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N-Heterocyclic carbene-stabilized nitric oxide radicals were prepared by direct addition of nitric oxide to two N-heterocyclic carbenes in solution phase. The compounds were fully characterized by X-ray crystallography and EPR. The nitric oxide moiety in the solid compounds obtained can be thermally transferred to another N-heterocyclic carbene, suggesting potential applications to NO delivery.

Herein, a coumaraz-2-on-4-ylidene (1) as a new example of an ambiphilic N-heterocyclic carbene, having electronic properties that can be fine-tuned, is reported. The N-carbamic and aryl groups on the carbene carbon center provide exceptionally high electrophilicity and nucleophilicity simultaneously to the carbene center, as evidenced by the Se NMR chemical shifts of their selenoketone derivatives and the CO stretching strengths of their rhodium carbonyl complexes. Since the precursors of 1 could be synthesized from various functionalized Schiff bases in a practical and scalable manner, the electronic properties of 1 can be fine-tuned in a quantitative and predictable way by using the Hammett σ constant of the functional groups on aryl ring. The facile electronic tuning capability of 1 may be applicable to eliciting novel properties in main-group and transition-metal chemistry.

Here, we report the imidazolium-/pyrrolinium-substituted persistent zwitterionic ferrocenate derivatives, which were characterized by electron paramagnetic resonance (EPR) and 57 Fe Mössbauer spectroscopy. Additional theoretical studies on these zwitterionic ferrocenate derivatives clearly explain the origin of their thermal stability and the orbital interactions between iron and imidazolium-/pyrrolinium-substituted zwitterionic cyclopentadienyl ligand. Exploiting the facile Fe(II/I) redox chemistry, we successfully demonstrated that the ferrocene derivative can be applied as an example of derivatized ferrocene anolyte for redox-flow batteries. These zwitterionic ferrocenate derivatives will not only deepen our understanding of the intrinsic chemistry of ferrocenate but have the potential to open the way for the rational design of metallocenate derivatives for various applications. electrochemistry based on Fe(I)/Fe(II) ferrocene redox couples is feasible in practical battery operation and is enabled by the stereoelectronic stabilization of pyrrolinium-substituted zwitterionic ferrocenate derivative. ConclusionsFerrocene, one of the most representative compounds in the discipline of organometallic chemistry, attracted much attention because of its extremely high thermodynamic, chemical, and Fe(II/III) redox stability. However, the applications using the extremely negative voltage of ferrocene have not been reported to date due to the fast dissociation of the cyclopentadienyl anion ligands from Fe(I). Thus, an Fe(I) biscyclopentadiene complex remained elusive for a long time. Here, we have shown that zwitterionic ferrocenate derivatives 2a and 2b can be prepared by the one-electron reduction of 1a and 1b at ambient conditions, as confirmed by several complementary spectroscopic and theoretical methods. Additionally, a potential application in a redox-flow battery was successfully demonstrated using 1b as an example of ferrocene anolyte, in part due to the high degree of stabilization of low-valent state by the cAAC moiety. These imidazolium-/pyrrolinium-substituted ferrocene derivatives may deepen our understanding of the electrochemical behavior of ferrocene compounds and open the way to the rational design of ferrocenate derivatives. ASSOCIATED CONTENTSupporting Information. Experimental details, X-ray crystallographic analysis, Computational details, and other characterization data. The crystallographic data for 1a and 1b are deposited in the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers CCDC 1940623 and 1940622. This material is available free of charge via the Internet at http://pubs.acs.org.

Stable organic radicals have been of great academic interest not only in the context of fundamental understanding of reactive intermediates but also because of their numerous applications as functional materials. Apart from the early examples of triphenylmethyl and TEMPO derivatives, reports on air- and water-stable organic radicals are scarce, and their development remains a challenge. Herein, we present the design and synthesis of a novel organic radical based on a 1,2-dicarbonyl scaffold supported by N-heterocyclic carbenes (NHCs). The presented radical cations exhibit remarkable stability toward various harsh conditions, such as the presence of reactive chemicals (reductants, oxidants, strong acids, and bases) or high temperatures, by far exceeding the stability of triphenylmethyl and TEMPO radicals. In addition, physiological conditions including aqueous buffer and blood serum are tolerated. The steric and electronic stabilization provided by the two NHC moieties enabled the successful design of the highly stable radical.