Poly[(μ6-4-amino-3,5,6-trichloropyridine-2-carboxylato)aquacaesium]

In the structure of the title complex, [Cs(C6H2Cl3N2O2)(H2O)]n, the caesium salt of the commercial herbicide picloram, the Cs+ cation lies on a crystallographic mirror plane, which also contains the coordinating water molecule and all non-H atoms of the 4-amino-3,5,6-trichloropicolinate anion except the carboxylate O-atom donors. The irregular CsCl4O5 coordination polyhedron comprises chlorine donors from the ortho-related ring substituents of the picloramate ligand in a bidentate chelate mode, with a third chlorine bridging [Cs—Cl range 3.6052 (11)–3.7151 (11) Å] as well as a bidentate chelate carboxylate group giving sheets extending parallel to (010). A three-dimensional coordination polymer structure is generated through the carboxylate group, which also bridges the sheets down [010]. Within the structure, there are intra-unit water O—H⋯Ocarboxylate and amine N—H⋯Npyridine hydrogen-bonding interactions.


Graham Smith
Comment 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 picloramato ligands in the crystallographic literature. Examples include picloram as a bidentate chelate ligand [with Mn II (Smith et al., 1981a) and Cu II (O′Reilly et al., 1983)], while in the Mg complex (Smith et al., 1981b], picloram acts as a counter-anion. A caesium complex derived from dipicolinic acid has also been reported (Santra et al., 2011).
The reaction of picloram with caesium hydroxide in aqueous ethanol gave crystals of the title compound [Cs(C 6 H 2 Cl 3 N 2 O 2 )(H 2 O)] n and the structure is reported herein. In this structure, the Cs + cation lies on a crystallographic mirror plane which also contains the coordinating monodentate water molecule and all non-H atoms of the picloramate ligand except the carboxyl O-atom donors (Fig. 1) (x), -x + 1, y -1/2, -z + 2; for (xi), -x + 1, y + 1/2, -z + 2], as well as a bidentate chelate and bridging carboxyl group.
Although in most structures containing caesium and related ligands, the Cl atom is anionic rather than coordinating, an example of a coordinating carbon-bound Cl is known in which 1,2-dichloroethane acts as a bidentate chelate ligand (Levitskaia et al., 2000). The Cs-Cl bond lengths in that structure are shorter than those in the title complex (3.46-3.56 Å).
In the present complex, sheets are formed parallel to (010) (Fig. 2) and these are extended into a three-dimensional coordination polymer structure through the carboxyl group of the picloram ligand which bridges the sheets down [010] (Fig. 3). The amine group gives weak intramolecular N-H···Cl5 and ···Cl6 interactions and as well forms inter-complex N-H···N pyridine hydrogen bonds which accompany water O-H···O carboxyl hydrogen-bonding interactions in the structure (Table 2).

Experimental
The title compound was synthesized by heating together under reflux for 10 minutes, 0.5 mmol of caesium hydroxide and 0.5 mmol of 4-amino-3,5,6-trichloropicolinic acid in 20 ml of 10% ethanol-water. Room temperature evaporation of the solution to incipient dryness gave colourless crystal plates of the title complex from which a specimen was cleaved for the X-ray analysis.

Refinement
Hydrogen atoms of the coordinating water molecule and the amine group were located in a difference-Fourier synthesis but were allowed to ride in the refinement with U iso (H) = 1.2U eq (N) or 1.5U eq (O).

Figure 1
The molecular configuration and atom-numbering scheme for the title compound, with non-H atoms drawn as 50% probability ellipsoids. For symmetry codes: see Table 1.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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