Jean-Claude Bunzli - Non-covalent lanthanide podates with predetermined physicochemical properties: Iron(II) spin-state equilibria in self-assembled heterodinuclear d-f supramolecular complexes

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  Piguet,C., RivaraMinten,E., Bernardinelli,G., Bunzli,J.C.G.,   Hopfgartner,G. Journal of the Chemical Society-Dalton   Transactions 1997 (3) 421-433

  Abstract: The reaction of the segmental compound   2-[6-(diethylcarbamoyl)pyridin-2-yl]-1,1'-dimethyl-2'-(5-methylpyridin-2-yl)-5, 5'-methylenebis(1H-benzimidazole)   (L) with a stoichiometric mixture of Fe-II and Ln(III) (Ln = La,   Nd, Eu, Gd, Tb, Yb, Lu, Y or Sc) or Ca-II in acetonitrile   produced selectively the heterodinuclear non-covalent podates   [LnFeL(3)](5+) and [CaFeL(3)](4+). Proton NMR and electronic   spectroscopy and electrochemistry showed that the ligands are   helically wrapped around the metal ions leading to a   C-3-triple-helical structure with Fe-II occupying the   pseudo-octahedral co-ordination site produced by the three   bidentate binding units and Ln(III) lying in the remaining   pseudo-tricapped trigonal-prismatic site defined by the three   tridentate binding units. In this chemical environment Fe-II   sustains a thermally induced low-spin --> high-spin transition   around room temperature in acetonitrile,the thermodynamic   parameters Of which can be finely controlled by the size of the   co-ordinated Ln(III). Thermodynamic investigations of the   assembly process suggest that the stability of the final   complexes [LnFeL(3)](5+) depends on the Size of Ln(III), small   metal ions leading to intricate mixtures of complexes. The   crystal structure of [LaFeL(3)][ClO4](0.5)[CF3SO3](4.5). MeCN .   4H(2)O at 170 K is isostructural with that of   [EuZnL(3)][ClO4][CF3SO3](4) . 4MeCN and indicates that (i) the   Fe-N bonds are in the range expected for essentially low-spin   Fe-II and (ii) [LaFeL(3)](5+) adopts the regular triple-helical   structure found in solution. Magnetic measurements in the solid   state showed smooth spin transitions similar to those observed in   solution, while photophysical studies suggested that Eu-III   --> Fe-II (low-spin) energy transfers are responsible for the   complete quenching of the Eu-centred emission

  Address (URL): WOS:A1997WJ49600023