Noncovalent interactions in metal complexes, 8

Lanthanoid complexes with 1-l-menthyloxy-3-benzoylacetone, [Ln(l-moba)3(H2O)sub(n)] (Ln=La, Pr, Nd, Eu, Dy, Ho, Tm; n=0, 1 or 2), have been synthesized and characterized by elemental analyses, molecular weight determinations, and electronic, circular dichroism, and luminescence spectra. The molecular weight determinations suggest that the complexes are in a monomer-dimer equilibrium in nondonating solvents while they are monomeric in donating solvents. The complexes exhibit fairly intense circular dichroisms at the f-f band region, demonstrating the stereoselective formation of an enantiomer. The stereoselectivities have been discussed in terms of the intramolecular interligand CH/π-interaction operating between the phenyl and the menthyl groups of the ligands. [Eu(l-moba)3(H2O)] shows luminescence at the 5D0→7F1 and 5D0→7F2 transitions in benzene. Based on the concentration dependence of the luminescence, it is demonstrated that dimerization facilitates nonradiative relaxation and greatly reduces the quantum efficiency of luminescence. (author)