Structural characterization of two modifications of tris(tetrahydrofuran)(tris(trimethylsilyl)silyl)lithium: a compound with a silicon-29-lithium-7 NMR coupling

The crystal structures of (MesSi),Si.THF (a = b = 12.310(1) A, c = 17.308(2) A, tetragonal, space group P4/ncc (No. 130), Z = 4), [(Me3Si)&i][(Me3Si)3SiLi(THF)3](a~ = b = 15.146(8) A, c = 21.338(10) A, trigonal, space groupP3cl (No. 165), Z = 2), (Me3Si)3SiLi(THF)3 (a = b = 31.342(4) A, c = 22.000(6) A, rhombohedral, space groupR3c (No. 161), Z = 24), and (Me3Si)3SiSi(SiMe3)3aTHF (a = 11.360(3) A, b = 17.131(6) A, c = 18.952(6) A, 0 = 91.21(3)', monoclinic, space group P21/c (No. 14), Z = 4), have been determined by low-temperature X-ray analysis. Whereas the small S i S i S i angle in (tris(trimethylsily1)silyl)lithium of 102.1 O supports the ionic character of the Si-Li bond, there must be at least some covalent contribution leading to a J[%,7Li] coupling constant of 38.6 Hz. A modified preparation for (Me3si)~siLi(THF)is~ reported giving the pure product rather than the cocrystallization product [(Me3Si)4Si] [(Me3Si)3SiLi(THF)3]2, where the starting material is still present in the ratio 1:2. The pure product is far more reactive than the cocrystallized material and is as pyrophoric as 'BuLi powder.