl-alkyl ionic liquids) (Ahrens et al., 2009). The idea is to tune the properties of the ionic liquids through modification of the aryl and alkyl substituents of an imidazole cation (Scheme 1). The new cations can still be combined with the previously used anions in ILs. These workers have demonstrated that electrondonating para-substituents on the aryl group lower the melting point, while electron-withdrawing para-substituents raise the melting point. Thus one can tune the IL properties through ISSN 2056-9890 the introduction of an electronic variation through parasubstitution on the aryl rings. This group has also extended the concept to the 1,2,4-triazolium cation core (Meyer & Strassner, 2011).
Our group became interested in learning how the aryl/alkyl substituents on the triazole ring affect the solid-state structures of the salts because strategic choice of substituents should allow tailorableinteractions as predicted by Strassner's group (Meyer & Strassner, 2011). Herein, the preparation and crystal structure analyses of salts (1)-(5) are discussed (Scheme 2). Cations (1)-(3) compare the inductive effects of the electronic para-substituents in the aryl group, while cations (3)-(5) contrast the steric bulk of the alkyl substituents. None of the compounds presented here are ILs because we used iodide or bromide counter-anions to facilitate crystal formation. Understanding interactions in the solid state may help better design systems where the triazolium cations are needed.

Structural commentary
Salts (1) and (2) crystallized in the orthorhombic space group Pccn, salt (3) in the monoclinic space group P2 1 /n, and salt (5) in the monoclinic space group C2/c. Salt (4) crystallized in the non-centrosymmetric space group Cc with a Flack parameter of À0.01 (2) indicating the absolute structure is well determined.

Supramolecular features
For all five salts, there is a predominant C-HÁ Á Áhalide intermolecular interaction between the hydrogen atoms in the triazolium ring and the counter ions, forming an extended network . For the asymmetric unit in salt (1), there are a total of four C-HÁ Á ÁI À intermolecular interactions with two neighboring molecules ( Fig. 1, Table 1). Each iodide ion interacts with two C-H moieties from the triazolium ring and two from the ortho C-H moieties of the aryl group. There is an additional C-HÁ Á ÁN interaction between the meta C-H of the aryl ring and the triazolium nitrogen atom. The fluorine substituent in the para-position of the aryl ring is not an acceptor in any of the C-H interactions in salt (1). The asymmetric unit of salt (2)  Extended sheet network viewed along the c axis of salt (1). H atoms not participating in the extended sheet network are not shown. For symmetry codes, see Table 1. Table 1 Hydrogen-bond geometry (Å , ) for salt (1). Symmetry codes: (i) Àx þ 1 2 ; y; z þ 1 2 ; (ii) Àx; Ày; Àz þ 1; (iii) x þ 1 2 ; Ày; Àz þ 3 2 . Table 2 Hydrogen-bond geometry (Å , ) for salt (2). three C-HÁ Á ÁI À intermolecular interactions with two neighboring molecules (Fig. 2, Table 2). Two C-H moieties from the triazolium ring and one ortho C-H of the aryl ring interact with one iodide ion. In the asymmetric unit of salt (3) (Fig. 3, Table 3), there are a total of three C-HÁ Á ÁI À intermolecular interactions, two from the triazolium C-H moieties, and one methine hydrogen atom from the isopropyl group because the aryl ring does not lie on the plane of the triazolium ring. For salts (4) and (5) Tables 4 and  5), there are only a total of two C-HÁ Á ÁI/Br À intermolecular interactions, both from the triazole ring's C-H groups. However, in salt (5), a water molecule is in the asymmetric unit along the plane of the triazole and phenyl rings and is also interacting with the Br À ion and the ortho C-H of the phenyl ring. A square-shaped hydrogen-bonding network is formed between two bromide ions and water molecules ( Fig. 6 and Table 5). Thus, each bromide ion has two acceptor interactions with water hydrogen atoms and one acceptor interaction with the C-H of the triazolium ring, and each water molecule has two donor interactions with the bromide ions and one Extended sheet network viewed along the c axis of salt (2). H atoms not participating in the extended sheet network are not shown. For symmetry codes, see Table 2. Table 3 Hydrogen-bond geometry (Å , ) for salt (3). Symmetry codes: (i) x þ 1; y; z; (ii) x À 1 2 ; Ày þ 1 2 ; z À 1 2 . Table 4 Hydrogen-bond geometry (Å , ) for salt (4). Symmetry codes: (i) x À 1 2 ; Ày þ 1 2 ; z À 1 2 ; (ii) x; Ày; z À 1 2 ; (iii) x; y; z À 1. Table 5 Hydrogen-bond geometry (Å , ) for salt (5).  (7) 3.341 (3) 170 (5) Symmetry codes: (i) x; y À 1; z; (ii) Àx þ 2; y; Àz þ 1 2 .

