Synthesis Of 8-hydroxyquinoline Hydrazide Schiff Bases And Theoretical Study Of Proton Transfer

Acyl hydrazone type Schiff bases(R-CO(S)-NH-N=CH-R)are formed by the condensation of aldehydes or ketones with hydrazides.They have very strong coordination ability because of containing multiple coordination atoms,which can form complexes with metal ions by ketone type configuration or enol type configuration.Many acyl hydrazone type Schiff bases and their complexes have good biological activity,catalytic activity,bacteriostatic activity,or anti-cancer activity.The introduction of 8-hydroxyquinoline unit in acyl hydrazone type Schiff bases not only enhances coordination ability,but also improves coordinating flexibility,which makes the study of acyl hydrazone type Schiff bases bearing 8-hydroxyquinoline unit significant.In this paper,new acyl hydrazone type Schiff bases bearing 8-hydroxyquinoline unit were synthesized and characterized by IR,1H NMR,and X-ray single crystal diffraction.The proton transfer reactions including phenol proton transfer reaction and ketone-enol tautomerism of acyl hydrazone structure unit of N'-(8-hydroxyquinoline-2-methylene)-2-furancarboxylic hydrazide are studied by theoretical calculation at the B3LYP/6-31G(d)level.The main research content includes the following several parts:1.Two kinds of acylhydrazone Schiff base type compounds,N'-(8-hydroxyquinoline-2-methylene)-2-hydroxy-2-phenyl acetic hydrazide and N'-(8-hydroxyquinoline-2-methylene)-2-furancarboxylic hydrazide were prepared by the condensation of 2-formyl-8-hydroxylquinoline and corresponding hydrazide compounds.Their structures were characterized by x-ray single crystal diffraction,IR,1H NMR.The crystal structure of N'-(8-hydroxyquinoline-2-methylene)-2-hydroxy-2-phenyl acetic hydrazide is monoclinic system,P2l/n space group,in which the Schiff base double bond C=N and the quinoline ring are coplanar(the dihedral angle being 1.51°).2.The structural properties and vibrational frequencies of N'-(8-hydroxyquinoline-2-methylene)-2-furancarboxylic hydrazide were studied with the B3LYP/6-31G(d)level,stable conformers were obtained.The bond lengths and angles,the vibrational spectra of the most stable conformer with minimum energy were analyzed and compared with the experimental results.It was shown that the structure parameters and the vibration frequencies of the theoretical calculations were in agreement with the experimental data.Then,the intramolecular proton transfer(PT),the water-assisted PT,the methanol-assisted PT,the ethanol-assisted PT,and the acetic acid-assisted PT of N'-(8-hydroxyquinoline-2-methylene)-2-furancarboxylic hydrazide were investigated by density-functional theory calculations at B3LYP/6-31+G(d)basis set level.The primary geometry structures,infrared frequencies,energies,as well as the NMR data were studied.The results illustrate that the water,or methanol,or ethanol,or acetic acid is involved in the assisted proton transfer processes through hydrogen bonds,and seven-membered cyclic transition states are formed,which were favorable for PT reactions owing to their capability to significantly reduce the PT energy barriers.While,in the process of the intramolecular PT a planar five-menmbered transition state was afforded.The H-bonds played a key role in decreasing the activation energy.3.The ketone-enol tautomerism of N'-(8-hydroxyquinoline-2-methylene)-2-furancarboxylic hydrazide was studied with the B3LYP/6-31G(d)level.In the mechanism of pure intramolecular ketone-enol tautomerism,a four-membered cyclic transition state derived of C=O and N-H bonds is afforded,which has highest energy in the process.But in the process of water-assisted,methanol-assisted,ethanol-assisted ketone-enol tautomerism,the water,or methanol,or ethanol was involved in the formation of six-membered cyclic transition states,which were favorable for ketone-enol tautomerism owing to their capability to significantly reduce the reaction energy barriers.While,in the process of acetic acid-assisted ketone-enol tautomerism,seven-membered transition state is afforded,the carboxyl group taking part in the transition state structure,which also reduce the reaction energy barriers significantly.The geometry structures and infrared frequencies of the reactants,transition states and products in these ketone-enol tautomerisms,the reaction energies,as well as the NMR data were studied.