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

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 [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 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 [LnFeL₃]⁵⁺ depends on the size of Ln^III, 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 Fe^II 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 Eu^III→Fe^II (low-spin) energy transfers are responsible for the complete quenching of the Eu-centred emission.

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