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.

中文翻译：

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单磺化偶氮染料：晶体学研究的游离酸，阴离子和双阴离子形式的分子结构。

单磺化偶氮染料的晶体学研究集中在含有偶氮阴离子的盐形式上。在这里，我们提出一项研究，比较这些阴离子与质子化的游离酸形式和双去质子化的二价阴离子形式的结构。为此，提出了13种系统相关的单磺酸化偶氮染料的游离酸形式的新单晶衍射结构，以及三个双去质子化形式的新结构和两个偶氮阴离子的Na盐形式的新结构。以前没有报道过双质子化的单磺化偶氮染料的结构，该论文还报道了带有with阳离子的偶氮染料的第一个晶体结构。将游离酸，阴离子和二价阴离子形式的几何形状与文献等效值进行比较。已表明，偶氮键的质子化可预测的键的延长和缩短，其幅度大于偶氮-互变异构所引起的类似作用，或者是基于O或N的取代基提供共振电子而产生的较小的影响。包含二价阴离子的结构具有扭曲的二价阴离子几何形状，这可以基于酚盐基团的共振效应以及结合至相对大量的金属阳离子的需要来理解。