Dimeric [3,3-Dimethyl-2-(trimethylsilyl)cyclopropenyl]- lithium−Tetramethylethylenediamine: Distortion of the Cyclopropenyl Geometry Due to Strong Rehybridization at the Lithiated Carbon

The reaction of 3,3-dimethyl-1-(trimethylsilyl)cyclopropene
with n-BuLi in the presence of 1 equiv of
tetramethylethylenediamine (TMEDA) affords
[3,3-dimethyl-2-(trimethylsilyl)cyclopropenyl]lithium (2).
While NMR
data reveal a monomer in tetrahydrofuran (THF) solution, 2
crystallizes as a dimeric TMEDA solvate,
(2·TMEDA)2.
The structure was determined by single-crystal X-ray diffraction
(crystal data:  monoclinic, space group
P21/n, a =
1879.9(2) pm, b = 1035.0(2) pm, c =
2045.5(2) pm, β = 113.198(9)°, V =
3.6580(8) nm3, Z = 4,
[C28H62Li2N4Si2]). Although dimeric unsolvated
cyclopropenyllithium was computed (Becke3LYP/6-31G*) to have two
planar
tetracoordinate carbon
(R1R2CLi2) fragments,
(2·TMEDA)2 adopts a perpendicular
(“tetrahedral”) structure due to
lithium solvation and the steric crowding of the lithium ligands.
Lithiation at C(α) of the cyclopropenyl ring in
2
lengthens the vicinal and shortens the distal C−C bonds due to the
rehybridization at the lithiated carbon. This is
confirmed both by the natural localized molecular orbital carbon
hybridizations and by the large coupling constant,
1J13C-6Li
= 17.6 Hz, observed in THF solution (the usual range for
organolithium monomers is 10 and 16 Hz).
Despite the strong rehybridization and their relationship to the
C(Li)X halogen and C(Li)O carbenoids, the lithiated
cyclopropenes do not have carbenoid nature.