Metal ion directed tautomeric polymorphism in a hydrazonamide/hydrozonate system

Abstract

Ortho and para‐aminophenyl substituted hydrazonamide ligands viz. 2‐amino‐benzoic acid (amino‐pyridin‐2‐yl‐methylene)‐hydrazide and 4‐amino‐benzoic acid (amino‐pyridin‐2‐yl‐methylene)‐hydrazide) can exist in two tautomeric forms. The hydrazonamide (keto) form of the former ligand is only observed in free ligand, while its enolate form exists in some of its metal complexes. Both hydrazonamide and hydrazonate tautomers possess a six membered intramolecular hydrogen bond between the o‐amino group and amide oxygen or imino nitrogen respectively. As a result, hydrazonamide and hydrazonate tautomeric forms exist in different molecular conformation and thus ‘tautomeric polymorphism’ is observed in this system. The evidence of tautomerism and consequent conformational change in metal complexes of 2‐amino‐benzoic acid (amino‐pyridin‐2‐yl‐methylene)‐hydrazide is well supported by both DFT and NBO studies.