CONFORMATIONAL PROPERTIES OF EU(III) COMPLEXES OF 3,3'-5,3''-POLYMETHYLENE BRIDGED DERIVATIVES OF 2,2'-6,2''-TERPYRIDINE

The complex [Eu(tpy)3](ClO4)3, where tpy = 2,2'; 6,2''-terpyridine, has been prepared and reexamined. The complex appears to be stable in acetonitrile solution with respect to decomplexation of the ligands but the addition of water does cause partial replacement of tpy. Analogous complexes have been prepared with 3,3'; 5,3''-polymethylene bridged derivatives of tpy having two or three carbons in the bridge. The bridging enforces a cisoid geometry of the ligand and prohibits its replacement by added water. An X-ray determination was carried out for [Eu(3b)3](ClO4)3, where 3b=3,3'; 5,3''-dimethylene tpy, which crystallizes in the monoclinic space group P2(1)/c with a=11.908(4), b=15.768(5), c=29.513(9) angstrom, beta=93.60(2)-degrees, mu=13.5 cm-1 and Z=4. The complex forms a tricapped trigonal prism with each of the ligands adopting the same dl conformation. Variable temperature NMR analysis of the bridged ligand complexes indicates that conformational inversion of the bound ligand is not a concerted process and barriers for inversion of individual methylene units can be estimated from coalescence of the signals from the geminal methylene protons. The luminescence properties of the bridged tpy complexes are similar to the parent unbridged system.