Figure 3
Extended sheet network viewed along the c axis of salt (3). H atoms not participating in the extended sheet network are not shown. For symmetry codes, see Table 3.

Figure 4
Extended sheet network viewed along the a axis of salt (4). H atoms not participating in the extended sheet network are not shown. For symmetry codes, see Table 4.

Figure 5
Extended sheet network of salt (5). H atoms not participating in the extended sheet network are not shown. For symmetry codes, see Table 5.

Figure 6
Donor-acceptor interactions of bromide ions and water molecules with each other, with the triazolium C-H, and the ortho C-H of the aryl ring found in salt (5). H atoms not participating in the interactions are not shown. For symmetry codes, see Table 5.
acceptor interaction with the ortho C-H of the aryl ring (Figs. 5 and 6, Table 5). Salts (1), (2), and (4) pack as layered sheets as shown in Fig. 7. In salt (1), there is an additional intermolecular interaction between the triazolium carbon and the iodide ion (C1Á Á ÁI1) with a distance of 3.546 (4) Å between layers along the unit cell c axis (Fig. 8). While an anion-interaction is not common, similar interactions have been reported in the literature, especially in supramolecular systems (Chifotides & Dunbar, 2013). Each cation in the sheet is further stabilized by an FÁ Á ÁF interaction with a distance of 2.889 (5) Å between neighboring cations (Fig. 8). C-FÁ Á ÁF-C contacts are reported in the literature to be weak but still relevant for crystal packing (Chopra, 2012). In salt (2), the iodide ion between layers is interacting with both the triazolium carbon [C1Á Á ÁI1 distance of 3.532 (4) Å , Fig. 9] and the methine hydrogen atom of the isopropyl group (C3-H3Á Á ÁI1, Fig. 9, Table 2), in addition to the three hydrogen-bonding interactions with the ortho hydrogen atom and triazolium hydrogen atom of a cation within the sheet (Figs. 2 and 9, Table 2). Salt (4) also demonstrates iodide ion interaction with both the triazolium carbon [C1Á Á ÁI1 distance of 3.503 (3) Å , Fig. 10] and the methyl hydrogen atom (C3-H3Á Á ÁI1; Fig. 10, Table 4) in alternating layers, in addition to the hydrogen bonding with the neighboring cation's triazolium hydrogen atoms (Fig. 3, Table 4). The structure is stabilized further byinteractions between aryl carbon atoms in alternating layers    Salt (2) showing intermolecular interactions between layers and neighboring cations. H atoms not participating in intermolecular interactions are not shown. [Symmetry codes: (iv) Àx + 1 2 , y, z + 1 2 ; (v) Àx + 1 2 , y, z À 1 2 .]

Figure 10
Salt (4)  [C6Á Á ÁC9 with a distance of 3.384 (5) Å ] and an aryl carbon atom with a triazolium carbon atom [C1Á Á ÁC8 with a distance of 3.282 (4) Å ], also in alternating layers (Fig. 10). In salt (5) there are -interactions [C11Á Á ÁC11 with a distance of 3.220 (5) Å and C1Á Á ÁC12 with a distance of 3.335 (4) Å ] between triazolium and aryl rings in alternating layers which are closely associated with the donor-acceptor interactions of the bromide ions and water molecules (Figs. 6 and 11). Extending the layers further reveals anotherinteraction [C1Á Á ÁC13 with a distance of 3.370 (4) Å ] between the triazolium cation and aryl rings (Fig. 12), and ainteraction [C2Á Á ÁO1, 3.143 (5) Å ] between the carbon atom of the triazole ring and the oxygen atom of the water molecule ( Fig. 12a). This triazole-phenyl stacking is parallel with the c axis (Fig. 12b). The extended sheet network in salt (3) passes diagonally through the cell, but there are no significant intermolecular interactions between cations, as shown in Fig. 13.
Interestingly, when there is para-substitution on the aryl ring [salts (1) and (2)], there are no observedinteractions between the phenylene and triazole rings. The observed interactions are predominantly from the triazolium carbon atom with the iodide ion. The absence of the para-substituents allowsinteractions between the phenyl and triazole rings as demonstrated in salts (4) and (5). However, to facilitateinteraction, the aryl ring needs to be co-planar with the triazolium ring; thus there are nointeractions in salt (3). Salt (5) exhibited the lowest melting-point temperature, possibly due to the presence of water in the crystal lattice, and thus will not be included in the discussion here. The higher melting points of salts (1), (2) and (4) compared to salt (3) may reflect the layering of the triazolium-aryl cation core sheets and the resulting inter-layer interactions. As predicted by Strassner, the electron-withdrawing substituent in the aryl ring found in salt (1) increased the melting point when compared to salt (2), which contains an electron-donating substituent on the aryl ring (Meyer & Strassner, 2011). Theinteractions between the phenyl and triazole rings in salt (4) likely facilitate the increase in melting-point temperature.
In summary, for 1-alkyl-4-aryl-1,2,4-triazol-1-ium halide salts, the predominant intermolecular interaction is the C-HÁ Á Áhalide hydrogen bond between the hydrogen atoms in the triazolium cation and the halide ions forming extended sheets. For salts with para-substitution on the aryl ring,interactions between the triazolium carbon and the halide are present. The melting points of these salts agree with substituent inductive effects predictions. For salts without the parasubstitution on the aryl ring,interactions displayed by the layers are between the triazolium and aryl rings.
(4) is a carbene-precursor to phosphorescent platinum(II)-NHC complexes; the crystal structure as a carbene ligand is also reported (Tenne et al., 2013). Triazolium cation (5) was used in the investigation of kinetics and mechanism of azocoupling (Becker et al., 1991).

