STRUCTURES OF STERICALLY OVERCROWDED OR CHARGE-PERTURBED MOLECULES .28. [(H3C)(3)SI]C-3-C-C-C[SI(CH3)(3)](3) - A PI-SYSTEM CONCEALED BY A HYDROCARBON VEIL
H. Bock et al., STRUCTURES OF STERICALLY OVERCROWDED OR CHARGE-PERTURBED MOLECULES .28. [(H3C)(3)SI]C-3-C-C-C[SI(CH3)(3)](3) - A PI-SYSTEM CONCEALED BY A HYDROCARBON VEIL, Journal of organometallic chemistry, 462(1-2), 1993, pp. 31-37
The kinetically shielded di(tris(trimethylsilyl)methyl)acetylene cryst
allizes in the triclinic space group P (1) over bar with 1 molecule pe
r unit cell in rather dense lattice packing. Its staggered skeletal co
nformation of close to D-3d symmetry represents a sterically advantage
ous cogwheel gearing of the inner methyl groups of the half-shells of
both substituents. The shortest nonbonded C...C distances of 400 pm be
tween them equal the sum of the van der Waals radii of two H3C groups.
A space-filling representation of the molecular structure shows the a
cetylenic pi-system to be almost completely covered by the peripheral
HC skin, in keeping with its unusual unreactivity, e.g. towards cataly
tic hydrogenation. The spatial and electronic effects of the bulky sub
stituents are discussed in terms of structural comparisons with chemic
ally related compounds such as [(H3C)(3)Si]Si-3-Si[Si(CH3)(3)](3), [(H
3C)(3)C]Si-3-Si[C(CH3)(3)](3) or [(H3C)(3)Si]C-3-C6H4-C[Si(CH3)(3)](3)
, which also adopt staggered conformations of their sterically congest
ed [(H3C)(3)X]Y-3-half shells along substituent axes of C-3-symmetry a
t different distances Y...Y.