Isolation and characterization of diazoolefins

Lithium Anionic Dicarbenes or Acetylides: What is in the Name?

The two-fold deprotonation of the C2-arylated 1,3-imidazolium salts (IPr-Ar)X (1-Ar)X (IPr-Ar = ArC{N(Dipp)CH}2; Ar = Ph, 4-Me2NC6H4 (DMP) or 4-PhC6H4 (Bp); Dipp = 2,6-iPr2C6H3) with nBuLi affords the so-called anionic dicarbenes Li(ADC) (2-Ar) (ADC = ArC{N(Dipp)C}2). 2-Ar can be used to prepare a variety of main group heterocycles, however their structures in the solid-state remained hitherto unknown. Herein reported single-crystal X-ray diffraction studies reveal an acetylide type [ArC{N(Dipp)}(Dipp)NC≡CLi)]n (3-Ar) dimeric (n = 2) or trimeric (n = 3) molecular structure for 2-Ar. Treatment of 3-Ph with Et3B cleanly yields the monoanionic carbene Li[(ADC)(BEt3)] (4-Ph) featuring a weakly coordinating anion embedded in the same molecular entity. 3-Ar readily undergo reactions with CO2 and N2O to form the ring-closing products Li[(ADC)(CO2)2] (5-Ar) and Li[(ADC)N2O] (6-Ar), respectively.

An Isolable Base-Stabilized Diazosilenyl Cation

In contrast to the emerging chemistry of stable diazoalkenes, there are not yet any studies devoted to heavier silicon analogues, diazosilenes. Here, we report the synthesis of a base-stabilized diazosilenyl cation 2 by the reaction of base-stabilized C-phosphonio-silyne 1 with N2O. This silicon analog of diazoalkenes 2 exhibits a remarkable stability thanks to the coordination of phosphine and phosphine oxide ligands at the cationic silicon center. In addition, DFT calculations predict that, due to a particular stabilizing effect of the electropositive silicon atom, for diazosilenes (R2Si=C=N2), the presence of π-donor substituents is not essential to prevent N2 dissociation, contrary to carbon analogues. Interestingly, diazosilenyl cation 2 tends to isomerize into (silylene)(phosphonio)diazomethane 7 via a 1,2-phosphine ligand migration and thus exhibits dual reactivity as a diazoalkene and as a diazo-substituted silylene.

Independent LUMO Reactivity in Mesoionic N-Heterocyclic Thiones: Synthesis of a Stable Radical Anion

Mesoionic compounds, with positive and negative charges, are expected to have dual-site highest occupied molecular orbital (HOMO, donor) and lowest unoccupied molecular orbital (LUMO, acceptor) reactivity. Herein, we report a novel class of air-stable mesoionic N-heterocyclic thiones (mNHTs) synthesized from abnormal N-heterocyclic carbenes (aNHCs). DFT studies revealed a highly polarized exocyclic thione moiety and computed Fukui function analysis suggests the dual-site HOMO/LUMO reactivity of mNHTs predicting donor property at the negatively charged 'S' center while acceptor property at the cationic imidazole ring. The independent LUMO reactivity of the mNHT was realized by its chemical reduction to an elusive radical anion, which was characterized by a single crystal X-raydiffraction study. Further, we explore the reactivity of radical anion for the activation of SO2 gas, C-Br bonds of aryl bromide and photocatalytic functionalization of C-X (X = F, Br) bonds. This work unlocks the independent LUMO reactivity of a mesoionic compound.

Cleavage of Carbodicarbenes with N2O for Accessing Stable Diazoalkenes: Two-Fold Ligand Exchange at a C(0)-Atom

The cleavage of carbophosphinocarbenes and carbodicarbenes with nitrous oxide (N2O) leads to the formation of room-temperature stable diazoalkenes. The utility of Ph3P/N2 and NHC/N2 ligand exchange reactions were demonstrated by accessing novel benzimidazole- and benzothiazole derived diazoalkenes, which are not accessible by the current state-of-the-art methods. The stable diazoalkenes subsequently allow further ligand exchange reactions at C(0) with carbon monoxide, isocyanide, or a diamidocarbene (DAC). Overall, the combination of hitherto unknown NHC/N2 and N2/L (L = DAC, CO, R-NC) ligand exchange reactions at a C(0) center allow the selective functionalization of the carbodicarbene ligand structure which represents a new methodology to rapidly assemble novel carbodicarbenes or cumulenic compounds.

