Determination of the Chelating Site Preferentially Involved in the Complex of Lead(II) with Caffeic Acid: A Spectroscopic and Structural Study

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Abstract
The complexation of lead(II) with mono-deprotonated caffeic acid in aqueous solution (pH = 6.50) has been investigated by UV−visible, fluorescence, and vibrational spectroscopies combined with quantum chemical calculations (DFT). The caffeate ion presents two chelating sites in competition: the carboxylate and the catechol functions. Electronic spectroscopies highlighted two different complexed forms with, respectively, 1:1 and 2:1 stoichiometry. The 1:1 complex predominates for low lead concentrations, even if the second complexed form appears before the first chelating site is fully occupied. Both spectroscopic data and calculations reveal that Pb(II) preferentially coordinates with the carboxylate function, in opposition with previous results found for the Al(III) complexation, where the catechol group presents the greater complexing power. The structural and vibrational modifications between the mono-deprotonated ligand and 1:1 complex engendered by the chelation are discussed. Water molecules have been added on the Pb ion to modify its coordination, and structures of Pb(H2CA)(H2O)n+ with n = 0−4 were optimized. Calculations of theoretical frequencies have permitted us to propose a tentative assignment of infrared and Raman spectra of complexed species.