MIXED-LIGAND IMIDAZOLE COMPLEXES OF ORGANOLANTHANIDES

Heptacoordinate trivalent lanthanide complexes containing mixed ligands were prepared and characterized. These complexes were of general structure M(THD)3-IM. In this formula M represents the lanthanide + 3 ions of europium, holmium, praseodymium, dysprosium, ytterbium and gadolinium. THD stands for 2,2,6,6-tetramethyl-3,5-heptane- dionato and IM for imidazole. The IR spectra of these materials were compared and we have demonstrated that the CH stretching frequency of the imidazole moiety exhibits a monotonic decrease as a function of increasing size of the lanthanide + 3 ion. The NH stretching frequency for imidazole shows a similar trend, except that the point for the europium occurs at a position quite far above that of the other compounds. These results suggest that the bond strength between the lanthanide ion and the imidazole is sensitive to the radius of the ion and that the chemical reactivity of the imidazole may be influenced by this effect. The crystal and molecular structures of Eu(THD)3-IM have been determined. The following cell constants were found: a = 19.40(1), b = 10.243(5), c = 21.76(1) Å, β= 108.70(5)° and Z = 4. The space group is P2 1/n. The europium ion is seven-coordinate with bonding to the six carbonyl groups of the three THD units and to one of the imidazole nitrogens, thereby forming a capped trigonal prism. Preliminary X-ray data from other members of the series indicate that all are isotructural with the europium compound. © 1990.