DMAP-stabilized bis(silyl)silylenes as versatile synthons for organosilicon compounds

DMAP-stabilized silylenes 1a–c are obtained from the reductive debromination of the corresponding dibromosilanes in the presence of DMAP. Their distinctly different thermal isomerization reactions via C–H bond activation, dearomative ring expansion and silyl migration are discussed. Furthermore, complexes 1 dissociate at elevated temperatures, providing the corresponding free silylenes in situ, which are even capable of single-site activation of H2. Additionally, a potassium-substituted silicon-centered radical 2 is isolated from overreduction of (tBu3Si)2SiBr2.


General Methods and Instrumentation
All manipulations were carried out under exclusion of water and oxygen under an atmosphere of argon 4.6 (≥99.996%) using standard Schlenk and glovebox techniques. The glassware used was heat dried under fine vacuum prior to use. All solvents were refluxed over sodium/benzophenone, freshly distilled under argon and deoxygenated prior to use. PTFE-based grease (Triboflon III from Freudenberg & Co. KG) was used as sealant. Deuterated benzene (C6D6) was obtained from Sigma-Aldrich, dried over Na/K alloy, flask-to-flask condensed, deoxygenated by three freezepump-thaw cycles and stored over 3 Å molecular sieves in a glovebox. All NMR samples were prepared under argon in J. Young PTFE valve NMR tubes. The NMR spectra were recorded on a δ( 13 C) = 128.1 ppm). [S1] The following abbreviations are used to describe signal multiplicities: s = singlet, d = doublet, dd = doublet of doublets, m = multiplet, br = broad. In some NMR spectra, signals from silicone oil (C6D6: δ( 1 H) = 0.29 ppm, δ( 13 C) = 1.4 ppm and δ( 29 Si) = -21.8 ppm), originating from the cannulas used (B. Braun Melsungen AG Sterican®), can be observed. EPR spectra were recorded on a Jeol jes-Fa200 esr spectrometer with a spectrometer frequency of (≥99.95%) were purchased from Westfalen AG and used as received. The compounds ((TMS)3Si)2SiBr2, [S2] ( t Bu2MeSi)2SiBr2, [S3] ( t Bu3Si)2SiBr2 [S4] and ((TMS)3Si)( t Bu3Si)Si←DMAP (1a) [S5] were prepared as described in the corresponding references. Potassium graphite (KC8) was synthesized following a literature reported procedure upon heating a 1:8 mixture of potassium and graphite in a thick-walled, PTFE-capped pressurize-able Schlenk flask to 500 °C until a homogenous bronze powder was obtained. [S6]
Crystals suitable for SC-XRD analysis were obtained from a cooled
Note: During the synthesis of 1c, the concomitant formation of hexakis(trimethylsilyl)trisilirane (4) and Si(TMS)4 was observed. Therefore, no sample of 1c with sufficient purity for elemental analysis was obtained and the yield was not determined.
Concomitantly formed KBr and graphite were separated by extracting the residue with toluene (3 × 4 mL). Evaporation of the solvent in vacuo and subsequent washing of the residue with n-hexane (3 × 2 mL) afforded compound 2 as orange-brown solid (54.1 mg, 88.5 µmol, 52%). Crystals suitable for SC-XRD analysis were obtained from a cooled (-35 °C) solution of 2 in toluene. m.p. = 60 °C (decomposition; color change to dark red) EPR (toluene, 286 K) g = 2.0056, a(α-29 Si) = 2.92 mT Note: Compound 2 is completely NMR silent. Elemental analysis was not matching, presumably because an unquantifiable amount of coordinating toluene was removed during drying compound 2 in fine vacuum. Due to its extreme air and moisture sensitivity and the fact, that it is not stable in toluene, no satisfactory spectroscopic data of 2 was obtained before addition of crown ether (18-C-6). With crown ether however, one signal in the EPR spectrum was observed (Fig. S7).
Hyperfine coupling with the β-29 Si nuclei was not visible.

Siliranes 6
The synthesis of siliranes 6 was conducted by a similar procedure than than that for hydrosilanes 5 (vide supra). Instead of H2, the DMAP-silylene complexes 1a and 1b were exposed to ethylene (1 bar). The compounds 6 were obtained as colorless solids.
Silirane 6a was identified by comparison of NMR spectral data with literature reports. [S5] Compound 6b was identified by multinuclear NMR spectroscopy.

General Information
The X-ray intensity data of 2 were collected on an X-ray single crystal diffractometer equipped with a CMOS detector , a rotating anode (Bruker TXS) with MoKα radiation (λ = 0.71073 Å) and a Helios mirror optic by using the APEX III software package. [S8] The X-ray intensity data of 1b and 7a were collected on an X-ray single crystal diffractometer equipped with a CMOS detector , an IMS microsource with MoKα radiation (λ = 0.71073 Å) and a Helios mirror optic by using the APEX III software package. [S8] The X-ray intensity data of 3 was collected on an X-ray single crystal diffractometer equipped with a CCD detector (Apex II CCD), a fine-focus sealed tube with MoKα radiation (λ = 0.71073 Å) and a Triumph monochromator by using the APEX II/III software package. [S8] The measurements were performed on single crystals coated with the perfluorinated ether Fomblin ® Y. The crystal was fixed on the top of a micro sampler, transferred to the diffractometer and frozen under a stream of cold nitrogen.
A matrix scan was used to determine the initial lattice parameters. Reflections were merged and corrected for Lorenz and polarization effects, scan speed, and background using SAINT. [S9] Absorption corrections, including odd and even ordered spherical harmonics were performed using SADABS. [S9] Space group assignments were based upon systematic absences, E statistics, and successful refinement of the structures. Structures were solved by direct methods with the aid of successive difference Fourier maps, and were refined against all data using the APEX III software in conjunction with SHELXL-2014 [S10] and SHELXLE. [S11] All H atoms were placed in calculated positions and refined using a riding model, with methylene and aromatic C-H distances of 0.99 and 0.95 Å, respectively, and Uiso(H) = 1.2·Ueq(C). Full-matrix least-squares refinements were carried out by minimizing ∆w(Fo 2 -Fc 2 ) [S9] with SHELXL-97 weighting scheme. [S12] Neutral atom scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from International Tables for Crystallography. [S13] The images of the crystal structures were generated by Mercury. [S14] The CCDC numbers CCDC-1967942 (1b), CCDC-1967943 (2), CCDC-1967944 (3) and CCDC-1967945 (7a) contain the supplementary crystallographic data for the structures. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.    where P=(Fo 2 +2Fc 2 )/3 where P=(Fo 2 +2Fc 2 )/3 where P=(Fo 2 +2Fc 2 )/3 Largest diff. peak and hole 0.560 and -0.742 eÅ -3 0.263 and -0.218 eÅ -3 0.366 and -0.261 eÅ -3 0.634 and -0.258 eÅ -3 R.M.S. deviation from mean 0.104 eÅ -3 0.040 eÅ -3 0.042 eÅ -3 0.058 eÅ -3