A ¹³⁹La and ¹⁷O nuclear magnetic resonance study of the nature of La(III) first coordination sphere in acetonitrile solutions of hydrated lanthanum nitrate

Abstract

ZHIGANG CHEN and CHRISTIAN DETELLIER. Can. J. Chem. 72, 1797 (1994). The nature of the first coordination sphere of the La(II1) cation in acetonitrile solutions of La(N03)3.6H20 was investigated by 139La and 170 NMR. All three nitrate anions are coordinated to La(II1). There is a competition between water and acetonitrile molecules for the occupancy of the La(II1) first coordination sphere. The ' 3 9~a and 1 7 0 NMR results, obtained for various concentrations of La(N03)3.6H20, were quantitatively interpreted through a chemical equilibrium model. It was shown that the presence of only two La(II1) species, (La(II1) (NO3-),)' and (La(II1) (N03-)3.H20), in rapid exchange in solution could account for all the NMR results. The variation of the 139La and 1 7 0 NMR parameters (chemical shifts and relaxation rates) with the concentration of La(N03)3.6H20 results from the population increase of the species (La(II1) (N03-)3.H20). The chemical exchange between free and coordinated water molecules is also fast on the 170 NMR time scale. The 170 NMR signal of water is considerably broadened when it coordinates to the La(II1) cation. This broadening was attributed to both direct and indirect interactions between the two quadrupolar nuclei I7O and 1 3 9~a .The direct interaction comes from an increase of the electric field gradient at the I7O nucleus when a water molecule coordinates to the La(II1) cation. The indirect interaction results from an 170-139La scalar coupling. The detailed analysis of the 170 NMR spectra also allowed the determination of the indirect coupling constant between coordinated 170H2 and '39La(III), I ' J ( '~~_ '~~L a)(: Lanthanide ions are increasingly used as spectroscopic probes in biochemical and biological systems (1, 2), and a more detailed knowledge of their coordination behavior in the presence of specific ligands in solutions is needed. However, many aspects of their coordination chemistry remain unclear. For example, the nature and the structure of the chemical species present in aqueous solutions of their salts are not yet fully understood (3-5).It is likely that ion pairs be formed in the solutions of the tripositive lanthanoid ions ("Ln3+": y3+, La3+-Lu3+), particularly in poorly coordinating solvents. Bunzli and co-workers (6, 7) have investigated the interactions between lanthanoid cations and the perchlorate anion in anhydrous acetonitrile solutions by FTIR spectroscopy. They found that inner sphere complexes of Ln3+ with C104-were formed in these systems (6, 7). The nature of the solutions of lanthanide chloride in methanol and methanol-water mixtures has been studied by several techniques, including X-ray diffraction (8, 9), NMR spectroscopy (8-1 l), ultrasonic absorption measurements (12), 'Author to whom correspondence may be addressed. Tel: (613) 564-2967, fax: (613) 564-6793, e-mail: DETSE@ACADVMl. UOTTAWA.CA.'Revision received April 7, 1994.and UV-Visible spectroscopy (13). On the basis of these studies, evidences were found that contact ion pairs are formed between Ln3+ and C1-in...