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

Abstract

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, 1 J 13 C - 6 Li = 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.

Rehybridization as a general mechanism for maximizing chemical and supramolecular bonding and a driving force for chemical reactions

“…For example, the carbon orbitals of the CF bonds are about sp 4 ‐hybridized in alkanes, sp 3 ‐hybridized in alkenes, and sp 2 ‐hybridized in alkynes. For an illustrative example of structural and energetic consequences of rehybridization by an electropositive (Li) substituent, see ref 14…”

Orbital hybridization: a key electronic factor in control of structure and reactivity

“…The excellent correlation between the hybridization at carbon and the electronegativity of Y in C–Y bonds of monosubstituted alkanes, alkenes and alkynes is well documented. Variable hybridization in combination with Bent's rule readily explains many geometric features of fluoroorganics that do not readily follow from the Valence shell electron pair repulsion (VSEPR) theory . An illustrative example is provided by difluoromethane where the F–C–F valence angle is smaller than the H–C–H angle (Fig.…”

Toward Planar Tetracoordinate Carbon in the Puckered Ladder Structures of Chelated Cyclopropenyllithium Aggregates

“…For the tetracoordinate Li1 and Li1A, the increased coordination number at the metals leads to longer C−Li bonds relative to the trigonal, inner Li2 and Li2A (217.3(3) versus 210.0(3) pm; cf. the 220.5(3)−221.9(3) pm values found in the dimeric TMEDA complex of [3,3-dimethyl-2-(trimethylsilyl)cyclopropenyl]lithium) …”

“…Unsolvated dimeric cyclopropenyllithium 6 was computed recently to prefer an “in plane” geometry with planar tetracoordinate R 1 R 2 CLi 2 carbons. ,2b However, as with the phenyllithium dimer 3 , , lithium solvation was shown to favor the “perpendicular” geometry in the X-ray structure of the dimeric TMEDA complex of [3,3-dimethyl-2-(trimethylsilyl)cyclopropenyl]lithium ( 7 ) . We now report the results of a deliberate attempt to realize planar tetracoordinate carbon arrangements in simple organolithium dimers.…”