Solid State and Solution Study on the Formation of Inorganic Anion Complexes with a Series of Tetrazine-Based Ligands

Four molecules (L1-L4) constituted by an s-tetrazine ring appended with two identical aliphatic chains of increasing length bearing terminal morpholine groups were studied as anion receptors in water. The basicity properties of these molecules were also investigated. Speciation of the anion complexes formed in solution and determination of their stability constants were performed by means of potentiometric (pH-metric) titrations, while further information was obtained by NMR and isothermal titration calorimetry (ITC) measurements. The crystal structures of two neutral ligand (L3, L4) and of their H₂L3(ClO₄)₂∙2H₂O, H₂L4(ClO₄)₂∙2H₂O, H₂L3(PF₆)₂ and H₂L3(PF₆)₂∙2H₂O anion complexes were determined by single crystal X-ray diffraction. The formation of anion-π interactions is the leitmotiv of these complexes, both in solution and in the solid state, although hydrogen bonding and/or formation of salt-bridges can contribute to their stability. A particular evidence of the ability of these ligands to form anion-π interactions is given by the observation that even the neutral (not-protonated) molecules bind anions in water, forming complexes of significant stability, including elusive OH- anions