A barium complex of a phenoxazinone sulfonate dye

An aqueous reaction of barium hydroxide and 3-amino-4-hy-droxy-benzene-sulfonic acid yielded a very small amount of an unexpected product, bis-(2-amino-3-oxo-3<i>H</i>-phenoxazine-8-sulfonato)-tetra-aqua-barium, [Ba(C₁₂H₇N₂O₅S)₂(H₂O)₄], (I). The compound crystallizes in the triclinic space group <i>P</i>1 with all atoms on general positions. The barium cation is 8-coordinate with three sulfonate O atoms, one carbonyl O atom, and four water mol-ecules at Ba-O distances of 2.716 (2) to 2.807 (1) Å. The two independent anions have essentially planar phenoxazinone systems with an angle of <i>ca</i> 2.4° between them. The extended structure has columns of nearly parallel anions that are bridged by the hydrated barium cations so that the overall motif consists of alternating layers of inorganic cations and organic anions. The packing is reinforced by O-H⋯O and N-H⋯O hydrogen bonds involving the coordinated water mol-ecules, amine groups, and sulfonate groups. This is the first reported structure of a metal salt of a phenoxazinone sulfonate derivative. The condensation of 3-amino-4-hy-droxy-benzene-sulfonic acid to form the phenoxazinone is known to occur in an enzyme-catalyzed reaction, though how or when it occurred here is uncertain.