Jean-Claude Bunzli - Thermodynamic parameters governing the self-assembly of head-head-head lanthanide bimetallic helicates

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  Jensen,T.B., Scopelliti,R., Bunzli,J.C.G. Chemistry-A   European Journal 2007 13 (30)   8404-8410

  Abstract: The heterobitopic ligands L-ABX (X =   1, 2, 3, 4 or 5), differing only by a Cl or NEt2 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 Delta H and AS, which vary in   opposite direction along the lanthanide series for complexes with   the same ligand in a way that keeps the value of Delta G almost   constant. The results are interpreted in terms of differences in   interstrand stacking, ion-dipole interactions and metal-metal   repulsion

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