Research ArticleChemistry

Ultrafast photoactivation of C─H bonds inside water-soluble nanocages

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Science Advances  22 Feb 2019:
Vol. 5, no. 2, eaav4806
DOI: 10.1126/sciadv.aav4806
  • Fig. 1 Strategy for ultrafast C─H bond activation followed by photooxidation in water.

    Our bioinspired host-guest CT paradigm for C─H bond oxidation inside a cationic Pd6L412+ nanocage using visible light (Vis), molecular O2, and water. The Pd6L412+ nanocage encapsulates organic substrates in water and provides a C─H photoactivation route through the ultrafast PCET reaction to oxidize the substrate C─H bond under ambient conditions.

  • Fig. 2 Spectroscopic detection of charge transfer intermediates and ultrafast generation of neutral benzylic radicals.

    (A) Absorption spectra of the host-guest complexes with 9-MA (1), 1-MeNap (2), and Tol (3) inside the Pd6L412+ nanocage. The inset shows the visible color of the aqueous complexes. (B) Transient absorption spectra of guest cation radicals inside the cavity at ~1 ps after photoexcitation to host-guest CT. (C) Comparative decay kinetics of the radical cation state of 2 ⊂ Cage in D2O versus H2O, tracked by plotting the excited-state population decay with time, derived from pump-probe transient absorption data on 2 ⊂ Cage, excited by a 400-nm pump. (D) Comparative lifetimes of all the three alkyl aromatic hydrocarbons in their radical cation states and corresponding KIEs for the benzylic PT reactions from radical cations to the solvent to form neutral radicals. (E) Scheme for PCET reaction for 2 ⊂ Cage and 3 ⊂ Cage, leading to the generation of the long-lived (>2.5 ns) neutral benzylic radicals. PT time scales for 2 ⊂ Cage and 3 ⊂ Cage are 354 ± 25 ps and 46 ± 4 ps respectively. OD, optical density.

  • Fig. 3 Visible light–triggered photoreactions inside the Pd6L412+ cage and the corresponding photocatalytic cycle.

    (A) Photooxidation of 9-MA (1) inside the cage with aerial O2 for 1.5 hours generates four photoproducts dominated by the 9,10-anthraquinone. The nanocage-mediated photooxidation of substrates mimics the action of natural oxidase enzymes, such as cytochrome P450. (B) Cage-incarcerated 1-MeNap (2) leads to two oxidized products under aerobic photooxidation for 7.5 hours. (C) Tol (3) gives rise to a single photooxidized product, benzaldehyde, from the photoreaction under high O2 pressure (~1.5 atm) for 8.5 hours. (D) Gas chromatography–mass spectrometry (GC-MS) chromatograms showing photocatalytic conversion of Tol to benzaldehyde in a time-lapsed manner in the presence of a catalytic (5 mol %) amount of nanocage. (E) Higher alkyl analog of Tol, cumene, 4 (3° benzylic site), forms three photooxidized products upon 400-nm light-emitting diode (LED) illumination for 33 hours in the presence of 5 mol % cage catalyst. Isolation of cumene hydroperoxide proves the generation of benzylic hydroperoxides from the initial carbon-centered radicals at the benzylic site after reaction with molecular O2. (F) Catalytic cycle for Tol photooxidation starts with the activation half where, after uptake of the substrate, PCET-induced selective sp3 C─H photoactivation takes place in water. In the oxidation part of the catalytic cycle, benzaldehyde formation from neutral benzyl radical goes via benzyl hydroperoxide (this 1° hydroperoxide could not be isolated because of its unstable nature) generation and subsequent heterolytic cleavage of the O─O bond. Then, excess Tol molecules, present outside the cage, displace the benzaldehyde product from the cavity and replenish the Tol ⊂ Cage complex for the next catalytic cycle.

  • Fig. 4 Tuning the substrate and cavity electronics for photocatalytic oxidation of Tol.

    (A) Product distribution for Tol and each of the Tol derivatives at 5 mol % catalyst loading of the Pd6L412+ nanocage and under high O2 pressure (~1.5 atm) with 400-nm LED illumination. All the reactants showed benzaldehyde as the major product with >94% yields. Minor products of alcohol and acid are also observed for certain derivatives. (B) Comparison of photocatalytic reaction time (gray bars) and percentage conversion (orange bars) for Tol, para-methoxy Tol, para-Fluoro Tol, para-xylene, meta-xylene, and ortho-xylene inside the Encage. (C) Reaction rates for conversion of Tol to benzaldehyde using two different Pd6L412+ nanocages, Encage and Bipycage, are plotted.

  • Fig. 5 Molecular packing of Tol inside the nanocage and correlation with photocatalytic rates.

    (A) 2D ROESY (rotating frame nuclear Overhauser effect spectroscopy) spectrum of Tol ⊂ Cage complex in D2O in 500 MHz. Cross-correlation peaks are marked by i (Hβ, Tol-CH3), ii (Hα, Tol-CH3), iii (Hβ, Tol-aromatic), and iv (Hα, Tol-aromatic). Distances between protons are calculated by considering the cross peak (Hα, en-CH2) as a reference peak and distance between them (r = 5.35 ± 0.36 Ǻ) as a reference distance. (B) Optimized geometry of single Tol incarcerated inside Pd6L412+ cage. (C) Correlation of methyl-to-triazine distances (1H-1H ROESY NMR distances) with the photocatalytic oxidation rates of the para-substituted Tol derivatives.

  • Fig. 6 Ground-state polarization of the preorganized sp3 C─H bond.

    (A) Steady-state Raman traces for free Tol (top, blue) and caged Tol (bottom, black) along with the fit (red line) after an excitation at 532 nm. Phenylic and methyl C─H stretches are both redshifted because of cage confinement. (B) Ground-state Raman frequency shifts (zoomed in) of methyl C─H stretches (three modes are color-coded) in the presence of E↑↑ (top, E-field parallel to the C3 axis of the Tol molecule) and E↑↓ (bottom, E-field opposite to the C3 axis of the Tol molecule) full spectra are presented in figs. S58 and S59. (C) Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of Tol are plotted in the absence (first column) and presence of E↑↑ (at 10 and 25 V/Å) (middle pair of columns) and E↑↓ (at 5 and 25 V/Å) (last pair of columns) to show the polarization effect on the frontier orbitals due to an external E-field. a.u., arbitrary units.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaav4806/DC1

    Section S1. Materials and methods

    Fig. S1. Calibration distance for 2D ROESY NMR.

    Fig. S2. Octahedral nanocage.

    Fig. S3. 1H NMR spectrum of a synthesized Pd6L412+ nanocage in D2O.

    Fig. S4. 13C NMR spectrum of a synthesized Pd6L412+ nanocage.

    Fig. S5. Visible charge transfer complex of 9-MA.

    Fig. S6. Host-guest complexation and NMR upfield shifts.

    Fig. S7. Temperature dependence of population heterogeneity of caged 9-MA.

    Fig. S8. Charge transfer complex with 1-MeNap.

    Fig. S9. Host-guest complexation of 1-MeNap.

    Fig. S10. Temperature-dependent 1H NMR of methyl protons.

    Fig. S11. Complexation of Tol and nanocage.

    Fig. S12. 13C NMR spectrum of Tol ⊂ Cage.

    Fig. S13. Optimized structure of Tol inside a nanocage.

    Fig. S14. Substituent-dependent steady-state absorption of host-guest CT complexes of Tol derivatives.

    Fig. S15. Formation of ortho-xylene ⊂ Cage complex.

    Fig. S16. 2D ROESY spectrum of ortho-xylene ⊂ Cage complex in D2O in 800 MHz.

    Fig. S17. Formation of para-xylene ⊂ Cage complex.

    Fig. S18. ROESY spectrum of para-xylene ⊂ Cage complex in D2O (800 MHz).

    Fig. S19. Formation of meta-xylene ⊂ Cage complex.

    Fig. S20. 1H-1H 2D ROESY NMR spectrum of meta-xylene ⊂ Cage complex in D2O in 800 MHz.

    Fig. S21. Complexation of para-OMeTol with nanocage.

    Fig. S22. 2D 1H-1H ROESY NMR spectrum of para-OMeTol ⊂ Cage complex in D2O.

    Fig. S23. Complexation of para-FluoroTol with nanocage.

    Fig. S24. 2D 1H-1H ROESY NMR spectrum of para-FluoroTol ⊂ Cage complex in D2O (800 MHz).

    Fig. S25. Electronics- and regioisomerism-dependent molecular packing of Tol derivatives inside the nanocage.

    Fig. S26. 1H NMR spectra of Tol ⊂ Bipycage (top) in D2O and its comparison with Tol ⊂ Encage in D2O.

    Fig. S27. 2D 1H-1H ROESY NMR spectrum of Tol ⊂ Bipycage complex in D2O (800 MHz).

    Fig. S28. Transient absorption of radical cation of 9-MA ⊂ Cage and corresponding decay.

    Fig. S29. PCET reaction for 9-MA ⊂ Cage complex.

    Fig. S30. Substrate isotope dependence on C─H vs. C─D bond cleavage and primary KIE.

    Fig. S31. C─H versus C─D bond cleavage and primary KIE.

    Fig. S32. Tol radical cation decay inside the nanocage.

    Fig. S33. Excited-state absorption of coupled host-guest CT state.

    Fig. S34. Solvation relaxation of the excited host-guest CT state.

    Fig. S35. Photoreaction on encapsulated 9-MA, illuminated by cheap green LED (530 nm).

    Fig. S36. Rate of photoreaction is dependent on O2 concentration.

    Fig. S37. Photooxidized product identification.

    Fig. S38. Both benzylic and ring oxidation take place for 9-MA.

    Fig. S39. Photoreaction under high O2 pressure for 30 min.

    Fig. S40. UV light–mediated photoreaction under ambient O2.

    Fig. S41. Photocatalysis on caged 9-MA.

    Fig. S42. Control photoreactions for mechanism elucidation.

    Fig. S43. Selective photooxidation products of 1-MeNap.

    Fig. S44. Spectral characteristics of the photoproducts.

    Fig. S45. Mass spectrum of photooxidation product, benzaldehyde.

    Fig. S46. Concomitant decay of the substrate and rise of the oxidized product with a lag phase.

    Fig. S47. Benzyl alcohol to benzaldehyde formation.

    Fig. S48. Photooxidation of secondary benzylic radicals.

    Fig. S49. Photoreactivity of cage incarcerated tertiary benzylic radicals under O2.

    Fig. S50. Photooxidation of para-xylene inside the nanocage.

    Fig. S51. Ortho-xylene oxidation by light.

    Fig. S52. Photocatalytic oxidation of meta-xylene.

    Fig. S53. Oxidation of para-OMeTol with blue light.

    Fig. S54. Electron-withdrawing substituent effect on photocatalytic oxidation of para-FluoroTol.

    Fig. S55. Photocatalytic oxidation rates are strongly dependent on molecular packing of the substrates inside the cage.

    Fig. S56. Cage electronics perturb the photocatalytic reaction rates.

    Fig. S57. Cage behavior like a prototypical photoenzyme: Tuning photoproduct selectivity by changing the cage electronics.

    Fig. S58. Parallel E-field and C─H Raman stretches.

    Fig. S59. Antiparallel E-field effects on C─H Raman stretches.

    Fig. S60. E-field effects on C─H bond lengths.

    Fig. S61. Effects of aromaticity on E-field induced C─H bond polarization.

    Scheme S1. Making of the nanocage.

    Scheme S2. Plausible mechanism for photooxidation of 9-MA inside cage.

    Scheme S3. Oxidation of 1-MeNap by light.

    Table S1. Comparison between 2D NMR and molecular modeling–obtained relative distances.

    Table S2. Geometric separation of Tol methyl protons from the triazine walls for different derivatives.

