Synthesis, Characterization, and Structural Analysis of AM[Al(NONDipp)(H)(SiH2Ph)] (AM = Li, Na, K, Rb, Cs) Compounds, Made Via Oxidative Addition of Phenylsilane to Alkali Metal Aluminyls

We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NONDipp)}]2 (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)n][Al(NONDipp)(H)(SiH2Ph)] (AM = Li, Na, K, Rb, L = Et2O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)4}2(Rb{Al(NONDipp)(H)(SiH2Ph)}2)]+ [Rb{Al(NONDipp)(H)(SiH2Ph)}2]−, was isolated during an attempted recrystallization of Rb[Al(NONDipp)(H)(SiH2Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of μ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.

For compounds prepared at the University of Strathclyde: 1-Rb·(Et2O), 1-Cs·(THF)2 and 2 Hexane, THF and diethyl ether were dried by heating to reflux over sodium benzophenone ketyl and then distilled under nitrogen prior to use. Pentane, benzene, and toluene were degassed with nitrogen, dried over activated aluminium oxide (Innovative Technology, Pure Solv 400-4-MD, Solvent Purification System), and then stored under inert atmosphere over activated 4 Å molecular sieves. Benzene-d6, toluene-d8 and THF-d8 were degassed by freeze-pump-thaw methods and stored over activated 4 Å molecular sieves. NMR spectra were recorded on a Bruker AV3 or AV 400 MHz spectrometer operating at 400.13 MHz for 1 H, 100.62 MHz for 13 C. All 13 C spectra were proton decoupled. 1 H and 13 C{ 1 H} chemical shifts are expressed in parts per million (δ, ppm) and referenced to residual solvent peaks.
Infrared spectra of starting materials and selected products were obtained as Nujol mulls on NaCl plates. Mulls were prepared in the glove box using anhydrous Nujol, which was dried and stored over activated 4 Å molecular sieves under argon, and then transferred to the spectrometer in a desiccator.
Spectra were recorded on a Nicolet 360 FTIR spectrometer spanning 4000-400 cm -1 . The melting points of selected products and starting materials were determined as follows. A small sample of crystalline/powdered material was loaded into a melting point tube in the glove box. This tube was then sealed with Plasticine® before removal from the glove box. The melting point was then determined in the usual manner using a Buchi Melting Point B-545 apparatus. Elemental analysis was conducted by the Elemental Analysis Service at London Metropolitan University. Crystallographic data for complexes 1-Rb·(Et2O) and 1-Cs·(THF)2 were collected on an Oxford Diffraction Gemini S instrument with graphite-monochromated Mo−Kα (λ 0.71073 Å) radiation or on Rigaku XtaLAB Synergy-S with monochromated Cu−Kα (λ 1.54184 Å) radiation. The measured data was processed with the CrysAlisPro [S11] software package. Using Olex2, [S12] the structure was solved with the ShelXT [S8] structure solution program using Intrinsic Phasing and refined with the ShelXL [S13] refinement package using Least Squares minimization or by the full-matrix least-squares method using SHELXL-2018 implemented within WINGX. [S8] All non-hydrogen atoms were refined using anisotropic thermal parameters unless noted otherwise.

Preparation of [Cs(THF)2][Al(NON Dipp )(H)(SiH2Ph)]: 1-Cs·(THF)2
In a 25 mL Schlenk flask [Cs{Al(NON Dipp )}]2 (378 mg, 0.588 mmol) was dissolved in hexane (15 mL) to give a yellow solution. PhSiH3 (90 μL, 0.729 mmol, 1.24 equiv.) was added resulting in the precipitation of a white solid within 10 minutes of stirring at room temperature. After stirring the reaction mixture for additional 16 hours at room temperature the precipitate was collected via filtration and subsequently washed with n-hexane (4 x 2 mL). The crude product was then dissolved in THF (4 mL) and layered with n-hexane. Storing this solution at -20 °C for two days yielded colourless crystals suitable for single X-ray diffraction. Isolation of the crystalline material and drying at high vacuum the target compound was obtained as white solid in 64 % (336 mg, 0.375 mmol) yield.

Preparation of [{Rb(THF)4}2(Rb{Al(NON Dipp )(H)(SiH2Ph)}2)][Rb{Al(NON Dipp )(H)(SiH2Ph)}2] (2)
In a 25 mL Schlenk flask [Rb{Al(NON Dipp )}]2 (245 mg, 0.412 mmol) was dissolved in hexane (14 mL) to give a yellow solution. PhSiH3 (56 μL, 0.454 mmol, 1.10 equiv.) was added resulting in the precipitation of a white solid within one hour of stirring at room temperature. After stirring the reaction mixture for S8 additional 16 hours at room temperature the precipitate was collected via filtration and subsequently washed with n-hexane (4 x 2 mL). The crude product was then dissolved in THF (2 mL) and layered with n-hexane. Storing this solution at -20 °C for one day yielded colourless crystals suitable for single X-ray diffraction. Isolation of the crystalline material and drying at high vacuum the target compound was obtained as white solid in 69 % (239 mg, 0.141 mmol) yield.

