A low-temperature modification of hexa-tert-butyldisilane and a new polymorph of 1,1,2,2-tetra-tert-butyl-1,2-diphenyldisilane

Abstract

Crystals of hexa-tert-butyldisilane, C24H54Si2, undergo a reversible phase transition at 179 (2) K. The space group changes fromIbca(high temperature) toPbca(low temperature), but the lattice constantsa,bandcdo not change significantly during the phase transition. The crystallographic twofold axis of the molecule in the high-temperature phase is replaced by a noncrystallographic twofold axis in the low-temperature phase. The angle between the two axes is 2.36 (4)°. The centre of the molecule undergoes a translation of 0.123 (1) Å during the phase transition, but the conformation angles of the molecule remain unchanged. Between the two tri-tert-butylsilyl subunits there are six short repulsive intramolecular C—H...H—C contacts, with H...H distances between 2.02 and 2.04 Å, resulting in a significant lengthening of the Si—Si and Si—C bonds. The Si—Si bond length is 2.6863 (5) Å and the Si—C bond lengths are between 1.9860 (14) and 1.9933 (14) Å. Torsion angles about the Si—Si and Si—C bonds deviate by approximately 15° from the values expected for staggered conformations due to intramolecular steric H...H repulsions. A new polymorph is reported for the crystal structure of 1,1,2,2-tetra-tert-butyl-1,2-diphenyldisilane, C28H46Si2. It has two independent molecules with rather similar conformations. The Si—Si bond lengths are 2.4869 (8) and 2.4944 (8) Å. The C—Si—Si—C torsion angles deviate by between −3.4 (1) and −18.5 (1)° from the values expected for a staggered conformation. These deviations result from steric interactions. Four Si—C(t-Bu) bonds are almost staggered, while the other four Si—C(t-Bu) bonds are intermediate between a staggered and an eclipsed conformation. The latter Si—C(t-Bu) bonds are about 0.019 (2) Å longer than the staggered Si—C(t-Bu) bonds.