Syntheses and crystal structures of the anhydride 4-oxa-tetra-cyclo-[5.3.2.02,6.08,10]dodec-11-ene-3,5-dione and the related imide 4-(4-bromo-phen-yl)-4-aza-tetra-cyclo-[5.3.2.02,6.08,10]dodec-11-ene-3,5-dione.

The syntheses and crystal structures of the two title compounds, C11H10O3 (I) and C17H14BrNO2 (II), both containing the bi-cyclo-[2.2.2]octene ring system, are reported here [the structure of I has been reported previously: White & Goh (2014 ▸). Private Communication (refcode HOKRIK). CCDC, Cambridge, England]. The bond lengths and angles of the bi-cyclo-[2.2.2]octene ring system are similar for both structures. The imide functional group of II features carbonyl C=O bond lengths of 1.209 (2) and 1.210 (2) Å, with C-N bond lengths of 1.393 (2) and 1.397 (2) Å. The five-membered imide ring is nearly planar, and it is positioned exo relative to the alkene bridgehead carbon atoms of the bi-cyclo-[2.2.2]octene ring system. Non-covalent inter-actions present in the crystal structure of II include a number of C-H⋯O inter-actions. The extended structure of II also features C-H⋯O hydrogen bonds as well as C-H⋯π and lone pair-π inter-actions, which combine together to create supra-molecular sheets.

Computational and dynamic NMR investigation of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane.

The structure and rotational barrier for the mesityl-silicon bond of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane have been investigated by 1 H- and 13 C-variable temperature nuclear magnetic resonance (NMR) as well as by density functional theory structural calculations. The calculations show that the lowest energy structure has C2 symmetry with nonequivalent ortho methyl groups, consistent with the crystal structure and solution NMR. The nonequivalent ortho methyl groups exchange through a Cs transition state with a calculated relative free energy of 11.0 kcal mol-1 . The barrier for this rotation found by dynamic NMR is 13.4 ± 0.2 kcal mol-1 at 298 K.

Syntheses and crystal structures of 2-methyl-1,1,2,3,3-penta-phenyl-2-sila-propane and 2-methyl-1,1,3,3-tetra-phenyl-2-silapropan-2-ol.

The sterically hindered silicon compound 2-methyl-1,1,2,3,3-penta-phenyl-2-sila-propane, C33H30Si (I), was prepared via the reaction of two equivalents of di-phenyl-methyl-lithium (benzhydryllithium) and di-chloro-methyl-phenyl-silane. This bis-benzhydryl-substituted silicon compound was then reacted with tri-fluoro-methane-sulfonic acid, followed by hydrolysis with water to give the silanol 2-methyl-1,1,3,3-tetra-phenyl-2-silapropan-2-ol, C27H26OSi (II). Key geometric features for I are the Si-C bond lengths that range from 1.867 (2) to 1.914 (2) Šand a τ4 descriptor for fourfold coordination around the Si atom of 0.97 (indicating a nearly perfect tetra-hedron). Key geometric features for compound II include Si-C bond lengths that range from 1.835 (4) to 1.905 (3) Å, a Si-O bond length of 1.665 (3) Å, and a τ4 descriptor for fourfold coordination around the Si atom of 0.96. In compound II, there is an intra-molecular C-H⋯O hydrogen bond present. In the crystal of I, mol-ecules are linked by two pairs of C-H⋯π inter-actions, forming dimers that are linked into ribbons propagating along the b-axis direction. In the crystal of II, mol-ecules are linked by C-H⋯π and O-H⋯π inter-actions that result in the formation of ribbons that run along the a-axis direction.

Crystal structures of 2-bromo-1,1,1,3,3,3-hexa-methyl-2-(tri-methyl-sil-yl)tris-ilane and 2-bromo-1,1,1,3,3,3-hexa-isopropyl-2-(triiso-propyl-sil-yl)tri-silane.

The synthesis and crystal structures of two tris-(tri-alkyl-sil-yl)silyl bromide compounds, C9H27BrSi4 (I, HypSiBr) and C27H63BrSi4 (II, TipSiBr), are described. Compound I was prepared in 85% yield by free-radical bromination of 1,1,1,3,3,3-hexa-methyl-2-(tri-methyl-sil-yl)tris-ilane using bromo-butane and 2,2'-azobis(2-methyl-propio-nitrile) as a radical initiator at 333 K. The mol-ecule possesses threefold rotational symmetry, with the central Si atom and the Br atom being located on the threefold rotation axis. The Si-Br bond distance is 2.2990 (12) Šand the Si-Si bond lengths are 2.3477 (8) Å. The Br-Si-Si bond angles are 104.83 (3)° and the Si-Si-Si bond angles are 113.69 (2)°, reflecting the steric hindrance inherent in the three tri-methyl-silyl groups attached to the central Si atom. Compound II was prepared in 55% yield by free-radical bromination of 1,1,1,3,3,3-hexa-isopropyl-2-(triiso-propyl-sil-yl)tris-ilane using N-bromo-succinimide and 2,2'-azobis(2-methyl-propio-nitrile) as a radical initiator at 353 K. Here the Si-Br bond length is 2.3185 (7) Šand the Si-Si bond lengths range from 2.443 (1) to 2.4628 (9) Å. The Br-Si-Si bond angles range from 98.44 (3) to 103.77 (3)°, indicating steric hindrance between the three triiso-propyl-silyl groups.