Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent‐Silyliumylidene Cation

The trapping of a silicon(I) radical with N‐heterocyclic carbenes is described. The reaction of the cyclic (alkyl)(amino) carbene [cAAC_Me] (cAAC_Me=:C(CMe₂)₂(CH₂)NAr, Ar=2,6‐i Pr₂C₆H₃) with H₂SiI₂ in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAAC_Me)₂Si:^.]⁺I⁻ ( 1 ), which features a cationic cAAC–silicon(I) radical, and [cAAC_Me−H]⁺I⁻. In addition, the reaction of the NHC–iodosilicon(I) dimer [I_Ar(I)Si:]₂ (I_Ar=:C{N(Ar)CH}₂) with 4 equiv of I_Me (:C{N(Me)CMe}₂), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(I_Me)₂SiH]⁺I⁻ ( 2 ) comprising the first NHC–parent‐silyliumylidene cation. Its further reaction with fluorobenzene afforded the C_Ar−H bond activation product [1‐F‐2‐I_Me‐C₆H₄]⁺I⁻ ( 3 ). The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 . Compounds 1 – 3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X‐ray crystallography.     

A Room-Temperature Stable Distonic Radical Cation

Distonic radical cations (DRCs) with spatially separated charge and radical sites have, so far, largely been observed by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Herein, we disclose the isolation of a crystalline dicarbondiphosphide-based β-distonic radical cation salt 3^.+ (BARF) (BARF=[B(3,5-(CF₃)₂C₆H₃)₄)]⁻) stable at room temperature and formed by a one-electron-oxidation-induced intramolecular skeletal rearrangement reaction. Such a species has been validated by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray diffraction, UV/Vis spectroscopy and density functional theory (DFT) calculations. Compound 3^.+ (BARF) exhibits a large majority of spin density at a two-coordinate phosphorus atom (0.74 a.u.) and a cationic charge located predominantly at the four-coordinate phosphorus atom (1.53 a.u.), which are separated by one carbon atom. This species represents an isolable entity of a phosphorus radical cation that is the closest to a genuine phosphorus DRC to date.     

An N‐Heterocyclic‐Carbene‐Derived Distonic Radical Cation

We present the discovery of a novel radical cation formed through one‐electron oxidation of an N‐heterocyclic carbene–carbodiimide (NHC–CDI) zwitterionic adduct. This compound possesses a distonic electronic structure (spatially separate spin and charge regions) and displays persistence under ambient conditions. We demonstrate its application in a redox‐flow battery exhibiting minimal voltage hysteresis, a flat voltage plateau, high Coulombic efficiency, and no performance decay for at least 100 cycles. The chemical tunability of NHCs and CDIs suggests that this approach could provide a general entry to redox‐active NHC–CDI adducts and their persistent radical ions for various applications.     

An Antitumor Bis(N‐Heterocyclic Carbene)Platinum(II) Complex That Engages Asparagine Synthetase as an Anticancer Target

New anticancer platinum(II) compounds with distinctive modes of action are appealing alternatives to combat the drug resistance and improve the efficacy of clinically used platinum chemotherapy. Herein, we describe a rare example of an antitumor Pt^II complex targeting a tumor‐associated protein, rather than DNA, under cellular conditions. Complex [(bis‐NHC)Pt(bt)]PF₆ ( 1 a ; Hbt=1‐(3‐hydroxybenzo[b]thiophen‐2‐yl)ethanone) overcomes cisplatin resistance in cancer cells and displays significant tumor growth inhibition in mice with higher tolerable doses compared to cisplatin. The cellular Pt species shows little association with DNA, and localizes in the cytoplasm as revealed by nanoscale secondary ion mass spectrometry. An unbiased thermal proteome profiling experiment identified asparagine synthetase (ASNS) as a molecular target of 1 a . Accordingly, 1 a treatment reduced the cellular asparagine levels and inhibited cancer cell proliferation, which could be reversed by asparagine supplementation. A bis‐NHC‐ligated Pt species generated from the hydrolysis of 1 a forms adducts with thiols and appears to target an active‐site cysteine of ASNS.     

