Cadmium(II) and lead(II) complexes with novel macrocyclic receptors derived from 1,10-diaza-15-crown-5

Two novel macrocycles, N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 (L-1) and a non-symmetric cryptand incorporating a pyridinyl Schiff-base spacer ( L-3), both derived from 1,10-diaza-15-crown-5, have been shown to act as re ceptors for lead(II) and cadmium(ll) guests. The X-ray crystal structures r eveal an anti conformation for the free ligand L1, but a syn arrangement in the lead(II) complex, [PbL1](ClO4)(2). The corresponding cryptates of L-3 can be prepared by simple transmetallation reactions starting from the bari um cryptate, showing the preference of L-3 for the heavy metal ions. The le ad(II) ion, unlike cadmium(II), has been shown to be capable of acting as a template agent in the synthesis of L-3. The X-ray crystal structures of th e metal cryptates allow an insight into the influence of the nature of the metal ion guest on the conformation of the macrobicyclic receptor L-3; the most significant changes are a different folding of the crown and variation s in the distance between the two pivotal nitrogen atoms and the imine nitr ogen atoms. The cryptand can thus expand or contract its cavity in order to accommodate the different metal ions.