A Mono‐Substituted Silicon(II) Cation: A Crystalline “Supersilylene”

Abstract Mono‐coordinated silicon(II) cations are predicted to be reactive ambiphiles, combining the typically high Lewis acidity of silicon cations with nucleophilicity due to the presence of an electron pair at the same atomic centre. Here, a carbazole‐derived scaffold was used to isolate salts with a mono‐coordinated silicon(II) cation, [RSi]+ (R=bulky carbazolyl substituent), by halide abstraction from a base‐free halosilylene, RSiI, with Ag[Al(OtBuF)4]. Despite the bulk of the carbazolyl moiety, the silylenylium cation [RSi]+ retains high reactivity. It was shown to react with an amine to form three bonds at the silicon atom in one reaction which conforms with the notion of a “supersilylene”. The resulting silylium cation [RSi(H)NR′2]+ (in the formal oxidation state SiIV) obtained by oxidative addition of an NH bond at [RSi]+ is even more acidic than the silylenylium cation (SiII) due to the absence of a lone pair of electrons the silicon atom.

Starting materials were prepared according to literature protocols: Potassium carbazolide (RK) [33] and Ag[Al(OC4F9)4] [36] were prepared according to established procedures. NMR spectra were acquired on a Bruker Avance 400 MHz spectrometer. Reported chemical shifts are referenced to the 1 H and 13 C NMR resonances of the deuterated solvent. [37] Coupling constants J are given in Hertz as positive values regardless of their real individual sign. 1 H, 11 B, 13 C, 15 N, 19 F, 29 Si NMR spectra were obtained at 400.1, 128.4, 100.6, 40.6, 376.5, 79.5 MHz, respectively. IR spectra were recorded on a Bruker Alpha spectrometer using the attenuated total reflection (ATR) technique on powdered samples.
Elemental analyses were obtained with a Vario Micro Cube (Elementar Analysensysteme GmbH) in the institutional technical laboratories of the Karlsruhe Institute of Technology (KIT).
Single crystals were mounted in perfluoropolyalkyl ether oil on a cryo loop and then brought into the cold nitrogen stream of a low-temperature device (Oxford Cryosystems Cryostream unit) so that the oil solidified. Diffraction data were collected using a Stoe IPDS II diffractometer and graphitemonochromated Mo-Kα (0.71073 Å) radiation or a Stoe STADIVARI diffractometer and Ga-Kα (1.34134 Å) radiation. The structures were solved by direct methods with SHELXS [38] or intrinsic phasing with SHELXT [39] followed by full-matrix least-squares refinement using SHELXL-2018/3 [40] and the ShelXle GUI. [41] All non-hydrogen atoms were refined anisotropically. The contribution of the hydrogen atoms, in their calculated positions, was included in the refinement using a riding model.

RSiCl3 (1Cl)
To a solution of 0.2 ml SiCl4 (approx. 300 mg, 1.76 mmol) in 3 ml toluene a solution of 480 mg (0.692 mmol) RK in 10 ml toluene was added dropwise at ambient temperature. The yellow mixture was then heated to 70 °C and stirred overnight. Then, all volatiles were removed in vacuo, affording a pale yellow residue. The residue was extracted with 10 ml of toluene and filtered. The filtrate was concentrated to incipient crystallisation in an oil bath at 50 °C (approx. 2 ml) and then left to cool down overnight, affording colourless crystals. The supernatant was discarded and the crystals were dried in vacuo (418 mg, 0.529 mmol, 77%).

RSiBr3 (1Br)
To a solution of 375 mg SiBr4 (1.08 mmol) in 3 ml toluene a solution of 680 mg (0.980 mmol) RK in 15 ml toluene was added dropwise at ambient temperature. The yellow mixture was then heated to 60 °C and stirred overnight. Then, all volatiles were removed in vacuo, affording a pale yellow residue. The residue was extracted with 15 ml of toluene and filtered. The filtrate was concentrated to incipient crystallisation in an oil bath at 50 °C (approx. 3 ml) and then left to cool down overnight, affording pale yellow crystals. The supernatant was discarded and the crystals were dried in vacuo (642 mg, 0.696 mmol, 71%).

