Selective oxidation of silanes into silanols with water using [MnBr(CO)5] as a precatalyst

The development of earth-abundant catalysts for the selective conversion of silanes to silanols with water as an oxidant generating valuable hydrogen as the only by-product continues to be a challenge. Here, we demonstrate that [MnBr(CO)5] is a highly active precatalyst for this reaction, operating under neutral conditions and avoiding the undesired formation of siloxanes. As a result, a broad substrate scope, including primary and secondary silanes, could be converted to the desired products. The turnover performances of the catalyst were also examined, yielding a maximum TOF of 4088 h−1. New light was shed on the debated mechanism of the interaction between [MnBr(CO)5] and Si–H bonds based on the reaction kinetics (including KIEs of PhMe2SiD and D2O) and spectroscopic techniques (FT-IR, GC-TCD, 1H-, 29Si-, and 13C-NMR). The initial activation of [MnBr(CO)5] was found to result from the formation of a manganese(i) hydride species and R3SiBr, and the experimental data are most consistent with a catalytic cycle comprising a cationic tricarbonyl Mn(i) unit as the active framework.


General Considerations
Unless stated otherwise, all air-sensitive experiments were conducted under an argon atmosphere using standard Schlenk techniques or an MBraun inert-gas glovebox. Solvents for air-and moisture-sensitive experiments were purified through a MBraun-SPS-7 system or dried over activated molecular sieves (3 or 4Å) and degassed according to standard laboratory procedure. Starting materials were purchased from Sigma Aldrich, Abcr, Alfa Aesar, or TCI Europe and used as received. [MnBr(CO)5] has been used as received from Sigma Aldrich or Alfa Aesar. NMR spectra were recorded on a Bruker AVANCE NEO 400 MHz, a Bruker AVANCE III HD 500 MHz NMR spectrometer with a Bruker Prodigy probe, or Bruker AVANCE NEO 600 MHz NMR spectrometer with a BBO cryoprobe. The reaction kinetics were studied on a Bruker AVANCE III HD 400 MHz NMR spectrometer equipped with a BBO probe. The chemical shifts () are given in ppm (parts per million) relative to TMS and were assigned taking as a reference the residual solvent signals (CDCl3: H = 7.26 ppm, C = 77.16 ppm; DMSO-d6: H = 2.50 ppm, C = 39.52 ppm; THF-d8: H = 3.58 ppm, C = 25.31 ppm). 13 C spectra were generally acquired with broadband proton decoupling. 29 Si-NMR spectra were recorded using a DEPT pulse sequence, and the chemical shifts are reported in ppm with respect to TMS (Si = 0 ppm). The peak patterns are indicated as follows: s = singlet; d = doublet; t = triplet; q = quartet; h = sextet: m = multiplet; br = broad. Gas-phase analyses have been performed on a Shimadzu GC Nexis 2030 via manual injection (100 µL) equipped with a Restek Q-Bond column (Length: 30m; inner Diameter: 0.32 mm; film thickness: 10 µm) and a TCD detector using He(g) as a carrier gas. HR-MS have been recorded via an "LTQ-FT-Ultra" provided by Thermo Scientific. The flow FT-IR was a Bruker Vertex 70v with a Harrick Scientific high-pressure demountable liquid cell equipped with diamond windows with a path length of 100 µm. A 1/16'' tubing connected the IR device to a Fischer-Porter tube. The sample was continuously pumped through the loop to the FT-IR spectrometer during sample measurement with a flow rate of 3 mL/min by a WADose Lite HP HPLC pump. The blue light "Evoluchem TM " (450 nm) was purchased from Hepatochem and used in combination with the PhotoRedOx Box TC.

General Method and screening of catalyst loading
[Mn(CO)5Br] (see Table S1), mesitylene (20 μL, 0.14 mmol), and dimethylphenylsilane (68.1 mg, 0.5 mmol) were weighed in a 4 mL vial equipped with a screw cap and a magnetic stirrer inside the glovebox.
The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. H2O (45 μL, 2.5 mmol) and THF (0.5 mL) were added to the vial under argon flush. The reaction mixture was heated at 50 °C for 1 hour. The reaction vessel was allowed to cool down to RT. An NMR tube was filled with the crude mixture (ca 0.05 mL), DMSO-d6 (0.5 mL), and 1 H NMR spectroscopy was conducted to determine the conversion and yield of the reaction. The Si-OH integral (5.88 ppm, 1H) was set at 1.00 and compared with the characteristic peak value of mesitylene (6.76 ppm, 3H).

