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The catalytic activity of a Ru complex 1, bearing a fused π‐conjugated imidazo[1,2–a][1,8]naphthyridine‐based mesoionic carbene (MIC) ligand, is examined for the oxidation of primary amines. Complex 1 affords nitrile or imine depending on the nature of the terminal oxidants and solvents used in the reactions. Primary amines are converted to nitriles using NaIO4 in EtOAc/H2O mixture, whereas imines are obtained under O2 balloon pressure in toluene. A variety of nitriles and imines are accessed with high yields and selectivity. A set of control experiments, reaction profiles, and kinetic studies are undertaken to disclose the mechanistic details for nitrile and imine formation. The catalytic reactions illustrate a subtle dependency on the choice of oxidants and solvents in the oxidation of primary amines. [ABSTRACT FROM AUTHOR]

We describe an iridium-catalyzed ortho-selective C–H borylation of unprotected anilines. The employing strong σ-donating cyclometalated mesoionic carbene as C^C bidentate ligand is crucial for the high ortho-selectivity. Computational studies support outer-sphere X–H···Oboryl hydrogen bond interactions, which are strengthened by bidentate C^C ligand introduction. The method shows broad substrate scope and high functional group tolerance.

Classical N‐heterocyclic carbenes (NHCs) featuring the carbene center at the C2‐position of 1,3‐imidazole framework (i.e. C2‐carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ‐donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so‐called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ‐donors than C2‐carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5‐anionic dicarbenes (ADCs), also based on an 1,3‐imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main‐group heterocycles, radicals, molecular catalysts, ligands sets, and more. [ABSTRACT FROM AUTHOR]

Mesoionic carbenes (MIC) are a popular class of compound that are heavily investigated at the moment. The access to cationic MICs, and the ability of MICs to stabilize radicals are two highly attractive fields that have hardly been explored until now. Here the synthesis and characterisation of three different cationic azide‐substituted 1,2,3‐triazolium salts, used as building blocks for studying their reactivity towards triphenylphosphine are reported, where the reactivity is dependent on the nature of the starting triazolium salt. Furthermore, the cationic triazolium salts were used to develop a series of unsymmetrical MIC‐triazene‐NHC/MIC' compounds, which can be readily converted to the radical form either by electrochemical or chemical methods. These radicals, which display NIR electrochromism, were investigated using a battery of techniques such as electrochemistry, UV/Vis/NIR and EPR spectroelectrochemistry, and theoretical calculations. Interestingly, the MIC plays an important role in the stabilization of the triazenyl radical, particularly in a competitive role vis‐à‐vis their NHC counterparts. These results shed new light on the ability of MICs to stabilize radicals, and possibly also on their π‐accepting ability. [ABSTRACT FROM AUTHOR]

Ketones are among the most useful functional groups in organic synthesis, and they are commonly encountered in a broad range of compounds with various applications. Herein, we describe the mesoionic carbene‐catalyzed coupling reaction of aldehydes with non‐activated secondary and even primary alkyl halides. This metal‐free method utilizes deprotonated Breslow intermediates derived from mesoionic carbenes (MICs), which act as super electron donors and induce the single‐electron reduction of alkyl halides. This mild coupling reaction has a broad substrate scope and tolerates many functional groups, which allows to prepare a diversity of simple ketones as well as bio‐active molecules by late‐stage functionalization. [ABSTRACT FROM AUTHOR]

This work tackles the photochemistry of a series of mononuclear Cr0, Mo0 and W0 carbonyl complexes containing a bidentate mesoionic carbene ligand of the 1,2,3‐triazol‐5‐ylidene type. FTIR spectroscopy, combined with density functional theory calculations, revealed a clean photo‐induced reaction in organic solvents (acetonitrile, pyridine, valeronitrile) to give mainly one photoproduct with monosubstitution of a carbonyl ligand for a solvent molecule. The highest photodissociation quantum yields were reached for the Cr0 complex under UV irradiation (266 nm). Based on previous investigations, the kinetics of the dark reverse reactions have now been determined, with reaction times of up to several hours in pyridine. Photochemical studies in the solid state (KBr matrix, frozen solution) also showed light‐induced reactivity with stabilization of the metastable intermediate with a free coordination site at very low temperature. The identified reactive species emphasizes a mechanism without ligand–sphere reorganization. [ABSTRACT FROM AUTHOR]

