Ab initio study of structure, protonation and complex formation of novel pyrazolone-5 derivatives

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Abstract The protonation of two novel pyrazolone-5 derivatives and their complex formation with rare-earth metals have been studied by ab initio methods at HF level of theory. The full geometry optimization of the ligands, their protonated forms and model fragments of the complexes with Pr3+ and Sc3+ ions was carried out. The carbonyl group of the pyrazolone cycle is the most preferable site to the proton attack that is indicated by the analysis of the relative stability of the different protonated forms, composition of the frontier molecular orbitals and the electrostatic potential (ESP) calculations. In contrast, the bidentate type of coordination via both carbonyl groups is realized at the complex formation with transition metals. The protonation and complex formation are accompanied by significant conformational changes of the substituent at fourth position of the pyrazolone cycle (R) in spite of the existence of the high potential barrier (ca. 2–2.5 eV) for the rotation of R.