Crystal structures and hydrogen bonding in the anhydrous tryptaminium salts of the isomeric (2,4-dichlorophenoxy)acetic and (3,5-dichlorophenoxy)acetic acids

The anhydrous tryptaminium salts of isomeric (2,4-dichlorophenoxy)acetic acid and (3,5-dichlorophenoxy)acetic acid give one-dimensional hydrogen-bonded chain structures which differ both in their cation–anion conformations and modes of inter-ion interaction.

The anhydrous salts of 2-(1H-indol-3-yl)ethanamine (tryptamine) with isomeric (2,4-dichlorophenoxy)acetic acid (2,4-D) and (3,5-dichlorophenoxy)acetic (3,5-D), both C 10 H 13 N 2 + ÁC 8 H 5 Cl 2 O 3 À [(I) and (II), respectively], have been determined and their one-dimensional hydrogen-bonded polymeric structures are described. In the crystal of (I), the aminium H atoms are involved in three separate inter-species N-HÁ Á ÁO hydrogen-bonding interactions, two with carboxylate O-atom acceptors and the third in an asymmetric three-centre bidentate carboxylate O,O 0 chelate [graph set R 1 2 (4)]. The indole H atom forms an N-HÁ Á ÁO carboxylate hydrogen bond, extending the chain structure along the b-axis direction. In (II), two of the three aminium H atoms are also involved in N-HÁ Á ÁO carboxylate hydrogen bonds similar to (I) but with the third, a threecentre asymmetric interaction with carboxylate and phenoxy O atoms is found [graph set R 1 2 (5)]. The chain polymeric extension is also along b. There are noring interactions in either of the structures. The aminium side-chain conformations differ significantly between the two structures, reflecting the conformational ambivalence of the tryptaminium cation, as found also in the benzoate salts.

Chemical context
2-(1H-Indol-3-yl)ethanamine (tryptamine) is an alkaloid found in plants and fungi and is a possible intermediate in the biosynthetic pathway to the plant hormone indole-3-acetic acid (Takahashi, 1986). It is also found in trace amounts in the mammalian brain, possibly acting as a neuromodulator or neurotransmitter (Jones, 1982). As a relatively strong base (pK a = 10.2), it readily forms salts with a number of organic acids. To investigate the modes of hydrogen-bonding interaction in crystals of the tryptaminium salts of ring-substituted phenoxyacetic acid analogues, the reaction of tryptamine with two isomeric homologues, the herbicidally active (2,4-dichlorophenoxy)acetic acid (2,4-D) (Zumdahl, 2010) and (3,5dichlorophenoxy)acetic acid (3,5-D), gave the anhydrous salts, C 10 H 13 N 2 + ÁC 8 H 5 Cl 2 O 3 À , (I) and (II), respectively. Their structures and hydrogen-bonding modes are reported herein. The structure of the anhydrous salt with phenoxyacetic acid (Koshima et al., 1999) represents the only reported example of a salt from this acid series. In that crystal, chirality was generated through hydrogen bonding, giving cation-anion units related along a 2 1 screw axes. A similar phenomenon was also observed in the tryptaminium 4-chlorobenzoate crystal (Koshima et al., 2005).

Figure 2
The atom-numbering scheme and the molecular conformation of the TRYP + cation (A) and the 3,5-D À anion (B) in (II) with displacement ellipsoids drawn at the 40% probability level. The cation-anion hydrogen bond is shown as a dashed line.
atom gives an N-HÁ Á ÁO carboxylate hydrogen bond, extending the chain structure down the [010] axis (Fig. 3). In the crystal of (II), as with (I), two of the three aminium N-HÁ Á ÁO interactions are with single carboxylate O atoms [(O13B, O14B iii ) but the third differs in that it forms a three-centre asymmetric interaction with carboxylate and phenoxy O atoms of the anion (O13B ii , O11B ii ) [graph set R 2 1 (4)] ( Table 2).
The chain polymeric N1-HÁ Á Á O14B extension is also along [010] (Fig. 4). The present pair of structures of salts of tryptamine with isomeric (2,4-dichlorophenoxy)acetic acid and (3,5-dichlorophenoxy)acetic acid provide examples which further reflect the conformational ambivalence of the cationic alkylaminium side chain of the tryptamine cation, shown also in the benzoate salts.

Synthesis and crystallization
The title compounds (I) and (II) were prepared by warming together for 2 min, solutions containing equimolar quantities of (2,4-dichlorophenoxy)acetic acid (2,4-D) or (3,5-dichlorophenoxy)acetic acid (3,5-D) (138 mg) with 100 mg of tryptamine in ethanol. Room temperature evaporation of the solutions gave in both cases, colourless needles of (I) and (II) from which specimens were cleaved for the X-ray analyses.

Figure 3
The one-dimensional hydrogen-bonded polymeric structure of (I) extending along [010], with non-associative H atoms omitted. For symmetry codes, see Table 1.

Figure 4
The one-dimensional hydrogen-bonded polymeric structure of (II) extending along [010], with non-associative H-atoms omitted. For symmetry codes, see Table 2. Acta Cryst. (2015). E71, 671-674 research communications  (7) Computer programs: CrysAlis PRO (Agilent, 2013), SIR92 (Altomare et al., 1993), SHELX97 (Sheldrick, 2008) within WinGX (Farrugia, 2012) and PLATON (Spek, 2009 (Agilent, 2013); program(s) used to solve structure: SIR92 (Altomare et al., 1993). Program(s) used to refine structure: SHELX97 (Sheldrick, 2008) within WinGX (Farrugia, 2012) for (I); SHELXL97 (Sheldrick, 2008) within WinGX (Farrugia, 2012) for (II). For both compounds, molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: PLATON (Spek, 2009).  (15) Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles 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 > 2sigma(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

(II) 2-(1H-Indol-3-yl)ethanaminium (3,5-dichlorophenoxy)acetate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.22 e Å −3 Δρ min = −0.24 e Å −3 Absolute structure: Flack (1983) Absolute structure parameter: 0.01 (7) Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell esds are taken into account in the estimation of distances, angles and torsion angles 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 > 2sigma(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 Cl3B 0.88773 (13