Molecular rearrangements controlled by pH-driven Cu2+ motions

Ligands 1-3 contain two coordinatively distinct compartments: AH(2), featuring a bis-amino bis-amido donor set, and B, featuring a bis amino-heterocycle donor set. Their coordinative tendencies towards Cu2+ have been studied in dioxane-water solution (4 : 1 v/v) by means of potentiometric titration experiments. The complex species which form in the pH 2-12 range have been individuated and their formation constants calculated. At neutral or slightly acidic pH values, the Cu2+ cation forms a 1 : 1 complex with the neutral ligands, residing in the B compartment. If the pH is then raised to distinctly basic values, the AH(2) compartment loses its two amide protons and Cu2+ moves inside it, forming neutral complexes with a bis-amino bis-imide donor set. The nature of these complexes has been ascertained by means of UV/Vis spectroscopy and by comparison with results for ligand 5, which also binds Cu2+ by losing its two amide protons, but does not possess further binding groups appended to the amine nitrogens. With ligands 1-3, when Cu2+ is coordinated by the bis-amino bis-imide donor set, one of the two appended heterocycles of the B compartment is still bound to it, in one of its apical positions, so that, on changing the pH between neutral and basic values, the Cu2+ movement induces an overall molecular rearrangement, which was found to be reversible.