STRUCTURES IN SOLUTION AND IN THE SOLID-STATE OF THE COMPLEXES OF LANTHANUM(III) WITH 1,10-PHENANTHROLINE - X-RAY CRYSTALLOGRAPHIC AND H-1, C-13, O-17, AND LA-139 SOLUTION NMR-STUDIES

The reaction between hexahydrated lanthanum(III) nitrate and 1,10-phenanthroline (phen) in acetonitrile solution was investigated by H-1, C-13, O-17, and La-139 NMR spectroscopy. The crystal structure of trinitratobis(1,10-phenanthroline)lanthanum(III) (La-(NO3)3(C12H8N2)2) was determined by X-ray diffraction: monoclinic; space group C2/c; a = 11.2252 (8) angstrom, b = 18.1844 (14) angstrom, c = 13.1278 (8) angstrom; Z = 4; beta = 100.416 (5)-degrees. At convergence, R(f) = 0.031 and R(w) = 0.032 for 2059 reflections. In solution, La-139 NMR spectra have shown up to three sites in slow exchange with variable intensities. They were attributed to solvated La(III) ions and to two complex species with one or two phen ligands in the La(III) coordination shell. A more complete structural analysis by O-17 NMR spectroscopy indicated that during the course of the complexation by phen, coordinated water was totally expelled from the La(III) coordination sphere, whereas all three nitrate groups remained coordinated. Proton NMR data have confirmed the total displacement of water. In addition, the proton chemical shifts of phen, in comparison with the C-13 NMR data, clearly showed an anisotropic shielding contribution when two phen ligands were coordinated to La(III). This indicates that the two phen molecules are in a close vicinity in the coordination sphere. From the NMR results, one can conclude that the structure in solution of the 2:1 complex, La(NO3)3(phen)2, is very similar to the solid-state one. From La-139 NMR spectroscopy, the populations of the various La(III) species were obtained and the apparent formation constants were calculated.