Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture

Abstract Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N‐heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring‐opened and ring‐closed states is accompanied by substantial pK a shifts of the NHIs by up to 8.7 units. Since only the ring‐closed isomers are sufficiently basic to activate CO2 via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light‐controlled capture and release of CO2.

: 1     (2-Methyl-5-phenylthiophen-3-yl)boronic acid (3) 3-bromo-2-methyl-5-phenylthiophene (2.0 g, 7.9 mmol, 1 eq) was dissolved in THF (40 mL) and cooled down to -78 °C. n-BuLi (5.5 mL, 1.6 M solution in hexane, 8.7 mmol, 1.1 eq) was added dropwise. The reaction mixture was stirred for 1 h at -78 °C. Tributyl borate (2.76 mL, 10 mmol, 1.3 eq) was added rapidly, then the solution was stirred for 15 min at -78 °C. The reaction was warmed up to room temperature and was stirred for 30 min at room temperature. The reaction was stopped by adding HCl solution (20 mL, 1 M). The phases were separated and the organic phase was washed with 1 M NaOH solution. The collected aqueous phases were evaporated to remove the rest of THF. Then solution was acidified at pH 1 with 6 N HCl at 0 °C. The resulting solid was filtrated and washed with npentane. After drying the product was isolated as a white solid with a yield of 93% (1.6 g, 7.33 mmol).

Preparation of 7HBF4
A Schlenk flask was charged with 6 (300 mg, 533 µmol, 1.00 eq.), anhydrous KF (186 mg, 3.20 mmol, 6.00 eq.) and MeNH3Cl (72.0 mg, 1.07 mmol, 2.00 eq.). MeCN (5 mL) was added and the resulting suspension was stirred at room temperature for 2 d. CHCl3 (5 mL) was added and the suspension was stirred for another 5 min at room temperature. The solid components were filtered off and the solution was transferred into a separation funnel. A diluted aq. solution of NaBF4 (293 mg, 2.67 mmol, 5.00 eq.) was added to the MeCN/CHCl3 suspension followed by vigorous shaking. The organic phase was separated and the aqueous phase was extracted once with a small amount of CHCl3. The combined organic fractions were dried (anhydrous MgSO4), filtered and the solvent was removed under reduced pressure to afford a colorless solid. After drying at 80 °C in vacuo overnight, 7HBF4 was obtained in 90% yield as a white solid (268 mg, 481 µmol).

Preparation of 8HBF4
A Schlenk flask was charged with 6 (160 mg, 284 µmol, 1.00 eq.), anhydrous KF (98.8 mg, 1.70 mmol, 6.00 eq.) and tert-butylamine (62.3 mg, 43.6 µL, 852 µmol, 3.00 eq.). MeCN (5 mL) was added and the resulting suspension was stirred at room temperature for 3 d. CHCl3 (5 mL) was added and the suspension was stirred for another 5 min at room temperature. The solid components were filtered off and the solution was transferred into a separation funnel. A diluted aq. solution of NaBF4 (156 mg, 1.42 mmol, 5.00 eq.) was added to the MeCN/CHCl3 suspension followed by vigorous shaking. The organic phase was separated and the aqueous phase was extracted once with a small amount of CHCl3. The combined organic fractions were dried (anhydrous MgSO4), filtered and the solvent was removed under reduced pressure to afford a colorless solid. After drying at 80 °C in vacuo overnight, 8HBF4 was obtained in 82% yield as a white solid (143 mg, 239 µmol).

Preparation of 7
A Schlenk flask was charged with a solution of the 7HBF4 (75.0 mg, 135 µmol, 1.00 eq.) in THF (5.00 mL). A diluted THF solution of KOtBu (14.8 mg, 132 µmol, 0.98 eq.) was added dropwise and the resulting suspension was stirred at room temperature for 2h. The volatiles were evaporated and 7 was extracted twice with n-hexane. The solvent was removed under reduced pressure to afford 7 in 94% yield as a pale yellow oil (59.2 mg, 126 µmol).

Preparation of 8
A Schlenk flask was charged with a solution of the 8HBF4 (130 mg, 217 µmol, 1.00 eq.) in THF (5.00 mL). A diluted THF solution of KOtBu (23.9 mg, 213 µmol, 0.98 eq.) was added dropwise and the resulting suspension was stirred at room temperature for 2h. The volatiles were evaporated and 8 was extracted twice with nhexane. The solvent was removed under reduced pressure to afford 8 in 94% yield as a white solid (103 mg, 201 µmol).   Preparation of 8CO 2 8 (8.00 mg, 15.6 µmol) was dissolved in MeCN-d3 and the NMR tube was pressurized with two bar 13 CO2 pressure.

X-ray Diffraction Studies
General: Single-crystal X-ray diffraction data were collected on a Bruker AXS detector using Mo-K radiation ( = 0.71073 Å). Crystals were selected under oil, mounted on nylon loops and then immediately placed in a cold stream of N2 on a diffractometer. Using Olex2, [5] the structures were solved with the Superflip [6] structure solution program using Charge Flipping and refined with the ShelXL [7]

Single-crystal X-ray structure analysis of 7HBF4:
Single crystals were obtained by cooling down a hot and saturated CHCl3 solution of 7HBF4. A Singlecrystal X-ray structure analysis revealed that 7HBF4 crystallizes in the monoclinic space group P21/c. The asymmetric unit contains one molecule of 7HBF4.

Single-crystal X-ray structure analysis of 8o
Single crystals were obtained by cooling down a saturated n-pentane solution of 8o to -34 °C. A Singlecrystal X-ray structure analysis revealed that 8o crystallizes in the triclinic space group P-1. The asymmetric unit contains one molecule of 8o and a disordered n-pentane molecule.

Single-crystal X-ray structure analysis of 8c
Single crystals were obtained by storing a MeCN solution of 8c at room temperature for two months. A Single-crystal X-ray structure analysis revealed that 8c crystallizes in the orthorhombic space group Pna21. The asymmetric unit contains one molecule of 8c. Figure S45: Molecular view of 8c in the solid state with thermal ellipsoid plot at the 50% levels of probability. Hydrogen atoms are omitted for clarity.

DFT Calculations
All structures were optimized without geometry constraints using the TPSS meta GGA functional 8 and an atom-pairwise dispersion correction (D3) 9 . A flexible triple zeta basis set (def2-TZVP) 10 was used in all calculations. The minimum character of all optimized stationary points was proven by the absence of imaginary vibrational frequencies. For the calculation of the free energy contributions from vibrational frequencies using the rigid-rotor-harmonic-oscillator approach (G RRHO (298K)), a rotor approximation was applied to vibrational modes with wave numbers below 100 cm -1 . 11 Electronic energies were recalculated with the hybrid functional PW6B95(-D3) 12 using the structures optimized with TPSS-D3. Solvation free energies were obtained with the COSMO-RS model 13 for implicit solvation with CH3CN as solvent at 298.15 K. All quantum chemical calculations were performed with the TURBOMOLE 7.5 program. 14 Gas phase basicities (GB) were calculated from the values in Table S5 (with G298(H + )=-6.22 kcal/mol): Proton affinities (PA) were calculated from the values in Table S5 (with H298(H + )=1.48 kcal/mol):  Table S6.