2) Crystal structure data [Li(Et2O)][Al(NON Dipp )(H)(SiH2Ph)]: 1-Li·(Et2O)
A colorless crystal of [Li(Et2O)][Al(NON Dipp )(H)(SiH2Ph)] was covered in inert oil and mounted. The crystal was then flash cooled to 120 K in a nitrogen gas stream and kept at this temperature during the experiment. Data were collected using focused micro-source Cu Kα radiation at 1.54184 Å. The measured data was processed with the CrysAlisPro software package. The structure was solved using direct methods with SHELXS and refined to convergence against F 2 for all independent reflections. All non-hydrogen atoms were refined using anisotropic thermal parameters. Figure S1. ORTEP plot of the asymmetric unit of 1-Li·(Et2O) (ellipsoids drawn at 30% probability). S10

[Na(Et2O)][Al(NON Dipp )(H)(SiH2Ph)]: 1-Na·(Et2O)
A colorless crystal of [Na(Et2O)][Al(NON Dipp )(H)(SiH2Ph)] was covered in inert oil and mounted. The crystal was then flash cooled to 120 K in a nitrogen gas stream and kept at this temperature during the experiment. Data were collected using focused micro-source Cu Kα radiation at 1.54184 Å. The measured data was processed with the CrysAlisPro software package. The structure was solved using direct methods with SHELXS and refined to convergence against F 2 for all independent reflections. All non-hydrogen atoms were refined using anisotropic thermal parameters. Figure S2. ORTEP plot of the asymmetric unit of 1-Na·(Et2O) (ellipsoids drawn at 30% probability). S12

A colorless crystal of [Rb(Et2O)][Al(NON Dipp )(H)(SiH2Ph)] was embedded in inert
perfluoropolyalkylether (viscosity 1800 cSt; ABCR GmbH) and mounted using a glass fiber. The crystal was then flash cooled to 100 K in a nitrogen gas stream and kept at this temperature during the experiment. The crystal structure was measured with a Rigaku XtaLAB Synergy-i with monochromated Cu−Kα (λ = 1.54184 Å) radiation. The measured data was processed with the CrysAlisPro software package. Using Olex2, the structure was solved with the ShelXT structure solution program and refined to convergence against F 2 for all independent reflections. All non-hydrogen atoms were refined using anisotropic thermal parameters. Figure S3. ORTEP plot of the asymmetric unit of 1-Rb·(Et2O) (ellipsoids drawn at 30% probability).

A colorless crystal of [Cs(THF)2][Al(NON Dipp )(H)(SiH2Ph)] was embedded in inert
perfluoropolyalkylether (viscosity 1800 cSt; ABCR GmbH) and mounted using a glass fiber. The crystal was then flash cooled to 100 K in a nitrogen gas stream and kept at this temperature during the experiment. The crystal structure was measured with an Oxford Diffraction Gemini E instrument with graphite-monochromated Mo−Kα (λ = 0.71073 Å) radiation. The measured data was processed with the CrysAlisPro software package. Using Olex2, the structure was solved with the ShelXT structure solution program and refined to convergence against F 2 for all independent reflections. All nonhydrogen atoms were refined using anisotropic thermal parameters. One of the THF ligands was modelled as disordered over two sites. Restraints and constraints were added to this disordered group to ensure that geometry and displacement ellipsoids approximated normal behavior. The occupancies of the disordered sites refined to 66.4(6):33.6(6). Figure S4. ORTEP plot of 1-Cs·(THF)2 (ellipsoids drawn at 30% probability).

A colorless crystal of [{Rb(THF)4}2(Rb{Al(NON Dipp )(H)(SiH2Ph)}2)][Rb{Al(NON Dipp )(H)(SiH2Ph)}2] was
embedded in inert perfluoropolyalkylether (viscosity 1800 cSt; ABCR GmbH) and mounted using a glass fiber. The crystal was then flash cooled to 100 K in a nitrogen gas stream and kept at this temperature during the experiment. The crystal structure was measured with an Oxford Diffraction Gemini S instrument with graphite-monochromated Mo−Kα (λ = 0.71073 Å) radiation. The measured data was processed with the CrysAlisPro software package. Using Olex2, the structure was solved with the ShelXT structure solution program and refined to convergence against F 2 for all independent reflections. All non-hydrogen atoms were refined using anisotropic thermal parameters. One of the THF ligands was modelled as disordered over two sites. Restraints and constraints were added to this disordered group to ensure that geometry and displacement ellipsoids approximated normal behavior.

4) Selected infrared spectra
Infrared vibrational spectra, shown in Figures S74 and S75, were obtained by Fourier transform infrared (FT-IR) recording on solid samples, using a Bruker Tensor 27 FT-IR spectrometer. Figure S74. IR vibrational spectrum of 1-Li·(Et2O) Figure S75. IR vibrational spectrum of 1-Na·(Et2O) Infrared vibrational spectra, shown in Figures S76 -S78, were obtained as Nujol mulls on NaCl plates. Mulls were prepared in the glove box using anhydrous Nujol, which was dried and stored over activated 4 Å molecular sieves under argon, and then transferred to the spectrometer in a desiccator. Spectra were recorded on a Nicolet 360 FTIR spectrometer spanning 4000-400 cm -1 at room temperature. Figure S76. IR vibrational spectrum of 1-Rb·(Et2O) Figure S77. IR vibrational spectrum of 1-Cs·(THF)2.