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

Abstract

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.