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Effect of Protonation and Interaction with Anions on a Lead(II) Complex with a Lateral Macrobicycle Containing a Phenol Schiff‐Base Spacer
Author(s) -
EstebanGómez David,
PlatasIglesias Carlos,
Avecilla Fernando,
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.200601003
Subject(s) - chemistry , protonation , acetonitrile , schiff base , intramolecular force , imine , titration , proton nmr , metal , nuclear magnetic resonance spectroscopy , hydrogen bond , cryptand , photochemistry , crystallography , stereochemistry , inorganic chemistry , ion , molecule , organic chemistry , catalysis
Abstract The macrobicycle receptor L 4 , derived from 1,10‐diaza‐15‐crown‐5 incorporating a phenol Schiff‐base spacer, forms stable complexes with lead(II). In [Pb(L 4 )(ClO 4 )](ClO 4 ) · CH 3 CN ( 1 ), the lead(II) ion is asymmetrically placed at the one end of the macrobicyclic cavity, because of the intramolecular hydrogen bonding interaction that occurs between an imine nitrogen atom and the phenol group. This asymmetric position of the metal ion inside the macrobicyclic cavity induces chirality in this system. Variable temperature 1 H NMR spectroscopic experiments indicate that the asymmetric coordination of the metal ion inside the macrocyclic cavity is maintained in acetonitrile solution, but a translocation of the Pb II ion from one end of the macrobicyclic cavity to the second one occurs. This dynamic behaviour, which corresponds to the interconversion between the two possible enantiomeric forms of the complex, is fast on the NMR timescale at 320 K but slow at low temperatures, and we have estimated an activation barrier of Δ G ‡ = 68 ± 2 kJ mol –1 . We have also studied the effect of the protonation on 1 by NMR and UV/Vis spectroscopy in CH 3 CN solutions at room temperature, finding that diprotonation causes demetallation of the complex without receptor destruction, recovering L 4 in its protonated form. On the other hand, the interaction of compound 1 with anions such as NO 3 – and SCN – has been evaluated by using spectrophotometric titrations in acetonitrile solution, finding log K values of 5.83(1) for NO 3 – and 6.27(2) forSCN – . The [Pb(L 4 )] 2+ , [Pb(L 4 )(NO 3 )] + , [Pb(L 4 )(NCS)] + and [Pb(L 4 )(SCN)] + systems were characterized by means of density functional theory calculations (DFT) performed by using the B3LYP model, and the coordination geometry as well as the role of the Pb II lone pair has been investigated and discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)