Risedronate complexes with Mg2+, Zn2+, Pb2+, and Cu2+: Species thermodynamics and sequestering ability in NaCl(aq) at different ionic strengths and at T = 298.15 K

In this paper, potentiometry and calorimetry were used to determine the thermodynamics of interaction between risedronate and four bivalent metal cations, namely: Mg2+, Zn2+, Pb2+, and Cu2+ in aqueous NaCl solutions at different ionic strengths and at T = 298.15 K. The data analysis allowed us to ascertain that the main species formed were the MLH2, MLH, ML and M2L; however scarcely soluble species precipitated at acidic pH values, between 4 and 7 depending on the metal cation involved, probably due to the formation of the neutral M2L(s) species. Comparison of the stability constants with other similar ligands suggests that metal complexation occurs through the phosphonate with an additional stabilizing contribution due to the pyridine moiety. The dissection of the free energy term revealed that the entropic change is the main contribution to the stability of the complexes for all equilibria, except those regarding the formation of the M2L species, and the main forces involved in these processes are entropic in nature, such as the desolvation of both the ligand and the metal cations. The sequestering ability was evaluated computing the pL0.5 parameter at different pH and ionic strength values; the highest ones were found for Cu2+ in acidic solutions and at low ionic strength, whereas pL0.5 values for both Pb2+ and Zn2+ were slightly lower. Results reported in this work may be helpful in the assessment of the use of risedronate as effective chelating agent and for understanding the mechanism of action of this molecule as a drug.