First triazenyl radicals stabilized and characterized

Over the past decade, researchers have succeeded in making radical compounds of main group elements by stabilizing them with N-heterocyclic carbenes. Nitrogen-containing triazenyl radicals are attractive for their potential reactivity and physical properties. Free triazenyl radicals have previously been detected by electron paramagnetic resonance (EPR) spectroscopy and as ligands in transition-metal complexes, but they’ve been challenging to stabilize and isolate. Researchers led by Eunsung Lee of the Institute for Basic Science and Pohang University of Science & Technology have now synthesized and characterized a pair of triazenyl radicals stabilized by N-heterocyclic carbenes (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b08753). The researchers reacted chloroimidazolium chlorides with azides to form triazenyl cations, which they then reduced to the corresponding triazenyl radicals via one-electron reduction with potassium metal in toluene. The researchers characterized the radicals by single-crystal X-ray diffraction, EPR spectroscopy, and UV-vis absorption spectroscopy. Density functional theory calculations suggest that the unpaired electron in both radicals is strongly delocalized over the entire conjugated π system, which may be how the N-heterocyclic carbenes stabilize the triazenyl radical. The researchers demonstrated a potential application by using one of the triazenyl radicals as a cathode material in a lithium-ion battery.