Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy

Reactive surface species immobilized inside porous materials with suitably small windows can be studied by DNP SENS.


General Procedure
All experiments were carried out under controlled atmosphere. Treatments of the surface species were carried out using high vacuum lines (1.34 Pa) and glove box techniques. n-Pentane was distilled on Nabenzophenone and degassed through freeze pump thaw cycles.
Typical procedure of synthesis of mesoporous material Synthesis of SBA-15 with 6.0 nm pore size. Mesoporous silica was synthesized according to the literature 1 using a molecular ratio of the sol of 0.03 P123: 1.0 TEOS: 8.2 HCl: 300 H2O. In 500 mL glass, 8.0 g of pluronic P123 (Poly(ethyleneglycol)) was dissolved in 250 mL 1.9 M HClaq.. The jar was sealed and heated to 40˚C with stirring for up to 3 hours. Then, 17 g of TEOS were added dropwise and the solution is stirred under this conditions for 24 hours. The mixture was transferred into an autoclave and reacted for another 24h at 100˚C. Then SBA-15 is filtered and washed, dried and calcined at 500˚C for 9h, applying a heat rate of 1˚C per minute.
Synthesis of MCM-41 with 3.0 nm pore size. The material was synthesized according to literature 2 using a molecular ratio of the sol of 0.11 CTAB: 1.0 TEOS: 0.11 NaOH: 111 H2O. First, 4.05 g of cetyltrimethylammonium bromide (CTAB) was dissolved in 200 mL H2O and stirred for 1 hour. Then, NaOH (440 mg) was added and the mixture was stirred for 15 more minutes. TEOS (20.6 mg) was added and the reaction mixture was stirred for 1 more hour. The reaction mixture was transferred in an autoclave and hydrothermally treated at 80˚C for 24 h. The next day, the mixture was filtered and washed with 0.1 M HCl in ethanol (3x 500 mL). The recovered material was dried at 130˚C for at least 24 hours and finally calcined at 500˚C for 9 hours applying a heat rate of 1˚C per hour.
Synthesis of MCM-41 with 2.5 nm pore size. The material was synthesized according to literature 3 using a molecular ratio of the sol of 0.14 CTAB: 1.0 TEOS: 1.7 NH4NO3: 150 H2O. In a glass jar, cetyltetrammonium bromid (CTAB) (2.4 g) was dissolved in H2O (120 mL). After 10 min, 30 wt% NH4OH(aq) (10.5 mL) were added and stirred for another 15 min. Then, TEOS (10.5 mL) was slowly added and the mixture was stirred another hour. The precipitate was recovered by filtration without washing followed by hydrothermal treatment at 80 °C for 24 h. The material was dried in vacuo before using an extraction protocol 4 for removing the surfactant by NH4 + /CTMA + exchange. Typically, 1 g of MCM-41 was dispersed in ethanol (95%) containing 0.3 g of NH4NO3 (150 mL). The mixture was stirred at 60˚C for 20 minutes. Then the solids were recovered by filtration and washed with cold ethanol, and the above treatment could be repeated twice. Finally, the MCM-41 was dried well. To be sure that the surfactant is fully removed, the material was subsequently calcined at 550˚C for 3 hours with and heating rate of 1˚C per hour.
Dehydroxylation at 500˚C. All three materials SBA-15 (1) and MCM-41 (2 and 3) with specific surface areas were dehydroxylated at 500°C for 24 h under dynamic vacuum (10 -5 mbar) to generate SBA-15500 (1) and MCM-41500, (2,3) respectively, as the well-ordered hexagonal structure is maintained under these conditions. 5 Titration of Silanol. The concentration of silanol was determined by titration of Si-OH with MeLi. 6 Passivation of external surface. One of our strategies was to perform external passivation. 7 Unfortunately, passivation is limited to a dehydroxylation temperature of only 200˚C as the trimethylsilyl groups decompose at 500˚C. 8,9 A dehydroxylation temperature of 200˚C, however, would lead to a mixture of monopodal and bipodal SOMFs, which is not consistent with our study (MCM-41500).
For 1B, 2B and 3B, 1, 2 or 3 (500 mg) were suspended in n-pentane (20 mL) in a double Schlenk. A solution of WMe6 was added and reacted at -40˚C for 6h. Then, the solvent was removed by filtration, the precipitate was washed with n-pentane, until the remaining B was fully removed. Elemental Analyzer from Thermo Scientific.
The small-angle X-ray powder diffraction (XRD) data were acquired on a Bruker D8 advance diffractometer using Cu Kα monochromatic radiation (λ=1.054184Å) to confirm the hexagonal ordered structure of the samples.
Transmission-electron microscopy (TEM). The primary particle size and morphology of sample 3 was examined by conventional TEM. A microscope of model Titan, G 80-300 from FEI Company (Hillsboro, OR) was employed to carry out the analysis. Samples were loaded in argon-filled glove box onto a vacuum sealed holder of model 648 from Gatan, Inc. In this way, the samples were loaded into the microscope without their exposure to air. Conventional bright-field TEM (BF-TEM) as well as energy- Structure of silica particles was found to be well preserved after their loading with W-containing ligand.
Hence both channels and walls of the silica particles could be identified easily. The presence of W in the channels was proved by the W maps and Si-maps. It is imperative to note that the maximum intensity in the tungsten map cannot always be assigned to the regions having maximum tungsten in the channels (edge or center) for three reasons: 1) the applied energy loss of W at 35 eV is close to the zero-loss peak; 2) the pore size of the material is very small and 3) the sample might be slightly tilted. The white signal is presumed to emanating from the W ligand. The measurements on the white dot matched with channel width and hence it corroborates our argument that the W was present inside the channels.
Infrared spectra were recorded on a Nicolet 6700 FT-IR spectrometer equipped with a cell under controlled atmosphere.
Nitrogen adsorption-desorption isotherms at 77 K were measured using a Micromeritics ASAP 2024 physisorption analyzer. Specific surface areas were calculated following typical BET procedures. Pore size distribution was obtained using BJH pore analysis applied to the desorption branch of the nitrogen adsorption/desorption isotherm.

