Disulfonated azo dyes: metal coordination and ion-pair separation in twelve M ^II compounds of Ponceau Xylidine and Crystal Scarlet

Abstract

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 environment, whilst the Zn species is a one‐dimensional coordination polymer based on octahedral Zn centres.

Monosulfonated Azo Dyes: A Crystallographic Study of the Molecular Structures of the Free Acid, Anionic and Dianionic Forms

“…Here the changes in length of the azo fragment are larger than those seen above, and fit well with the presence of some proportion of hydrazone tautomer [45]. It is interesting that for all known structures of sulfonated azo dyes with OH ortho to the azo bond, those based on phenol rings are found to exist as the azo form whilst those based on napthol ring systems exist as hydrazone forms [5,10,15]. It has been noted that for related hydrazone species the largest bond length change comes not for the azo bond but for the C7-N2 bond [10,15]; this is also true for Na5.…”

“…The azo bonds are thus approximately the same length as those in the equivalent free acid forms, but do not have the large decreases in N-C7 bond length seen for the free acid forms. Both on protonation and with respect to tautomerisation effects, the N-C7 bond changes more than the azo bond [10]. However, on dideprotonation, it is the azo bond that is most changeable.…”

“…It is interesting that for all known structures of sulfonated azo dyes with OH ortho to the azo bond, those based on phenol rings are found to exist as the azo form whilst those based on napthol ring systems exist as hydrazone forms [5,10,15]. It has been noted that for related hydrazone species the largest bond length change comes not for the azo bond but for the C7-N2 bond [10,15]; this is also true for Na5. The significant changes in bond length seen for Na5, Table 2, come about not from a push pull mechanism but through the tautomeric effects seen in Scheme 3.…”

“…The ability to quickly form many closely related molecules, that are all variations on a theme, also makes azo compounds a good choice for systematic structural studies. Relatively large-scale solid-state structural studies on salt forms of monosulfonated azo dyes have been published [5][6][7], as has work on salt forms of disulfonated azo dyes [8][9][10]. Crystallographic work on salt forms of sulfonated azo pigments is also available.…”

“…Crystallographic work on salt forms of sulfonated azo pigments is also available. However, the extra difficulty of working with materials that typically have 7], as has work on salt forms of disulfonated azo dyes [8][9][10]. Crystallographic work on salt forms of sulfonated azo pigments is also available.…”