Solvent screening
[Mn(CO)5Br] (1.3 mg, 0.005 mmol), mesitylene (20 μL, 0.14 mmol), and dimethylphenylsilane (68.1 mg, 0.5 mmol) were weighed in a 4 mL vial equipped with a screw cap and a magnetic stirrer inside the glovebox.. The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. H2O (45 μL, 2.5 mmol) and the selected solvent (0.5 mL, see Table S2) were added to the vial under argon flush. The reaction mixture was heated at 50 °C for 1 hour.
The reaction vessel was allowed to cool down to RT. An NMR tube was filled with the crude mixture (ca 0.05 mL), DMSO-d6 (0.5 mL), and 1 H NMR spectroscopy was conducted to determine the conversion and yield of the reaction. The Si-OH proton signal integral (5.88 ppm, 1H) was set to 1.00 and compared to the characteristic peak value in mesitylene (6.76 ppm, 3H).
The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. The selected amount of H2O (see Table S3) and THF (0.5 mL) were added to the vial under argon flush. The reaction mixture was heated at 50 °C for 1 hour. The reaction vessel was allowed to cool down to RT. An NMR tube was filled with the crude mixture (ca 0.05 mL), DMSO-d6 (0.5 mL), and 1 H NMR spectroscopy was conducted to determine the conversion and yield of the reaction. The Si-OH proton signal integral (5.88 ppm, 1H) was set to 1.00 and compared to the characteristic peak value in mesitylene (6.76 ppm, 3H).

Screening of other Manganese carbonyl complexes
Selected manganese complex (see Table S4), mesitylene (20 μL, 0.14 mmol), and dimethylphenylsilane (68.1 mg, 0.5 mmol) were weighed in a 4 mL vial equipped with a screw cap and a magnetic stirrer inside the glovebox. The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. H2O (45 μL, 2.5 mmol) and THF (0.5 mL) were added to the vial under argon. The reaction mixture was heated at 50 °C for 1 hour. The reaction vessel was allowed to cool down to RT. An NMR tube was filled with the crude mixture (ca 0.05 mL), DMSO-d6 (0.5 mL), and 1 H NMR spectroscopy was conducted to determine the conversion and yield of the reaction. The Si-OH proton signal integral (5.88 ppm, 1H) was set to 1.00 and compared to the characteristic peak value in mesitylene (6.76 ppm, 3H). Manganese complexes Mn-2, [1] Mn-3, [2] and Mn-4 [3] were prepared by following reported literature procedures. In contrast, Mn-1 was purchased from Sigma Aldrich and was used as received.

Screening of other Mn(X) complexes
Selected manganese complex and associated additives (see Table S4), mesitylene (20 μL, 0.14 mmol), and dimethylphenylsilane (68.1 mg, 0.5 mmol) were weighed in a 4 mL vial equipped with a screw cap and a magnetic stirrer inside the glovebox. The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. H2O (45 μL, 2.5 mmol) and THF (0.5 mL) were added to the vial under argon. The reaction mixture was heated at 50 °C for 1 hour. The reaction vessel was allowed to cool down to RT. An NMR tube was filled with the crude mixture (ca 0.05 mL), DMSO-d6 (0.5 mL), and 1 H NMR spectroscopy was conducted to determine the conversion and yield of the reaction.
The Si-OH proton signal integral (5.88 ppm, 1H) was set to 1.00 and compared to the characteristic peak value in mesitylene (6.76 ppm, 3H).

Procedure (a)
[Mn(CO)5Br] (1.3 mg, 0.005 mmol) and the indicated silane (0.5 mmol) were weighed in a 4 mL vial equipped with a screw cap and a magnetic stirrer inside the glovebox. The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. H2O (27 μL, 1.5 mmol) and the indicated solvent (0.5 mL) were added to the vial under argon flush. The reaction mixture was heated at the indicated temperature and time. The reaction vessel was allowed to cool down to RT. The vial was carefully opened to release the H2 overpressure and stirred open to the air for 10-15 min. The vial was closed and irradiated with blue light (450 nm) until the color of the solution changed from yellow to colorless with brown particles of MnO2(s). Activated 3Å molecular sieves were added to the reaction mixture to remove the excess water. The reaction mixture was filtered through a syringe filter, and the vial was washed with DCM (5 mL). The solvent was removed in vacuo to provide the target compound.

Procedure (b)
The indicated silane (0. was added. The reaction mixture was heated at the indicated temperature and time (see Table S7). The reaction vessel was allowed to cool down to RT. The vial was carefully opened to release the H2 overpressure and stirred open to the air for 10-15 min. The vial was closed and irradiated with blue light (450 nm) until the color of the solution changed from yellow to colorless with brown particles of MnO2(s).
Activated 3Å molecular sieves were added to the reaction mixture to remove the excess water. The reaction mixture was filtered through a syringe filter, and the vial was washed with DCM (5 mL). The solvent was removed in vacuo to provide the target compound.

