Stable crystalline adducts of 1,6-bis(o-hydroxyanilino)-1,5-hexadiene-3,4-dione withN,N-dimethyl carboxamides and urea derivatives

The red colored and poorly soluble 1,6-bis(o-hydroxyanilino)-1,5-hexadiene-3,4-dione (3f) forms stable adducts5 and7 of exact stoichiometric ratios withN,N-dimethyl carboxamides4 and ureas6. The adducts are yellow colored and easily soluble in organic solvents. The crystal structure of3f (monoclinic space groupP2 ₁/c witha=7.903(2),b=10.941(2),c=8.976(3) Å, =90.13(2)°) indicates planarity with extensive delocalization of the -electrons. The poor solubility is referred to the formation of strong intermolecular hydrogen bonds (H(O)...O3 andH(O)...O3=1.79 Å). The crystal structure of the 12 DMF adduct5a (monoclinic space groupP2 ₁/c witha=6.068(1),b=19.668(2),c=10.645(1) Å, =107.791(8)°) shows a less pronounced delocalization of the -electrons which might be the explanation for the color change from red to yellow. In5a the intermolecular hydrogen bonds of3f are interrupted by forming new hydrogen bonds from the hydroxyl group to the carbonyl group of DMF (H(O)...O(L) andH(O)...O(L)=1.86 Å), whereby the solubility is markedly changed. The thermal stability of the addition products5 and7 was determined by thermogravimetry.