Tris[2-(2-thienylmethylamino)ethyl]ammonium triiodide

In the title compound, C21H33N4S3 3+·3I−, three secondary amines are protonated, while the central amine remains unprotonated. One thiophene is disordered with an occupancy ratio of 0.868 (6)/0.132 (6). Each protonated amine is involved in N—H⋯I hydrogen-bonding interactions with the iodide anions.


Comment
Anions play a key role in many chemical and biological processes. In particular, structural characterization of an anion complex is important in achieving selective hosts for anions (Hossain, 2008, Saeed et al., 2010. Among the numerous systems, trigonal receptors are of interest because of their synthetic simplicity and capability for anion binding though hydrogen bonding interactions. Tris(aminoethyl)-amine is an excellent building block for synthesizing fuctionalized tripodal hosts for anion binding (Burgess et al., 1991;Hossain et al., 2004;Bazzicalupi, et al., 2009). These molecules have been shown to bind a variety of anion including nitrate, phosphate and sulfate (Bianchi et al., 1997;Kang et al., 2005). Herein, we report the molecular structure of the title compound in which three iodides are held by hydrogen bonding with protonated secondary amines.
Single crystal analysis of the title compound reveals that the molecule crystallizes in its orthorhombic space group forming a cavity. The tren unit is triply charged, where all three secondary N atoms are protonated. The central amine is not protonated.
The three arms form a cavity, and one thiophene unit is disordered. In the complex, the protonated amines are involved in hydrogen bonding interactions with iodide anions having N···I distances 3.460 (3) to 3.553 (4)Å ( Fig. 1 and Table 1). One iodide (I1) accepts two hydrogen bonds from two protonated amines (N3 and N4), while each of the other two iodides accepts one hydrogen bond from N2 and N3. Therefore, one secondary nitrogen (N3) donates two hydrogen bonds to two iodides (I1 and I3). The N···I distances are comparable with those observed in an iodide complex of an azacryptand (3.476 (4)Å and 3.632 (4)Å) reported earlier (Hossain et al., 2002).
The disorder of the thiophene ring containing S3 involves two conformations, differing by rotation about two different bonds. One is a twofold rotation about C17-C18, which swaps S3 and C19. Refinement of this type of model resulted in elongated ellipsoids in the plane of the ring for all atoms of the thiophene, as well as unacceptable residual densities.
This was interpreted as a second conformational difference involving a difference in rotation about the N4-C17 bond, amounting to a torsional difference of 11.7°.

sup-2
The salt was redissolved in water and ethanol (1:2 v/v, 1 ml) and crystals suitable for X-ray analysis were grown from slow evaporation of the solvent at room temperature.

Refinement
H atoms based on C were placed in idealized positions with C-H distances 0.95Å-0.99Å, N-H distances 0.92Å, and thereafter treated as riding. U iso for H was assigned as 1.2 times U eq of the attached atom. The largest residual density peak was 0.81Å from I2, and the deepest hole was 0.59Å from I2. The disorder in the thiophene ring containing S3 was modeled with two orientations having populations 0.868 (6) and 0.132 (6), their geometries being restrained to be the same as that of the thiophene containing S1. This required 30 restraints. Full anisotropic refinement was not successful for the disordered region, and it was necessary to treat nine atoms as isotropic, with a common displacement parameter for the five atoms of the minor contributor thiophene ring. The absolute structure was determined by refinement of the Flack (1983) parameter, based on 4708 Friedel pairs. Six low-angle reflections were given zero weight in the refinement. Fig. 1

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( supplementary materials sup-9 Fig. 1