Lanthanide(III) complexation by the ligand 1,3,5-triamino-1,3,5-trideoxy-cis-inositol: an unusual thermodynamic behaviour across the rare-earth series

1,3,5-Triamino-1,3,5-trideoxy-cis-inositol (taci) gives trinuclear sandwich-type complexes of trivalent lanthanide cations [Ln3(taciH-3)2] 3+ in aqueous solution, whose stability constants experience a very great increase across the rare-earth series. To have a better insight into factors controlling lanthanide recognition, the enthalpy changes of the complexation reaction were measured by microcalorimetry. This thermodynamic analysis demonstrates that the discrimination is mainly due to enthalpic effects; the entropic contribution shows little variation. The X-ray structure of the lutetium complex [Lu3(taciH-3)2(H2O)6](SO4)1.5 is also presented and gives crucial structural information for the smallest lanthanide cations.