A DIRECT N-15 NMR-STUDY OF PRASEODYMIUM(III)-NITRATE COMPLEX-FORMATION IN AQUEOUS SOLVENT MIXTURES

A direct, low-temperature nitrogen-15(N-15) NMR technique has been app lied to the study of inner-shell complex formation between praseodymiu m(III) and nitrate ion in aqueous solvent mixtures. In water-acetone m ixtures at -95-degrees-C, ligand exchange is slow enough to permit the observation of N-15 NMR signals for uncomplexed and coordinated nitra te ion, but satisfactory resolution is obtained only by the addition o f Freon-12 to these systems for study at -110 to -115-degrees-C. Four coordinated nitrate signals are generally observed and a very small si gnal for an additional complex, or an isomer of one of the others, app ears at the highest nitrate concentrations. Signals for the mono- and dinitrato complexes are unambiguously identified, but with the excepti on of the trinitrato complex, several possibilities exist for the rema ining peaks. To overcome excessive viscosity signal broadening, measur ements in methanol and ethanol are possible only with praseodyminum tr ifluoromethanesulfonate (triflate). Coordinated nitrate signals in aqu eous and anhydrous methanol are observed only for the mono- and dinitr ato species, and signal areas indicate a maximum of two moles of nitra te per Pr(III) are complexed. A third signal is evident in the ethanol solution spectra. and the presence of this higher complex was confirm ed by area measurement of the fraction of bound nitrate. The extent of complex formation in these solvent systems is attributed to differenc es in the dielectric constant. A comparison of the complexing tendenci es of Pr(III) to other ions studied by this NMR method suggests the po ssibility of a coordination number change across the lanthanide series . Preliminary N-15 NMR results for metal-ion complexes with the isothi ocyanate ion are presented.