STRUCTURE OF DILITHIO-(E)-1,4-BIS(TRIMETHYLSILYL)BUT-2-ENE BY X-RAY CRYSTALLOGRAPHY AND H-1-LI-7 HOESY

H-1-Li-7 HOESY spectra of good spectral quality were obtained on [{Li(TMEDA)}2{(Me3SiCH=CH)2}] (1) and [{Li(TMEDA)121(Me3SiCH=CMe)2}] (2) in solution at room temperature. Fluxionality leads to motional averaging of the environment around the Li nucleus and consequently a reduction in the quadrupolar contribution to Li-7 relaxation. Measured Li-7 relaxation times in 1 and 2 were temperature dependent, but relatively long T1 values (ca. 500 ms near ambient temperature) permitted the observation of Li-7-H-1 HOESY spectra. This is the first report of Li-7-H-1 HOESY NMR for obtaining structural information. X-ray diffraction data for 1 established a planar (Z)-butenediyl unit, with essentially double-bond character across C2 and C3 (1.34 (3) A) and concentration of charge on the terminal carbons (Si-C1 = 1.76 (1) angstrom, C1-C2 = 1.47 angstrom), with the lithium atoms residing symmetrically over the vector between these carbon atoms. Crystals of 1 are orthorhombic, space group Pbca, a = 10.711 (1) angstrom, b = 15.393 (3) A, c = 19.941 (3) angstrom, V = 3287.8 (9) angstrom3, Z = 4. Cross-peak volumes for the Li-butenediyl fragment of 1 measured in Li-7-H-1 HOESY spectra in solution are consistent with the X-ray data. The combination of NMRs spectroscopy and X-ray structural analysis established a double-bridging lithium structure with similar geometries for both I and 2.