Hydrogen-bonded supramolecular structures in co-crystals of β- or ζ-diketone enols with 2,6-diaminopyridine or 2,4-diaminopyrimidine

Abstract

Preparation and single-crystal X-ray structure determination of five co-crystals of conjugated β- or ζ-diketoenolate anions (1,3-cyclopentanedionate, 1,3-cyclohexanedionate, 5-methyl-1,3-cyclohexanedionate and tropolonate) with aromatic protonated nitrogen bases (2,6-diaminopyridinium and 2,4-diaminopyrimidinium) are reported. Their packings are discussed in terms of the relative energies of the intermolecular hydrogen bonds formed, calculated by the Lippincott and Schroeder method, showing that such bonds largely determine the supramolecular net of interconnections within the crystal. The β- and ζ-diketoenolate anions are shown to have specific properties, being π-conjugated fragments with two terminal oxygens sharing a single negative charge that is partially displaced on one side or another according to the strength of the hydrogen bonds formed. The β-cyclohexanedionate anion displays, moreover, the ability to form a dimeric association with its neutral form through a very strong central [O–H⋯O]⁻ bond of charge-assisted nature.