Over the past decade, numerous stable organic and main-group radicals have been synthesized from N-heterocyclic carbenes (NHCs). The structure of NHCs, in particular, offers electronic stabilization that helps to delocalize the unpaired electron over the molecule. In addition, the sterically bulky substituents of NHCs protect the radical center to prevent detrimental reactions such as dimerization. These advantages enable the straightforward synthesis and characterization of various interesting organic radicals starting from NHCs, which are, these days, widely available. NHCs have enabled the structural characterization of various organic radicals over the past decade, including α-carbonyl, propargyl, oxyallyl, and triazenyl radicals as notable examples. This minireview summarizes the advances in this field, mainly focusing on the preparation and stability as well as the properties and applications of NHC-derived organic radicals.

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Binding of Lewis acidic metal ions and Brønsted acid at the metal–oxo group of high-valent metal–oxo complexes enhances their reactivities significantly in oxidation reactions. However, such a binding of Lewis acids and proton at the metal–oxo group has been questioned in several cases and remains to be clarified. Herein, we report the synthesis, characterization, and reactivity studies of a mononuclear manganese­(IV)–oxo complex binding triflic acid, {[(dpaq)­MnIV(O)]–HOTf}+ (1–HOTf). First, 1–HOTf was synthesized and characterized using various spectroscopic techniques, including resonance Raman (rRaman) and X-ray absorption spectroscopy/extended X-ray absorption fine structure. In particular, in rRaman experiments, we observed a linear correlation between the Mn–O stretching frequencies of 1–HOTf (e.g., νMn–O at ∼793 cm–1) and 1–M n+ (M n+ = Ca2+, Zn2+, Lu3+, Al3+, or Sc3+) and the Lewis acidities of H+ and M n+ ions, suggesting that H+ and M n+ bind at the metal–oxo moiety of [(dpaq)­MnIV(O)]+. Interestingly, a single-crystal structure of 1–HOTf was obtained by X-ray diffraction analysis, but the structure was not an expected Mn­(IV)–oxo complex but a Mn­(IV)–hydroxide complex, [(dpaq)­MnIV(OH)]­(OTf)2 (4), with a Mn–O bond distance of 1.8043(19) Å and a Mn–O stretch at 660 cm–1. More interestingly, 4 reverted to 1–HOTf upon dissolution, demonstrating that 1–HOTf and 4 are interconvertible depending on the physical states, such as 1–HOTf in solution and 4 in isolated solid. The reactivity of 1–HOTf was investigated in hydrogen atom transfer (HAT) and oxygen atom transfer (OAT) reactions and then compared with those of 1–M n+ complexes; an interesting correlation between the Mn–O stretching frequencies of 1–HOTf and 1–M n+ and their reactivities in the OAT and HAT reactions is reported for the first time in this study.