Structure of Dilithio-(E)-1,4-bis(trimethylsilyl)but-2-ene by X-ray Crystallography and ¹H–⁷Li HOESY

¹H–⁷Li HOESY spectra of good spectral quality were obtained on [{Li(TMEDA)}₂{(Me₃SiCH=CH)₂}] (1) and [{Li(TMEDA)}₂{(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 ⁷Li relaxation. Measured ⁷Li relaxation times in 1 and 2 were temperature dependent, but relatively long T₁ values (ca. 500 ms near ambient temperature) permitted the observation of ⁷Li–¹H HOESY spectra. This is the first report of ⁷Li–¹H 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) Å) and concentration of charge on the terminal carbons (Si–C1 = 1.76 (1) Å, C1–C2 = 1.47 Å), 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) Å, b = 15.393 (3) Å, c = 19.941 (3) Å, V = 3287.8 (9) ų, Z = 4. Cross-peak volumes for the Li-butenediyl fragment of 1 measured in ⁷Li–¹H HOESY spectra in solution are consistent with the X-ray data. The combination of NMR spectroscopy and X-ray structural analysis established a double-bridging lithium structure with similar geometries for both 1 and 2.