Crystal structure of poly[[di-μ2-aqua-aquasodium] 4-amino-3,5,6-trichloropyridine-2-carboxylate trihydrate], the sodium salt of the herbicide picloram

The trihydrated sodium salt of the herbicide picloram comprises a cationic μ2-aqua-bridged chain structure that is linked to the picloramate anions through amine N—H⋯O and water O—H⋯O and O—H⋯N hydrogen bonds.


Chemical context
4-Amino-3,5,6-trichloropyridine-2-carboxylic acid (picloram) is a commercial herbicide (Mullinson, 1985) introduced by Dow Chemicals as Tordon (O'Neil, 2001). Although it has potential as a metal-chelating ligand similar to picolinic acid, there are only five metal complexes with picloramate anions in the crystallographic literature. Examples include picloram as a bidentate N,O chelating ligand with Mn II (Smith et al., 1981a) and Cu II (two structures, one a mixed-ligand complex with 2aminopyrimidine; O'Reilly et al., 1983) and caesium (Smith, 2013). In the Mg complex (Smith et al., 1981b), the picloramate anions act as counter-ions to the [Mg(H 2 O) 6 ] 2+ cation. Although the structure of picloram has not been reported, that of the guanidinium salt is known (Parthasarathi et al., 1982). The reaction of picloram with sodium bicarbonate in aqueous ethanol gave crystals of the title complex salt {[Na(H 2 O) 3 ] + ÁC 6 H 2 Cl 3 N 2 O 2 À Á3H 2 O} n , and the structure is reported herein. ISSN 2056-9890

Figure 2
The three-dimensional hydrogen-bonded structure, with inter-species hydrogen bonds and intramolecular N-HÁ Á ÁCl associations shown as dashed lines. For symmetry codes, see Fig. 1 and Table 2. in a large number of reported crystal structures. However, with few exceptions, these are based on NaO 6 polyhedra, with octahedral or distorted octahedral stereochemistry, having two non-bridging water molecules [Na 2 (H 2 O) 8

Synthesis and crystallization
The title compound was synthesized by briefly heating together 0.5 mmol of 4-amino-3,5,6-trichloropicolinic acid (picloram) with excess NaHCO 3 in 10 ml of 10% (v/v) ethanol-water. Room temperature evaporation of the solution to dryness gave minor colourless crystal blocks of the title complex from which a specimen was cleaved for the X-ray analysis.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms of the water molecules and the amine group were located in a difference-Fourier synthesis but were subsequently constrained in the refinement with the isotropic displacement parameters allowed to ride, with U iso (H) = 1.2U eq (N) or 1.5U eq (O).

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 e.s.d.'s 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 > 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.