0022-4766/13/5405-0952 © 952 Journal of Structural Chemistry. Vol. 54, No. 5, pp. 952-959, 2013 Original Russian Text Copyright © 2013 by S. I. Levchenkov, L. D. Popov, I. N. Shcherbakov, G. G. Aleksandrov, A. A. Zubenko, V. A. Kogan TAUTOMERISM OF SUBSTITUTED SALICYLALDEHYDE AND 2-DIPHENYLPHOSPHINEBENZALDEHYDE 1′-PHTHALAZINYLHYDRAZONES: X-RAY CRYSTALLOGRAPHY AND QUANTUM CHEMICAL MODELING S. I. Levchenkov, 1 L. D. Popov, 2 I. N. Shcherbakov, 2 G. G. Aleksandrov, 3 A. A. Zubenko, 4 and V. A. Kogan 2 UDC 539.27:544.18:547.556.93 1′-Phthalazinylhydrazones of salicylaldehyde, its substituted derivatives, and 2-diphenylphosphine- benzaldehyde are synthesized and studied. A description is given of the structures salicylaldehyde 1′- phthalazinylhydrazone (1а) and 2-diphenylphosphinebenzaldehyde 1′-phthalazinylhydrazone (2), which exist in the crystal in the hydrazonophthalazone tautomeric form. Molecules of hydrazone 1а form in the crystal infinite stacks of hydrogen bonded dimers with intermolecular π-stacking interactions. A quantum chemical calculation is made of the geometry and total energy of the possible tautomers in vacuum and in aqueous and chloroform solutions. The hydrazonophthalazone tautomers are shown to be the most stable in all cases. The X-ray crystallography results are compared with the calculated data. DOI: 10.1134/S0022476613050168 Keywords: hydrazones, tautomerism, X-ray crystallography, hydrogen bond, density functional theory. The reason for the interest in 1-hydrazinophthalazine (hydralazine) and its derivatives and their complexes with transition metals is the inherent biological activity of these compounds, especially their hypotensive, antiinflammatory, and anticancer activity [1-5]. An additional incentive is the fact that hydralazine metabolism involves the interaction with the carbonyl compounds formed in the Krebs cycle [6-8]. The literature describes several hydralazine hydrazones with a number of mono- and dicarbonyl compounds [9-16]. However, until recently there have been only two papers on salicylaldehyde 1′- phthalazinylhydrazones [17, 18] although hydrazones of salicylaldehyde and its substituted derivatives are “classical” ligand systems for modern coordination chemistry. It should be noted that these, as well as other studies, proceed from the assumption that 1′-phthalazinylhydrazones exist in the hydrazone tautomeric form. However, a number of studies using X-ray crystallography and quantum mechanical calculations have shown that the hydrazonophthalazone (diazine) tautomer is the main tautomeric form of this type of compounds [11, 14-16, 19, 20]. This paper presents the results of physicochemical studies and quantum chemical modeling of 1′-phthalazinyl- hydrazones of substituted salicylaldehydes (1) and 2-diphenylphosphinebenzaldehyde (2). The aldehyde component of the 1 Southern Scientific Center, Russian Academy of Sciences, Rostov-on-Don, Russia; s.levchenkov@gmail.com. 2 Chemistry Department, Southern Federal University, Rostov-on-Don, Russia. 3 N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia. 4 North-Caucasian Zonal Research Veterinary Institute, Novocherkassk, Russia. Translated from Zhurnal Strukturnoi Khimii, Vol. 54, No. 5, pp. 914-922, September-October, 2013. Original article submitted February 2, 2013.