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Lithol Red: a systematic structural study on salts of a sulfonated azo pigment
Alan R Kennedy1, Heather Stewart, Katherine Eremin
1WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL UK. a.r.kennedy@strath.ac.uk
Chemistry (Weinheim an Der Bergstrasse, Germany)
|February 3, 2012
Summary
This study presents the first crystal structures of Lithol Red azo lake pigments with various cations. Their structures can be predicted from related azo dye models, except for solvates.
Area of Science:
- Materials Science
- Crystallography
- Coordination Chemistry
Background:
- Azo lake pigments, like Lithol Red, are commercially and culturally significant.
- Understanding their crystal structures is crucial for predicting their properties and applications.
- Previous studies focused mainly on calcium-based pigments.
Purpose of the Study:
- To systematically determine and analyze the single-crystal diffraction structures of Lithol Red pigments with diverse cations (Mg(II), Ca(II), Sr(II), Ba(II), Na(I), Cd(II)).
- To investigate the predictability of these pigment structures based on a database of related sulfonated azo dye structures.
- To explore the coordination chemistry and tautomeric forms of Lithol Red anions.
Main Methods:
- Single-crystal X-ray diffraction was employed to determine the structures of various Lithol Red pigment salts.
- A database of related sulfonated azo dye structures was constructed for comparative analysis and prediction.
- Structural comparison between different cation salts and with known azo dye models.
Main Results:
- The study presents the first non-calcium Lithol Red pigment structures, including Mg(II), Sr(II), Ba(II), Na(I), and Cd(II) salts.
- Structural behavior of most Lithol Red pigments is predictable from related sulfonated azo dye models, with Mg(II) forming solvent-separated ion pairs and heavier alkaline-earth metals forming contact ion pairs.
- All Lithol Red anions exist as planar hydrazone tautomers, and a common packing mode of inorganic layers and organic bilayers was observed, except for the Na(I) DMF solvate.
Conclusions:
- The crystal structures of Lithol Red pigments can be largely predicted using a database of related sulfonated azo dyes, highlighting the utility of model compounds.
- The Mg(II) and Cd(II) salts exhibit distinct structural characteristics compared to Ca(II), Sr(II), and Ba(II) salts.
- Linked eight-membered [MOSO](2) rings represent a fundamental coordination motif in sulfonated azo pigments lacking competing carboxylate groups.
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