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 for HL2 + I = (HL2)I and ΔG° = -13.4(2) kJ mol for HL2 + I = [(HL2)I]). When solutions of [(HL2)I] are left in contact with air, crystals of the oxidation product (HL2)(I)I·4HO are formed. Unfortunately, the low solubility of I complexes prevents the determination of their stability constants. The crystal structures of HL2I·HO (1), HL2(I)·2HO (2) and (HL2)(I)I·4HO (3) were determined by means of X-ray diffraction analyses. In all crystal structures, it was found that the interaction between I and I with HL2 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 I 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 I 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.

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