Stoichiometry-dependent structures: an X-ray and neutron single-crystal diffraction study of the effect of reaction stoichiometry on the crystalline products formed in the potassium–cyanurate system

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Reaction of cyanuric acid (C(3)H(3)N(3)O(3); neutral form CYH(3); anionic form CYH_2;-; dianionic form CYH(2-)) with K(2)CO(3) or KOH in aqueous solution gave three different crystalline products, according to the reaction stoichiometry used. The structures of two products were easily determined by single-crystal X-ray diffraction: [K(CYH(2))(H(2)O)], (1), of which the crystal structure is already known [Marsh, R. E. et al. (2002). Acta Cryst. B58, 62-77], and [K(2)(CYH)], (2), a highly symmetrical and dense structure. Further stoichiometric variation produced another new complex, (3), and reaction of Rb(2)CO(3) with CYH(3) yielded a further product, (4), which was found to be isostructural with (3). Determination of the structures of (3) and (4) by X-ray diffraction gave a result that was chemically unreasonable owing to a charge imbalance, with the asymmetric unit apparently containing 2- against 1.5+ (partial charges as the result of crystallographic mirror symmetry). A single-crystal neutron diffraction analysis carried out on a fully deuterated sample of (3) revealed the presence of a supercell, with the c axis doubled compared with the X-ray result. Subsequent refinement of the structure with this supercell showed that it is the result of just two D atoms breaking crystallographically imposed mirror symmetry, which is otherwise essentially observed by the rest of the structure. This minor deviation from pseudo-symmetry could not be identified by X-ray diffraction. Thus, single-crystal neutron diffraction data revealed that the true chemical formula of the structure of (3) [and presumably also of the isostructural (4) with Rb instead of K and H instead of D], is [K(3)(C(3)D(2)N(3)O(3))(3)(C(3)D(3)N(3)O(3))(D(2)O)(4)], the deuterated form of [K(3)(CYH(2))(3)(CYH(3))(H(2)O)(4)].