Synthesis, Characterizations, Magnetism and Thermal Degradation of a 2-Fold Interpenetrated 3D Cobalt-Organic Framework

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 ligands (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. 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 to form complexes of significant stability, including elusive OH- anions.

Halide and hydroxide anion binding in water

The formation of halide and hydroxide anion complexes with two ligands L1 (3,6-bis(morpholin-4-ylmethyl)-1,2,4,5-tetrazine) and L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) was studied in aqueous solution, by means of potentiometric and ITC procedures. In the solid state, HF₂^-, Cl^- and Br^- complexes of H₂L2²⁺ were analysed by single crystal XRD measurements. Further information on the latter was obtained with the use of density functional theory (DFT) calculations in combination with the polarizable continuum model (PCM). The presence of two halide or bifluoride HF₂^- (F-H-F^-) anions forming anion-π interactions, respectively above and below the ligand tetrazine ring, is the leitmotiv of the [(H₂L2)X₂] (X = HF₂, Cl, Br, I) complexes in the solid state, while hydrogen bonding between the anions and protonated morpholine ligand groups contributes to strengthen the anion-ligand interaction, in particular in the case of Cl^- and Br^-. In contrast to the solid state, only the anion : ligand complexes of 1 : 1 stoichiometry were found in solution. The stability of these complexes displays the peculiar trend I^- > F^- > Br^- > Cl^- which was rationalized in terms of electrostatic, hydrogen bond, anion-π interactions and solvent effects. DFT calculations performed on [(H₂L2)X]⁺ (X = F, Cl, Br, I) in PCM water suggested that the ligand assumes a U-shaped conformation to form one anion-π and two salt bridge interactions with the included anions and furnished structural information to interpret the solvation effects affecting complex formation. The formation of hydroxide anion complexes with neutral (not protonated) L1 and L2 molecules represents an unprecedented case in water. The stability of the [L(OH)]^- (L = L1, L2) complexes is comparable to or higher than the stability of halide complexes with protonated ligand molecules, their formation being promoted by largely favourable enthalpic contributions that prevail over unfavourable entropic changes.