Combining Distibene, Diazoolefins, and Visible Light: Synthesis and Reactivity of Inorganic Rings

The chemistry of heterocycles containing "diaza" units has been extensively studied due to their applications ranging from pharmaceuticals to advanced materials. In contrast, heterocycles incorporating heavier elements, such as Sb and Bi, remain exceedingly rare and lack straightforward synthetic methodologies. Herein, we present a comprehensive experimental and theoretical investigation of the first diazadistiboylidenes (1a, 1b), synthesized via a [3 + 2]-cycloaddition between a distibene and diazoolefins. These stiboylidenes are key intermediates to promote selective nucleophilic substitution, leading to a rare example of diantimonyl anion. Furthermore, upon visible-light irradiation, we could isolate the first example of methylenedistibiranes, heavier analogs of methylenediaziridine (C2H4N2). These findings offer a novel platform for heavy dipnictogen chemistry, showcasing that diazoolefins, in combination with visible light, can facilitate the formation of unprecedented heavy heterocycles and serve as a platform to promote CO2 activation.

An Acceptor-Substituted N-Heterocyclic ortho-Quinodimethane: Pushing the Boundaries of Polarization in Donor-Acceptor-Substituted Polyenes

We report the synthesis, isolation, and characterization of a stable donor-acceptor substituted ortho-quinodimethane (oQDM). This system with an imidazolidine scaffold as the donor can also be referred to as acceptor-substituted ortho-N-heterocyclic quinodimethane (oNHQ). We have examined the extent of polarization of the conjugated π-system using single-crystal X-ray diffraction, NMR and UV/vis spectroscopy, cyclic voltammetry, and DFT computations. The bond lengths in the phenyl linker do not exhibit the alternation typical of oQDMs. In addition, the 13C and 15N NMR shifts suggest significant charge separation, an interpretation supported by the diatropic ring current determined by NICSZZ(r) computations, which is characteristic of aromatic compounds. DFT calculations show that polarization is an electronic effect that is amplified by steric influences. More strikingly, the oxidation and reduction potentials of the push-pull substituted oQDM are virtually identical to those of authenticated anionic and cationic derivatives. The results therefore indicate that an aromatic zwitterionic structure represents the electronic structure more accurately than a neutral quinoidal Lewis structure, which indicates that the acceptor-substituted oNHQ is a rare example of an organic zwitterion in which the centers of charge are in conjugation. The ambiphilic reactivity of the acceptor-substituted oNHQ, which is evidenced by the dehydrogenation of ammonia borane and the addition of phenylacetylene via heterolytic C-H bond cleavage, further supports its notation as an organic zwitterion and is reminiscent of frustrated Lewis pairs (FLPs). Thus, the acceptor-substituted oNHQ can be considered to be an intramolecular carbogenic FLP in terms of its reactivity.

α-(N-Alkyl-N-heteroarenium)-α-diazoacetates: synthesis and reactivity of a novel class of 'onium' diazo compounds

Treatment of alkyl α-(N-heteroaryl)-α-diazoacetates with alkylating reagents affords diazoacetate N-heteroarenium salts. These novel 'onium' diazo compounds are mostly yellow solids, displaying increased thermal and acid stability. Their tetrafluoroborates undergo rhodium catalyzed [2 + 1] and Doyle-Kirmse reactions under mild conditions, suggesting the N-quaternization an effective means of elimination of N-coordination caused catalyst toxicity.

