A direct hydrogen-1, carbon-13, and nitrogen-15 NMR study of lutetium(III)-isothiocyanate complexation in aqueous solvent mixtures

Abstract

A direct, low-temperature hydrogen-1, carbon-13, and nitrogen-15 nuclear magnetic resonance study of lutetium(III)-isothiocyanate complex formation in aqueous solvent mixtures has been completed. At −100°C to −120°C in water-acetone-Freon mixtures, ligand exchange is slowed sufficiently to permit the observation of separate¹H,¹³C, and¹⁵N NMR signals for coordinated and free water and isothiocyanate ions. In the¹³C and¹⁵N spectra of NCS⁻, resonance signals for five complexes are observed over the range of concentrations studied. The¹³C chemical shifts of complexed NCS⁻ varied from −0.5 ppm to −3 ppm from that of free anion. For the same complexes, the¹⁵N chemical shifts from free anion were about −11 ppm to −15 ppm. The magnitude and sign of the¹⁵N chemical shifts identified the nitrogen atom as the binding site in NCS⁻. The concentration dependence of the¹³C and¹⁵N signal areas, and estimates of the fraction of anion bound at each NCS⁻:Lu³⁺ mole ratio, were consistent with the formation of [(H₂O)₅Lu(NCS)]²⁺ through [(H₂O)Lu(NCS)₅]²⁻. Although proton and/or ligand exchange and the resulting bulk-coordinated signal overlap prevented accurate hydration number measurements, a good qualitative correlation of the water¹H NMR spectral results with those of¹³C and¹⁵N was possible.