Molecular rearrangements controlled by pH-driven Cu2+ motions

Abstract

Ligands 1–3 contain two coordinatively distinct compartments: AH₂, 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 AH₂ 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.