A direct carbon-13 and nitrogen-15 NMR study of praseodymium(III)- and neodymium(III)-isothiocyanate complex formation in aqueous solvent mixtures

Multinuclear magnetic resonance spectroscopic studies of the trivalent lant hanide complexes with isothiocyanate have been completed For the praseodymi um(III) and neodymium(III) ions. In water-acetone-Freon mixtures, at temper atures low enough to slow ligand exchange, usually -85 to -125 degreesC for isothiocyanate, separate carbon-13 and nitrogen-15 NMR signals can be obse rved for free anion and NCS- in each metal-ion complex. For both metal ions , N-15 NMR signals are observed for four complexes, displaced about +1500 p pm downfield from free NCS- for pr(3+) and about +2000 ppm for Nd3+. In the C-13 NMR spectra, only three peaks are observed for the complexes of both metal anions, with signal overlap obscuring the resonance for the fourth co mplex. However, the metal ion coordination numbers, obtained by integration of the resonance signals, are comparable in the 15N and 13C spectra, appro aching a maximum value of about 3. These spectral data indicate the formati on of Ln(NCS)(2+) through Ln(NCS)(4)(1-) occurs for both lanthanides in the se solvent systems, a result also observed previously for Ce3+, Sm3+, and E u3+ in our laboratory. Attempts to study these complexes in water-methanol were unsuccessful, due to the inability to achieve low enough temperatures to slow ligand exchange sufficiently Results for NCS- and Cl- competitive-b inding studies by Cl-35 NMR for both metal ions will also be described.