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

Abstract

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