NONCOVALENT LANTHANIDE PODATES WITH PREDETERMINED PHYSICOCHEMICAL PROPERTIES - IRON(II) SPIN-STATE EQUILIBRIA IN SELF-ASSEMBLED HETERODINUCLEAR D-F SUPRAMOLECULAR COMPLEXES

The reaction of the segmental compound lpyridin-2-yl)-5,5'-methylenebi s(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 acetonitri le produced selectively the heterodinuclear non-covalent podates [LnFe L(3)](5+) and [CaFeL(3)](4+). Proton NMR and electronic spectroscopy a nd electrochemistry showed that the ligands are helically wrapped arou nd the metal ions leading to a C-3-triple-helical structure with Fe-II occupying the pseudo-octahedral co-ordination site produced by the th ree bidentate binding units and Ln(III) lying in the remaining pseudo- tricapped trigonal-prismatic site defined by the three tridentate bind ing units. In this chemical environment Fe-II sustains a thermally ind uced low-spin --> high-spin transition around room temperature in acet onitrile,the thermodynamic parameters Of which can be finely controlle d by the size of the co-ordinated Ln(III). Thermodynamic investigation s of the assembly process suggest that the stability of the final comp lexes [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 isostruct ural with that of [EuZnL(3)][ClO4][CF3SO3](4) . 4MeCN and indicates th at (i) the Fe-N bonds are in the range expected for essentially low-sp in Fe-II and (ii) [LaFeL(3)](5+) adopts the regular triple-helical str ucture found in solution. Magnetic measurements in the solid state sho wed smooth spin transitions similar to those observed in solution, whi le photophysical studies suggested that Eu-III --> Fe-II (low-spin) en ergy transfers are responsible for the complete quenching of the Eu-ce ntred emission.