Atmospheric-Pressure Conversion of CO2 to Cyclic Carbonates over Constrained Dinuclear Iron Catalysts

The conversion of CO2 and epoxides to cyclic carbonates over a silica-supported di-iron(III) complex having a reduced Robson macrocycle ligand system is shown to proceed at 1 atm and 80 °C, exclusively producing the cis-cyclohexene carbonate from cyclohexene oxide. We examine the effect of immobilization configuration to show that the complex grafted in a semirigid configuration catalytically outperforms the rigid, flexible configurations and even the homogeneous counterparts. Using the semirigid catalyst, we are able to obtain a TON of up to 800 and a TOF of up to 37 h–1 under 1 atm CO2. The catalyst is shown to be recyclable with only minor leaching and no change to product selectivity. We further examine a range of epoxides with varying electron-withdrawing/donating properties. This work highlights the benefit arising from the constraining effect of a solid surface, akin to the role of hydrogen bonds in enzyme catalysts, and the importance of correctly balancing it.


Materials and Methods
The pro-ligand (H2L) and di-iron complex were prepared according to literature procedures. 1,2 The chemicals required for all these syntheses were purchased from Aldrich and used as received unless otherwise noted. The deuterated solvent for NMR studies, i.e. CDCl3 was acquired from Aldrich and used as received. The common reagents and common solvents were acquired locally and used as received. All 1 H and 13 C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 25 o C with chemical shifts given in parts per million (ppm) using the residual solvent peak as reference, located at 7.26 ppm in the case of CDCl3 and at 2.5 ppm in the case of d6-DMSO. Fourier transform infrared spectra (FTIR) were recorded on a Nicolet 8700 FTIR spectrometer equipped with an attenuated total reflection (ATR) stage. ICP-OEM analyses were done on a Thermo Scientific ICP Spectrometer ICAP 6300 DUO. Samples were digested in 1 ml conc. HCl by stirring at 65 °C for 12 h, and the resulting solution was diluted with deionized water prior to ICP-OES analysis. TGA-MS measurements were performed on a SETARAM Labsys-Evo coupled to a Hiden QGA-pro using synthetic air as the carrier gas (20% O2 in Ar, 30 ml/min, 5 °C/min). High resolution X-ray photoelectron spectroscopy (HR-XPS) measurements were performed in an analysis chamber (UHV -2*10 -10 Torr during analysis) using a Versaprobe III -PHI Instrument (PHI, USA). The sample was irradiated with a Focused X-Ray AlKα monochromated X-ray source (1486.6eV) using an X-ray beam (size 200 micron, 50 W, 15 kV). The outcoming photoelectrons were directed to a Spherical Capacitor Analyzer (SCA). The sample charging was compensated by a Dual Beam charge neutralization based on a combination of a traditional electron flood gun and a low energy argon ion beam. Diffuse reflectance UV-vis spectra were recorded on a Agilent Cary 5000 UV-vis spectrophotometer with a DRA-2500 integrating sphere. Electron paramagnetic resonance (EPR) spectra were obtained on a Bruker EMX EPR spectrometer controlled with a Bruker ER 041 XG microwave bridge at 15 or 77 K. The EPR spectrometer was equipped with a continuous-flow liquid He cryostat and an ITC503 temperature controller made by Oxford Instruments, Inc.

Synthesis Procedures: Synthesis of LFe2Cl4:
Reduced Robson macrocycle ligand (LH2) 1 and the corresponding di iron (III) complex (LFe2Cl4) 2 were synthesized by slight modification of reported procedure. The clear dry THF solution of ligand LH2 (933 mg, 1.69 mmol) in dry flask was kept in a glove box freezer for 0.5 h. KH (147 mg, 3.4 mmol) was slowly added by small portions to this cold THF solution. After 12 h of stirring, the reaction mixture was centrifuged, then [FeCl3(DME)] (860 mg, 3.4 mmol) was added to this colorless solution, and immediately the reaction mixture changed to dark blue. The resulting mixture was left to stir at RT for 24 h. The white KCl precipitates were eliminated by centrifugation, and the dark blue THF supernatant was removed in vacuum. The dark blue solid residue was finally washed with hexane (3 × 15 mL), then dried under vacuum for 24 h.

Synthesis of LFe2-NH/SiO2:
A clear solution of LFe2Cl3ClO4 (755 mg, 0.87 mmol) in 20 mL of dry acetonitrile was transferred into a Schlenk tube and to this a suspension of 3-aminopropyl silica (0.7g) in 15mL acetonitrile was added. The reaction mixture was vigorously stirred at 85 o C for 48 h under argon. The resulting solid product was separated by centrifugation and washed seven times with fresh acetonitrile, three times THF, two times diethyl ether and the obtained solid product was dried under vacuum (yield: 0.589 g).

