PH-Controlled metal translocation outside/inside the cavity of a polyamine macrocycle

Abstract

The synthesis of the macrocyclic ligand 4,4′-(2,6,10,13,18-pentaaza26)-2,2′-bipyridylcylophane (L), which contains a penta-amine chain linking the 4,4′ positions of a 2,2′-bipyridyl moiety, is reported. The heteroaromatic nitrogens of L are located outside the macrocyclic cavity. Ligand protonation, as well as Cu(II) and Ni(II) complexation by L, were studied by potentiometric and UV-Vis techniques in aqueous solution. Only mononuclear complexes are formed. In [CuL]2+, the metal is encapsulated inside the cavity, not coordinated by the bipyridyl unit. Protonation of the complex occurs on the aliphatic polyamine chain and gives rise to translocation of the metal outside the cavity, bound to the heteroaromatic nitrogens. Conversely, Ni(II) is always bound to the bipyridyl nitrogens in both protonated and unprotonated complexes.