1,3,5-Tris[(trimethylstannyl)ethynyl]- 1,3,5-trimethyl-1,3,5-trisilacyclohexane

2.1 Synthesis of 2

The title compound 2, 1,3,5-tris[(trimethylstannyl)ethynyl]-1,3,5-trimethyl-1,3,5-trisilacyclohexane, was synthesised in analogy to a reaction described by Wrackmeyer et al. [17]. The all-cis-1,3,5-triethynyl-1,3,5-trimethyl-1,3,5-trisilacyclohexane scaffold was converted with three equivalents of (dimethylamino)trimethyltin (Scheme 1) in n-pentane solution. After removing all volatiles in vacuo compound 2 was obtained as a colorless solid in quantitative yield. Compound 2 was characterized by elemental analysis, single crystal X-ray diffraction, FT-IR and multinuclear NMR spectroscopy.

Scheme 1:

Reaction of 1,3,5-triethynyl-1,3,5-trimethyl- 1,3,5-trisilacyclohexane (1) with (dimethylamino)trimethyltin.

2.2 Spectroscopic data of 2

Multinuclear solution NMR spectra of 2 were recorded in C₆D₆. The ¹H NMR spectrum shows signals for protons of methyl (ring and tin, two singlets) and methylene groups (two doublets) in the range between δ = 0.63 and 0.09 ppm. Compared to 1,3,5-triethynyl-1,3,5-trimethyl-1,3,5-trisilacyclohexane (1) the terminal metalation of ethynyl groups causes only a small shift at resonances of methylene and methyl groups. The ¹³C{¹H} NMR spectrum shows the same insignificant effect on the chemical shifts of methylene and methyl groups, but a significant change of chemical shifts is found for the ethynyl groups. The resonance of the metal-bound carbon atoms is observed at δ = 113.2 ppm (δ = 94.5 ppm [18], 1). The silicon-bound carbon atoms cause a resonance at δ = 118.8 ppm (δ = 90.1 ppm [18]). The chemical shift of the silicon atoms of 2 has a similar value (δ = –21.1 ppm) as that of 1 (–18.5 ppm [18]), confirming that the silicon atoms bear intact ethynyl groups. The ¹¹⁹Sn NMR spectrum of compound 2 shows a single resonance at –76.6 ppm. This value is comparable with that of literature values for Me₃Sn groups bound to ethynyl groups [17].

Further proof for the identity of C≡C units stems from FT-IR spectra. Bands were observed at 2086 and 2036 cm^–1 for compound 2. Elemental analysis data confirm the composition.

2.3 Crystal structure of 2

Compound 2 crystallizes in the trigonal space group R3c with six molecules per unit cell. Selected structural parameters are listed in Table 1. The molecular structure of 2 (Fig. 1) is comparable with that of [CH₂Si(Me)(CCSiMe₃)]₃. The trisilacyclohexane ring features a chair conformation and the Si–C bond lengths (Si(1)–C(1) 1.876(3) Å) of the ring (Table 1) are comparable with the values of 1 (mean: 1.869 Å) [18], [CH₂Si(Me)(CCSiMe₃)]₃ (mean: 1.870) [18] and [SiCl₂CH₂]₃ (mean: 1.855 Å) [19]. The ethynyl groups are arranged equatorially like in the structures of 1 and [CH₂Si(Me)(CCSiMe₃)]₃ [18]. Due to the functionalization with a trimethylstannyl group, the C≡C distance is longer (C(3)–C(4) 1.211(5) Å) in comparison with the C≡C bond of 1 (mean: C≡C 1.194 Å). Furthermore the Sn–C bonds to the methyl groups are slightly longer than the Sn–C bond to the carbon-carbon triple bond. The distance between the tin atoms (Table 1) specifies the cavity of the molecule with the potential to bind Lewis bases by expansion of their coordination sphere.

Fig. 1:

Molecular structure of 2 in the crystal. Displacements ellipsoids are drawn at the 50 % probability level. Hydrogen atoms are omitted for clarity. Symmetry codes used: ¹1+y–x, 1–x, +z; ²1–y, +x–y, +z.

4 Experimental section

All manipulations were performed under dried argon or nitrogen Schlenk techniques. n-Pentane was dried with LiAlH₄ and distilled before use. all-cis-1,3,5-Triethynyl- 1,3,5-trimethyl-1,3,5-trisilacyclohexane (1) was prepared along the lines of established protocols [18]. (Dimethylamino)trimethyltin (Sigma-Aldrich) was distilled before use. NMR measurements were carried out with Bruker Avance III 500 and Bruker DRX 500 instruments. NMR spectra were referenced to the residual signal of used protonated solvents (¹H, ¹³C) (²⁹Si: TMS; ¹¹⁹Sn: SnMe₄, external standard). EI mass spectra were recorded using an Autospec X magnetic sector mass spectrometer with EBE geometry (Vacuum Generators, Manchester, UK) equipped with a standard EI source. FT-IR spectra were collected on a Bruker ALPHA FT-IR spectrometer. Elemental analyses were performed with a CHNS elemental analyser HEKAtech EURO EA.