An Isolable, Photoswitchable N‐Heterocyclic Carbene: On‐Demand Reversible Ammonia Activation

Abstract

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.

Photoswitching of Copper(I) Chromophores with Dithienylethene‐Based Ligands

“…[10][11][12][13][14][15][16][17] Yam and co-workers in 2004 first reported 1,10-phenanthroline functionalizedd ithienylethene as al igand for metalc oordination. In these compounds, the thienyl groups are attached to five-or six-memberedr ings such as the cyclopentene moiety to prevent unwanted cis-trans isomerization.M ost modificationso f this basic motif have involvedc hanges to the thienylu nits, but now significant efforts have been extended to replacet he fivemembered ring systems with various heterocycles.…”

“…The optimized geometry of complex 1 in DCM shows as ignificant change in the bond length betweenC (13) and C(14) from 1.36 in the open form to 1.49 in the closed form, consistent with the conclusion that photoirradiation can lead to ring expansion in these metallacycles. [7][8][9][10][11] We wanted to explore applications of these complexes as photoswitchable catalysts for hydroboration since it has been shown that copper(I) complexes can catalyze this reaction with high regioselectivity and yield under mild conditions. In the closed form (1c), the HOMO corresponds to a p orbital of the dithienylethene ring system,w hereas the LUMO corresponds mainly to a p*o rbital Interest in the developmento fa rtificial switchable catalysts is burgeoning because they provide an oninvasive,r eversible methodt oc ontrol reactionr ates.…”

“…[1][2][3][4] Many potentiala pplications in optoelectronics such as optical recording and data storage, switchable catalysis, as well as chemical and biological sensing and imaging are envisioned. [4][5][6][7][8][9][10][11] In severala pplication areas, the molecular design of photoswitchable diarylethenes has to meet divergent criteria for the quantum yields (QYs) of the photocyclization and photocycloreversion reactions. For example,f or super-resolution fluorescencem icroscopy,t he QY of the photocycloreversion reaction mustb em uch lower than that of the photocyclizationr eaction to ensure sufficientp hoton collection is possible to construct an image.…”

“…In these compounds, the thienyl groups are attached to five-or six-memberedr ings such as the cyclopentene moiety to prevent unwanted cis-trans isomerization.M ost modificationso f this basic motif have involvedc hanges to the thienylu nits, but now significant efforts have been extended to replacet he fivemembered ring systems with various heterocycles. [10][11][12][13][14][15][16][17] Yam and co-workers in 2004 first reported 1,10-phenanthroline functionalizedd ithienylethene as al igand for metalc oordination. [16] Since then, there have been extensivee fforts to coordinate metals including Ru, Re, Pt, Zn, Au, Pd, Fe, and Eu [9,[18][19][20] to dithienylethene-based ligands.…”

“…In the open form 1o,t he HOMO is delocalized over the copper do rbitals and a p orbital of the dithienylethene phosphine ligand, whereas the metal center does not contribute significantly to the LUMO. In the closed form (1c), the HOMO corresponds to a p orbital of the dithienylethene ring system,w hereas the LUMO corresponds mainly to a p*o rbital [7][8][9][10][11] We wanted to explore applications of these complexes as photoswitchable catalysts for hydroboration since it has been shown that copper(I) complexes can catalyze this reaction with high regioselectivity and yield under mild conditions. [27] We used 6 as ap recatalyst and generated ar eactive copper hydride species in situ by treating it with potassium tert-butoxide and pinacolborane in toluene.…”