Intermolecular interactions of sulfonated azo dyes: crystal structures of the diammonium, dilithium, magnesium and calcium salts of 7-hydroxy-8-(phenylazo)-1,3-naphthalenedisulfonic acid (Orange G)

The crystal structures of four salts containing the Orange G dianion have been determined, three at low temperature. Molecular geometry and conformation of the dye molecule are closely similar from structure to structure. In each case, the molecule exits as the hydrazo tautomer rather than as the azo tautomer. A recurring close intermolecular contact is a bridging interaction in which two water molecules coordinated to a given metal atom are hydrogen bonded to two atoms of a single sulfonate group