Synthesis and crystallization
General Methods. All salts were synthesized in two steps. The first step is an intramolecular transamination pathway similar to literature methods (Meyer & Strassner, 2011;Naik et al., 2008;Holm et al., 2010). The products of this transamination step are 4-(4-fluorophenyl)-1,2,4-triazole as the salt (1) precursor, 4-(4-methylphenyl)-1,2,4-triazole as the salt (2) precursor, and 4-(phenyl)-1,2,4-triazole as salts (3), (4) and (5) precursor. In our attempts, we utilized a microwave reactor to shorten the reaction time from 24 hrs to roughly 15-30 mins with 20-70% yields (Meyer & Strassner, 2011;Naik et al., 2008;Holm et al., 2010). The second step is a nucleophilic substitution between the first-step products, 4-aryl-1,2,4-triazoles, and an alkyl halide (2-iodopropane, iodomethane, and benzyl bromide). This synthetic approach was used in the literature (Meyer & Strassner, 2011;Holm et al., 2010), but in our attempts we again used the microwave reactor to shorten the reaction time from 48 hrs to 10-30 mins with 10-70% yields (Meyer & Strassner, 2011;Holm et al., 2010). N,N-dimethylformamide azine dihydrochloride (DMFAÁ-2HCl) was synthesized following literature methods (Naik et al., 2008;Holm et al., 2010). All other reagents and solvents were purchased from Sigma-Aldrich. Tetrahydrofuran (THF) and isopropanol were dried with molecular sieves (4Å ). A Biotage microwave reactor was used for all synthetic preparations. All NMR spectra were recorded on a JEOL 400 MHz spectrometer. 1 H and 13 C NMR chemical shifts were determined by reference to residual 1 H and 13 C solvent peaks. All thermal analysis experiments were performed on a TA model TGA Q500 thermal gravimetric analyzer and TA model DSC Q100 differential scanning calorimeter. For TGA experiments, crystal samples with masses between 0.4 to 1.4 mg were loaded onto platinum pans. Dry grade nitrogen gas was used for all samples with a balance purge rate of 40.00 mL/min and a sample purge rate of 60.00 mL/min. The temperature was ramped at 20.00 K per minute until a final temperature of 673.00 or 773.00 K was reached. For DSC experiments, crystal samples with masses between 3 and 9 mg were loaded onto platinum pans. Dry grade nitrogen gas was used for all samples with a sample purge range of 50.00 mL/ min. The samples were subjected to a heat/cool/heat cycle with a temperature ramp rate of 10.00 K per minute until a final temperature of 473-523 K was reached for the heating cycle, and a temperature ramp rate of 5.00 K per minute until a final temperature of 273 or 248 K was reached for the cooling cycle.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 6. H atoms for salts (1)-(4) were placed in calculated positions and allowed to ride on their parent atoms at C-H distances of 0.95 Å for the triazolium and aryl rings, 0.98 Å for the methyl groups, and 1.00 Å for the methine group. H atoms for salt (5) were treated with a mixture of independent and constrained refinement. The C-H distances are 0.95 Å for the triazolium and aryl rings, 0.99 Å for the methylene group, and 0.95 (6) Å and 0.92 (7) Å for water. Salt (4) crystallized in the non-centrosymmetric space group Cc with a Flack parameter of À0.01 (2) indicating the absolute structure is well determined. For all compounds, data collection: CrystalClearSM Expert (Rigaku, 2011); cell refinement: CrystalClearSM Expert (Rigaku, 2011); data reduction: CrystalClearSM Expert (Rigaku, 2011); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: CrystalStructure (Rigaku, 2010); software used to prepare material for publication: Mercury (Macrae et al., 2006).