Uncovering diverse reactivity of NHCs with diazoalkane: C-H activation, C[double bond, length as m-dash]C bond formation, and access to N-heterocyclic methylenehydrazine

N-heterocyclic carbenes (NHCs) have attracted significant attention due to their strong σ-donating capabilities, as well as their transition-metal-like reactivity towards small molecules. However, their interaction with diazoalkanes remains understudied. In this manuscript, we explore the reactivity of a series of stable carbenes, encompassing a wide range of electronic properties, with Me3SiCHN2. 5-SIPr activates the C-H bond of Me3SiCHN2, resulting in the formation of a novel diazo derivative (1), while carbenes such as 5-IPr, 6-SIPr, and diamido carbene yield N-heterocyclic methylenehydrazine derivatives (3, 4, and 8). The reaction of Me3SiCHN2 with 5-I t Bu unexpectedly leads to the formation of a triazole ring linked with the imidazole moiety via a C[double bond, length as m-dash]C double bond (6) alongside the azine product (7). Substituting the diazoalkane with diazoester consistently yields azine derivatives (9-12 and 14). Only in the case of 5-I t Bu, an imidazolium salt with tetrazenide anion (13) was obtained as a side product. The reaction of 4 with HCl resulted in the desilylprotonation to form a salt, 5a, which undergoes deprotonation upon using bases such as Et3N and KHMDS to form N-heterocyclic methylene hydrazine, 5. Theoretical calculations have been conducted to elucidate the diverse mechanisms underlying product formation.

Modular Synthesis of Azines Bearing a Guanidine Core from N-Heterocyclic Carbene (NHC)-Derived Selenoureas and Diazo Reagents

N-Heterocyclic carbene (NHC)-derived selenoureas comprise a fundamentally important class of NHC derivatives, with key applications in coordination chemistry and the determination of NHC electronic properties. Considering the broad reactivity of chalcogen-containing compounds, it is surprising to note that the use of NHC-derived selenoureas as organic synthons remains essentially unexplored. The present contribution introduces a novel, straightforward transformation leading to azines bearing a guanidine moiety, through the reaction of a wide range of NHC-derived selenoureas with commercially available diazo compounds, in the presence of triphenylphosphine. This transformation offers a new approach to such products, having biological, materials chemistry, and organic synthesis applications. The guanidine-bearing azines are obtained in excellent yields, with all manipulations taking place in air. A reaction mechanism is proposed, based on both experimental mechanistic findings and density functional theory (DFT) calculations. A one-pot, multicomponent transesterification reaction between selenoureas, α-diazoesters, alcohols, and triphenylphosphine was also developed, providing highly functionalized azines.

Cycloadditions of Diazoalkenes with P4 and tBuCP: Access to Diazaphospholes

Diazoalkenes readily react with tert-butylphosphaalkyne (tBuCP) and white phosphorus (P4) to afford novel phosphorus heterocycles, 3H-1,2,4-diazamonophospholes and 1,2,3,4-diazadiphospholes. Both species represent rare examples of neutral heterophospholes. The mechanism of formation and the electronic structures of these formal (3+2) cycloaddition products were analyzed computationally. The new phospholes form structurally diverse coordination compounds with transition metal and main group elements. Given the growing number of stable diazoalkenes, this work offers a straightforward route to neutral aza(di-)phospholes as a new ligand class.

An Azide-Free Synthesis of Metallodiazomethanes Using Nitrous Oxide

Diazo compounds are valuable reagents in synthesis but usually require the use of potentially explosive or toxic starting materials. Here, we report the synthesis and isolation of alkali metal diazomethanides by the reaction of metalated ylides with nitrous oxide, resulting in a formal exchange of the phosphine ligand by dinitrogen. The reaction proceeds through a Wittig-like mechanism via a [3 + 2] cycloaddition of N2O across the ylide bond with release of phosphine oxide. The metalated diazomethanes exhibit an increased thermal stability due to the stronger binding of N2 compared to neutral diazomethanes. This is reflected in short C-N distances and red-shifted N-N vibrations and enables versatile applications such as for the preparation of transition metal diazomethanide complexes and the synthesis of 1,2,3-triazoles from nitriles, diazoacetates from carbon dioxide, or alkynes from aldehydes.

