Cryptates. XXV. Stability and selectivity of cation inclusion complexes of polyaza‐macrobicyclic ligands. Selective complexation of toxic heavy metal cations

We have measured the stability constants of the cryptate complexes formed by ligands 1 – 4 with alkali, alkaline‐earth, transition metal and toxic heavy metal cations. Stabilities and selectivities of complexation of the alkali and alkaline‐earth cations are less pronounced in 1 – 4 than in the parent compounds 5 and 6 and decrease as the number of nitrogen sites increase. Remarkable complexation properties are found towards transition metal and toxic heavy metal cations. The intramolecular cavity of ligands 1 – 3 is too large for small cations like Co 2+ , Ni 2+ , Zn 2+ so that the complexes formed are comparatively weak; however these cations are strongly complexed by ligand 4 whose intramolecular cavity has a much smaller size, compatible with their ionic radius. On the other hand, ligands 1 – 4 all form highly stable cryptates with Cd 2+ , Hg 2+ , Pb 2+ . Thus by the combined operation of the two structural parameters, cavity size and nature of the binding sites, cryptands 2 et 3 present very high selectivities for the complexation of these toxic heavy metal cations with respect to the biologically important ones Na + , K + , Mg 2+ , Ca 2+ , Zn 2+ . The selectivities of ligand 2 for Cd 2+ , Hg 2+ and Pb 2+ with respect to Zn 2+ are as high as 10 6 , 10 18 and 10 9 respectively. They are much more pronounced than those of previously known complexing agents. Cryptands like 2 and 3 thus present a unique selectivity sequence of special interest in detoxication (decorporation, depollution). Further structural elaboration may allow to design ligands which present a given selectivity pattern of potential use in “ cryptatotherapy ” and “ environment pollution control ”. The results also provide evidence for the existence, at low pH, of protonated complexes which probably participate in an acid catalysed process for dissociation of the complexes.