Anion Complexes with Tetrazine-Based Ligands: Formation of Strong Anion−π Interactions in Solution and in the Solid State

Ligands L1 and L2, consisting of a tetrazine ring decorated with two morpholine
pendants of different lengths, show peculiar anion-binding behaviors.
In several cases, even the neutral ligands, in addition to their protonated
HL+ and H2L2+ (L = L1 and L2) forms, bind anions such as F–, NO3–, PF6–, ClO4–, and SO42– to form stable complexes in water. The crystal structures of H2L1(PF6)2·2H2O, H2L1(ClO4)2·2H2O, H2L2(NO3)2, H2L2(PF6)2·H2O, and H2L2(ClO4)2·H2O show that anion−π interactions
are pivotal for the formation of these complexes, although other weak
forces may contribute to their stability. Complex stability constants
were determined by means of potentiometric titration in aqueous solution
at 298.1 K, while dissection of the free-energy change of association
(ΔG°) into its enthalpic (ΔH°) and entropic (TΔS°)
components was accomplished by means of isothermal titration calorimetry
measurements. Stability constants are poorly regulated by anion–ligand
charge–charge attraction. Thermodynamic data show that the
formation of complexes with neutral ligands, which are principally
stabilized by anion−π interactions, is enthalpically
favorable (−ΔG°, 11.1–17.5
kJ/mol; ΔH°, −2.3 to −0.5
kJ/mol; TΔS°, 9.0–17.0
kJ/mol), while for charged ligands, enthalpy changes are mostly unfavorable.
Complexation reactions are invariably promoted by large and favorable
entropic contributions. The importance of desolvation phenomena manifested
by such thermodynamic data was confirmed by the hydrodynamic results
obtained by means of diffusion NMR spectroscopy. In the case of L2, complexation equilibria were also studied in a 80:20 (v/v)
water/ethanol mixture. In this mixed solvent of lower dielectric constant
than water, the stability of anion complexes decreases, relative to
water. Solvation effects, mostly involving the ligand, are thought
to be responsible for this peculiar behavior.