Poly[[diaquabis[μ-2-(4-fluorophenoxy)acetato-κ2 O 1:O 1′]magnesium] 0.4-hydrate]

In the title compound, {[Mg(C8H6FO3)2(H2O)2]·0.4H2O}n, slightly distorted octahedral MgO6 complex units have crystallographic inversion symmetry, the coordination polyhedron comprising two trans-related water molecules and four carboxyl O-atom donors, two of which are bridging. Within the two-dimensional complex polymer which is parallel to (100), coordinating water molecules form intermolecular O—H⋯O hydrogen bonds with carboxylate and phenoxy O-atom acceptors, as well as with the partial-occupancy solvent water molecules.

In the title compound, {[Mg(C 8 H 6 FO 3 ) 2 (H 2 O) 2 ]Á0.4H 2 O} n , slightly distorted octahedral MgO 6 complex units have crystallographic inversion symmetry, the coordination polyhedron comprising two trans-related water molecules and four carboxyl O-atom donors, two of which are bridging. Within the two-dimensional complex polymer which is parallel to (100), coordinating water molecules form intermolecular O-HÁ Á ÁO hydrogen bonds with carboxylate and phenoxy O-atom acceptors, as well as with the partial-occupancy solvent water molecules.
The title complex, [Mg(H 2 O) 2 (C 8 H 6 FO 3 ) 2 ] n (0.4H 2 O) n was obtained from the reaction of 4-fluorophenoxyacetic acid with MgCO 3 in aqueous ethanol and the structure is reported herein. In this structure ( Fig. 1 (100), the coordinated water molecules from intermolecular O-H···O hydrogen-bonding interactions (Table 2), with carboxyl and phenoxy O-atom acceptors as well as with the partial water molecules of solvation (S.O.F. = 0.2) (Fig. 2). Except for the presence of the partial water molecules, the structure is similar to the those of the isomorphous Mg complexes with phenoxyacetate and 4-chlorophenoxyacetate (Smith et al., 1980). In the present complex, the 4-fluorophenoxyacetate ligand is essentially planar, with the carboxyl group rotated slightly out of the plane [benzene ring to acetate dihedral angle = 12.26 (12)°].

Experimental
The title compound was synthesized by the addition of excess MgCO 3 to 15 ml of a hot aqueous ethanolic solution (10:1) of 4-fluorophenoxyacetic acid (0.1 g). After completion of the reaction, the excess MgCO 3 was removed by filtration and the solution was allowed evaporate to incipient dryness at room temperature, giving thin colourless plates of the title compound from which a specimen was cleaved for the X-ray analysis.

Refinement
Hydrogen atoms on the coordinated water molecule were located by difference methods and both positional and isotropic displacement parameters were initially refined but these were then allowed to ride, with U iso (H) = 1.5U eq (C). Other Hatoms were included in the refinement at calculated positions [C-H(aromatic) = 0.93 Å, 0.98 Å (methylene)] or O-H = 0.84-0.94 Å, with U iso (H) = 1.2U eq (C) or 1.5U eq (O), also using a riding-model approximation. The site occupancy factor for the partial water molecule of solvation was determined as 0.196 (4) and was subsequently fixed as 0.20.

Figure 1
The molecular structure of the title compound, including the partial water molecules of solvation (O2W), with displacement ellipsoids drawn at the 50% probability level. For symmetry codes, see Table 1.  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.29 e Å −3 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.