Metal-Mediated Synthesis of a Mixed Arduengo-Fischer Carbodicarbene Ligand

Carbodicarbenes are strong C-donor ligands, which have found numerous applications in organometallic and main group element chemistry. Herein, we report a structurally distinct carbodicarbene ligand, which is formed by dinitrogenative coupling of a Fischer carbene complex with an N-heterocyclic diazoolefin. The resulting carbonyl complex serves as a stable source for the mixed Arduengo-Fischer carbodicarbene ligand. Facile ligand transfer reactions were demonstrated to occur with gold(I), copper(I), palladium(II), and rhodium(I) complexes.

Nitrous oxide as diazo transfer reagent

Nitrous oxide, commonly known as "laughing gas", is formed as a by-product in several industrial processes. It is also readily available by thermal decomposition of ammonium nitrate. Traditionally, the chemical valorization of N2O is achieved via oxidation chemistry, where N2O acts as a selective oxygen atom transfer reagent. Recent results have shown that N2O can also function as an efficient diazo transfer reagent. Synthetically useful methods for synthesizing triazenes, N-heterocycles, and azo- or diazo compounds were developed. This review article summarizes significant advancements in this emerging field.

NHC-Supported 2-Sila and 2-Germavinylidenes: Synthesis, Dynamics, First Reactivity and Theoretical Studies

2-tetrelavinylidenes (C=EH2; E=Si, Ge) are according to quantum chemical studies the least stable isomers on the [E,C,2H] potential energy hypersurface isomerizing easily via the trans-bent tetrelaacetylenes HE≡CH to the thermodynamically most stable 1-tetrelavinylidenes (E=CH2). Consequently, experimental studies on 2-tetrelavinylidenes (C=ER2) and their derivatives are lacking. Herein we report experimental and theoretical studies of the first N-heterocyclic carbene (NHC) supported 2-silavinylidene (NHC)C=SiBr(Tbb) (1-Si: NHC=C[N(Dipp)CH]2, Dipp=2,6-diisopropylphenyl, Tbb=2,6-bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl) and the isovalent 2-germavinylidenes (NHC)C=GeBr(R) (1-Ge, 1-GeMind: R=Tbb, Mind (1,1,3,3,5,5,7,7-octamethyl-s-hydrindacene-4-yl)). The NHC-supported 2-tetrelavinylidenes were obtained selectively from the 1,2-dibromoditetrelenes (E)-(R)BrE=EBr(R) using the diazoolefin (NHC)CN2 as vinylidene transfer reagent. 1-E (E=Si, Ge) have a planar vinylidene core, a bent-dicoordinated vinylidene carbon atom (CVNL), a very short E=CVNL bond and an almost orthogonal orientation of the NHC five-membered ring to the vinylidene core. Quantum chemical analysis of the electronic structures of 1-E suggest a significantly bent 1-tetrelaallene and tetrelyne character. NMR studies shed light into the dynamics of 1-E involving NHC-rotation around the CVNL-CNHC bond with a low activation barrier. Furthermore, the synthetic potential of 1-E is demonstrated by the synthesis and full characterization of the unprecedented NHC-supported bromogermynes BrGe=C(EBr2Tbb)(NHC) (2-SiGe: E=Si; 2-GeGe: E=Ge).

