Polymorphism in an unexpected caesium complex of 5‐hydroxy­hydurilic acid

Vigorous reaction of barbituric acid with caesium hydroxide in water resulted in an unexpected coupling product, 5‐hydroxy­hydurilic acid, complexed to caesium, giving poly[[caesium‐μ 5 ‐5‐hydroxy­hydurilato] hemihydrate], {[Cs(C 8 H 5 N 4 O 7 )]·0.5H 2 O} n . This was obtained in two different polymorphic forms, depending on the method of crystal growth. Slow solvent evaporation yielded an ortho­rhombic polymorph, (I), which crystallized in the space group Pnab (non‐standard setting of Pbcn ), with the uncoordinated water mol­ecule lying on a crystallographic twofold axis. The water mol­ecule is sandwiched into cavities within the structure and is securely held in place by N—H⋯O and O—H⋯O hydrogen bonding. Polymorph (II) is monoclinic, although the unit‐cell parameters are similar to those of polymorph (I), and it crystallizes in the space group C 2/ c , with the uncoordinated water mol­ecule again lying on the twofold axis and secured by hydrogen bonding. The differences between the two polymorphs, both of which have nine‐coordinate caesium and a similar first‐shell environment of all structural components, are in the overall arrangement of the cations, anions and water mol­ecules, and in the shape of the cavities, revealed by inspecting and comparing crystal‐packing diagrams.