A direct carbon-13 and nitrogen-15 nmr study of europium(iii)-isothiocyanate complex formation in aqueous solvent mixtures

A continuation of the contact ion-pairing studies of the trivalent lanthanides by direct, low-temperature, multinuclear magnetic resonance techniques has been completed for the europium(III)-isothiocyanate system. In water-acetone-Freon-22 solvent mixtures, ligand exchange is sufficiently slow at — 100°C to - 125°C to permit the observation of¹³C and¹⁵N NMR signals for Eu³⁺-NCS^- contact ion-pair complexes. With each nuclide, signals for four complexes are observed, displaced approximately 250 ppm upfield from free anion in the^13C spectra, and 2,500 ppm upfield from bulk NCS^- in the¹⁵N spectra. The concentration dependence of the signal areas is consistent with the formation of Eu(NCS)²⁺ through Eu(NCS) ₄ ^1- , with water molecules completing the solvation shell. In the¹⁵N NMR spectra, the large chemical shifts identified the nitrogen atom as the NCS^- binding site. Also, the observation of two¹⁵N NMR signals for isomers of Eu(NCS) ₂ ¹⁺ was possible in several spectra. In methanol, a medium of higher dielectric constant, complex formation was diminished, with signal area integrations confirming the dominance of Eu(NCS) ₁ ²⁺ . A comparative binding study of Cl^- and NCS^- also was made by³⁵C1 NMR chemical shift and linewidth measurements in water-methanol mixtures. The much stronger coordinating ability of NCS^- was evident in these experiments, but there is a strong possibility of Eu³⁺-Cl^- ion-pairing in the absence of this anion.