Transforming atmospheric CO2 into alternative fuels: a metal-free approach under ambient conditions

The present work established the metal-free approach to capture CO2 from air and its reduction into alternative fuel under ambient conditions.


Methoxyborane formation with compound 3
In a glovebox, borane (0.07 mmol, 10 equiv) was added in a 5 mL glass vial containing compound 3 (5.0 mg, 0.007 mmol) and finally the vial was closed with proper cap and taken outside. To this glass vial, 2 mL C 6 D 6 was added after quickly opening the vial in air and then closed with the cap and kept it at RT. Immediately, we observed a gas evolution (please see video clip), which was confirmed as hydrogen gas by 1

Catalytic hydroboration of CO 2 with compound 3
Under an argon atmosphere, a 25 mL Schlenk tube equipped with a J. Young valve was charged with 3 (5.0 mg, 0.007 mmol), BH 3 .SMe 2 (0.14 mmol, 20 equiv.) and C 6 D 6 (2 mL). To this solution, hexamethylbenzene was added as an internal standard. The mixture was degassed by a freeze-pump-thaw cycle and placed under 1 atm. of CO 2 (99.995% pure CO 2 from a CO 2 cylinder) at room temperature. The progress of the reaction was monitored by 1 H NMR spectroscopy and the conversion was calculated based on the integration of methoxy group of (CH 3 OBO) 3 with respect to internal standard. All reported conversion of product was an average of at least two runs.

Computational details
All optimization were performed using M062X 3-4 /6-31+G(d,p) 5 level of theory as M062X is reported to be highly efficient for main group elements. Gaussian 09 6 suit of programs is employed for all calculation. As reported earlier, the ground state of aNHCcarbene (1) is singlet and the first triplet state is 49.8 kcal/mol higher in energy. 7   generates 9BBN-OH (14) along with the release of the carbene salt (15). The activation energy for the decomposition is computed to be 25.6 kcal/mol and it goes through a S13 transition state namely TS6. The overall exergonicity of the process is 28.7 kcal/mol (∆G =-28.7 kcal/mol). All the free energies in the bracket are given in kcal/mol. Relevant bond distances are shown in Å.

Fig. S4
Optimized structures for conversion of compound 10 to 14 and 15 via TS6.

Details for preparation of video clipping of the carbon dioxide reduction from air with compound 3
With the help of a glovebox,9-BBN(0.035 mmol, 10 equiv) was added in a 5 mL glass vial containing compound 3 (5.0 mg, 0.007 mmol) and the vial was taken out of the glovebox.
To the vial, quickly 2 mL C 6 D 6 was added in open air and the cap of the vial was closed and kept. Immediately, we observed a gas evolution, which was confirmed as hydrogen by 1 H S14 NMR spectroscopy (δ = 4.46 ppm). 14 Although the total time of the reaction to consume the substrates is about 6 h, we have presented the edited video file capturing visual gas evaluation during the reaction initial thirty seconds when gas evolution rate was very high.
In this video, especially we have captured the gas evolution first few seconds after addition of 9-BBN, when the rate of gas evolution was very high and the rate becomes slow with time (after 30 min. the rate of evolution becomes very slow).

X-ray crystal structure measurements
Suitable single crystals of 2, 3 and 5 were selected and mounted under nitrogen atmosphere using the X-TEMP2 and intensity data were collected on a Super Nova, Dual, Cu at zero, Eos diffractometer. The crystal was kept at 100.00(10) K during data collection.
Using Olex2, 15 the structure was solved with the Superflip 16 structure solution program using Charge Flipping and refined with the ShelXL 17 refinement package using Least Squares minimization. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined isotropically on calculated positions using a riding model. Disordered moieties were refined using bond lengths restraints and isotropic displacement parameters restraints. Crystallographic data (excluding structure factors) for the structures have been deposited with the Cambridge Crystallographic Data Centre.
Single-crystal X-ray structural studies of compound 4 were performed on an Oxford Diffraction XCALIBUR-EOS CCD equipped diffractometer with an Oxford Instruments low temperature attachment. Crystal data were collected at 293(2) K using graphitemonochromated Mo Kαradiation (λα = 0.71073 Å). The strategy for the data collection was evaluated by using the CrysAlisPro CCD software. The data were collected by the standard S15 '-' scan method and were scaled and reduced using CrysAlisPro RED software. The structures were solved by direct methods (SHELXS) and refined by full-matrix leastsquares calculations on F 2 (SHELXL). 18 Copies of the data can be obtained free of charge       (2).