Two high and low symmetry europium complexes with L-proline: spectroscopy and structure

Lanthanide ions are widely applied as spectroscopic probes in studies of biomolecular structures. However, in all known crystal structures of lanthanide complexes with amino acids and peptides, dimers and polymers are formed with one or more symmetry sites of the metal ions; this strongly affects the spectroscopic properties. In order to explain the role of the type of amino acid, the Ln/ligand ratio and ligand chirality in the structure, two types of Eu(III) complexes with L-proline of formule [Eu2(C5H9O2N)4(H2O)8](ClO4)6.4H2O (I) and [Eu(C5H9O2N)3(H2O)3] (ClO4)3 (II) were obtained and characterised by X-ray and electron spectroscopic methods. Compound I crystallizes in a tetragonal space group (P4212) and forms dimers with two nonequivalent europium sites, with square antiprism polyhedra. Compound II (triclinic P1) forms polymeric chains in a crystal lattice, with two coordination modes of the proline molecules (mono- and bridging bidentate) and with dodecahedron polyhedra. Moreover, differences in the Eu-O bond lengths between two crystallographically nonequivalent metal ions are smaller in structure II. High resolution absorption, excitation and emission spectra of the compounds were measured at 4-293 K and analysed in order to explain how subtle structural changes affect the intensity of the electronic transitions, splitting of levels and vibronic coupling. (orig.)