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Thermodynamic Parameters Governing the Self-Assembly of Head-Head-Head Lanthanide Bimetallic Helicates.

Jensen TB, Scopelliti R, Bünzli JC

Laboratory of Lanthanide Supramolecular Chemistry, École Polytechnique Fédérale de Lausanne (EPFL), LCSL-BCH 1401 Lausanne, Switzerland, Fax: (+41) 21-693-9825.

The heterobitopic ligands L(ABX) (X=1, 2, 3, 4 or 5), differing only by a Cl or NEt(2) substituent, have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln(2)(L(ABX))(3)](6+). The percentage of HHH (head-head-head) isomer, in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit, is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25 % and ranges from 6-20 % for L(AB2) complexes to 93-96 % for L(AB4) complexes. The equilibrium between HHT (head-head-tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DeltaH and DeltaS, which vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DeltaG almost constant. The results are interpreted in terms of differences in interstrand stacking, ion-dipole interactions and metal-metal repulsion.

Published 10 October 2007 in Chemistry, 13(30): 8404-8410.
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