Ph3PCN2: A stable reagent for carbon-atom transfer

Precise modification of a chemical site in a molecule at the single-atom level is one of the most elegant yet difficult transformations in chemistry. A reagent specifically designed for chemoselective introduction of monoatomic carbon is a particularly formidable challenge. Here, we report a straightforward, azide-free synthesis of a crystalline and isolable diazophosphorus ylide, Ph3PCN2, a stable compound with a carbon atom bonded to two chemically labile groups, triphenylphosphine (PPh3) and dinitrogen (N2). Without any additives, the diazophosphorus ylide serves as a highly selective transfer reagent for fragments, including Ph3PC, to deliver phosphorus ylide-terminated heterocumulenes and CN2 to produce multisubstituted pyrazoles. Ultimately, even exclusive carbon-atom transfer is possible. In reactions with aldehydes and acyclic and cyclic ketones (R2C=O), the carbon-atom substitution forms a vinylidene (R2C=C:) en route to alkynes or butatrienes.

Light-Dependent Reactivity of Heavy Pnictogen Double Bonds

The chemistry of light dipnictenes has been widely investigated in the last century with remarkable achievements especially for azobenzene derivatives. In contrast, distibenes and dibismuthenes are relatively rare and show very limited reactivity. Herein, we have designed a protocol using visible light to enhance the reactivity of heavy dipnictenes. Exploiting the distinctive π-π* transition, we have been able to isolate unique examples of dipnictene-cobalt complexes. The reactivity of the distibene complex was further exploited using red light in the presence of a diazoolefin to access an unusual four-membered bicyclo[1.1.0]butane analog, containing only a single carbon atom. These findings set the bases to a conceptually new strategy in heavy element double bonds chemistry where visible light is at the front seat of bond activation.

Gold(I)-catalyzed homologation of aryl aldehydes with trimethylsilyldiazomethane

A methodology is described herein for the synthesis of 2-aryl acetaldehydes from aryl aldehydes using TMSCHN2 under Au(I) catalysis. A diazoorgano Au(I) complex was shown to be an intermediate that reacts with ArCHO to give ArCH2CHO products. This homologation protocol was used to synthesize a wide range of 2-aryl acetaldehydes with high functional group compatibility.

An N-Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity

We report the preparation of a new organic σ-donor with a C6H4-linker between an N-heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N-heterocyclic quinodimethane (NHQ). The aromatization of the C6H4-linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N-heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head-to-head dimerization by C-C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open-shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N-heterocyclic bis-quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E1/2=-1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet-triplet gap of ΔES→T=4.4 kcal mol-1.

Synthesis and isolation of a cyclic bis-vinyl germylene via a diazoolefin adduct of germylene dichloride

Since the successful isolation of various stable diazoolefins, an array of complexes containing these promising ligands have been synthesized. We herein report the synthesis, characterization, and structures of neutral group 14 diazoolefin complexes and the subsequent transformation into a new cyclic bis-vinyl germylene.

Diazoalkenes: From an Elusive Intermediate to a Stable Substance Class in Organic Chemistry

Over decades diazoalkenes (R2 C=C=N2 ) were postulated as reactive intermediates in organic chemistry even though their direct spectroscopic detection proved very challenging. In the 1970/80ies several groups probed their existence mainly indirectly by trapping experiments or directly by matrix-isolation studies. In 2021, our group and the Severin group reported independently the synthesis and characterization of the first room-temperature stable diazoalkenes, which initiated a rapidly expanding research field. Up to now four different classes of N-heterocyclic substituted room-temperature stable diazoalkenes have been reported. Their properties and unique reactivity, such as N2 /CO exchange or utilization as vinylidene precursors in organic and transition metal chemistry are presented. This review summarizes the early discoveries of diazoalkenes from their initial postulation as transient, elusive species up to the recent findings of the room-temperature stable derivatives.

