Non-covalent 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 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.