Synthesis and characterisation of unsymmetrical Schiff bases derived from 3,4-diaminopyridine. Crystal and molecular structure of hydrogen-bonded dimers of 3-{[(4-aminopyridin-3-yl)amino]methylene}pentane-2,4-dione

3,4-Diaminopyridine reacts with 3-ethoxyvinylidene-2,4-pentanedione to form mainly the product of 1:2 substitution 1. 3-{[(4-Aminopyridin-3-yl)amino]methylene}pentene-2,4-dione 2 is isolated as the monosubstitution product. Reaction of this 'half-unit' with salicylaldehyde results in the formation of the unsymmetrical Schiff base 3. The molecular structure of 3-{[(4-Aminopyridin-3-yl)amino]methylene}pentene-2,4-dione 2 has been determined by the single crystal X-ray method. Compound 2 forms intermolecular hydrogen-bonded dimers in the crystalline state. The crystal structure of 2 has clearly established that the reaction occurs at the amino group at 3-position of pyridine ring. The dominant tautomeric form of the compounds in solution has been established.