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Macrocyclic Receptor Showing Improved Pb II /Zn II and Pb II /Ca II Selectivities
Author(s) -
FerreirósMartínez Raquel,
PlatasIglesias Carlos,
de Blas Andrés,
EstebanGómez David,
RodríguezBlas Teresa
Publication year - 2010
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.200901219
Subject(s) - chemistry , ligand (biochemistry) , crystallography , metal , potentiometric titration , nmr spectra database , crystal structure , stability constants of complexes , titration , stereochemistry , inorganic chemistry , ion , spectral line , aqueous solution , receptor , organic chemistry , biochemistry , physics , astronomy
Herein we report on the macrocyclic receptor N , N ′‐bis[(6‐carboxy‐2‐pyridyl)methyl]‐1,10‐diaza‐15‐crown‐5 ( H 2 bp15c5 ) and its coordination properties towards Zn II , Cd II , Pb II , and Ca II . The stability constants of these complexes determined by pH‐potentiometric titration at 25 °C in 0.1 M KNO 3 vary in the following order: Pb II > Cd II >> Zn II > Ca II . As a result, bp15c5 presents very important Pb II /Zn II and Pb II /Ca II selectivities. These results are in contrast to those reported for the related receptor derived from 1,7‐diaza‐12‐crown‐4, which provides very similar complex stabilities for Zn II and Pb II . The X‐ray crystal structure of [Cd( Hbp15c5 )] + shows heptadentate binding of the ligand to the metal ion, with two oxygen atoms of the macrocyclic unit remaining uncoordinated. The 1 H NMR spectra of the complexes formed with Pb II , Zn II , and Ca II (D 2 O) show very broad peaks in the region 2–5 ppm, indicating an important degree of flexibility of the crownmoiety in these complexes. On the contrary, the 1 H and 13 C NMR spectra recorded for the Cd II complex are well resolved and could be fully assigned. A detailed conformational investigation using theoretical calculations performed at the DFT (B3LYP) level predict a minimum energy conformation for [Cd( bp15c5 )] that is very similar to that observed in the solid state. Analogous calculations performed on the [M( bp15c5 )] (M = Zn or Pb) systems predict hexadentate binding of the ligand to these metal ions. In the case of the Pb II complex our calculations indicate that the 6s lone pair is stereochemically active, which results in a hemidirected coordination geometry around the metal ion. The minimum energy conformations calculated for the Zn II , Cd II , and Pb II complexes are compatible with the experimental NMR spectra obtained in D 2 O solution.