Structural tautomerism of 4-acylpyrazolone schiff bases and crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one

The Schiff base derivatives prepared from 4-acetyl-5-methyl-2-phenyl-2,4-dihydro-pyrazol-3-one and alkyl amines are shown to remain exclusively in the amine-one(I) tautomeric form in chloroform solutions at room temperature using a combination of (1)H, (13)C, APT, COSY, HMQC, and HMBC NMR spectroscopic methods. The crystal structure of 5-methyl-2-phenyl-4-{1-[(pyridin-2-ylmethyl)-amino]-ethylidene}-2,4-dihydro-pyrazol-3-one showed that this 4-acylpyrazolone Schiff base stays in the amine-one(I) form in the solid state as well, and the solid state structure supports the fact that strong hydrogen bonding between amine hydrogen and the pyrazolone C(3) carbonyl oxygen helps to stabilize the amine-one(I) tautomer.