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, ¹J_¹³C-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.