Diverse coordination of aroylhydrazones toward iron(III) in solid state and in solution: spectrometric, spectroscopic and computational study

The coordination properties of N′-(2-hydroxy-3-methoxyphenylmethylidene)-3-pyridinecarbohydrazide (H2L1), N′-(2-hydroxy-4-methoxyphenylmethylidene)-3-pyridinecarbohydrazide (H2L2) and N′-(2-hydroxy-5-methoxyphenylmethylidene)-3-pyridinecarbohydrazide (H2L3) toward Fe(III) ions were studied by computational, spectrometric (MS) and spectroscopic methods (UV–Vis, IR and Raman spectroscopy) in solid state and in solution. Free ligands were present in keto-amine form with intramolecular H-bond. In MeOH:H2O 1:1 system, the 1:1 complexes with Fe(III) were formed, characterized by lgK ≥ 6. The coordination to the metal ion was achieved via oxygen and azomethine nitrogen since the hydrolysis of hydrazone bond was suppressed. Unlike the 1:1 stoichiometry in methanolic solution, the composition of the complexes extracted to chloroform was Fe(L)(HL). The release of three protons upon complexation was determined by independent spectrophotometric measurements. The complexes isolated from MeOH/EtOH solution have also stoichiometry 1:2. However, depending on the position of the methoxy substituent, two types of complexes were formed. In Fe(H2L1)2Cl3 and Fe(H2L3)2Cl3, hydrazones acted as neutral ligands, while in Fe(HL2)2Cl the keto-enol tautomeric interconversion and release of one proton per ligand took place. All complexes were analyzed in gas phase as well, using triple quadrupole, ion trap and H/D exchange for determination of labile hydrogens. Based on the fragmentation pathways, the structural isomers were distinguished.