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 FeII and LnIII (Ln = La, Nd, Eu, Gd, Tb, Yb, Lu, Y or Sc) or CaII in acetonitrile produced selectively the heterodinuclear non-covalent podates [LnFeL3]5+ and [CaFeL3]4+. Proton NMR and electronic spectroscopy and electrochemistry showed that the ligands are helically wrapped around the metal ions leading to a C3-triple-helical structure with FeII occupying the pseudo-octahedral co-ordination site produced by the three bidentate binding units and LnIII lying in the remaining pseudo-tricapped trigonal-prismatic site defined by the three tridentate binding units. In this chemical environment FeII 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 LnIII. Thermodynamic investigations of the assembly process suggest that the stability of the final complexes [LnFeL3]5+ depends on the size of LnIII, small metal ions leading to intricate mixtures of complexes. The crystal structure of [LaFeL3][ClO4]0.5[CF3SO3 ]4.5·MeCN·4H2O at 170 K is isostructural with that of [EuZnL3][ClO4][CF3SO3]4 · 4MeCN and indicates that (i) the Fe–N bonds are in the range expected for essentially low-spin FeII and (ii) [LaFeL3]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 EuIIIFeII (low-spin) energy transfers are responsible for the complete quenching of the Eu-centred emission