On the isomerism/tautomerism of hydrazones. Crystal structures, study in solution and theoretical calculations of new series of alpha-N-heterocyclic hydrazones

The isomerism/tautomerism of new series of alpha -N-heterocyclic hydrazones with chelating properties towards metal ions and potential biological acti vity have been studied in the solid state by X-ray crystallography and in s olution by nuclear magnetic resonance. Among possible syn-Z/anti-E isomers and different tautomeric structures, the molecular structures of dimethylqu inolylhydrazone 5 and its phthalazinyl analogue 6 have shown that both are anti-E isomers. In 5 the mobile proton resides on the hydrazone nitrogen, b ut in the phthalazine derivative 6 it is located on one endocyclic nitrogen . The same isomeric structures are also confirmed in the solution by NMR ex periments. While for 6 no other isomer is detected, the quinoline derivativ es 1-5 show chemical equilibrium between the imino and amino tautomers in t he solution, the latter being present in a very high amount (> 90%). The st udy has also been extended to potential energy calculations of isolated mol ecules and of molecules in the crystal lattice.