Ca2+ vs. Ba2+ electrochemical detection by two disubstituted ferrocenyl chalcone chemosensors. Study of the ligand–metal interactions in CH3CN

We show here that the disubstituted ferrocenyl chalcones 1 and 2 are good electrochemical sensors for calcium and barium in CH3CN. However, these two triflate salts are detected in a different way by both ligands. To clarify this point, a thorough and informative NMR study of the ligand–salt interactions is presented. The unusual shapes of the titration curves obtained depend on both the ligand and cation used. For example, they illustrate that ligand 1 mainly interacts with the metal by its CO functions, while ligand 2 also interacts by its azacrown groups. These curves also reflect complex equilibriums in solution involving several ligand–salt adducts detected by mass spectrometry. To evaluate the strength of these interactions, the association constants of all the species formed have been determined by fitting the NMR data. It is noteworthy that changing the diethylamino groups in molecule 1 by the azacrown residue enhances the selectivity for the calcium salt, as pointed out by the value of the association constant of the 2Ca2+ species. The synthesis of the protonated counterparts 3 and 4 was useful to clarify the electrochemical behaviour of 1 and 2. Although the two ligand–salt interactions present several common points, the whole results obtained allow us to propose an original explanation for the difference observed between the Ca2+ and Ba2+ electrochemical sensing.