Synthesis,molecular structure and reactivity of sodium 5-sulfosalicylate dihydrate and sodium[triaqua(5-sulfosalicylato)copper(II)] 2 hemihydrate

The crystal and molecular structure of sodium 5-sulfosalicylate dihydrate, Na(H₂Ssal)(H₂O)₂], (1) (H₃Ssal=5-sulfosalicylic acid) has been determined through X-ray diffraction analysis. The 5-sulfosalicylate anion has lost the proton at the SO₃H group but retains the usual intermolecular hydrogen bond between phenolic and carboxylic oxygen. The reaction in water of 1 with Cu(II)(H₂O)₄]SO₄·H₂O, gives rise to the green sodiumtriaqua(5-sulfosalicylato)copper(II)] 2 hemihydrate, Na(H₂O)₃(Ssal)Cu(II)]·2×0.5H₂O, (2). The 5-sulfosalicylate anion, (Ssal³⁻), coordinates rather unusually in the syn–syn coordination mode since it binds bidentately the Cu(II) ion through the carboxylic and the phenolic oxygens, with Cu(II)O_carboxylic=1.909(4) Å and Cu(II)O_phenolic=1.885(4) Å distances. Copper(II) completes its square-planar coordination with two water molecules and in addition, perpendicularly to the square-planar coordination plane, another two water molecules with long bonds are present (Cu(II)O=2.518 and 2.912 Å). The green complex 2 reacts easily with adenine in water at pH 7 giving rise to the violet tetraadeninato(diaqua)-bis(copper(II)) dihydrate, Cu₂(Ade)₄(H₂O)₂])]·2H₂O, (3) (Ade⁻=adeninato monoanion). This complex, that geometrically resembles copper(II) acetate monohydrate, was already described by Sletten. Finally, on the basis of the present results a possible mechanism for the anticancer activity of complex 2 and of other Cu(II)–salicylate complexes is proposed and discussed.