A Schiff-base bibracchial lariat ether selective receptor for lanthanide(III) ions

The bibracchial lariat ether N,N′-bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6 (L), in its di-deprotonated form, behaves as a selective receptor for lanthanide(III) ions . It only forms stable complexes with the three lightest members of the lanthanide series, lanthanum(III), cerium(III) and praseodymium(III). The X-ray crystal structures of these three complexes show the metal ion ten-coordinated and deeply buried in the cavity of the dianionic receptor. In all three structures the lariat ether presents a syn arrangement. However, the different size of the encapsulated metal ion modifies the conformation of the receptor, particularly in the fold of the crown, resulting in a very different coordination polyhedron for La(III) (distorted tetracapped trigonal prism) with respect to that of Ce(III) and Pr(III) (bicapped square antiprism). Spectrophotometric titration of the dianionic lariat ether with anhydrous La(ClO4)3 in acetonitrile allowed us to calculate log K[La(L − 2H)] = 4.65(4).