Spectroscopic and structural investigations on [(DD18C6)H2]I8 formed in the reaction of N,N′-dibenzylated diazacrown ether (DD18C6) with iodine

Photometric titration measurements indicated in the reaction of diazacrown ether N,N′-dibenzyl-1,4,10,13-tetraoxo-7,16-diazacyclooctadecane (DD18C6, 2) with iodine in chloroform that a complex was formed in the molar ratio DD18C6:I2=1:4. This complex was also prepared on a preparative scale as dark brown compound and characterized by microanalysis, UV–Vis, IR, and Raman spectroscopy. By X-ray diffraction analysis the solid-state structure of the complex was shown to be [(DD18C6)H2]I8 ([3]I8) consisting of a doubly protonated macrocycle and an octaiodide dianion (I8 2−) in the typical (nearly planar) Z-shaped geometry. The macrocycle is Ci symmetric and the protonated nitrogen atoms adopt an endo–endo orientation that is stabilized by the three-center hydrogen bonds N–H(⋯O)2, where the oxygen atoms of the macrocycle act as hydrogen acceptors. The orientation of the phenyl groups of the benzyl sidearms are turned above and below the macrocycle. Quantum chemical calculations on the DFT level of theory of the non-protonated and the doubly protonated macrocycle (DD18C6, 2c) and [(DD18C6)H2]2+ (3c), respectively, were performed and discussed for 3c in terms of conformational strain of the macrocycle, the strength of the intramolecular N–H⋯O hydrogen bonds and cation–π interactions.