The desire to access increasingly complex molecular architectures demands the realization of enhanced control over chemical reaction pathways. Typically, a catalyst chosen for a particular transformation performs with a specific unalterable activity, selectivity, and substrate scope. Although many reports describe reactions that are catalyzed in a manner that provides excellent degrees of regio-, stereo-, and/or chemoselectivity, relatively few examples that feature catalysts which can respond to external stimuli have been disclosed to date. Indeed, the growing field of ‘switchable catalysis’ offers a means to utilize multiple active states displayed by some catalysts to manipulate activities or selectivities over the course of a single reaction.
N-heterocyclic carbenes (NHCs) are widely utilized as ligands in transition metal-based catalysts as well as organocatalysts and represent attractive targets for the development of switchable catalysts. Recently, the first isolable, photochromic carbene was synthesized through the incorporation of a diarylethene unit into a common N-heterocyclic carbenes scaffold. Exposure of the free carbene to UV radiation resulted in the electrocyclic ring-closure of the diarylethene backbone and provided a significantly more electrophilic carbene center, as determined by a series of spectroscopic techniques and molecular orbital calculations. Subsequent exposure to visible light reversed the ring-closing reaction and returned the N-heterocyclic carbenes to its initial electronic state. The photoswitchable N-heterocyclic carbenes was also successfully toggled between the ring-opened and ring-closed states with minimal observed decomposition over multiple switching cycles.
The modular electronic structure of the aforementioned N-heterocyclic carbenes was subsequently utilized to control a chemical reaction. While the ring-closed form of the N-heterocyclic carbenes was found to readily activate ammonia, an abundant chemical feedstock, the ring-opened isomer was incapable of promoting the same transformation. This orthogonal reactivity in conjunction with selective photoirradiation was used to control the reaction of the NHC with ammonia on-demand. For example, UV irradiation of a solution containing the ring-opened N-heterocyclic carbenes and ammonia initially resulted in the formation of a ring-closed N-heterocyclic carbenes, which in turn effectively captured ammonia. The process was reversed upon subsequent exposure of the reaction mixture to visible light, which released free ammonia and returned the NHC back to its ring-opened form.
The ability to use light as a means to selectively capture ammonia represents the first example of its kind and provides an unprecedented degree of control over a challenging transformation. Given the magnitude of chemical reactions catalyzed by N-heterocyclic carbenes and their corresponding transition metal complexes, the work described is expected to facilitate to the design and development of new classes of switchable catalysts that provide externally-controlled activities and/or selectivities.
About the Authors
Teator AJ1, Tian Y2, Chen M2, Lee JK3, Bielawski CW4,5.Show Affiliations
- Department of Chemistry, The University of Texas at Austin, 1 University Station, A1590, Austin, TX 78712 (USA).
- Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Rd, Piscataway, NJ 08854 (USA).
- Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Rd, Piscataway, NJ 08854 (USA). [email protected]
- Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 689-798 (Republic of Korea). [email protected]
- Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798 (Republic of Korea). [email protected]
The first isolable, photoswitchable N-heterocyclic carbene was synthesized and found to undergo reversible electrocyclic isomerization upon successive exposure to UV and visible radiation. The UV-induced ring closure afforded substantial changes to the electronic structure of the dithienylethene-based NHC, as evidenced by changes in the corresponding UV/Vis absorption and (13)C NMR spectra. Likewise, molecular orbital calculations revealed diminished electron density at the carbene nucleus upon photocyclization, consistent with the enhanced electrophilicity displayed by the ring-closed form. The photoswitchable NHC was successfully switched between its ring-opened and ring-closed states with high fidelity over multiple cycles. Furthermore, the ring-closed isomer was found to undergo facile N-H bond activation, allowing for the controlled capture and release of ammonia upon cycling between its isomeric states.
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