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 H2 L22+ + I- = [(H2 L2)I]+ ). When solutions of [(H2 L2)I]+ are left in contact with air, crystals of the oxidation product (H2 L2)2 (I3 )3 I·4H2 O are formed. Unfortunately, the low solubility of I3 - complexes prevents the determination of their stability constants. The crystal structures of H2 L2I2 ·H2 O (1), H2 L2(I3 )2 ·2H2 O (2) and (H2 L2)2 (I3 )3 I·4H2 O (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 H2 L22+ 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.