Alkali metal salts of 4-hy­droxy­benzoic acid: a structural and educational study

As part of an educational exercise designed to introduce school students to the technique of single-crystal X-ray diffraction and enhance their understanding of primary and secondary bonding, a group of nine secondary school students was given the opportunity to prepare new com­pounds and to solve and refine data collected on the crystalline materials they had prepared. Their investigation of the alkali metal salts of 4-hy­droxy­benzoic acid (H(2)hba) yielded nine new com­pounds and their structures are described in this article. Whilst the salts might be expected to have similar atomic arrangements, there are significant differences in their structures. Although H(2)hba is a relatively simple organic mol­ecule, it displays remarkable coordinative flexibility, forming ionic solids containing the uncharged mol­ecule, the monoanion Hhba(−) or the dianion hba(2−). A common feature of the structures is their layered arrangement: alternating hydro­philic layers made up of closely packed metal–oxygen polyhedra separated by the hydro­phobic com­ponent of the hy­droxy­benzoate linking units. Close packing of these units seems to be a dominant influence in determining the overall structure. The hy­droxy­benzoate units are usually both parallel and anti­parallel with their immediate neighbours, with packing that can be edge-to-face, face-to-face or a mixture of the two. Hydrogen bonding plays a key role in the structure of most com­pounds and a short strong hydrogen bond (SSHB) is observed in two of the networks. The com­pounds of 4-hy­droxy­benzoic acid, C(7)H(6)O(3), described here are: poly[di-μ-aqua-μ-4-oxidobenzoato-dilithium], [Li(2)(C(7)H(4)O(3))(H(2)O)(2)]( n ), 1, poly[tri­aqua-μ-4-oxidobenzoato-dilithium], [Li(2)(C(7)H(4)O(3))(H(2)O)(3)]( n ), 2, poly[μ-4-hy­droxy­benzoato-lithium], [Li(C(7)H(5)O(3))]( n ), 3, catena-poly[4-hy­droxy­benzoate [[di­aqua­sodium]-di-μ-aqua]], {[Na(H(2)O)(4)](C(7)H(5)O(3))}( n ), 4, poly[di-μ-aqua-aqua-μ-4-hy­droxy­benzoato-potassium], [K(C(7)H(5)O(3))(H(2)O)(3)]( n ), 5, poly[μ-aqua-μ-4-hy­droxy­benzoato-potassium], [K(C(7)H(5)O(3))(H(2)O)]( n ), 6, poly[aqua-μ-4-hy­droxy­benzoato-rubidium], [Rb(C(7)H(5)O(3))(H(2)O)]( n ), 7, poly[aqua-μ-4-hy­droxy­benzoato-caesium], [Cs(C(7)H(5)O(3))(H(2)O)]( n ), 8, poly[[μ-aqua-aqua­(μ-4-hy­droxy­benzoato)(4-hy­droxy­benzoic acid)sodium] monohydrate], {[Na(C(7)H(5)O(3))(C(7)H(6)O(3))(H(2)O)(2)]·H(2)O}( n ), 9, poly[[(μ-4-hy­droxy­benzoato)(μ-4-hy­droxy­benzoic acid)rubidium] monohydrate], {[K(C(7)H(5)O(3))(C(7)H(6)O(3))]·H(2)O}( n ), 10, and poly[[(μ-4-hy­droxy­benzoato)(μ-4-hy­droxy­benzoic acid)rubidium] monohydrate], {[Rb(C(7)H(5)O(3))(C(7)H(6)O(3))]·H(2)O}( n ), 11.