title: 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 from Ibca (high temperature) to Pbca (low temperature), but the lattice constants a, b and c do 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.
authors.affiliation: Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Scholz
authors.affiliation: Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.
authors.forename: Hans-Wolfram
authors.initials: HW
authors.lastname: Lerner
authors.affiliation: Institut für Organische Chemie und Chemische Biologie, Universität Frankfurt, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.
authors.forename: Jan W
authors.initials: JW
authors.lastname: Bats
keywords: 1,1,2,2-tetra-tert-butyl-1,2-diphenyldisilane; bond elongation; crystal structure; hexa-tert-butyldisilane; polymorphs; reversible phase transition; steric interactions