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Structural characterization of a series of alkali metal coordination polymers of gallic acid
Ava Kreider-Mueller1, James A Golen2, Thomas A Morris1
1Department of Chemistry, Hampden-Sydney College, 62 Via Sacra, Hampden-Sydney, VA 23943, USA.
Abstract:
This work aims to reveal the rich coordination chemistry of gallic acid (H4GA), a ubiquitous plant polyphenol, toward alkali metals. To date, only three crystal structures have been reported in the literature that feature an alkali metal, namely Na, K, and Cs, in combination with a deprotonated form of gallic acid. Of those three structures, only the Cs compound, which is an alkali metal-based coordination polymer of gallic acid, features direct coordination between the deprotonated acid and the alkali metal center. This work reports the crystal structures of six new alkali metal-based coordination polymers of gallic acid, including poly[diaqua(μ-3,4,5-trihydroxybenzoato)(μ-3,4,5-trihydroxybenzoic acid)lithium], [Li(C7H5O5)(C7H6O5)(H2O)2]n, 1, poly[diaqua-μ-aqua-(μ-3,4,5-trihydroxybenzoato)sodium], [Na(C7H5O5)(H2O)3]n, 2, poly[[tetraaquabis(μ-3,4,5-trihydroxybenzoato)bis(3,4,5-trihydroxybenzoic acid)disodium] tetrahydrate], {[Na2(C7H5O5)2(C7H6O5)2(H2O)4]·4H2O}n, 3, poly[[μ-acetato-(μ-3,4,5-trihydroxybenzoic acid)potassium] monohydrate], {[K(C7H6O5)(C2H3O2)]·H2O}n, 4, poly[[μ-acetato-(μ-3,4,5-trihydroxybenzoic acid)rubidium]monohydrate], {[Rb(C7H6O5)(C2H3O2)]·H2O}n, 5, poly[diaquabis(μ-3,4,5-trihydroxybenzoato)-bis(μ-3,4,5-trihydroxybenzoic acid)dicaesium], [Cs2(C7H5O5)2(C7H6O5)2(H2O)2]n, 6. Single crystals suitable for X-ray diffraction studies were obtained from ethanolic solutions of H4GA that were treated with a series of aqueous solutions of alkali metal acetates, MOAc (M = Li, Na, K, Rb, and Cs). Seven different molar ratios of MOAc to H4GA (1:1, 1:2, 1:3, 1:4, 2:1, 3:1, and 4:1) were tested for each reaction combination. The dimensionality of the six coordination polymers reported varies across this series of compounds, ranging from one-dimensional chains (3) to two-dimensional sheets (1 and 2), and complex three-dimensional coordination network solids (4-6). Interestingly, the K (4) and Rb (5) compounds are isomorphous, while the sodium salt crystallizes in two forms (2 and 3).
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