Synthesis of LFe2-O/SiO2:
A solution of N, N-diisopropylethylamine (112 mg, 0.87 mmol) in 5 mL of dry acetonitrile was transferred into a Schlenk tube containing a suspension of silica support (SiO2-200, 0.7 g) in 10 mL acetonitrile, and the resulting mixture was stirred for two hours at room temperature. To this, the clear solution of LFe2Cl3ClO4 (755 mg, 0.87 mmol) in 20 mL of acetonitrile was added. The reaction mixture was vigorously stirred at 85 o C for 48 h under argon. The resulting solid product was purified in the same manner as above (yield: 0.625 g).

Synthesis of LFe2-O-NH2/SiO2:
A solution of N, N-diisopropylethylamine (224 mg, 1.74 mmol) in 5 mL of dry acetonitrile was transferred into a Schlenk tube containing a suspension of 3-aminopropyl silica (0.7 g) in 10 mL acetonitrile, and the resulting mixture was stirred for two hours at room temperature. To this, the clear solution of LFe2Cl3ClO4 (755 mg, 0.87 mmol) in 20 mL of acetonitrile was added. The reaction mixture was vigorously stirred at 85 o C for 48 h under argon. The resulting solid product was purified in the same manner as above (yield: 0.611 g).

Typical procedure for the synthesis of cyclic carbonate from an epoxide and 1 atm CO2
The catalytic conversion of CO2 to cyclic carbonate was carried out in a 25 mL glass flask refluxing with a CO2 balloon. In a typical catalytic cycloaddition, dry cyclohexene oxide (CHO, 9.2 mmol) and the dry di iron (III) catalyst LFe2-O-NH2/SiO2 (0.036 g, 9.210 -3 mmol, according to the amount of LFe2Cl3ClO4) followed by bis(triphenylphosphino)iminium chloride (PPNCl) (10.6 mg, 18.410 -3 mmol) were placed in a 25 mL glass flask equipped with a magnetic stirrer. After being sealed, three times the flask was carefully evacuated and refilled with CO2. After that, the reaction was carried out at 80 o C temperature by refluxing with a CO2 balloon for the desired period of time. After the reaction, the pressure was released and the resulting mixture was diluted with CH2Cl2. After which, the reaction mixture was centrifuged to remove the solid catalyst and evaporated and dried in vacuo overnight. The product was analyzed by 1 H NMR spectroscopy without further purification as the vacuum was sufficient to remove unreacted cyclohexene oxide. Turn-overnumber (TON) was calculated as [number of moles of epoxide consumed based on epoxide conversion/moles of solid catalyst]. These number were obtained by: [(isolated cyclic carbonate yieldweight of PPNCl) /142.1]/moles of solid catalyst. The calculated error is based on three separate reaction runs.
Typical procedure for the synthesis of cyclic carbonate from an epoxide and 15 atm. CO2

S-5
For high-pressure cycloaddition reactions, the di-iron (III) catalyst LFe2-O-NH2/SiO2 (0.036 g, 9.210 -3 mmol), PPNCl (10.6 mg, 18.410 -3 mmol) and cyclohexene oxide (CHO, 0.092 mol) were placed in a 25 mL stainless-steel Parr reaction vessel (which was dried in an oven at 140 o C overnight) under argon. After being sealed, the reactor was carefully evacuated and refilled with CO2. The reaction was carried out at the specified temperature and CO2 pressure for the desired period of time. After the reaction, the reactor was cooled in an ice-water bath and slowly depressurized. The product purification was carried out in the same manner as above.       Table S1. Interpretation of proposed models by comparing the experimental and theoretical peak areas from XPS.
H.B = higher binding energy; L.B = lower binding energy; N'= electron deficient, N= electron rich. a Oxygen atomic composition reflects contributions of the grafted materials and of the (dominating) signal from the SiO2 oxygen.  Table S1A), which leads to non-equivalent nitrogen atoms in a symmetrical macrocycle.    Figure S10. Illustrates the proposed pathways for formation of active anionic species, ironalkoxide species and iron-carbonate species via dual-activation mechanism by two iron metal centers. Figure S11. Backbiting reactions from carbonate and alkoxide to form cis-cyclohexene carbonate and trans-cyclohexene carbonate respectively. 3  S-20 Figure S17. The reaction setup used for the conversion of CO2 to cyclic carbonate under 1 atm CO2 pressure.