Procedure (d)
The indicated silane (0.5 mmol, substrate 2s; 0.25 mmol substrates 2q, 2r, and 2t) was weighed in a 4 mL vial equipped with a screw cap and a magnetic stirrer inside the glovebox. The reaction vessel was taken outside the glovebox, placed in a Schlenk tube previously evacuated, and filled with argon three times. H2O added to the reaction mixture to remove the excess water. The reaction mixture was filtered through a syringe filter, and the vial was washed with THF (5 mL). The solvent was concentrated in vacuo, and products 2r, 2s, and 2t were precipitated with pentane and washed with cold pentane (5 mL, 3 times).
Product 2q was purified via silica gel column chromatography using a Pentane:EtOAc = 3:1 mixture of solvents.

Synthesis of PhMe2SiD
The synthesis was carried out following slightly changed literature procedures. [4]* LiAlD4 (44.9 mg, 1.0 mmol) was placed in a Schlenk tube equipped with a magnetic stirrer and a screw cap.
PhMe2SiCl (494.6 mg, 2.9 mmol) was weighed in a vial inside the glovebox. Et2O (3 mL) was added to LiAlD4 to form a suspension. PhMe2SiCl was added dropwise to the suspension. The vial was washed with Et2O (4*0.5 mL) and added to the Schlenk tube. The resulting mixture was heated at 36 °C for 16 hours. A NaOH(aq) solution (3 mL, 10% w/w) was added dropwise to the crude reaction mixture and stirred for 15 minutes. The aqueous phase turned into an emulsion and was extracted with Et2O (4*5 mL). The solvent was evaporated, and PhMe2SiD was isolated via vacuum transfer as a colorless oil (69.6 mg, 0.51 mmol, 18%).

Synthesis of 13 C-enriched [MnBr(CO)5]
[MnBr(CO)5] (56.4 mg, 0.2 mmol) was dissolved in the minimum amount of THF (ca. 1-1.5 mL). The solution was frozen with N2(l), evacuated, and refilled with an atmospheric pressure of 13 CO(g). The solution was stirred overnight at 40 °C, cooled down at room temperature, and the solvent was removed in vacuo until a dry solid was obtained. S15 Figure S1: FT-IR spectrum of [MnBr( 13 CO)n(CO)5-n] 2 mM in THF.

Characterization Data
The different NMR spectra are consistent with those previously reported in the literature.

S21
Diphenylsilanediol (2r) [5] C12H12O2Si, white solid. 1  Diethylsilanediol (2s) [5] C4H12O2Si, white solid. Methylphenylsilanediol (2t) [ [MnBr(CO)5] (10.9 mg, 0.04 mmol) and PhMe2SiH (5.5 mg, 0.04 mmol) were dissolved in THF-d8 (0.6 mL) and placed in an oven-dried (120 °C) J-Young tube. 1 H-and 29 Si-NMR were recorded over time. First, the "hydride-bromide exchange" can be observed. Indeed, after 15 minutes, it is possible to observe the formation of PhMe2SiBr and the corresponding manganese hydride complex ( Figure S4). Over time ( Figure   S5). It is possible to observe that after only 2 hours, PhSiMe2Br is completely consumed to afford different silicon-based species. Details of the NMR spectra are reported in the following figures.     The volume of the solution has been brought to 0.5 mL by further addition of dry and degassed THF-d8. The J-Young tube was closed, and the sample was taken outside the glovebox and immediately placed in a Dewar filled with dry ice. The tube was placed in the probe-head of an NMR spectrometer pre-heated at 35 °C, and 1 H-NMR spectra were recorded until full conversion of PhMe2SiH was achieved. The order with respect to each component was determined by an initial rate method and confirmed visually via VTNA. [11] The kinetic S29 isotope effect (KIE) was determined by preparing the samples according to the same procedure but using stock solutions of D2O and PhMe2SiD in dry and degassed THF-d8.        Figure S14). The data were also analyzed with the initial rates method ( Figure S15).     (bottom right graph, Figure S16). The data were also analyzed with the initial rates method ( Figure S17).

Order in H2O
The initial reaction rate (r0) for each run was determined as the slope of the concentration of silanol over time. An initial reaction order of -0.71 with respect to H2O was obtained as the slope of the natural logarithm of the initial reaction rate of each run over the natural logarithm of the concentration of H2O of the corresponding run. S38 Figure S17: Method used to determine the order of H2O in the initial reaction rate using the initial rates method.

Kinetic Isotope Effect of PhMe2SiD
Graph S1: Concentration profiles of PhMe2SiOH with time scale normalized with respect to the water content. The starting concentration for each component is given in Table S8: Run 2 (in black) and Run 10 (in red).

