Metal–salen molecular cages as efficient and recyclable heterogeneous catalysts for cycloaddition of CO2 with epoxides under ambient conditions

Metal-salen molecular cages are efficient and recyclable heterogeneous catalysts for cycloaddition of CO2, achieving full conversion at ambient conditions.

S4 analysis (TGA) was performed on a TA Instruments Q500 thermogravimetric analyser by measuring the weight loss while heating at a rate of 10 °C min −1 from room temperature to 800 °C under nitrogen. Nitrogen sorption isotherms were measured at 77 K with an ASAP 2020 Automatic High Resolution Micropore Physisorption Analyser. Before measurement, the samples were degassed in vacuum at 100 °C for at least 10 h. The Brunauer−Emmett−Teller (BET) surface area was evaluated using N2 adsorption data in the relative pressure (P/P0) range of 0.01 -0.14. Powder X-ray Diffraction (PXRD) patterns of samples were collected over the 2Ѳ range 5° to 90° on a Bruker AXS GADDS X-ray diffractometer with Cu-Ka radiation (λ = 1.54056 Å ). The scanning electron micrographs (SEM) were taken with a JEOL FESEM JSM6700F. XPS spectra were measured on powder samples using VG Thermo Escalab 220i -XL X-ray photoelectron spectroscopy system and XPS data was analyzed using Thermo Avantage v4.12. The ee of propylene carbonate was determined via gas chromatography (Shimadzu GC 2010 plus) using Astec Chiraldex BTA with the column temperature set at 160°C (isothermal) while the detector and injector temperatures were set at 180°C. The ee of styrene carbonate was determined by HPLC (Shimadzu LC-20AD) using a Chiralcel OD-H column (eluent 80:20 Hexane:iPrOH). S7 reaction mixture was refluxed for 11 h. The reaction mixture was cooled to room temperature and 50 mL of ethyl acetate and 50 mL of 1N HCl was added. The resulting organic phase was separated and washed with 1 N HCl (2 x 100 mL) and water (3 x 100 mL), dried over MgSO4, and concentrated in vacuum. The residue was purified by silica gel column chromatography (CH2Cl2/hexane 1:3) to give target product as a pale yellow solid (1.17 g, 82%  197.30, 161.13, 142.03, 139.30, 133.42, 132.33, 130.45, 124.54, 120.93, 35.27, 29.39

Synthesis of salen@cage
In an oven dried round-bottomed flask, compounds (C) (436 mg, 0.72 mmol, 2 equiv.) and (D) (136 mg, 1.08 mmol, 3.3 equiv.) were dissolved in dry DMF (100 mL) and a solution of trifluoroacetic acid (TFA) in dry DMF (2 mol%, 0.05 M, 200 μL) was added. The solution turned cloudy upon addition of TFA and the mixture was stirred at 120°C for 5 days. The resultant yellow suspension was allowed to cool to room temperature, filtered and the solid obtained was washed with dry DMF (3 x 25 mL), diethyl ether (3 x 25 mL) and hexane (3 x 25 mL), and dried in vacuo for 24 h to obtain salen@cage as a bright yellow solid (300 mg, 58%).
Glacial acetic acid (0.35 mL, ~30 equiv.) dissolved in dry CH2Cl2 (6 mL) was added slowly to the mixture and stirred under an oxygen atmosphere overnight. The colour of the mixture turned from dark red to dark brown which indicated that the Co(II) metal centres have been S11 successfully oxidized to Co(III) as these colour changes are associated with Co(II) salen and  Table S1).
Al(III)@cage. salen@cage (0.3 mmol, 435 mg, 1 equiv.), Al(OEt)3 (1.08 mmol, 175 mg, 3.6 equiv.) and dry toluene (120 mL) were added into an oven dried round-bottomed flask and stirred vigorously. The yellow suspension was heated to 110 °C and stirred for 3 days. After cooling to room temperature, the yellow solid was filtered and washed with toluene to remove any residues of the metal salt. Further purification of the compound was done by  ppm, which correspond to the carbonyl C=O for aldehydes, also indicated that the trisaldehyde starting material was clearly absent from salen@cage. S15 Figure S4a. IR spectra of tris-aldehyde (C) and salen@cage. The spectra were recorded using KBr pellets. strong C=N imine stretching bands at ῦ = 1630 and 1591 cm -1 . The C=O S16 stretching band of the tris-aldehyde at ~ 1651 cm -1 was not detected which further showed that no starting material was left in salen@cage. Figure S4b. IR spectra of the cage complexes. The spectra were recorded using KBr pellets. Upon metalation of the salen@cage with Al or Co, the C=N imine stretch at 1630 cm -1 shifts to lower frequencies to indicate successful metalation. Figure S5. N 1s XPS spectra of salen@cage, Al(III)@cage, Co(II)@cage and Co(III)@cage.   Figure S7. 13 C CP MAS NMR spectra of salen@cage, Co(III)@cage and Al(III)@cage.    Figure S11. Molecular dimensions of Al(III)@cage based on DFT B3LYP/6-31G (Refer to   Table S4).  Figure S14. SEM of (a) salen@cage at 10,000x, (b) Co(II)@cage at 5,000x, (c) Co(III)@cage at 5,000x, (d) Al(III)@cage at 5,000x.

S3. Catalytic cycloaddition of CO2 with epoxides and catalyst recycling
General procedure for cycloaddition of CO2 with styrene oxide at room temperature and pressure.
The reaction was typically carried out in an oven-dried 25 mL Schlenk tube using styrene oxide (5 mmol, 0.60 g) in solvent free environment under a CO2 balloon. The catalyst and tetra-n-tertbutylammonium bromide (TBAB) was first added into the Schlenk tube and the tube was evacuated and backfilled with CO2 three times. Then styrene oxide was added and the reaction was stirred at the required temperature for a certain amount of time. After the reaction was complete, a small amount of the reaction mixture was sampled, diluted with CDCl3 and filtered through a small pad of Celite to remove the solid catalyst. The solution was then analysed by 1 H NMR spectroscopy to calculate the conversion of styrene oxide to styrene carbonate.
General procedure for cycloaddition of CO2 with epoxide substrates at room temperature and pressure using Al(III)@cage as catalyst.
The reaction was typically carried out in an oven-dried a 10mL Biotage Microwave Reaction V-shaped vial with the epoxide (1 mmol) in solvent free environment under a CO2 balloon.
The catalyst and tetra-n-tertbutylammonium bromide (TBAB) was first added into the vial and sealed with a rubber septum. The vial was evacuated and purged with CO2 three times.
Then 1 mmol of the epoxide was transferred into the vial via micro-syringe and the reaction was stirred at the room temperature for 48 h. The reaction mixture was diluted with methylene chloride and filtered through a short pad of silica to remove the catalyst and TBAB. The pure cyclic carbonates were obtained via flash chromatography using hexane:ethyl acetate (9:1 to 3:1).

Recycling experiments
For catalyst recycling experiments, the catalyst was recycled by filtration, washed with a copious amount of CH2Cl2 and then dried under vacuum overnight. The recycled catalyst was then reused for the next run without further purification.