Metal Ion Complementarity: Effect of Ring‐Size Variation on the Conformation and Stability of Lead(II) and Cadmium(II) Complexes with Pendant‐Armed Crowns

Abstract

AbstractThe binding tendencies of the pendant‐armed crown ethers L1–L3 [L1 = N,N′‐bis(benzimidazol‐2‐ylmethyl)‐1,7‐diaza‐12‐crown‐4, L2 = N,N′‐bis(benzimidazol‐2‐ylmethyl)‐1,10‐diaza‐15‐crown‐5) and L3 = N,N′‐bis(benzimidazol‐2‐ylmethyl)‐4,13‐diaza‐18‐crown‐6] towards PbII and CdII have been investigated. The X‐ray crystal structure of [Cd(L3)](ClO4)2·EtOH shows that, in the solid state, the CdII ion is eight‐coordinate and fits quite well into the crown hole, favouring an anti arrangement of the organic receptor. NMR measurements recorded in acetonitrile solution indicate that increasing the crown size induces a conformational change in the series of CdII complexes. The conformation goes from a syn arrangement for L1 to an anti arrangement for L3, passing through a syn [lrarr2] anti equilibrium in the complex derived from L2. On the contrary, no conformational change was observed for the corresponding PbII complexes, which have a syn conformation in all cases. These results have been confirmed by means of density functional theory (DFT) calculations performed by using the B3LYP model. The binding constants obtained from UV/Vis titration experiments in DMSO solution demonstrate that a decrease in the crown size provokes a 102‐fold enhancement of the stability for this series of CdII complexes, whereas for PbII a gradual decrease of the binding constants is observed. Receptor L1 shows a certain degree of selectivity for CdII over PbII, with a selectivity factor > 102. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)