A direct nitrogen-15 NMR study of praseodymium(III)-nitrate complex formation in aqueous solvent mixtures

A direct, low-temperature nitrogen-15([sup 15]N) NMR technique has been applied to the study of inner-shell complex formation between praseodymium(III) and nitrate ion in aqueous solvent mixtures. In water-acetone mixtures at [minus]95[degrees]C, ligand exchange is slow enough to permit the observation of [sup 15]N NMR signals for uncomplexed and coordinated nitrate ion, but satisfactory resolution is obtained only by the addition of Freon-12 to these systems for study at [minus]110 to [minus]115[degrees]C. Four coordinated nitrate signals are generally observed and a very small signal for an additional complex, or an isomer of one of the others, appears at the highest nitrate concentrations. Signals for the mono- and dinitrato complexes are unambiguously identified, but with the exception of the trinitrato complex, several possibilities exist for the remaining peaks. To overcome excessive viscosity signal broadening, measurements in methanol and ethanol are possible only with praseodymium trifluoromethanesulfonate (triflate). Coordinated nitrate signals in aqueous and anhydrous methanol are observed only for the mono- and dinitrato species, and signal areas indicate a maximum of two moles of nitrate per Pr(III) are complexes. A third signal is evident in the ethanol solution spectra, and the presence of this higher complex was confirmed by area measurement of the fraction of bound nitrate. The extent of complex formation in these solvent systems is attributed to differences in the dielectric constant. A comparison of the complexing tendencies of Pr(III) to other ions studied by this NMR method suggests that possibility of a coordination number change across the lanthanide series. Preliminary [sup 15]N NMR results for metal-ion complexes with the isothiocyanate ion are presented. 63 refs., 3 figs., 1 tab.