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Metal Ion Complementarity: Effect of Ring‐Size Variation on the Conformation and Stability of Lead(II) and Cadmium(II) Complexes with Pendant‐Armed Crowns
Author(s) -
RegueiroFigueroa Martín,
EstebanGómez David,
PlatasIglesias Carlos,
de Blas Andrés,
RodríguezBlas Teresa
Publication year - 2007
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200601220
Subject(s) - chemistry , acetonitrile , metal , crystallography , stability constants of complexes , titration , ring size , 15 crown 5 , metal ions in aqueous solution , stereochemistry , conformational change , ring (chemistry) , ion , crown ether , organic chemistry
The binding tendencies of the pendant‐armed crown ethers L 1 –L 3 [L 1 = N , N′ ‐bis(benzimidazol‐2‐ylmethyl)‐1,7‐diaza‐12‐crown‐4, L 2 = N , N′ ‐bis(benzimidazol‐2‐ylmethyl)‐1,10‐diaza‐15‐crown‐5) and L 3 = N , N′ ‐bis(benzimidazol‐2‐ylmethyl)‐4,13‐diaza‐18‐crown‐6] towards Pb II and Cd II have been investigated. The X‐ray crystal structure of [Cd(L 3 )](ClO 4 ) 2 · EtOH shows that, in the solid state, the Cd II 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 Cd II complexes. The conformation goes from a syn arrangement for L 1 to an anti arrangement for L 3 , passing through a syn [lrarr2] anti equilibrium in the complex derived from L 2 . On the contrary, no conformational change was observed for the corresponding Pb II 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 10 2 ‐fold enhancement of the stability for this series of Cd II complexes, whereas for Pb II a gradual decrease of the binding constants is observed. Receptor L 1 shows a certain degree of selectivity for Cd II over Pb II , with a selectivity factor > 10 2 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)