Liquid-phase synthesis of multidecker organoeuropium sandwich complexes and their physical properties

Multidecker organoeuropium sandwich complexes were synthesized with the organic ligands 1,3,5,7-cyclooctatetraene (COT) and 1,4- and 1,6-bis(trimethylsilyl)cyclooctatetraenes (COT″) in a liquid ammonia solution of europium (Eu) metal. Infrared (IR) absorption and Raman spectroscopies and magnetic measurements were used to identify local structures and charge distributions. It was found that the organoeuropium complexes have Eu²⁺-COT^2- (Eu²⁺-COT″^2-) charge distributions, including a local structure of ligand-Eu-ligand sandwiches. Together with elemental analysis by inductively coupled plasma atomic emission spectroscopy, it is shown that the organoeuropium complexes form a multidecker sandwich structure that is insouble in many solvents. In addition to their paramagnetic behaviors at 150-300 K, the organoeuropium complexes of Eu-COT and Eu-COT″ showed red and orange-red emissions, respectively, under UV irradiation. The introdution of bulky trimethylsilyl groups onto COT prominently enhanced the emission intensity by more than a factor of 10 compared to that of Eu-COT, and similar emission enhancement was observed when ethylenediamine vapor was contacted with Eu-COT solid. The intensity enhancement and the shifts in the emission wavelength are discussed from the viewpoint of orbital interactions between Eu²⁺ and COT^2- (COT″^2-).