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Two azo pigments based on beta-naphthol.

Martin U Schmidt1, Jürgen Brüning, Daniela Wirth

  • 1Institute of Inorganic and Analytical Chemistry, University of Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany. m.schmidt@chemie.uni-frankfurt.de

Acta Crystallographica. Section C, Crystal Structure Communications
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PubMed
Summary
This summary is machine-generated.

Azo-hydrazone tautomerism in pigments was investigated. The red pigments studied exist predominantly in the azo form in solid states, challenging previous assumptions and revealing unique crystal structures.

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Area of Science:

  • Pigment chemistry
  • Solid-state chemistry
  • Organic chemistry

Background:

  • Azo pigments commonly exhibit hydrazone tautomerism in the solid state.
  • Specific red pigments, 1-[4-(dimethylamino)phenyldiazenyl]-2-naphthol (1a) and 1-[4-(diethylamino)phenyldiazenyl]-2-naphthol (1b), have been an exception, with reports suggesting azo tautomerism or mixed forms.
  • Understanding tautomerism is crucial for pigment properties and applications.

Purpose of the Study:

  • To synthesize and structurally elucidate two specific red pigments: 1-[4-(dimethylamino)phenyldiazenyl]-2-naphthol (1a) and 1-[4-(diethylamino)phenyldiazenyl]-2-naphthol (1b).
  • To definitively determine the tautomeric form (azo vs. hydrazone) of these pigments in the solid state.
  • To investigate the crystal packing and structural characteristics of these compounds.

Main Methods:

  • Synthesis of compounds (1a) and (1b).
  • Recrystallization for purification and crystal growth.
  • Single-crystal X-ray diffraction analysis.
  • Difference electron-density mapping to locate hydrogen atoms.

Main Results:

  • Crystal structure analysis confirmed that the hydrogen atoms of the hydroxyl groups are bonded to the oxygen atoms, indicating the azo tautomeric form.
  • A small but detectable presence of the hydrazone tautomer was observed, suggesting these compounds exist as predominantly azo tautomers.
  • Compound (1a) exhibits a herring-bone crystal structure, while compound (1b) displays a rare double herring-bone packing.

Conclusions:

  • The studied red pigments (1a and 1b) are predominantly in the azo tautomeric form in the solid state, contrary to some prior reports.
  • These compounds represent near 'true' azo compounds, offering insights into azo-hydrazone tautomerism.
  • The distinct crystal structures (herring-bone and double herring-bone) provide valuable information on molecular packing in solid-state organic materials.