The behaviour of triazolylidene ligands coordinated at a {Fe2(CO)5(µ-dithiolate)} core related to the active site of [FeFe]-hydrogenases have been considered to determine whether such carbenes may act as redox electron-reservoirs, with innocent or non-innocent properties. A novel complex featuring a mesoionic carbene (MIC) [Fe2(CO)5(Pmpt)(µ-pdt)] (1; Pmpt = 1-phenyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene; pdt = propanedithiolate) was synthesized and characterized by IR, 1H, 13C{1H} NMR spectroscopies, elemental analyses, X-ray diffraction, and cyclic voltammetry. Comparison with the spectroscopic characteristics of its analogue [Fe2(CO)5(Pmbt)(µ-pdt)] (2; Pmbt = 1-phenyl-3-methyl-4-butyl-1,2,3-triazol-5-ylidene) showed the effect of the replacement of a n-butyl by a phenyl group in the 1,2,3-triazole heterocycle. A DFT study was performed to rationalize the electronic behaviour of 1, 2 upon the transfer of two electrons and showed that such carbenes do not behave as redox ligands. With highly perfluorinated carbenes, electronic communication between the di-iron site and the triazole cycle is still limited, suggesting low redox properties of MIC ligands used in this study. Finally, although the catalytic performances of 2 towards proton reduction are weak, the protonation process after a two-electron reduction of 2 was examined by DFT and revealed that the protonation process is favoured by S-protonation but the stabilized diprotonated intermediate featuring a {Fe-H⋯H-S} interaction does not facilitate the release of H2 and may explain low efficiency towards HER (Hydrogen Evolution Reaction). [ABSTRACT FROM AUTHOR]

N-Heterocyclic carbenes (NHCs) have seen more and more use over the years. The go-to systems that are usually considered are derivatives of benzimidazole or imidazole. Caffeine possesses an imidazole unit and was already utilized as a carbene-type ligand; however, its use within a tridentate bis-NHC system has—to the best of our knowledge—not been reported so far. The synthesis of the ligand is straightforward and metal complexes are readily available via silver-salt metathesis. A platinum(II) and a palladium(II) complex were isolated and a crystal structure of the former was examined. For the Pt(II) complex, luminescence is observed in solid state as well as in solution. [ABSTRACT FROM AUTHOR]

A novel complex featuring a mesoionic carbene [Fe2(CO)5(trz)(μ-pdt)] (1) (trz = 1-phenyl-l,3-methyl,4-butyl-1,2,3-triazol-5-ylidene), was synthesized and spectroscopically and structurally characterized. The reductive behaviour of this compound in the presence and in the absence of acid (CH3CO2H) was examined by cyclic voltammetry (CV) that revealed the lack of efficient activity towards proton reduction.

A simple experimental procedure for scaling carbene Brønsted basicity is described. The results highlight the strong basicity of pyrazol‐4‐ylidenes, a type of mesoionic carbene, also named cyclic‐bentallenes (CBA). They are more basic (pKaH >42.7 in acetonitrile) than the popular proazaphosphatrane Verkade bases, and even the Schwesinger phosphazene superbase P4(tBu). The basicity of these compounds can readily be tuned, and they are accessible in multigram quantities. These results open new avenues for carbon centered superbases. [ABSTRACT FROM AUTHOR]

An evidence for the formation of a rare meta-metalated inherently chiral calix[4]arene is described. Our strategy involved using a mesoionic carbene to direct C–H activation, but proved to form an unexpectedly unstable intermediate that was identified through high-resolution mass spectrometry. On route to our target, a new optimized method to mononitrocalix[4]arenes was developed, including optimized and high yielding transformations to azide and 1,2,3-triazole derivatives which may have application in other areas of research.

Mesoionic carbenes (MIC) of the 1,2,3‐triazol‐5‐ylidene type are currently popular ligands in organometallic chemistry. Their use in main group chemistry has been rather limited. In this contribution we present mono‐ and di‐MIC‐boranes with MICs based on triazolylidenes. The synthesis involves in‐situ deprotonation of the corresponding triazolium salts and their reaction with boranes to form the desired compounds. Whereas this reaction route worked well for all triazolium salts derived from the 1,4‐regioisomer of the triazoles, for the methlyene‐bridged bi‐triazolium salt derived from a 1,5‐substiuted triazole, we observed the unexpected decomposition of the bi‐triazolium and the formation of a triazole‐borane with a new N−B bond. All compounds were characterized via multinuclear NMR spectroscopy, mass spectrometry, and single crystal X‐ray diffraction. Furthermore, the MIC‐boranes were used as reducing agents for the reduction of the C=O of aldehydes to the corresponding alcohols. [ABSTRACT FROM AUTHOR]

