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; systematic name of dication: 4‐[2‐(3,4‐dimethylphenyl)hydrazin‐1‐ylidene]‐3‐oxo‐3,4‐dihydronaphthalene‐2,7‐disulfonate}, also known as Acid Red 26, CI 16150, and of five divalent metal cation compounds of Crystal Scarlet {CS; systematic name of dication: 8‐[2‐(naphthalen‐1‐yl)hydrazin‐1‐ylidene]‐7‐oxo‐7,8‐dihydronaphthalene‐1,3‐disulfonate}, also known as Acid Red 44, CI 16250, are presented. These are hexaaquamagnesium(II) PX dimethylformamide (DMF) monosolvate, [Mg(H2O)6](C18H14N2O7S2)·C3H7NO, (I); heptaaquacalcium(II) PX 2.5‐hydrate, [Ca(H2O)7](C18H14N2O7S2)·2.5H2O, (II); catena‐poly[aqua(μ‐DMF)tris(DMF)bis(μ3‐PX)distrontium(II)], [Sr(C18H14N2O7S2)(C3H7NO)2(H2O)0.5]n, (III); the transition‐metal series hexaaquametal(II) PX DMF monosolvate, [M(H2O)6](C18H14N2O7S2)·C3H7NO, where M (metal) = Co, (IV), Ni, (V), Cu, (VI), and Zn, (VII); heptaaquacalcium(II) CS monohydrate, [Ca(H2O)7](C20H13N2O7S2)·H2O, (VIII); octaaquastrontium(II) CS monohydrate, [Sr(H2O)8](C20H13N2O7S2)·H2O, (IX); catena‐poly[[triaqua(DMF)barium(II)]‐μ‐CS], [Ba(C20H13N2O7S2)(C3H7NO)(H2O)3]n, (X); tetrakis(DMF)(CS)copper(II) monohydrate, [Cu(C20H13N2O7S2)(C3H7NO)4]·H2O, (XI); and catena‐poly[[[aquatris(DMF)zinc(III)]‐μ‐CS] diethyl ether hemisolvate], {[Zn(C20H13N2O7S2)(C3H7NO)3(H2O)]·0.5C4H10O}n, (XII). In all cases, the structures obtained were solvates with dimethylformamide (DMF) and/or water present. The disulfonated naphthalene‐based azo anions adopt hydrazone 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 the formula [M(H2O)6][PX]·DMF. The Ca salt of PX also has an SSIP structure, but has a higher hydration state, [Ca(H2O)7][PX]·2.5H2O. In contrast, the Sr salt of PX, [Sr(PX)(DMF)2(H2O)0.5]n forms a one‐dimensional coordination polymer. Both the Ca and the Sr salt of CS have an SSIP structure, namely [Ca(H2O)7][CS]·H2O and [Sr(H2O)8][CS]·H2O, whilst the heavier Ba analogue, [Ba(CS)(DMF)(H2O)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(H2O)]·0.5Et2O}n. 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 enviro