4-Thiocarbamoylpyridin-1-ium iodide

The title salt, C6H7N2S+·I−, crystallizes with two independent cations and two anions in the asymmetric unit. In one of the cations, the dihedral angle between the pyridinium ring and the thioamide group is 28.9 (2)°; in the other it is 33.5 (2)°. In the crystal, N—H⋯S and C—H⋯S hydrogen bonds link the independent cations into pairs. These pairs form a three-dimensional network through additional N—H⋯I and C—H⋯I hydrogen bonds to the anions.


Comment
Crystal structures of pyridine rings carrying a thioamide functional group (specifically, 4-thiocarbamoylpyridines) have been investigated primarily because of their use as anti-tuberculosis medications. Thus Colleter and Gadret determined the structure of neutral 4-thiocarbamoylpyridine (Colleter & Gadret, 1967). Ethionamide (2-ethyl-4-thiocaraboylpyridine) is a second line drug used in regimens to treat multi-drug-resistant tuberculosis; its in vivo activation has been investigated (Vannelli et al., 2002). The crystal structure of both the HCl and HBr salts of ethionamide have been reported (Colleter & Gadret, 1968a, 1968b as has the hexafluorsilicate (Gel'mbol'dt et al., 2010); clinically, ethionamide is administered as the hydrochloride. In order to investigate solubility issues related to drug delivery and the search for similar therapeutic agents, the structures of neutral ethionamide (Alléaume et al., 1973) and the closely related 2-methyl derivative have also been determined (Gadret & Goursolle, 1969). Similarly the 2-propyl (Colleter et al., 1970) and 2butyl analogues (Colleter et al., 1973) have also been determined as neutral compounds. More recently, the structure of a secondary amine derivative of ethionamide has been determined (Cardoso, et al., 2008). The vibrational spectra of the parent compound have been measured and compared to DFT calculated results (Wysokiński et al., 2006). There is also one reported structure wherein a (substituted) neutral 4-pyridylthioamide coordinates to a copper(II) glyoximato complex (Revathi et al., 2009); this same group earlier undertook the structure determination of the free ligand which is also a secondary amine derivative (Kavitha et al., 2008).
The structure of (I) determined from the X-ray diffraction analysis contains two independent cations and anions ( Figure   1). Compared to the parent compound, the average S-C, N2-C and C1-C2 bond distances are shortened in (I); all the other bonds are longer than in the neutral species. There is an extensive network of H-bonds linking the cations and anions in the lattice. One of the independent Iions interacts with one pyridinium NH donor and two thioamide NH 2 donors (one from each cation). The second Iinteracts strongly with two NH 2 donors and one of the aromatic ring CH, and only very weakly with a pyridinium NH. In addition, one thioamide S atom is H-bonded to both a pyridinium NH and the ortho CH. The strongest N-H···I interaction (donor-acceptor 3.48 (2) Å), which corresponds to 0.49 Å less than the sum of the v.d·W. radii of N and H, is between one of the two Iions and one of the pyridinium N atoms. In close second place at 0.45 Å less than the sum of the v.d·W. radii is the contact between an NH 2 nitrogen and the other Iion, which involves the same thioamide ring. This ring also forms an H-bond with the S atom as acceptor towards the pyridinium NH of the second thioamide in the cluster. In all, there are nine H-bonds which exceed 0.1 Å less than the sums of the corresponding v.d·W. radii.
The H-bonding is comparable in strength (shortest contact 0.49 Å < sum v.d·W. radii) to that reported by Colleter & Gadret (1968a, 1968b for the HCl and HBr adducts of ethionamide (shortest contacts 0.34; 0.31 Å < sum of v.d.W. radii). However, compared to these isostructural salts, in (I) the pattern is quite unusual (Figure 2) wherein pyridinium nitrogen N4-H4 are H-bonded to I1, but I2 is linked to the N3 amino rather than to a pyridinium group; the H-bond that forms from this second pyridinium unit involves an S···HN link. There is in fact an extensive network of H-bonds present

Experimental
Following a method designed for the oxidation of the thioamide (Liebscher & Hartmann, 1977), 4-pyridine thioamide (1.00 g, 10 mmol) and iodine (2.00 g, 10 mmol) in 7.00 ml glacial acetic acid were heated for about 10 minutes to the boil. The brownish colour of iodine in acetic acid changed to dark red on adding 4-pyridine thioamide. Initially, there were undissolved solids in the mixture, but these dissolved during heating. A dark precipitate was formed upon cooling which was collected on a Buchner funnel and rinsed. After drying on the pump to get rid of acetic acid, the solid was recrystallized from acetonitrile, affording 2.17 g (84%) of dark-brown X-ray quality needles, mp. 426.6-426.8 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. Crystal data, data collection and structure refinement details are summarized in Table 1. H atoms were located on a difference Fourier map, but for purposes of refinement those bonded to C atoms are treated as riding with C-H(aromatic) = 0.95 Å and U iso (H) = 1.2U eq C. H atoms bonded to N atoms in the cations were refined independently but with isotropic displacement parameters set to 1.2U eq of the attached N atom.