Crystal structures of the potassium and rubidium salts of (3,5-dichlorophenoxy)acetic acid: two isotypic coordination polymers

The two compounds are isotypic and the two-dimensional polymeric structure is based on centrosymmetric dinuclear bridged complex units. Within the layers, which lie parallel to (100), the coordinating water molecule forms an O—H⋯O hydrogen bond to the single bridging carboxylate O atom.


Chemical context
The phenoxyacetic acids are a particularly useful series of compounds since certain members having specific ringsubstituents have herbicidal activity, resulting in their being used commercially. Of these, the most common have been the chlorine-substituted analogues (2,4-dichlorophenoxy)acetic acid (2,4-D), (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T) and (4-chloro-2-methylphenoxy)acetic acid (MCPA) (Zumdahl, 2010). As such, the active members have received considerable attention, particularly with respect to health aspects resulting from residual breakdown components after environmental exposure. Compounds formed from their reaction with a wide range of metals have provided a significant number of crystal structures, e.g. for 2,4-D, there are 60 examples of metal complexes, contained in the Cambridge Structural Database (CSD; Groom & Allen, 2014), e.g. with Ca II (Song et al., 2002) and with Zn II (Kobylecka et al., 2012).
Metal complex formation with the phenoxyacetic acids has been facilitated by their versatility as ligands, showing various interactive modes with common metals including monodentate and bidentate-bridging coordinations involving the O carboxyl , O 1 phenoxy [(O,O) 1 ] chelate interaction, first reported for the monomeric copper(II) phenoxyacetate complex (Prout et al., 1968) and also found in the potassium-2,4-D salt (Kennard et al., 1983) as well as in the caesium complexes with 4-fluorophenoxyacetate and (4-chloro-2-methyl)phenoxyacetate (Smith, 2015a). In the caesium complex-adduct with 2,4-D (Smith & Lynch, 2014), a tridentate chelate interaction ISSN 2056-9890 variant is found which includes, in addition to the O,O 1chelate, a Cs-Cl bond to the ortho-Cl ring substituent of the ligand. Only occasional examples of the bidentate carboxylate O,O 0 -chelate interaction are found, e.g. with the previously mentioned caesium 4-fluorophenoxyacetate.
To investigate the nature of the coordination complex structures formed in the potassium and rubidium salts of the 2,4-D isomer, reactions of (3,5-dichlorophenoxy)acetic acid (3,5-D) with K 2 CO 3 and Rb 2 CO 3 in aqueous ethanol were carried out, affording the isotypic polymeric title compounds [K 2 (C 8

Figure 2
The molecular configuration and atom-numbering scheme for the isomeric K and Rb complexes with 3,5-D [(I) and (II)], with displacement ellipsoids drawn at the 40% probability level (with data taken from the potassium structure). For symmetry codes, see Table 1.
The present isotypic potassium and rubidium salts of (3,5dichlorophenoxy)acetic acid provide an example of isotypism which extends to the ammonium salt (Smith, 2015b). Isotypism is also found in the analogous NH 4 + , K + and Rb + hemihydrate salts of isomeric 2,4-D (Table 5). It may also be possible that a similar series exists with MCPA for which the structure of only the ammonium hemihydrate salt (NH 4 + MCPA À Á0.5H 2 O) is known (Smith, 2014b). It is of note that the sodium salts are not included in the sets, the structures for which are not known.

Synthesis and crystallization
Compounds (I) and (II) were synthesized by the addition of 0.5 mmol of K 2 CO 3 (65 mg) [for (I)] or Rb 2 CO 3 (115 mg) (for (II)] to a hot solution of (3,5-dichlorophenoxy)acetic acid (3,5-D) (220 mg) in 10 ml of 50% (v/v) ethanol/water. After heating for 5 min, partial room temperature evaporation of the solutions gave in all two cases, colourless needles from which specimens were cleaved for the X-ray analyses.

Refinement details
Crystal data, data collection and structure refinement details for (I) and (II) are summarized in Table 6. Hydrogen atoms were placed in calculated positions [C-H aromatic = 0.95 Å or C-H methylene = 0.99 Å ] and were allowed to ride in the refinements, with U iso (H) = 1.2U eq (C). The water H-atom in both structures was located in a difference Fourier map and was allowed to ride in the refinements with an O-H distance restraint of 0.90AE0.02 Å and with U iso (H) = 1.5U eq (O). Table 3 Hydrogen-bond geometry (Å , ) for (I). (2) 174 (2) Symmetry code: (iv) Àx þ 1; Ày; Àz þ 1. Table 4 Hydrogen-bond geometry (Å , ) for (II).

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.

(II) Poly[µ-aqua-bis[µ 3 -(3,5-dichlorophenoxy)acetato]dirubidium]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.003 Δρ max = 0.98 e Å −3 Δρ min = −1.00 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 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. Geometric parameters (Å, º)