A DIRECT C-13 AND N-15 NMR-STUDY OF EUROPIUM(III)-ISOTHIOCYANATE COMPLEX-FORMATION IN AQUEOUS SOLVENT MIXTURES

A continuation of the contact ion-pairing studies of the trivalent lan thanides by direct, low-temperature, multinuclear magnetic resonance t echniques has been completed for the europium(III)-isothiocyanate syst em. In water-acetone-Freon-22 solvent mixtures, ligand exchange is suf ficiently slow at -100 degrees C to -125 degrees C to permit the obser vation of C-13 and N-15 NMR signals for Eu3+-NCS- contact ion-pair com plexes. With each nuclide, signals for four complexes are observed, di splaced approximately 250 ppm upfield from free anion in the C-13 spec tra, and 2,500 ppm upfield from bulk NCS- in the N-15 spectra. The con centration dependence of the signal areas is consistent with the forma tion of Eu(NCS)(2+) through Eu(NCS)(4)(1-), with water molecules compl eting the solvation shell. In the N-15 NMR spectra, the large chemical shifts identified the nitrogen atom as the NCS- binding site. Also, t he observation of two N-15 NMR signals for isomers of Eu(NCS)(2)(1+) w as possible in several spectra. In methanol, a medium of higher dielec tric constant, complex formation was diminished, with signal area inte grations confirming the dominance of Eu(NCS)(2)(1+). A comparative bin ding study of Cl- and NCS- also was made by Cl-35 NMR chemical shift a nd linewidth measurements in water-methanol mixtures. The much stronge r coordinating ability of NCS- was evident in these experiments, but t here is a strong possibility of Eu3+-Cl- ion-pairing in the absence of this anion.