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

Alan R Kennedy,Olivier Puissegur,John E Warren,Edward Staunton,Linda K Conway,Karen M Mccarney,Jennifer B A Kirkhouse,Simon J Teat

doi:10.3390/cryst10080662

Alan R Kennedy, Olivier Puissegur + Show 6 more

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https://doi.org/10.3390/cryst10080662

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Abstract

Crystallographic studies of monosulfonated azo dyes have concentrated on the salt forms that contain the azo anion. Here we present a study that compares the structures of these anions with protonated free acid forms and with doubly deprotonated dianion forms. To this end, the new single crystal diffraction structures of 13 systematically related free acid forms of monosulfonated azo dyes are presented, together with three new structures of doubly deprotonated forms and two new structures of Na salt forms of azo anions. No structures of dideprotonated monosulfonated azo dyes have previously been reported and this paper also reports the first crystal structure of an azo dye with a hydronium cation. The geometries of the free acid, anion and dianion forms are compared to literature equivalents. It is shown that protonation of the azo bond gives predictable bond lengthening and shortening, which is of a greater magnitude than similar effects caused by azo-hydrazone tautomerisation, or the smaller again effects caused by the resonance electron donation from O or N based substituents. The dianion containing structures have twisted dianion geometries that can be understood based on the resonance effects of the phenoxide groups and upon the needs to bond to a relatively high number of metal cations.

Highlights

Azo colourants are one of the most widespread and widely used class of dyes
We present neware crystal we present 13 new crystal structures of free acid forms of monosulfonated azo dyes. 13
Azo dyes were either obtained from commercial suppliers (H4, H7, H8, H10, and H11 were supplied by Fujifilm and Na12 by Sigma Aldrich) or synthesised by the well-known azo coupling supplied by Fujifilm and Na12 by Sigma Aldrich) or synthesised by the well-known azo coupling method [1,4,6,17]

Summary

Introduction

Azo colourants are one of the most widespread and widely used class of dyes. They often bear sulfonate substituents, which impart enhanced aqueous solubility and help decrease toxicity [1,2,3].The popularity of azo compounds as dyes and pigments is at least partly because of the ease with which the azo coupling reaction can accommodate a wide range of different functionalities—allowing simple and aqueous access to a large variety of compounds all with different material and colour properties [4].The ability to quickly form many closely related molecules, that are all variations on a theme, makes azo compounds a good choice for systematic structural studies. Azo colourants are one of the most widespread and widely used class of dyes. They often bear sulfonate substituents, which impart enhanced aqueous solubility and help decrease toxicity [1,2,3]. The popularity of azo compounds as dyes and pigments is at least partly because of the ease with which the azo coupling reaction can accommodate a wide range of different functionalities—allowing simple and aqueous access to a large variety of compounds all with different material and colour properties [4]. Crystallographic work on salt forms of sulfonated azo pigments is available. The extra difficulty of working with materials that typically have

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