Y-89 AND SI-29 NMR-SPECTRA AS PROBES OF SOLUTION DYNAMICS - LIGAND AND METAL POLYHEDRAL REARRANGEMENTS IN YTTRIUM AGGREGATES

NMR (H-1, Y-89, and Si-29) studies of yttrium alkoxide, siloxide, and acetylacetonate compounds, including aggregates, have been carried out with special attention to solution structure and integrity (compared to solid-state structure) and fluxionality. Y(OSiPh3)3(THF)3 shows Y/Si coupling (8.1 Hz) in the Si-29 NMR spectrum only under conditions where exchange of OSiPh3 ligands with free Ph3SiOH has been halted. Y10(OC2H4OMe)30 dissolves in toluene to give only one Y-89 signal, consistent with retention of the decameric structure. Y2(OSiPh3)6 shows two Si-29 chemical shifts, consistent with the two bridging and four terminal ligands found in the solid state; only the terminal ligands exhibit detectable Si/Y coupling (7.7 Hz). The fluxionality of inequivalent siloxide ligands in cis-Y(OSiPh3)4(MeOC2H4OMe)- was shown to be intramolecular by observation of Si/Y coupling (7 Hz) above the coalescence temperature. Yttrium site exchange in square-pyramidal Y5O(O(i)Pr)13 was shown to be promoted by (i)PrOH via an associative mechanism. Features of the structure of the alcohol adduct naturally account for site exchange of the two distinct metal sites. Y3(acac)4(OC2H4OMe)5 shows no exchange among inequivalent yttrium sites, but H-1 NMR studies show site exchange of the ether ends of the two mu(3)-eta(2)-OC2H4OMe ligands.