Binuclear Lanthanide(III) Complexes with Chiral Ligands: Dynamic Equilibria in Solution and Binding with Nucleotides Studied by Spectroscopic Methods

Binuclear lanthanide complexes of Eu(III) and Sm(III) were obtained in the presence of chiral ligand 1,2-(R,R+S,S)-N,N'-bis(2-pyridylmethylene),2-diamine. An unusual structure of the Eu(III) compound with two lanthanide atoms connected through two chlorines was determined by X-ray crystallography. In solution, the dimer coexists with a monomeric complex, and the stability of the binuclear form depends on the solvent and concentration. The dimer-monomer equilibrium was monitored by circularly polarized luminescence (CPL) measured on a Raman optical activity (ROA) spectrometer, where both forms provided large CPL anisotropic ratios of up to 5.6×10-2 . Monomer formation was favored in water, whereas the dimer was stabilized in methanol. When mixed with adenosine phosphates, AMP gave much smaller CPL than ADP and ATP, indicating a high affinity of the Eu (III) complex for the phosphate group, which in connection with the ROA/CPL technique can be developed into a bioanalytical probe.