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

Leslie D. Field, Michael G. Gardiner, Barbara A. Messerle, Colin L. Raston

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Abstract

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) Å3, 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.

Original languageEnglish
Pages (from-to)3566-3570
Number of pages5
JournalOrganometallics
Volume11
Issue number11
DOIs
Publication statusPublished - 1 Nov 1992
Externally publishedYes

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