Molecular Recognition Study Based on a Multi-point Interaction Utilizing Metal Complexes

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Some 4-acyl-5-pyrazolones have been synthesized and the association reaction between their copper or zinc chelates and neutral ligands with one or two functional groups has been investigated spectrophotometrically. The binding constants in chloroform for neutral ligands such as aminoalcohols, hydroxypyridines and pyridylcarbinols which have both nitrogen and hydroxy group were measured.The binding constants depend on the acidity of acylpyrazolones, the basicity of nitrogen atom and the steric effect. Concurrently, the ligands having a hydroxy group were found to show larger binding constants, indicating the hydrogen bonding between the hydroxy group and the donating oxygens of acylpyrazolones. It was evidently shown that the strength of this hydrogen bond depends on the number of the methylene group between the nitrogen atom and the hydroxy group and the electron withdrawing substituent at the 4-position of acylpyrazolones.The values of DELTAH and DELTAS for amines and pyridines with a hydroxy group in the reaction with copper acylpyrazolonates were more negative as compared to those for with no hydroxy group. It was corifirmed that the ^1H-NMR signal of the methylene protons adjacent to the hydroxy group shifted to the high field in the reaction of a bifunctional ligand and zinc acylpyrazolohate. This high field shift indicates that the methylene protons come on close to 4-benzoyl or 4-naphthoyl group of acylpyrazolones on forming the hydrogen bond. The two point interaction between metal acylpyrazolonates and the bifunctional ligands could be confirmed by the thermodynamic study and ^1H-NMR spectra.