Iodide and triiodide anion complexes involving anion–π interactions with a tetrazine-based receptor

Protonated forms of the tetrazine ligand L2 (3,6-bis(morpholin-4-ylethyl)-1,2,4,5-tetrazine) interact with iodide in aqueous solution forming relatively stable complexes (ΔG° = −11.6(4) kJ mol−1 for HL2+ + I− = (HL2)I and ΔG° = −13.4(2) kJ mol−1 for H2L22+ + I− = [(H2L2)I]+). When solutions of [(H2L2)I]+ are left in contact with air, crystals of the oxidation product (H2L2)2(I3)3I·4H2O are formed. Unfortunately, the low solubility of I3− complexes prevents the determination of their stability constants. The crystal structures of H2L2I2·H2O (1), H2L2(I3)2·2H2O (2) and (H2L2)2(I3)3I·4H2O (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I− and I3− with H2L22+ is dominated by anion interactions with the π electron density of the receptor. Only in the case of 1, the iodide anions involved in close anion–π interactions with the ligand tetrazine ring form an additional H-bond with the protonated morpholine nitrogen of an adjacent ligand molecule. Conversely, in crystals of 2 and 3 there are alternate segregated planes which contain only protonated ligands hydrogen-bonded to cocrystallized water molecules or I3− and I− forming infinite two-dimensional networks established through short interhalogen contacts, making these crystalline products good candidates to behave as solid conductors. In the solid complexes, the triiodide anion displays both end-on and side-on interaction modes with the tetrazine ring, in agreement with density functional theory calculations indicating a preference for the alignment of the I3− molecular axis with the molecular axis of the ligand. Further information about geometries and structures of triiodide anions in 2 and 3 was acquired by the analysis of their Raman spectra.