Disulfonated azo-dyes: metal coordination and ion-pair separation in twelve MII compounds of ponceau xylidine and crystal scarlet

The structures of seven divalent metal cation compounds of Ponceau Xylidine (PX) aka Acid Red 26, CI 16150, and of five divalent metal cation compounds of Crystal Scarlet (CS) aka Acid Red 44, CI 16250, are presented. In all cases the structures obtained were solvates with DMF and/or water present. The disulfonated naphthalene based azo anions adopt hydrozone tautomeric forms. The structures of the Mg salt and of four transition metal forms (M = Co, Ni, Cu and Zn) of PX are found to form an isostructural series. All have solvent-separated ion-pair (SSIP) type structures and formula [M(OH2)6][PX]·DMF. The Ca salt of PX also has a SSIP structure, but has a higher hydration state, [Ca(OH2)7] [PX]·2.5H2O. In contrast, the Sr salt of PX [Sr(PX)(DMF)2(OH2)0.5]n forms a one-dimensional coordination polymer. Both the Ca and Sr salts of CS have SSIP structures, namely [Ca(OH2)7][CS]·H2O and [Sr(OH2)8][CS]·H2O whilst the heavier Ba analogue [Ba(CS)(DMF)(OH2)3]n forms a one-dimensional coordination polymer. Unlike PX, two CS structures containing transition metals are found to be coordination complexes, [Cu((CS)(DMF)4]·H2O and [Zn(CS) (DMF)3(OH2)]n.n/2 Et2O. This suggests that CS is a better ligand than PX for transition metals. The Cu complex forms discrete molecules with Cu in a square pyramidal environment, whilst the Zn species is a one-dimensional coordination polymer based on octahedral Zn centres.