Molecular and crystal structure of sodium diatrizoate, an X-ray contrast agent

The single-crystal structure of the X-ray contrast agent sodium diatrizoate has been determined by X-ray crystallography using Sodium-3,5-diacetamido-2,4,6-triiodobenzoate, sodium C11H8I3NaN2O4 . 4H(2)O, M(W) = 707.96 g mol(-1), crystallizes in the triclinic space group <P(1)over bar>, Z=4 with unit-cell dimensions a=11.754(2), b=13.091(3), c=15.129(4) Angstrom, alpha=76.32(2), beta=84.56(2), gamma=63.50(1)degrees and V=2024(1) Angstrom(3). R=0.027 for 7678 unique reflections. There are two crystallographically independent diatrizoate ions in the crystal which have essentially the same conformation. Both ions have an endo,endo conformation (the carbonyl moiety of the acetanilido group pointing towards the ring, endo, or away from it, exo). The carbonyl moieties of the acetanilido groups are located on the same side of the aromatic ring, cis, for both ions. This gives one side that is more hydrophilic, where also the sodium ions are joined to diatrizoate through a carboxylate oxygen atom. As a consequence of disordered water oxygen atoms in the coordination sphere, one of the sodium ions (Na1) is either five- or six-coordinated, while the latter sodium ion (Na2) is four- or five-coordinated. The mean sodium-oxygen bond length is 2.37(2) Angstrom. There is an extensive intermolecular hydrogen-bonding network in the crystal.