Synthetic Strategies of N-Heterocyclic Olefin (NHOs) and Their Recent Application of Organocatalytic Reactions and Beyond

N-heterocyclic olefin (NHO) derivatives have an electron-rich as well as highly polarized carabon-carbon (C=C) double bond because of the electron-donating nature of nitrogen and sulphur atoms. While NHOs have been developing as novel organocatalysts and ligands for transition-metal complexes in various organic compound syntheses, different research groups are currently interested in preparing imidazole and triazolium-based chiral NHO catalysts. Some of them have been used for enantioselective organic transformations, but were still elusive. N-heterocyclic olefins, the alkylidene derivatives of N-heterocyclic carbenes (NHC), have shown promising results as effective promoters for numerous organic syntheses such as asymmetric catalysis, hydroborylation, hydrosilylation, reduction, CO2 sequestration, alkylation, cycloaddition, polymerization and the ring-opening reaction of aziridine and epoxides, esterification, C-F bond functionalization, amine coupling, trifluoromethyl thiolation, amination etc. NHOs catalysts with suitable structures can serve as a novel class of Lewis/Bronsted bases with strong basicity and high nucleophilicity properties.These facts strongly suggest their enormous chemical potential as sustainable catalysts for a wide variety of reactions in synthetic chemistry. The synthesis of NHOs and their properties are briefly reviewed in this article, along with a summary of the imidazole and triazole core of NHOs' most recent catalytic uses.

Synthesis and Reactivity of an Anionic Diazoolefin

Bent (hetero)allenes such as carbodicarbenes and carbodiphosphoranes can act as neutral C-donor ligands, and diverse applications in coordination chemistry have been reported. N-Heterocyclic diazoolefins are heterocumulenes, which can function in a similar fashion as L-type ligands. Herein, we describe the synthesis and the reactivity of an anionic diazoolefin. This compound displays distinct reactivity compared to neutral diazoolefins, as evidenced by the preparation of diazo compounds via protonation, alkylation, or silylation. The anionic diazoolefin can be employed as an ambidentate, X-type ligand in salt metathesis reactions with metal halide complexes. Extrusion of dinitrogen was observed in a reaction with PCl(NiPr2 )2 , resulting in a stable phosphinocarbene.

Arenes participate in 1,3-dipolar cycloaddition with in situ-generated diazoalkenes

The venerable 1,3-dipolar cycloaddition has been widely used in organic synthesis for the construction of various heterocycles. However, in its century-long history, the simple and omnipresent aromatic phenyl ring has remained a stubbornly unreactive dipolarophile. Here we report 1,3-dipolar cycloaddition between aromatic groups and diazoalkenes, generated in situ from lithium acetylides and N-sulfonyl azides. The reaction results in densely functionalized annulated cyclic sulfonamide-indazoles that can be further converted into stable organic molecules that are important in organic synthesis. The involvement of aromatic groups in the 1,3-dipolar cycloadditions broadens the synthetic utility of diazoalkenes, a family of dipoles that have been little explored so far and are otherwise difficult to access. The process described here provides a route for the synthesis of medicinally relevant heterocycles and can be extended to other arene-containing starting materials. Computational examination of the proposed reaction pathway revealed a series of finely orchestrated bond-breaking and bond-forming events that ultimately lead to the annulated products.

Gold-Catalyzed Anti-Markovnikov Oxidation of Au-Allenylidene to Generate Alkylidene Ketene

It remains a long-standing challenge to directly convert alkynes to carboxylic derivatives. Herein, a unexpectedly anti-Markovnikov oxidation of a unique Au-allenylidene pathway instead of a traditional α-oxo gold carbene routine is disclosed for in situ formation and transformation of highly unsaturated alkylidene ketenes, which are subsequently trapped by broad nucleophiles such as alcohols, phenols, water, amines, and sulfoximines to easily access α,β-unsaturated drugs and natural product derivatives by a multicomponent reaction. Based on this scenario, polyacrylate and polyacrylamide are efficiently afforded by corresponding multicomponent polymerization.

