Azo pigments and their intermediates: A CP MAS 13C-NMR study of the tetrazotized products of 1,5-diamino-4,8-dihydroxyanthraquinone

Abstract

An attempt to use 1,5-diamino-4,8-dihydroxyanthraquinone ( 1) as a precursor to synthesize bisazo pigments, by an azotization-coupling reaction sequence, is reported. The tetrazotization of 1, by a slight excess of sodium nitrite, is effective both in concentrated hydrochloric acid and concentrated sulfuric acid. In concentrated hydrochloric acid, the tetrazonium product was isolated as the tetrafluoroborate salt, 3, by adding fluoroboric acid into the azotized solution. In concentrated sulfuric acid, the tetrazonium product was isolated as the sulfate salt, 4, by simply diluting the azotized solution with ice-water. The structures of 3 and 4 were characterized by IR and crosspolarization magic angle spinning (CP MAS) 13C-NMR spectroscopy, as well as solution NMR measurements. Due to their high acidity, 3 and 4 deprotonate to form 5, an insoluble purple-brown solid in dilute aqueous solution or in organic solvents. Compound 5 only dissolves in acidic solutions and, since this involves a protonation reaction, it can only be characterized by solid-state techniques. In this work, the structure of 5 was established by IR and CP MAS 13C-NMR spectroscopy. Two characteristic carbonyl stretchings at 1670 and 1655 cm −1 are observed in the IR spectrum and two carbonyl carbons at 176.8 and 178.2 ppm are observed in the solid-state 13C-NMR spectrum. The protonation-deprotonation relationships between 3, 4 and 5 were established from their solution and solid-state NMR spectra. The facile deprotonation of 3 and 4 and the insolubility of 5 in any solvents are shown to be a hindered factor for the synthesis of bisazo pigments.