A. Fratiello et al., A DIRECT N-15 NMR-STUDY OF PRASEODYMIUM(III)-NITRATE COMPLEX-FORMATION IN AQUEOUS SOLVENT MIXTURES, Journal of solution chemistry, 22(6), 1993, pp. 519-538
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.