Azo‐MICs: Redox‐Active Mesoionic Carbene Ligands Derived from Azoimidazolium Dyes

Azoimidazolium dyes were used as precursors for mesoionic carbene ligands (Azo‐MICs). The properties of these ligands were examined by synthesizing Rh^I, Au^I, and Pd^II complexes. Experimental (NMR, IR) and theoretical investigations show that Azo‐MICs are potent σ‐donor ligands. Yet, they feature a small singlet–triplet gap and very low‐lying LUMO levels. The unique electronic properties of Azo‐MICs allow for reversible one‐electron reductions of the metal complexes, as evidenced by cyclic voltammetry.     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

Enantioselective N‐Heterocyclic Carbene Catalysis that Exploits Imine Umpolung

The catalytic umpolung of imines remains an underdeveloped approach to reaction discovery. Herein we report an enantioselective aza‐Stetter reaction that proceeds via imine umpolung using N‐heterocyclic carbene catalysis. The reaction proceeds with high levels of enantioselectivity (all ≥96:4 er) and good generality (21 examples). Mechanistic studies are reported and are consistent with turnover‐limiting addition of the NHC to the imine.     

The Viologen Cation Radical Pimer: A Case of Dispersion‐Driven Bonding

The π bonds between organic radicals have generated excitement as an orthogonal interaction for designing self‐assembling architectures in water. A systematic investigation of the effect of the viologen cation radical structure on the strength and nature of the pimer bond is provided. A striking and unexpected feature of this π bond is that the bond strength is unchanged by substitution with electron‐donating groups or withdrawing groups or with increased conjugation. Furthermore, the interaction is undiminished by sterically bulky N ‐alkyl groups. Theoretical modeling indicates that strong dispersion forces dominate the interaction between the radicals, rationalizing the insensitivity of the bonding interaction to substituents: The stacking of polarizable π radicals leads to attractive dispersion forces in excess of typical dispersion interactions of small molecules and helps overcome the Coulombic repulsion of bringing two cationic species into contact.     

Disproportionation Equilibrium of a μ‐Oxodiiron(III) Complex Giving Rise to C−H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct

μ‐Oxodiiron(III) species are air‐stable and unreactive products of autoxidation processes of monomeric heme and non‐heme iron(II) complexes. Now, the organometallic [(L^NHC)Fe^III‐(μ‐O)‐Fe^III(L^NHC)]⁴⁺ complex 1 (L^NHC is a macrocyclic tetracarbene) is shown to be reactive in C?H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) ( 2 ) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2 . Kinetic analysis gave an equilibrium constant for the disproportionation of 1 , which is very small (K_eq=7.5±2.5×10^?8 m ). Increasing K_eq might by a useful strategy for circumventing the formation of dead‐end μ‐oxodiiron(III) products during Fe‐based homogeneous oxidation catalysis.     

Stable Radical Cation and Dication of an N-Heterocyclic Carbene Stabilized Digallene: Synthesis,Characterization and Reactivity

One- and two-electron oxidation of a digallene stabilized by an N-heterocyclic carbene afforded the first stable gallium-based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu₃SnH and cyclo-S₈ was studied; a digallium–hydride cation salt containing a Ga−Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder-like Ga₄S₄ core structure were obtained, respectively.     

一例可分离的双核铁氢自由基阳离子

成功分离得到了一例双核铁氢自由基阳离子盐cis-[Fe₂Cp₂（μ-H）（μ-PPh₂）（CO）₂]^·+[Al（OC（CF₃）₃）₄]^-（cis-1^·+[Al（OC（CF₃）₃）₄]^-）晶体，并使用单晶X射线衍射、电子顺磁共振、红外光谱、紫外-可见光谱以及密度泛函理论对它进行了表征和理论计算。电子顺磁共振和密度泛函理论计算分析表明，自由基的自旋密度主要均等分布于2个铁原子上。     

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

A cross‐hydroalkenylation/rearrangement cascade (HARC), using a cyclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni(allyl)]BAr^F catalysts (NHC=N‐heterocyclic carbenes). By controlling the (NHC)Ni^IIH relative insertion reactivity with cyclopropene and alkyne, a broad scope of cyclopentadienes was obtained with highly selectively. The structural features of the new (NHC)Ni^II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni^II‐assisted vinylcyclopropane rearrangement reactivity.     

A Size‐Flexible Organometallic Box for the Encapsulation of Fullerenes

A palladium‐cornered molecular square with four pyrene‐bis(imidazolylidene) bridging ligands is reported. This metallo‐polygon can encapsulate C₆₀ and C₇₀. The X‐ray diffraction structures of the empty cage as well as the cages complexed with both fullerenes are described. The fullerene encapsulation produces perturbations in the structural parameters of the metallo‐square, showing that it can adjust the shape of its cavity to the size of each fullerene.