Structural and Anion Coordination Features of Macrocyclic Polyammonium Cations in the Solid,Solution and Computational Phases

Abstract

The crystal structures of two hexaaza macrocycles 1,4,7,12,15,18-hexaazacyclodocosane ([22]N₆:1-6H⁺, 6C1^?) and 1,13-dioxa-4,7,10,16,19,22-hexaazacyclotetraeicosane ([24]N₆O₂:2-6H⁺,6CI^?) as their hexa-hydrochloride salts have been determined. 1-6H⁺ binds specifically two CI^? anions above and below the almost planar hexaammonium macrocycle yielding a dinuclear CI^? complex. The hexacation 2-6H⁺ on the other hand interacts preferentially with three CI^? anions of the six present in the solid state. Among the three closest anions, one of them, interacting with four ammonium groups, is located in the centre of the macrocycle which adopts a “pocket-like” conformation. Potentiometric and ³⁵CI NMR experiments demonstrate that 2-6H⁺ also binds CI^? in aqueous solution. Subsequent extensive molecular dynamics computational studies starting from X-ray coordinates show that the solid state structure is representative of the solution conformations for I-6H⁺, whereas the conformations of 2-6H⁺ are strongly affected by intramolecular interactions between the ammonium centres and O-atoms of the ether linkage as well as by intermolecular interactions with H₂O molecules and CI^? counterions.