Improved synthesis and crystal structure of the parent 1,3,5-trisilacyclohexane

2.1 Crystal structure

Compound 1 crystallises in the orthorhombic space group Pmn2₁ with two molecules per unit cell. It lies on a crystallographic mirror plane and adopts a chair conformation in the solid state (Fig. 1) as well as in the gaseous phase [11] (gas electron diffraction, GED). Table 1 reveals, that the Si–C distances, in the solid state are as long as the in gas phase and an expansion of the C–Si–C angle compared to Si–C–Si angle has been observed in the crystal structure. The same relation between the angles has also been found in the gas phase structure.

Fig. 1:

Molecular structure of 1,3,5-trisilacyclohexane (1) in the crystalline state. Displacement ellipsoids are shown at the 50 % probability level. Selected bond lengths (Å): Si(1)–C(1) 1.871(1), Si(2)–C(1) 1.873(1), Si(2)–C(2) 1.873(1), Si(1)–H(1A) 1.39(3), Si(1)–H(1B) 1.40(3), Si(2)–H(2A) 1.38(2), Si(2)–H(2B) 1.45(3); angles (deg): C(1)–Si(1)–C(1′) 109.8(1), C(1)–Si(2)–C(2) 110.0(1), Si(1)–C(1)–Si(2) 113.5(1), Si(2)–C(2)–Si(2′) 112.4(1), C(2)–Si(2)–C(1)–Si(1) –54.9(1). Symmetry code used for primed atoms: –x, y, z.

The formal substitution of silicon by germanium atoms leads to the known isostructurally crystallising compound 1,3,5-trigermacyclohexane [15]. The structure 1,3,5-trigermacyclohexane in the solid state has – like 1 – a chair conformation and, expectedly, larger bond lengths (Ge–C, mean: 1.951 Å). The angle Ge–C–Ge (mean: 112.0°) and the angle C–Ge–C (mean: 109.8°) are comparable with those of compound 1 (Table 1).

3.1 Synthesis of 1,1,3,3,5,5-Trisilacyclohexane (1)

1,1,3,3,5,5-Hexamethoxy-1,3,5-trisilacyclohexane (8.56 g, 27.4 mmol), dissolved in diethyl ether (10 mL), was dropped to a suspension of LiAlH₄ (3.26 g, 85.9 mmol) in diethyl ether (80 mL) in a Young-tap ampoule. Afterwards the ampoule was cooled to –196 °C, evacuated, sealed and heated at 45 °C for 4.5 d. Workup: All volatile constituents of the reaction were condensed into another ampoule. This was cooled to a temperature between –50 °C and –45 °C and the diethyl ether was condensed off in dynamic vacuum. The product remained as colourless, acicular solid, with a melting point of 10 °C [lit. [8]: 10 °C]. Yield: 2.97 g (22.5 mmol, 82 %). –¹H NMR (500 MHz, C₆D₆, 298 K): δ = 4.18 (quint, ³J_H,H = 3.6 Hz, 6H, –CH₂SiH₂CH₂–, ¹J_Si,H = 196 Hz), –0.35 (quint, ³J_H,H = 3.6 Hz, 6H, –SiH₂CH₂SiH₂–, ²J_Si,H = 117 Hz). – ¹³C{¹H} NMR (126 MHz, C₆D₆, 298 K): δ = –10.3 (s, –SiCH₂Si–, ¹J_Si,C = 43 Hz). –²⁹Si{¹H} NMR (99 MHz, C₆D₆, 298 K): δ = –34.3 (s, –CH₂SiCH₂–). –GC/EI-MS: m/z (%) = 130 (100) [M–H₂]⁺, 101 [M–SiH₂]⁺, retention time: 2.6 min. – FT-IR (gas phase) ν (cm^–1) = 2153 (Si–H, s), 2143 (Si–H, s).

3.2 X-Ray diffraction experiments

Single crystals of compound 1 suitable for X-ray diffraction measurement were obtained by in situ crystallisation in a glass capillary on the goniometer of the diffractometer. This was achieved by first establishing a solid-liquid equilibrium at 288.2 K close to the melting point, then melting all solid but a tiny crystal seed (using a thin copper wire as external heat source) followed by chilling very slowly with 3 K h^–1 until the whole capillary was filled with a single crystalline specimen at 285 K, then chilling with 10 K h^–1 to 265 K and 25 K h^–1 to the temperature of measurement at 100 K. The measurement was carried out with MoKα radiation (λ = 0.71073 Å). The structure was solved by Direct Methods and refined by full-matrix least-squares cycles (programs: Shelxs/l-97) [16]. Further details of data collection and refinements are listed in Table 2.

CCDC 1419824 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.