We present herein anionic borate‐based bi‐mesoionic carbene compounds of the 1,2,3‐triazol‐4‐ylidene type that undergo C−N isomerization reactions. The isomerized compounds are excellent ligands for CoII centers. Strong agostic interactions with the "C−H"‐groups of the cyclohexyl substituents result in an unusual low‐spin square planar CoII complex, which is unreactive towards external substrates. Such agostic interactions are absent in the complex with phenyl substituents on the borate backbone. This complex displays a high‐spin tetrahedral CoII center, which is reactive towards external substrates including dioxygen. To the best of our knowledge, this is also the first investigation of agostic interactions through single‐crystal EPR spectroscopy. We conclusively show here that the structure and properties of these CoII complexes can be strongly influenced through interactions in the secondary coordination sphere. Additionally, we unravel a unique ligand rearrangement for these classes of anionic mesoionic carbene‐based ligands. [ABSTRACT FROM AUTHOR]

A strong σ-donor mesoionic carbene ligand has been synthesized and applied to four different palladium-catalyzed cross-coupling transformations, proving the catalyst/medium compatibility and the increased activity of this system over previous reports in Deep Eutectic Solvent medium. Some cross-coupling processes could be carried out at room temperature and using aryl chlorides as starting materials. The possible implementation of multistep synthesis in eutectic mixtures has also been explored. The presence of palladium nanoparticles in the reaction media has been evaluated and correlated to the observed activity.

A series of rhodium(I)-1,2,3-triazol-5-ylidene (trz) complexes are described, containing either a novel triarylated trz ligand A' (N1, N3-arylated, C4-ferrocenyl) {complexes 1, [Rh(A')Cl(cod)]; 4, [Rh(A')Cl(CO)2]}, or N3-alkylated triazolylidenes with a C4-ferrocenyl {2, [Rh(B')Cl(cod)]; 5, [Rh(B')Cl(CO)2]} or C4-phenyl substituent {3, [Rh(C')Cl(cod)]; 6, [Rh(C')Cl(CO)2]}. The free mesoionic carbene (MIC) A' is structurally characterized and its electronic properties evaluated by employing the complex [Pd(Br)2(iPr2-bimy)(A')] (7) in an NMR spectroscopic analysis method. The redox activity of A' is exploited, and the chemically oxidized precursor Aox and complex 4ox are isolated. The mesoionic carbene complexes 1-3, as well as in situ oxidized 1ox, are used as homogeneous catalysts for the hydroformylation of 1-octene for the first time, and the influence of chemical oxidation of the catalyst on the activity and chemoand regioselectivity of the catalyst precursor 1 is evaluated. [ABSTRACT FROM AUTHOR]

Cover Feature: Azide-Substituted 1,2,3-Triazolium Salts as Useful Synthetic Synthons: Access to Triazenyl Radicals and Staudinger Type Reactivity (Chem. Eur. J. 34/2023) Cationic azides, mesoionic carbene, radicals, spectroelectrochemistry, triazenes Keywords: cationic azides; mesoionic carbene; radicals; spectroelectrochemistry; triazenes EN cationic azides mesoionic carbene radicals spectroelectrochemistry triazenes 1 1 1 06/21/23 20230619 NES 230619 B An azido-1,2,3-triazolium salt b is a useful synthon that reacts either photochemically or thermally to deliver mesoionic carbene-stabilized imino-phosphoranes and mesoionic carbene-stabilized triazenes, which can be converted to stable radicals. [Extracted from the article]

A series of novel cationic and neutral Rh complexes with an N-donor-functionalized 1,2,3-triazol-5-ylidene (TRZ) ligand (in which the pendant N donor is NHBoc, NH2, or NMe2) is described. The catalytic activity of these complexes was evaluated in the hydrothiolation of alkynes. Among the catalysts, a neutral dicarbonyl complex featuring the tethered-NBoc amido-TRZ ligand proved very selective for alkyne hydrothiolation with an aryl thiol. Remarkably, the reaction could be carried out in the absence of pyridine or base additive. In addition, during the reaction, no evidence for oxidative addition of the thiol S−H bond was observed, strongly suggesting a reaction pathway in which a bifunctional ligand is involved. Experimental and theoretical mechanistic investigations suggest a ligand-assisted deprotonation of thiol, hemilabile dissociation of amine from the metal, and thiolate coordination, which is indicative of a different reaction mechanism to those previously reported for related alkyne hydrothiolation reactions. [ABSTRACT FROM AUTHOR]

(−) Menthol was converted to 1-menthyl-3-methyl-4-phenyltriazolium iodide and 1-neomethyl-3-methyl-4-phenyltriazolium iodide in four steps. These triazolium salts that are enantio pure but diastereo isomers, served as precursors for the synthesis of the corresponding enantio pure bis(1,2,3-triazol-5-ylidene)PdI 2 complexes with menthyl and neomenthyl wing tip groups. The palladium complexes were synthesized in high yields using a recently developed mild method under base free and silver free conditions at ambient temperature. These triazolium salts also served as precursors for the synthesis of the corresponding enantio pure PEPPSI type (1,2,3-triazol-5-ylidene) (pyridine)PdI 2 complexes. All the complexes were thoroughly characterized by spectroscopic and single crystal XRD data. The relative catalytic activities of these complexes were compared for Suzuki–Miyaura C–C coupling. [ABSTRACT FROM AUTHOR]