Kinetic Isotope Effect of D2O
Graph S2: Concentration profiles of PhMe2SiOH over time. The starting concentration for each component are given in Table S8: Run 2 (in black) and Run 11 (in red).

Product inhibition
Following the general procedure reported in Section 8.1, the kinetic run number 2 (Table S8) was repeated and followed after adding 25 mol% and 50 mol% of PhMe2SiOH with respect to the initial concentration of PhMe2SiH. All the concentration values are reported in Table S12. The results of the experiment are reported in Graph S3

Study on [MnBr(CO)5]
[Mn(CO)5Br] (5.5 mg, 0.02 mmol) and dimethylphenylsilane (133.8 mg, 1.0 mmol) were weighed in screw cap Schlenk tube inside the glovebox. The reaction vessel was taken outside the glovebox and covered with aluminum foil. H2O (36 µL, 2.0 mmol) and THF (1 mL) were added to the vial under argon flush. The reaction mixture was heated in an oil bath set to 50 °C for 1 hour. Subsequently, the reaction vessel was allowed to cool down to RT, and 100 µL of the gas phase was injected into the GC-TCD. The quantification of H2(g) and CO(g) has been performed using calibration curves corresponding to the respective integrated areas of the two gases.

Study on Mn-4
Mn-4 (3.1 mg, 0.005 mmol) and dimethylphenylsilane (76.6 mg, 0.5 mmol) were weighed in a screw cap Schlenk tube inside the glovebox. The reaction vessel was taken outside the glovebox. H2O (18 µL, 1.0 mmol) and 2-MTHF (0.5 mL) were added to the vial under argon flush. The reaction mixture was heated in an oil bath set to 80 °C for 1 hour. Subsequently, the reaction vessel was allowed to cool down to RT, and 100 µL of the gas phase was injected into the GC-TCD. No CO or H2 was detected. [Mn(CO)5Br] (28.5 mg, 0.1 mmol) and dimethylphenylsilane (15.7 mg, 0.1 mmol) were dissolved in THF (1 mL) in screw cap 4 mL vial inside the glovebox. The vial was closed with a septum screw cap, taken outside the glovebox, and stirred at RT for 5 minutes. Subsequently, 100 µL of the gas phase was injected into the GC-TCD. The quantification of H2(g) and CO(g) has been performed using calibration curves corresponding to the respective integrated areas of the two gases.

Key spectra and significant absorbance-time plots
Scheme S8: Selected spectra since the beginning of the reaction above.
Graph S4: Plot reporting the absorbance of a specific wavelength over time for the reaction reported in Scheme S8.  Subsequently, the reaction vessel was allowed to cool down to RT, and THF was removed in vacuo. Pentane (2 mL) was added to the crude reaction mixture leading to phase separation of the residual H2O ( Figure S22, panels A and B). H2O (1 mL, previously degassed) was added to the Schlenk tube, and the yellow water phase was extracted using pentane (3*2 mL) and once with Et2O (2 mL, Figure S22, panel C). The water was removed in vacuo, leading to a dark orange solid whose IR spectrum is reported in Graph S5. From this orange solid, it was possible to crystallize Mn-5.

X-ray Crystallography and Refinement of Structures
Two colorless single crystals of dimensions 0.26 × 0.07 × 0.07 mm 3 (CCDC 2184831, CCDC 2184832) and 0.27 × 0.12 × 0.10 mm 3 (CCDC 2184833) were selected in polarized light under a microscope and covered with polyfluorinated polyether. The crystals were picked with nylon loops and mounted in the nitrogen cold gas stream of the diffractometer at 100 K. A Bruker D8 Venture diffractometer equipped with an IS3 Diamond Mo-source (Mo-K radiation; =0.71073 Å) and an IS3 Cu-source (Co-K radiation; =1.54178 Å), INCOATEC Helios mirror optics, and Photon III detector was used for data collection. Final cell constants were obtained from least squares fits of setting angles of several thousand strong reflections.
Intensity data were corrected for absorption using intensities of redundant reflections using SADABS [12] .
The structures were readily solved by Direct methods and subsequent difference Fourier techniques. The Bruker APEX3 [13] software package was used to solve and refine the structures. All non-hydrogen atoms were anisotropically refined, and hydrogen atoms were placed at calculated positions and refined as riding atoms with isotropic displacement parameters.
The asymmetric unit of 2o (CCDC 2190210) contained two crystallographically independent molecules of C16H22O3Si2 and a severely disordered chloroform molecule. The scattering contributions of the solvent were removed using the Platon/SQUEEZE program package [14] since an attempt to model the disorder was not completely satisfying. A total of 58 electrons in a void volume of 144 Å3 was found, which perfectly corresponds to the calculated number of electrons for chloroform.
Crystallographic details of data collection and structure refinement are shown in Table S13.