Chromium-Salophen as a Soluble or Silica-Supported Co-Catalyst for the Fixation of CO2 Onto Styrene Oxide at Low Temperatures

Addition of a soluble or a supported CrIII-salophen complex as a co-catalyst greatly enhances the catalytic activity of Bu4NBr for the formation of styrene carbonate from styrene epoxide and CO2. Their combination with a very low co-catalyst:Bu4NBr:styrene oxide molar ratio = 1:2:112 (corresponding to 0.9 mol% of CrIII co-catalyst) led to an almost complete conversion of styrene oxide after 7 h at 80°C under an initial pressure of CO2 of 11 bar and to a selectivity in styrene carbonate of 100%. The covalent heterogenization of the complex was achieved through the formation of an amide bond with a functionalized {NH2}-SBA-15 silica support. In both conditions, the use of these CrIII catalysts allowed excellent conversion of styrene already at 50°C (69 and 47% after 24 h, respectively, in homogeneous and heterogeneous conditions). Comparison with our previous work using other metal cations from the transition metals particularly highlights the preponderant effect of the nature of the metal cation as a co-catalyst in this reaction, that may be linked to its calculated binding energy to the epoxides. Both co-catalysts were successfully reused four times without any appreciable loss of performance.


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Supplementary Data

Characterization techniques for the complexes and materials
IR spectra were obtained from KBr pellets on a Jasco FT/IR-4100 spectrometer with resolution better than 1 cm -1 . The 1 H NMR solution spectra were recorded in 5 mm o.d. tubes on Bruker Avance II 300 or 400 spectrometers equipped with a QNP probehead. Chemical shifts were referenced to tetramethylsilane (TMS) for 1 H and 13 C NMR. Prior to the N2 sorption analyses at -196°C using an ASAP-2020 Micromeritics apparatus, the samples were degassed at 200°C for 6 h. Thermogravimetric analyses (TGA/DSC) were performed under air with a TA-Instrument SDT Q600, between 20 and 700°C (air flow rate: 100.0 mL.min -1 , slope: 10°C.min -1 ). High-resolution mass spectra (HRMS) were measured by the Service de Spectrométrie de Masse of Sorbonne Université. Gas chromatography analyses were performed on a Shimadzu NEXIS-GC-2030 AF instrument equipped with a split/splitless (SPL) injector, a flame ionization detector and a GC Capillary Column SH-Rxi-5MS (ID: 0.25 mm; film thickness: 0.25 μm; Length: 30 m) using He as a vector gas (1.5 mL min -1 ) and the following temperature conditions: ramp from 70 to 250°C at 10°C min -1 , then an isotherm at 250°C during 10 min). The X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 diffractometer operating at 30 kV and 30 mA and using a Cu Kα radiation (α = 1.54184 Å) as X-ray source. For the low angles diffraction experiments, the data were collected in the 2θ range from 0.5 to 5° with a step of 0.02° and a counting time of 6 s/step.

Preparation of {NH 2 }-SBA-15.
SBA-15 was pre-formed via a classical sol-gel procedure described elsewhere. [i] SBA-15 was then functionalized with 3-aminopropyltriethoxysilane as described previously, targeting 3 mmol.g -1 ). [i] The obtained support was then characterized by TGA, XRD and N2 physisorption. The thermogravimetric curve of {NH2}-SBA-15 performed under air from room temperature up to 700°C shows two weight losses. The first one (ca. 3%), below 100°C, can be attributed to the loss of water molecules weakly adsorbed on the silica surface whereas the second one (13%, 100-700°C) was assigned to the loss of aminopropyl functions. This TGA analysis showed that {NH2}-SBA-15 would be functionalized with 2.3 mmol of {NH2}.g -1 (c.a. 77 % incorporation yield). Then, the solvent was evaporated until dryness and the resulting orange powder was washed successively with refluxing methanol and acetone using a Soxhlet, respectively for 3 days and 24 h. Salophen-tBu-Cr@{NH2}-SBA-15 (1.1 g) was thus obtained as a bright orange powder (Cr %weight = 0.8 % (5.6% mol Cr/mol -NH2) corresponding to a Cr grafting yield of 41 %).

Protocols for the catalysis tests
After each catalytic test, the resulting solutions or suspensions were analysed by gas chromatography (GC, see details in part 1 of this section) after dilution (20 L of the reaction mixture diluted in 10 mL of CH2Cl2).

Experiments in homogeneous conditions (including stability test)
In a 50 mL Teflon container, 0.7 mL of styrene oxide (6.1 mmol), 0.031 g (0.100 mmol) of n-Bu4NBr and 1 mL of p-xylene (internal standard) were dissolved in 13.3 mL of benzonitrile. Except for the tests performed in the absence of co-catalyst, 0.032 g of Salophen-CrCl (0.05 mmol)) was added and the resulting mixture was stirred for 5 min at room temperature. Then, the autoclave was pressurized at 11 bar of CO2 (corresponding to 41 mmol). The temperature was then increased up to 50-80°C in 20 min. Heating was then prolonged for 3, 7 or 23 h and the reaction was quenched by cooling the autoclave into a water-ice mixture.
For the stability test, the reaction was carried out for 7 h, then quenched as described above. After analysis by GC, a new batch of styrene oxide (0.7 mL, 6.1 mmol) was introduced in the recovered Teflon container (that still contained the n-Bu4NBr catalyst and the co-catalyst) and the reaction mixture was stirred for 5 min at room temperature. Then, the autoclave was pressurized with CO2 as described before. This procedure was repeated 3 times.

Experiments with the supported complexes (including recyclability test)
The protocols were identical with the exception of the mass of co-catalysts added. 0.325 g of Salophen-Cr@{NH2}-SBA-15, corresponding to 0.05 mmol of immobilized metal salophen complexes, was suspended in the benzonitrile solution. Then, the autoclave was pressurized as described before. After 3, 7 or 23 h, the reaction was quenched by cooling the autoclave into a water-ice mixture, and the reaction mixture was filtered on a büchner funnel, in order to separate the supported complex.
For the recyclability test, the reaction was carried out for 7 h and the solid recovered after each attempt was carefully washed by acetone, dried in an oven at 50°C for 12 h and weighted to check the mass after each run. The solid was then added in a Teflon vessel containing 0.7 mL of styrene oxide (6.1 mmol), 0.031 g (0.100 mmol) of n-Bu4NBr and 1 mL of p-xylene (internal standard) dissolved in 13.3 mL of benzonitrile. This procedure was repeated three times. Figure Figure S7). It is not meaningful to give an average pore diameter value in that case.  (100), (110) and (200)