What’s the Madder? Characterization of Old Fashioned Alizarin/Aluminum Red Pigments Using Liquid and Solid-State NMR

This work provides significant insight into the molecular structure of alizarin lake pigments used by artists in the past. To characterize two red powders, lakes 1 and 2, obtained by complexation of 1,2-dihydroxy anthraquinone (alizarin) with Al3+, a multi-analytical approach was designed based on solid and liquid state Nuclear Magnetic Resonance Spectroscopy (NMR), Fourier-Transform Infrared Spectroscopy (FTIR), Mass Spectrometry (MS) and Density Functional Theory (DFT) calculations. Lake 1 was synthesized according to literature and compared with lake 2, a reproduction of an artist’s pigment. FTIR showed Al3+ coordinated to oxygens in C1 and C9, and that in lake 2 the -OH groups in C2 are protonated, being responsible for its low solubility. 1H-NMR proved that lake 2 is formed by two tautomers [Al(Aliz-2-H-)2(OH)(H2O)] and [Al(Aliz-10-H-)2(OH)(H2O)], the latter being the major species. SS-NMR was the only technique that got insight into the Al3+ coordination, octahedral for both lakes. It confirmed the existence of two species in lake 2, in a 5:1 ratio. Both are amorphous “open structures”, resulting in fewer constraints for the ligands and in a large variety of geometries. SS-NMR allowed the analysis of the red pigments without preparation, which is a unique advantage for their study in artworks.