A novel synthetic route gives access to mesoionic carbene and cyclopropenylidene supported gold chloride complexes. The corresponding cationic MIC-gold complex obtained by chloride abstraction allows for the first transition metal-catalyzed functionalization of both nitrogens of parent hydrazine. [ABSTRACT FROM AUTHOR]

Group 6 metals, mesoionic carbene, metastable compounds, photochemistry, reversible reactions, time-resolved FTIR spectroscopy Keywords: group 6 metals; mesoionic carbene; metastable compounds; photochemistry; reversible reactions; time-resolved FTIR spectroscopy EN group 6 metals mesoionic carbene metastable compounds photochemistry reversible reactions time-resolved FTIR spectroscopy 1 1 1 09/15/22 20220912 NES 220912 B Kinetic and mechanistic investigations b of the photo-induced reversible binding and release of CO to Cr SP 0 sp , Mo SP 0 sp and W SP 0 sp complexes with pyridyl-mesoionic carbene ligands ([LM(CO) SB 4 sb ]) are reported. Front Cover: Mechanistic and Kinetic Investigations of ON/OFF (Photo)Switchable Binding of Carbon Monoxide by Chromium(0), Molybdenum(0) and Tungsten(0) Carbonyl Complexes with a Pyridyl-Mesoionic Carbene Ligand (Chem. Eur. J. 51/2022). [Extracted from the article]

1,2,3-Triazole derivatives containing a rigid dihydroanthracenyl skeleton are suitable precursors for both organometallic and organo-based catalysts. A Rh-carbene complex and the triazolium salt efficiently catalyzed the 1,4-additions of C- and heterodonor reagents to α,β-unsaturated carbonyl substrates, respectively. [ABSTRACT FROM AUTHOR]

Abstract: Copper(I)-catalyzed azide alkyne cycloaddition is now a widely used tool for the synthesis of elaborated compounds. Until now, few stable and efficient copper(I) catalysts are available despite the limitations inherent to approaches employing the in situ reduction of a CuII species to deliver the catalytic system. We report that the combination of a copper(I)-mesoionic N-heterocyclic carbene (MIC) with a phenanthroline derivative generates a highly active catalyst, functioning in aqueous alcoholic solvent, without the intervention of a reducing agent. This catalytic system surpasses related ‘normal’ N-heterocyclic carbene-based systems in their efficiency. [Copyright &y& Elsevier]

Synopsis : A series of half-sandwich RuⅡ complexes bearing carboxylato-anchored normal and abnormal NHCs (NHCs = N -heterocyclic carbenes) have been synthesized. The abnormal carbene species have exhibited superior catalytic performance compared to their normal carbene counterparts. A protonation-induced substrate binding/activation mechanism has been proposed. [Display omitted] Functionalized abnormal/mesoionic NHC (NHC = N -heterocyclic carbene) complexes have been much less investigated in contrast to their normal NHC counterparts. In this work, a series of [C carbene ^O carboxylate ]- and [C carbene ^C' phenyl ]-type Ru(II) complexes bearing normal and abnormal NHCs have been synthesized. Their spectroscopic features, solid-state structure and aquation reactivity have also been reported. All the as-synthesized complexes have served as precatalysts to catalyze intramolecular or intermolecular carboxylic acid-to-alkyne addition reactions. It was found that the catalytic behavior of abnormal NHC complexes is superior to that of the normal NHC analogs. An anti -Markovnikov addition reactivity, leading to the formation of an E-type enol ester as the major product, was observed. A few stoichiometric experiments have also been carried out to provide preliminary insight into the catalytic mechanism. [ABSTRACT FROM AUTHOR]

A novel PEPPSI (pyridine enhanced precatalyst preparation stabilization and initiation) type palladium(II) complex has been synthesized and fully characterized by NMR, HR-MS, as well as elemental analysis. The molecular structure was finally confirmed by single crystal X-ray diffraction, and it revealed that the pyridyl-armed 1,2,3-triazole ligand is coordinated to the palladium in bidentate fashion. This air and moisture insensitive complex has proven to be a suitable catalyst for the arylation of pyridine N -oxides and 2-bromopyrroles with a broad functional group tolerance, demonstrating that the robustness and high efficiency of our complex. Unlabelled Image • Design and synthesis of a PEPPSI type bidentate palladium complex. • The complex was applied to the arylation of pyridine N -oxides and bromopyrroles. • Mercury poisoning test was performed. • Pd(0) nanoparticles were generated in situ as highly catalytically active species. [ABSTRACT FROM AUTHOR]