Crystal structure of the magnesium salt of the herbicide 2,4-D: pentaaqua[(2,4-dichlorophenoxy)acetato-κO]magnesium (2,4-dichlorophenoxy)acetate hemihydrate

In the hydrogen-bonded structure of the Mg salt of the herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D), the metal atom is octahedrally coordinated by a monodentate 2,4-D ligand and five water molecules, with a 2,4-D− counter-anion and half a solvent water molecule completing the crystal structure.

In the crystal structure of the title magnesium salt of the phenoxy herbicide (2,4-dichlorophenoxy)acetic acid (2,4-D), [Mg(C 8 H 5 Cl 2 O 3 )(H 2 O) 5 ](C 8 H 5-Cl 2 O 3 )Á0.5H 2 O, the discrete cationic MgO 6 complex unit comprises a carboxylate O-donor from a monodentate 2,4-D anionic ligand and five water molecules, resulting in a slightly distorted octahedral coordination sphere. The free 2,4-D anions are linked to the complex units through duplex water-carboxylate O-HÁ Á ÁO hydrogen bonds through the coordinating water molecules. In the crystal, inter-unit O-HÁ Á ÁO hydrogen-bonding interactions involving coordinating water molecules as well as the solvent water molecule (occupancy 0.5) with carboxylate O-atom acceptors, give a layered structure lying parallel to (001), in whichligand-cation interactions [minimum ring centroid separation = 3.6405 (17) Å ] and a short O-HÁ Á ÁCl interaction are also found.

Structural commentary
In the title complex ( Fig. 1), the discrete MgO 6 complex units have, as expected, essentially octahedral stereochemistry [Mg-O bond length range = 2.031 (2)-2.094 (2) Å ], comprising a carboxylate O-donor from a monodentate 2,4-D À ligand and five water molecules. The free 2,4-D À counteranion is linked to the complex unit through an unusual duplex water-carboxylate O-HÁ Á ÁO hydrogen-bonding association involving the coordinating water molecules O1W and O2W (Table 1), giving a cyclic ring motif incorporating the Mg 2+ cation [graph set R 2 2 (8)]. Except for the presence of the hemihydrate molecule of solvation, the title complex is very similar to that of the Mg complex with the analogous phenoxy herbicide, (2,4,5-trichlorophenoxy)acetic acid (Smith et al., 1982).

Supramolecular features
In the crystal of the title compound, inter-unit O-HÁ Á ÁO hydrogen-bonding interactions (Table 1) involving all coordinating water molecules, as well as the hemihydrate solvent molecule, with carboxylate O-atom acceptors, give a layered structure lying parallel (001) (Fig. 2). Within these layers, weak interactions between centrosymmetrically related 2,4-D ligand-anion species AÁ Á ÁB i are also found. The 2,4-D À molecules lie parallel to (101)   Molecular configuration and atom-naming scheme for the title compound, with displacement ellipsoids drawn at the 40% probability level. Inter-species hydrogen bonds are shown as dashed lines.

Synthesis and crystallization
The title compound was synthesized by the addition of excess MgCO 3 to 15 ml of a hot aqueous solution of (2,4-dichlorophenoxy)acetic acid (0.1 mmol) in ethanol-water (1:10 v/v).
After completion of the reaction, excess MgCO 3 was removed by filtration and the solution was allowed to evaporate at room temperature, providing colourless prisms of the title compound from which a specimen was cleaved for the X-ray analysis.

Refinement details
Crystal data, data collection and structure refinement details are summarized in 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (