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. 2024 Aug 7;146(31):21196-21202.
doi: 10.1021/jacs.4c04846. Epub 2024 Jul 25.

Photoswitchable Catalysis by a Self-Assembled Molecular Cage

Ray G DiNardi  1 , Samina Rasheed  1 , Simona S Capomolla  1 , Man Him Chak  1 , Isis A Middleton  1 , Lauren K Macreadie  1 , Jake P Violi  1 , William A Donald  1 , Paul J Lusby  2 , Jonathon E Beves  1
Affiliations

Photoswitchable Catalysis by a Self-Assembled Molecular Cage

Ray G DiNardi et al. J Am Chem Soc. .

Abstract

A heteroleptic [Pd2L2L'2]4+ coordination cage containing a photoswitchable azobenzene-derived ligand catalyzes the Michael addition reaction between methyl vinyl ketone and benzoyl nitromethane within its cavity. The corresponding homoleptic cages are catalytically inactive. The heteroleptic cage can be reversibly disassembled and reassembled using 530 and 405 nm light, respectively, allowing catalysis within the cage to be switched OFF and ON at will.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Self-assembly of homoleptic cages [Pd2(E-1)4]4+, [Pd4(2)8]8+, [Pd3(2)6]6+ and photoswitchable heteroleptic cage catalyst [Pd2(E-1)2(2)2]4+.
Figure 2
Figure 2
Characterization of homoleptic and heteroleptic cages. a) 1H NMR spectra (600 MHz, DMSO-d6) of (i) photoswitchable ligand E-1; (ii) homoleptic [Pd2(E-1)4](BF4)4; (iii) heteroleptic [Pd2(E-1)2(2)2](BF4)4; (iv) a mixture of homoleptic [Pd4(2)8](BF4)8 and [Pd3(2)6](BF4)6, as reported; (v) ligand 2. b) Molecular mechanics model of homoleptic [Pd2(E-1)4]4+. c) Single crystal X-ray structure of heteroleptic [Pd2(E-1)2(2)2](BF4)4, CCDC: 2343886. Color codes: gray: carbon; white: hydrogen; blue: nitrogen; green: fluorine; teal: palladium. Anions and solvent molecules are omitted for the sake of clarity.
Figure 3
Figure 3
Partial 1H (600 MHz) and 19F (565 MHz) NMR spectra in DMSO-d6 showing photoswitching of heteroleptic cage [Pd2(E-1)2(2)2](BF4)4 (i) before irradiation; (ii) after 530 nm 10 min; (iii) 405 nm 5 min; (iv) 530 nm 10 min irradiation. The PSS composition (PSS530 = 88% Z-1, the same as that for free ligand 1) was confirmed by adding N,N-dimethyl-4-aminopyridine (DMAP) to displace the ligands. See Supporting Information S11.3 for details.
Figure 4
Figure 4
Catalysis of a Michael addition reaction by self-assembled cages. a) Substrate binding inside the heteroleptic cage via CH···O hydrogen bonds. b) Michael addition reaction between benzoyl nitromethane and methyl vinyl ketone. c) Michael addition reaction with different additives. Reaction conditions: CD2Cl2/CD3CN (11:1), benzoyl nitromethane (14 mM), methyl vinyl ketone (27 mM), and 18-crown-6 (11 mM). All cases with palladium [Pd] = 1.6 mM. Product formation was measured by 1H NMR spectroscopy. See Supporting Information S18.4.
Figure 5
Figure 5
Photoswitchable catalysis by heteroleptic cage [Pd2(E-1)2(2)2](BArF)4 in CD2Cl2:CD3CN 11:1 monitored by 1H NMR spectroscopy. a) ON/OFF/ON cycle and b) OFF/ON/OFF cycle. Reaction conditions: benzoyl nitromethane (17 mM), methyl vinyl ketone (33 mM), and 18-crown-6 (12 mM), [Pd] = 2.0 mM. Irradiation by 405 nm (5 min) and 530 nm (10 min) outside of the NMR instrument, with colored bars representing the time between NMR measurements; see Supporting Information S19 for details. The system can also be kept dormant by first irradiating [Pd2(E-1)2(2)2](BArF)4 with a 530 nm LED before the substrates are added (Figure 5b, Supporting Information S19.3). The reaction can then be activated at will by irradiating the sample with a 405 nm LED. The long thermal half-life of the photoswitch ensures that the cage remains in the state it is programmed after the irradiation is stopped. The responsiveness of the system to visible light demonstrates that using a molecular photoswitch to control self-assembly can lead to excellent control of the chemical reactivity. We also show that the system can be subjected to at least five cycles of photoswitching without any effect on catalytic performance (Supporting Information S19.4).
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