Disclosing Cyclic(Alkyl)(Amino)Carbenes as One‐Electron Reductants: Synthesis of Acyclic(Amino)(Aryl)Carbene‐Based Kekulé Diradicaloids

1H-1,2,3-Triazol-5-ylidenes as Catalytic Organic Single-Electron Reductants

Most of the known single-electron reductants are either metal based reagents, used in a stoichiometric amount, or a combination of an organic species and a photocatalyst. Here we report that 1H-1,2,3-triazol-5-ylidenes act not only as stoichiometric one-electron donors but also as catalytic organic reducing agents, without the need of a photocatalyst. As a proof of concept, we studied the reduction of quinones, which are well-known electron conveyors that are involved in various biological and industrial processes. This work also provides experimental evidence for the formation of a bis(triazolium)carbonate adduct, which acts as the resting state of the catalytic cycle and as the carbene reservoir.

Comment on "Disclosing Cyclic(Alkyl)(Amino)Carbenes as a One-Electron Reductant: Synthesis of Acyclic(Amino)(Aryl)Carbene-Based Kekulé Diradicaloids"

This correspondence addresses a misassignment of an EPR spectrum of 2 in a recent publication (Chem. Eur. J. 2022, 28, e202104567) by Dr. Jana and co-workers. The original authors have prepared this correspondence together with Dr. Korth.

A crystalline doubly oxidized carbene

The chemistry of carbon is governed by the octet rule, which refers to its tendency to have eight electrons in its valence shell. However, a few exceptions do exist, for example, the trityl radical (Ph3C∙) (ref. 1) and carbocation (Ph3C+) (ref. 2) with seven and six valence electrons, respectively, and carbenes (R2C:)-two-coordinate octet-defying species with formally six valence electrons3. Carbenes are now powerful tools in chemistry, and have even found applications in material and medicinal sciences4. Can we undress the carbene further by removing its non-bonding electrons? Here we describe the synthesis of a crystalline doubly oxidized carbene (R2C2+), through a two-electron oxidation/oxide-ion abstraction sequence from an electron-rich carbene5. Despite a cumulenic structure and strong delocalization of the positive charges, the dicoordinate carbon centre maintains significant electrophilicity, and possesses two accessible vacant orbitals. A two-electron reduction/deprotonation sequence regenerates the parent carbene, fully consistent with its description as a doubly oxidized carbene. This work demonstrates that the use of bulky strong electron-donor substituents can simultaneously impart electronic stabilization and steric protection to both vacant orbitals on the central carbon atom, paving the way for the isolation of a variety of doubly oxidized carbenes.

Aldiminium Cations as Countercations to Discrete Main Group Fluoroanions

The reactions of group 14 tetrafluorides (SiF4, GeF4, and SnF4) and group 15 pentafluorides (PF5, AsF5, and SbF5) with the CAAC-based trifluoride reagent [MeCAACH][F(HF)2] led to the isolation of salts containing discrete 5- or 6-coordinated fluoroanions. The syntheses of [MeCAACH][SiF5], [MeCAACH][GeF5], [MeCAACH][(THF)SnF5], and the structurally related [MeCAACH][(dioxane)SnF5], [MeCAACH][PF6], [MeCAACH][AsF6], and [MeCAACH][SbF6] are effective, selective and in high yield. All compounds were characterized by X-ray single-crystal structure analysis, NMR and Raman spectroscopy. It is worth noting that the synthesized [MeCAACH][GeF5] is a rare example of a structurally characterized compound with discrete [GeF5]- anion, while [MeCAACH][(THF)SnF5] and [MeCAACH][(dioxane)SnF5] represent the first compounds with discrete octahedrally coordinated tin fluoride anions with incorporated solvent molecules. Finally, the aldiminium-based cation [MeCAACH]+ proved to be suitable for the stabilization of rare discrete main group fluoride anions.

Enhancing Diradical Character of Chichibabin's Hydrocarbon through Fluoride Substitution

In this work, 5-SIDipp [SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene] (1) derived Chichibabin's hydrocarbon with an octafluorobiphenylene spacer (3) has been reported. The addition of two equivalents of 5-SIDipp with decafluorobiphenyl in presence of BF3 gives the double C-F bond activated imidazolium salt with two tetrafluoroborate anions, 2. Further reduction of 2 gives the fluorine substituted 5-SIDipp based Chichibabin's hydrocarbon, 3. Quantum chemical calculations suggested a singlet state of 3 with a singlet-triplet energy gap (ΔES-T ) of 3.7 kcal mol-1 , which is substantially lower with respect to the hydrogen substituted NHC-based Chichibabin's hydrocarbons (10.7 kcal mol-1 , B3LYP). As a result, the diradical character (y) of 3 (y=0.62) is also noticeably higher than the hydrogen substituted CHs (y=0.41-0.43). The ▵ES-T was found to be higher in CASSCF (22.24 kcal mol-1 ) and CASPT2 (11.17 kcal mol-1 ) for 3 and the diradical character (d) is 44.6 %.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.