A Novel Barium Complex of a Phenoxazinone Sulfonate Dye

A reaction of 3-amino-4-hydroxybenzenesulfonic acid and barium hydroxide in water has yielded an unexpected product, a bis(2-amino-3-oxo-3H-phenoxazinone-8-sulfonato)tetraaquabarium complex Ba(O3SC12H7N2O2)2(H2O)4. The compound crystallizes as small red needles in the triclinic space group P1-bar with two formula units in the cell. The barium cation is 8-coordinate with Ba–O distances to three sulfonate O atoms, one carbonyl O atom, and four water molecules ranging from 2.716(2) to 2.807(1) Å. The structure is built of layers of nearly parallel 2-amino-3- oxo-3H-phenoxazinone-8-sulfonate anions (interplanar separation ca. 3.5 Å) alternating with layers of the hydrated barium cations. The packing is further reinforced by O–H…O hydrogen bonds between coordinated water molecules and sulfonate O atoms. We subsequently learned that the condensation reaction that converted 3-amino-4-hydroxybenzenesulfonic acid to 2-amino-3-oxo-3H-phenoxazinone-8-sulfonic acid was reported to occur via an enzymatic pathway involving a fungus (Forte et al., 2010). The compound belongs to a large class of compounds that are of interest as dyes and are the subject of a US patent application. The barium complex was a trace product of our reaction and it is presently unknown whether the 2-amino-3-oxo-3H-phenoxazinone-8-sulfonic acid is present in the thirty year old 3-amino-4-hydroxybenzenesulfonic acid reagent or formed in situ during the reaction.