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

Publisher Website
Google Scholar
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 ][ClO 4 ] 0.5 [CF 3 SO 3 ] 4.5 ·MeCN·4H 2 O at 170 K is isostructural with that of [EuZnL 3 ][ClO 4 ][CF 3 SO 3 ] 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.

This publication has 0 references indexed in Scilit: