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 ˜ t t av ) and all-cis isomer ( ν ˜ c c a v ) indicates a significant decrement of Δtt-cc ν ˜ 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.