Metal-containing ditopic receptors for molecular recognition of diammonium cations

Two new ditopic receptors for α,ω-alkanediyldiammonium cations based on a tetraazamacrocyclic (cyclidene) nickel(II) complex bearing two crown-ether residues were synthesized. The studies of the host–guest interaction between the receptors and a series of α,ω-diammonium salts by NMR titration in acetonitrile-d3 showed that 1:1 complexes are formed with Kassoc∼103–105 M−1. Receptor with benzo-15-crown-5 arms showed substantial selectivity in binding of trimethylene- and tetramethylenediammonium dications, and 1–2 orders of magnitude weaker binding of shorter (C2) or longer (C5 and C6) diammonium cations. Receptor with benzo-18-crown-6 arms showed higher affinity to all studied diammonium cations, but the recognition of the length of α,ω-diammonium cations was less pronounced.