RSiI3 (1I)
A solution of RK (2.05 g, 2.95 mmol) in toluene was added to a suspension of SiI4 (2.06 g, 3.84 mmol) in toluene. The mixture was sonicated for 30 minutes, turning dark yellow, and then stirred for one day at 40 °C, affording a yellow suspension. The mixture was filtered through a sintered disk (G4). All volatiles of the filtrate were removed in vacuo. The yellow residue was then washed twice with 20 ml hexane, allowing the isolation of RSiI3 as a pale yellow solid. Recrystallisation from toluene at ambient temperature afforded single crystals suitable for structure elucidation (41%).      RSiBr3 (1250 mg, 1.35 mmol) and [( Mes BDI)Mg]2 (998 mg, 1.39 mmol) were combined as solids. To the mixture 40 ml of toluene were added. The mixture was sonicated for 1 hour and then stirred at 35 °C overnight. Afterwards, volatiles were removed in vacuo. The mixture was then extracted with 50 + 10 ml of hexane. The solution was concentrated to approx. 5 ml and left undisturbed overnight, resulting in the deposition of yellow crystals RSiBr. The supernatant was removed via syringe and the solid was dried in vacuo (516 mg, 0.81 mmol, 50%).

RSiI (2I)
RSiI3 (1700 mg, 1.60 mmol) and [( Mes BDI)Mg]2 (1450 mg, 1.64 mmol) were combined as solids. To the mixture 40 ml of toluene were added. The mixture was stirred overnight at room temperature, then volatiles were removed in vacuo. The mixture was then extracted with 60 + 10 ml of hexane. The solution was concentrated to approx. 5 ml and left undisturbed overnight, resulting in the deposition of orange RSiI. The supernatant was removed via syringe and the solid was dried in vacuo (830 mg, 1.02 mmol, 64%).

RSiI-II (2I-II)
Presumably a catalytic amount of HI formed on one occasion due to partial hydrolysis, and caused decomposition of the whole batch of 2I within minutes at the extraction with hexane and crystallisation step. The product was isolated after workup as pale yellow crystalline material (RSiI-II).

[RSi][Al(OC4F9)4] (3)
A mixture of Ag[Al(OC4F9)4] (560 mg, 0.521 mmol) and RSiI (425 mg, 0.525 mmol) was treated with 10 ml of fluorobenzene and then stirred at ambient temperature for 30 minutes. The initially orange mixture rapidly darkened. Afterwards, the suspension was filtered, affording an orange solution. The solution was slowly concentrated at ambient temperature to about 1 ml and left undisturbed overnight, which resulted in the deposition of X-ray quality orange crystals. The supernatant was transferred to another flask. In both fractions, volatiles were removed in vacuo, affording 486 mg crystalline and 272 mg amorphous product (combined 758 mg, 0.434 mmol, 83%).
The crystals darkened quickly in the perfluorinated ether. Solutions turned black upon exposure to air.

[RSi(H)(NH t Bu)NH2 t Bu][Al(OC4F9)4] (6)
A solution of t BuNH2 in 1.1 ml PhF (2.5 mg, 0.034 mmol) was added to a solution of [RSi][Al(OC4F9)4] (30 mg, 0.017 mmol) in 0.5 ml PhF. The mixture was concenctrated to approx. 0.1 ml. Afterwards 0.5 ml C6D6 were added to study the product by NMR spectroscopy. Attempts of large scale reactions and isolation of the product failed.

Computational Details
All computations wer performed using Gaussian16 [42] utilizing the PBE1PBE level of theory, Def2SVP basis sets and empirical dispersion correction (GD3). No solvent corrections were applied. All optimized molecular structures where checked to be minima on the energy hypersurface and possess no imaginary vibrational frequencies. Natural Bond Orbital Theory was applied to study the electronic states. [43] The SambVca server [44] was used to investigate buried volume [45] and steric maps. [46] Cone angles were calculated with Solid-G. [47] It is interesting to note that by utilising the NBO formalism, [43] the species [RSi] + is described as donor-acceptor compound with [R] -→[Si] 2+ and the Wiberg bond index (WBI) for the Si-N bond amounts to only 0.6573, indicating rather low covalency.       Figure S47: Isodesmic reactions.