Crystalline hosts derived from pyrazole carboxylic acids. X-ray crystal structures of 4-nitropyrazole-3,5-dicarboxylic acid and its sodium salt including dioxane molecules

Abstract The pyrazole rings in the title compounds show a standard geometry with the substituents either coplanar (carboxylic groups) or perpendicular to the ring (nitro groups). Of the three acidic hydrogens of compound 1, the anion of the sodium salt involves the 3-carboxylic group. The crystal packing of both compounds is dominated by strong O-H…O and N-H…O hydrogen bonds. In the case of the salt, the Na cation is six-coordinated in an octahedral environment. An analysis of 169 sodium coordination geometries, retrieved from 123 structures in the Cambridge Structural Database, reveals coordination numbers from 2 to 8 when only nitrogen and oxygen atoms are involved. By far, coordination number 6 appears to be the most common one, about 75% of the reported cases, 50% of them being octahedral.