    References (6972)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Materials and methods
    • Fig. S1. Calibration distance for 2D ROESY NMR.
    • Fig. S2. Octahedral nanocage.
    • Fig. S3. 1H NMR spectrum of a synthesized Pd6L412+ nanocage in D2O.
    • Fig. S4. 13C NMR spectrum of a synthesized Pd6L412+ nanocage.
    • Fig. S5. Visible charge transfer complex of 9-MA.
    • Fig. S6. Host-guest complexation and NMR upfield shifts.
    • Fig. S7. Temperature dependence of population heterogeneity of caged 9-MA.
    • Fig. S8. Charge transfer complex with 1-MeNap.
    • Fig. S9. Host-guest complexation of 1-MeNap.
    • Fig. S10. Temperature-dependent 1H NMR of methyl protons.
    • Fig. S11. Complexation of Tol and nanocage.
    • Fig. S12. 13C NMR spectrum of Tol ⊂ Cage.
    • Fig. S13. Optimized structure of Tol inside a nanocage.
    • Fig. S14. Substituent-dependent steady-state absorption of host-guest CT complexes of Tol derivatives.
    • Fig. S15. Formation of ortho-xylene ⊂ Cage complex.
    • Fig. S16. 2D ROESY spectrum of ortho-xylene ⊂ Cage complex in D2O in 800 MHz.
    • Fig. S17. Formation of para-xylene ⊂ Cage complex.
    • Fig. S18. ROESY spectrum of para-xylene ⊂ Cage complex in D2O (800 MHz).
    • Fig. S19. Formation of meta-xylene ⊂ Cage complex.
    • Fig. S20. 1H-1H 2D ROESY NMR spectrum of meta-xylene ⊂ Cage complex in D2O in 800 MHz.
    • Fig. S21. Complexation of para-OMeTol with nanocage.
    • Fig. S22. 2D 1H-1H ROESY NMR spectrum of para-OMeTol ⊂ Cage complex in D2O.
    • Fig. S23. Complexation of para-FluoroTol with nanocage.
    • Fig. S24. 2D 1H-1H ROESY NMR spectrum of para-FluoroTol ⊂ Cage complex in D2O (800 MHz).
    • Fig. S25. Electronics- and regioisomerism-dependent molecular packing of Tol derivatives inside the nanocage.
    • Fig. S26. 1H NMR spectra of Tol ⊂ Bipycage (top) in D2O and its comparison with Tol ⊂ Encage in D2O.
    • Fig. S27. 2D 1H-1H ROESY NMR spectrum of Tol ⊂ Bipycage complex in D2O (800 MHz).
    • Fig. S28. Transient absorption of radical cation of 9-MA ⊂ Cage and corresponding decay.
    • Fig. S29. PCET reaction for 9-MA ⊂ Cage complex.
    • Fig. S30. Substrate isotope dependence on C─H vs. C─D bond cleavage and primary KIE.
    • Fig. S31. C─H versus C─D bond cleavage and primary KIE.
    • Fig. S32. Tol radical cation decay inside the nanocage.
    • Fig. S33. Excited-state absorption of coupled host-guest CT state.
    • Fig. S34. Solvation relaxation of the excited host-guest CT state.
    • Fig. S35. Photoreaction on encapsulated 9-MA, illuminated by cheap green LED (530 nm).
    • Fig. S36. Rate of photoreaction is dependent on O2 concentration.
    • Fig. S37. Photooxidized product identification.
    • Fig. S38. Both benzylic and ring oxidation take place for 9-MA.
    • Fig. S39. Photoreaction under high O2 pressure for 30 min.
    • Fig. S40. UV light–mediated photoreaction under ambient O2.
    • Fig. S41. Photocatalysis on caged 9-MA.
    • Fig. S42. Control photoreactions for mechanism elucidation.
    • Fig. S43. Selective photooxidation products of 1-MeNap.
    • Fig. S44. Spectral characteristics of the photoproducts.
    • Fig. S45. Mass spectrum of photooxidation product, benzaldehyde.
    • Fig. S46. Concomitant decay of the substrate and rise of the oxidized product with a lag phase.
    • Fig. S47. Benzyl alcohol to benzaldehyde formation.
    • Fig. S48. Photooxidation of secondary benzylic radicals.
    • Fig. S49. Photoreactivity of cage incarcerated tertiary benzylic radicals under O2.
    • Fig. S50. Photooxidation of para-xylene inside the nanocage.
    • Fig. S51. Ortho-xylene oxidation by light.
    • Fig. S52. Photocatalytic oxidation of meta-xylene.
    • Fig. S53. Oxidation of para-OMeTol with blue light.
    • Fig. S54. Electron-withdrawing substituent effect on photocatalytic oxidation of para-FluoroTol.
    • Fig. S55. Photocatalytic oxidation rates are strongly dependent on molecular packing of the substrates inside the cage.
    • Fig. S56. Cage electronics perturb the photocatalytic reaction rates.
    • Fig. S57. Cage behavior like a prototypical photoenzyme: Tuning photoproduct selectivity by changing the cage electronics.
    • Fig. S58. Parallel E-field and C─H Raman stretches.
    • Fig. S59. Antiparallel E-field effects on C─H Raman stretches.
    • Fig. S60. E-field effects on C─H bond lengths.
    • Fig. S61. Effects of aromaticity on E-field induced C─H bond polarization.
    • Scheme S1. Making of the nanocage.
    • Scheme S2. Plausible mechanism for photooxidation of 9-MA inside cage.
    • Scheme S3. Oxidation of 1-MeNap by light.
    • Table S1. Comparison between 2D NMR and molecular modeling–obtained relative distances.
    • Table S2. Geometric separation of Tol methyl protons from the triazine walls for different derivatives.
    • References (6972)

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