Solid State NMR (SS NMR).
One-dimensional 1 H MAS and 13 C CP/MAS solid state NMR experiments were recorded on Bruker AVANCE III spectrometers operating at 400 MHz resonance frequencies for 1 H employing a conventional double-resonance 3.2 mm CP/MAS probe. In all cases the samples were packedinto zirconium rotors under an inert atmosphere inside gloveboxes. Dry nitrogen gas was utilized for sample spinning to prevent degradation of the samples. NMR chemical shifts are reported with respect to the external references TMS and adamantane. Potassium bromide (KBr) was used to calibrate the magic angle for the MAS probes. For 13 C CP/MAS NMR experiments, the following sequence was used: 90° pulse on the proton (pulse length 2.4 s), then a cross-polarization step with a contact time of typically 2 ms, and finally acquisition of the 13 C NMR signal under high-power proton decoupling. The delay between the scans was set to 5 s to allow the complete relaxation of the 1 H nuclei, and the number of scans of 60 000 for 13 C. An exponential apodization function corresponding to a line broadening of 80 Hz was applied prior to Fourier transformation.
A typical 13 C CP SS NMR spectra of 0A is shown in Figure S1. Even after 60 000 scans (which corresponds to 84 h experimental time), the carbyne is not observed. 10 Figure S1. One dimensional (1D) 13   factor of 0.57. 13 All 13 C NMR spectra were referenced to adamantane with the higher frequency peak set to 38.48 ppm with respect to TMS (0 ppm). More detailed information can be obtained from Table S1.

Nitrogen adsorption/desorption isotherms
Nitrogen adsorption/desorption isotherms of 1, 2 and 3 are shown in Figure S2 and Figure S3. The isotherms can be classified as type IV isotherms with a narrow pore size distribution. Furthermore, structural parameters of the mesoporous material were determined and are summarized in Table S2.

IR spectra
The IR spectra of these materials obtained after reaction are quite similar. Each one representative of A and B can be found in Figure S5 and Figure S6. For A (by the example of 1A),

DFT calculations
To estimate the size of TEKPol, W(≡CtBu)(CH2tBu)3 A and WMe6 B (see Figure S12), calculations were performed with the generalized gradient approximation (GGA) functional with the Gaussian09 software using the BP86 level of theory of Becke and Perdew. [14][15][16] The electronic configuration of the molecular systems are described with the standard split-valence basis set with a polarization function of Ahlrichs and co-workers for H, C, N and O (SVP keyword in Gaussian). 17 For W, the small-core, quasi-relativistic Studgard/Dresden effective core potential, with an associated contracted valence basis set (standard SDD keyword in Gaussian09) was used. 18 The coordinates for B, were taken from literature. 11 The coordinates can be found in Table S8, Table S9 and Table S10.