Complexation of lead(II) by chlorogenic acid: experimental and theoretical study

Density functional theory (DFT) structure calculations and time-dependent DFT electronic excitation calculations have been performed on chlorogenic acid (H(3)CGA), a polyphenolic compound, used as a model molecule of humic substances. The different deprotonated forms of H(3)CGA have also been investigated. H(3)CGA is a multisite ligand that presents several metal complexing sites in competition, notably the carboxylic and catechol moieties. In low acidic aqueous medium, the complexation of Pb(II) has been followed by electronic absorption spectrometry. The formation of two complexes of stoichiometry metal:ligand 1:1 (log beta(1:1) = 3.39) and 2:1 (log beta(2:1) = 7.12) has been highlighted with use of chemometric methods. The theoretical spectrum of the 1:1 complex obtained by TD-DFT methodology shows the formation of a chelate [Pb(H(2)CGA)(H(2)O)(3)](+) with the metal fixation at the level of the carboxylate function. The second complexing site, the catechol moiety, is rapidly involved in the formation of the 2:1 complex from molar ratios [metal]/[ligand] higher than 0.1. The electronic transitions calculated for both free ligand and complexes involved the same molecular orbitals, and no ligand-metal or metal-ligand charge transfer is observed.