Azobenzene Isomerization‐Induced Photomodulation of Electronic Properties of N‐Heterocyclic Carbenes.

Azobenzene‐based protonated N‐heterocyclic carbenes (NHCs), N,N'‐bis(azobenzene)imidazolium chlorides (IAz‐X⋅HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached azobenzene units along with the concomitant photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV light, a p‐methoxy‐azobenzene derivative get transformed from all‐trans isomer to nearly all‐cis isomer at the photostationary state. The extent of photomodulation of electronic properties in the NHC ring of the p‐methoxy‐azobenzene derivative is determined through the Tolman Electronic Parameter (TEP). Iridium complex, [(IAz‐OMe)IrCl(CO)2] was synthesized and infrared spectra were recorded in dichloromethane solution as film in NaCl crystals and by drop‐casting in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all‐trans isomer (ν˜ttav) and all‐cis isomer (ν˜ccav) indicates a significant decrement of Δtt–ccν˜av=2.7 cm−1 (film) and 3.8 cm−1 (ATR). Therefore, moderate to excellent enhancement of electron density (Δtt–cc TEP=2.3 cm−1 [film] and 3.2 cm−1 [ATR]) at the C‐2 position of the NHC is achieved through trans→cis isomerization of the remotely placed azobenzene units. This fine phototuning of electron‐donating ability at the catalytic center would pave the way to control NHC/NHC‐metal catalyzed organic transformations through external stimuli. [ABSTRACT FROM AUTHOR]