Bonding and Reactivity in Terminal versus Bridging Arenide Complexes of Thorium Acting as ThII Synthons

Thorium redox chemistry is extremely scarce due to the high stability of Th^IV . Here we report two unique examples of thorium arenide complexes prepared by reduction of a Th^IV -siloxide complex in presence of naphthalene, the mononuclear arenide complex [K(OSi(O^t Bu)₃ )₃ Th(η⁶ -C₁₀ H₈ )] (1) and the inverse-sandwich complex [K(OSi(O^t Bu)₃ )₃ Th]₂ (μ-η⁶ ,η⁶ -C₁₀ H₈ )] (2). The electrons stored in these complexes allow the reduction of a broad range of substrates (N₂ O, AdN₃ , CO₂ , HBBN). Higher reactivity was found for the complex 1 which reacts with the diazoolefin IDipp=CN₂ to yield the unexpected Th^IV amidoalkynyl complex 5 via a terminal N-heterocyclic vinylidene intermediate. This work showed that arenides can act as convenient redox-active ligands for implementing thorium-ligand cooperative multielectron transfer and that the reactivity can be tuned by the arenide binding mode.

Copper Complexes with Diazoolefin Ligands and their Photochemical Conversion into Alkenylidene Complexes

Homometallic copper complexes with alkenylidene ligands are discussed as intermediates in catalysis but the isolation of such complexes has remained elusive. Herein, we report the structural characterization of copper complexes with bridging and terminal alkenylidene ligands. The compounds were obtained by irradiation of Cu^I complexes with N-heterocyclic diazoolefin ligands. The complex with a terminal alkenylidene ligand required isolation in a crystalline matrix, and its structural characterization was enabled by in crystallo photolysis at low temperature.

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.

Activation of CS2 with the 2H-Phosphindole Complex to Construct P,S-Polycycles

The activation of CS₂ by the 2H-phosphindole complex with a low-coordinate phosphadiene moiety is reported. The successive hetero-Diels-Alder reaction between 2H-phosphindoles and CS₂ constructs two bridged rings and one spirocycle simultaneously, affording structurally complex P,S-polycyclic products. The two 2H-phosphindoles approach the C═S bond in a head-to-head disposition to minimize steric hindrance. This work reveals the unique reactivity of low-coordinate organophosphorus species and their potential applications in small molecule activation.

Recent advances of mesoionic N-heterocyclic olefins

This Perspective article highlights the recent development of mesoionic N-heterocyclic olefins (mNHOs), where the exo-cyclic olefinic carbon is not bonded to strongly electron-withdrawing groups. The unquenched basicity and nucleophilicity of the exo-cyclic olefinic carbon make mNHOs strong σ-donors and enable unique reactivity patterns.

Vanadium complexes with N-heterocyclic vinylidene ligands

The preparation and the structural characterization of vanadium complexes with terminal and bridging N-heterocyclic vinylidene ligands is reported. The synthesis of the complexes was enabled by utilization of diazoolefins as ligand precursors. Structural data and theoretical results show that N-heterocyclic vinylidenes can act as 6e^- donor ligands, leading to strong metal-carbon interactions.

Crystalline phosphino-functionalized mesoionic olefins (p-MIOs)

Phosphino-functionalized mesoionic olefins (p-MIOs), (iMIC)CHR (iMIC = PhC{N(Dipp)}2C(PPh2)C, Dipp = 2,6-iPr2C6H3; R = H 4a or Ph 4b) derived from a 1,3-imidazole based mesoionic carbene (iMIC) are reported. The p-MIOs...

Characterization of a Triplet Vinylidene

Singlet vinylidenes (R₂C=C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm^–1 and |E|/D = 0.028) were determined, and the ¹³C hyperfine coupling tensor was obtained by ¹³C-ENDOR measurements. Most strikingly, the isotropic ¹³C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Synthesis, structural characterization, and coordination chemistry of imidazole-based alkylidene ketenes

Alkylidene ketenes typically display high intrinsic reactivity, impeding isolation on a preparative scale. Herein, we report the synthesis of alkylidene ketenes by reaction of imidazole-based diazoolefins with carbon monoxide. The good thermal stability of these heterocumulenes allows for characterization by single crystal X-ray diffraction. N-Heterocyclic alkylidene ketenes can be used as C-donor ligands for transition and main group metals, as evidenced by the isolation of complexes with AuCl, RhCl(CO)₂, PdCl(C₃H₅) and GaCl₃.