Two air-stable organic radicals derived from oxalyl chloride and cAAC were synthesized, resulting in the unexpected formation of a known (amino)(carboxy) radical cation ([2]BF4) and the oxidative formation of a 1,2-dicarbonyl radical cation ([3]BF4) from a neutral 3-oxetanone compound (4). The highly strained and newly discovered 4 was obtained by a single-electron reduction of [3]BF4 with a mild reducing agent. This result differs from the generation of NHC-based 1,2-dicarbonyl radicals, indicating the uniqueness of cAAC.

The synthesis of air-persistent organic radicals (APORs) is challenging because of the presence of highly reactive species in air. It was found that N-heterocyclic carbenes (NHCs) are effective in stabilizing radical centers because of their bulky substituents and low-lying empty orbitals, thereby leading to the development of several APORs. Herein, we report the synthesis of two air-stable organic radicals derived from oxalyl chloride and cyclic (alkyl)(amino)carbene (cAAC), which included the unexpected formation of a known (amino)(carboxy) radical cation ([2]BF4) and a 1,2-dicarbonyl radical cation ([3]BF4). Most importantly, the reduced 3-oxetanone compound (4) was discovered as a new product originating from [3]BF4, which differs from the generation of NHC-based 1,2-dicarbonyl radicals. The highly strained 3-oxetanone 4 was obtained by a single-electron reduction of [3]BF4 with a mild reducing agent, chloride salt, and vice versa with a AgBF4 oxidant. Both [2]BF4 and [3]BF4 retained their intrinsic blue color and exhibited high air stability. Furthermore, [3]BF4 produced a temperature-dependent electron paramagnetic resonance spectrum.

Numerous developments in the electronic and steric modification of cyclopentadienyl (Cp) ligands have led to a corresponding growth in their applications. Herein, we present two new redox non-innocent Cp compounds, imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp 1a and PZCp 1b). The Cps were obtained in high yields and could be handled easily owing to their high stability in air and moisture, unlike other known Cps. The redox non-innocent behavior of the Cp ligands was demonstrated via experimental and computational analyses of their half-sandwich and sandwich complexes. During cyclic voltammetry, the molybdenum half-sandwich (3a, 3b) and ruthenium sandwich ([4a]PF6, [4b]PF6) complexes of IZCp and PZCp showed reversible one-electron reduction at a low potential (E1/2 ranging from −1.7 to −2.7 V vs. Fc/Fc+). This phenomenon has not been studied previously because of the high instability of the reduced complexes. DFT calculations for the reduced complexes ([3a]‾, [3b]‾, 4a, and 4b) confirmed that their spin density was mainly delocalized over the ligand center (70%–90%). EPR analysis of the isolated K[3b] and 4b indicated their ligand-centered radical structures. Furthermore, the CO stretching frequencies of K[3b] (νCO 1871.35, 1748.10, and 1698.80 cm−1) revealed that the redox non-innocent [PZCp]‾ ligand is the strongest electron-donating Cp ligand among previously reported CpMo(CO)3 derivatives (νCO > 1746 cm−1).

The synthesis of air-persistent organic radicals (APORs) is challenging because of the presence of highly reactive species in air. It was found that N-heterocyclic carbenes (NHCs) are effective in stabilizing radical centers because of their bulky substituents and low-lying empty orbitals, thereby leading to the development of several APORs. Herein, we report the synthesis of two air-stable organic radicals derived from oxalyl chloride and cyclic (alkyl)(amino)carbene (cAAC), which included the unexpected formation of a known (amino)(carboxy) radical cation ([2]BF4) and a 1,2-dicarbonyl radical cation ([3]BF4). Most importantly, the reduced 3-oxetanone compound (4) was discovered as a new product originating from [3]BF4, which differs from the generation of NHC-based 1,2-dicarbonyl radicals. The highly strained 3-oxetanone 4 was obtained by a single-electron reduction of [3]BF4 with a mild reducing agent, chloride salt, and vice versa with a AgBF4 oxidant. Both [2]BF4 and [3]BF4 retained their intrinsic blue color and exhibited high air stability. Furthermore, [3]BF4 produced a temperature-dependent electron paramagnetic resonance spectrum.