Search

International audience; New Blechert-type ruthenium complexes containing cyclic(alkyl)(amino)carbene (CAAC) ligands are reported. These catalysts demonstrate remarkable thermal stability in solution and excellent catalytic performances at low catalytic loading (up to 0.005 mol %) in ring-closing metathesis (RCM), macro-RCM, ring-closing enyne metathesis (RCEYM), cross-metathesis (CM), ethenolysis and ring-opening cross metathesis (ROCM). Moreover, up to 95% enantiomeric excess (ee) was obtained in asymmetric ring-opening cross metathesis (AROCM) and 57% ee was obtained in asymmetric cross-metathesis (ACM).

International audience; The development of highly efficient enantioselective olefin metathesis catalysts is a significant challenge. Using optically pure chiral cyclic (alkyl)(amino)carbene (ChiCAAC) ligands combined with preliminary mechanistic insights and density functional theory (DFT) computations, we show that catalytic performances in this field can be impaired by the formation of rotamers before the enantio-determining step. Using DFT, we also demonstrate that these results can help accelerate the process of ligand discovery by providing faster methods to discriminate potential candidates.

International audience; The synthesis and characterization of cyclic (alkyl)(amino)carbene, bicyclic (alkyl)(amino)carbene and cyclic (amino)(barrelene)carbene copper (I) complexes is reported. These complexes are obtained via a scalable, versatile, green, and cost/user-friendly strategy.

International audience; The first Z-stereoselective catechodithiolate ruthenium complexes containing cyclic(alkyl)(amino)carbene ligands are reported. Isolated in nearly quantitative yields or in situ generated, these catalysts demonstrated remarkable Z selectivity (Z/E ratio up to >98/2) in ring-opening metathesis polymerization (ROMP), ring-opening-cross metathesis (ROCM) and cross-metathesis (CM). Thanks to the efficient chiral HPLC resolution of racemic CAAC-complex precursors, optically pure dithiolated complexes were also synthesized allowing to produce enantioenriched Z-ROCM products in >99/1 Z/E with good levels of enantioselectivity.

Diapycnal mixing shapes the distribution of climatically important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation (AMOC). The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW, neutral density γn ≃ 27.6–28.15), with a diapycnal circulation of 0.5–8 Sv between 48°N and 32°S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to a vertical displacement of hundreds of meters in the vertical. This result, confirmed with a zonally averaged numerical model of the AMOC, indicates that mixing can alter where tracers upwell in the Southern Ocean, ultimately affecting their global pathways and ventilation timescales. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change. Key Points The cross-density mixing of water and tracers is quantified from observation-based estimates and numerical simulations in the Atlantic Ocean A net 0.5–8 Sv of North Atlantic Deep Water upwells diapycnally in the Atlantic Ocean (48°N–32°S), comprised of larger regional up/downwelling fluxes Tracer mixing in the deep Atlantic Ocean can significantly modify pathways and ventilation rates of tracers upwelling in the Southern Ocean Plain Language Summary The Atlantic Ocean meridional overturning circulation plays a key role in regulating the global heat and carbon budgets by inter-hemispheric transport of anthropogenic and natural tracers as well as water masses. While most of this transport occurs along nearly horizontal density surfaces in the ocean interior, vertical transport across density levels is key to bringing deep waters back to the surface. Such cross-density transport is facilitated mainly by the internal waves breaking into turbulence and near boundary processes. This work employs a host of observation-based estimates of turbulence in the Atlantic Ocean to (a) better quantify the contribution of cross-density mixing to the inter-hemispheric Atlantic circulation, and (b) discuss the potential implications for pathways and residence times of tracers carried from the North Atlantic to the Southern Ocean. This work calls for a more careful representation of turbulence-induced vertical mixing within the Atlantic Ocean in climate models to better understand and project the ongoing climate change.

International audience; An air-stable (amino)(amido)radical was synthesized by reacting a cyclic (alkyl)(amino)carbene with carbazoyl chloride, followed by one-electron reduction. We show that an adjacent radical center weakens the amide bond. It enables the amino group to act as a strong acceptor under steric contraint, thus enhancing the stabilizing-capto-dative effect.

International audience

International audience; An expedient access to the first optically pure ruthenium complexes containing C(1)-symmetric cyclic (alkyl)(amino)carbene ligands is reported. They demonstrate excellent catalytic performances in asymmetric olefin metathesis with high enantioselectivities (up to 92% ee). Preliminary mechanistic insights provided by density functional theory models highlight the origin of the enantioselectivity.

International audience; The precise preparation of monodisperse nanomaterials is among the most fundamental tasks in inorganic synthesis and materials science. Achieving this goal by galvanic exchange is hardly predictable and often results in major structural changes and polydisperse mixtures. Taking advantage of the enhanced stability imparted by ambiphilic carbenes, we report and rationalize the absolute templating, the complete exchange of metals in a template, of group 11 clusters across the entire coinage metal family by means of galvanic exchange. We further delineate that these species provide a molecular model for better understanding the reduction of CO 2 at M(111) coinage metal surfaces.

International audience; Cyclic (alkyl)- and (aryl)-(amino)carbenes (CAACs and CAArCs) are stronger σ-donors and π-acceptors than imidazol-2-ylidenes and imidazolidin-2-ylidenes, the well-known N-heterocyclic carbenes (NHCs). Consequently, they form strong bonds with coinage metals and stabilize both low and high oxidation states. This Review shows that CAACs and CAArCs have allowed for the isolation of copper and gold complexes that were believed to be only transient intermediates. This has not only allowed for a better understanding of the mechanism of known processes but has also led to the development of novel coinage metal-catalyzed reactions. In addition to their role in homogeneous catalysis, CAAC and CAArC coinage metal complexes have recently found applications in medicinal chemistry, as well as in materials science. When possible, the performance of CAAC and CAArC ligands are compared with those of classical NHCs.

International audience; Cyclic (Alkyl)(Amino)Carbenes (CAACs) have become forceful ligands for gold due to their ability to form very strong ligandmetal bonds. Inspired by the success of Auranofin and other gold complexes as antitumor agents, we have studied the cytotoxicity of bis-and mono-CAAC-gold complexes on different cancer cell lines: HeLa (cervical cancer), A549 (lung cancer), HT1080 (fibrosarcoma) and Caov-3 (ovarian cancer). Further investigations aimed at elucidating their mechanism of action are described. This includes quantification of affinities for TrxR, evaluation of their bioavailability and determination of associated cell death process. Moreover, Transmission Electron Microscopy (TEM) was used to study morphological changes upon exposure. Noticeably, a significant reduction in non-specific binding to serum proteins was observed with CAAC complexes when compared to Auranofin. These results confirm the potential of CAAC-gold complexes in biological environments, which may result in more specific drug-target interactions and decreased side effects.

Chiral acetyl-protected aminoalkyl quinoline (APAQ) ligands were recently discovered to afford highly active and enantioselective palladium catalysts for the arylation of methylene C(sp3)-H bonds, and herein, we investigate the origins of these heightened properties. Unprecedented amide-bridged APAQ-Pd dimers were predicted by density functional theory (DFT) calculations and were confirmed by single-crystal X-ray diffraction studies. Comparison of structural features between APAQ-Pd complexes and an acetyl-protected aminoethylpyridine APAPy-Pd complex strongly suggests that the high activity of the former originates from the presence of the quinoline ring, which slows the formation of the off-cycle palladium dimer. Furthermore, steric topographic maps for a representative subset of monomeric, monoligated palladium complexes allowed us to draw a unique parallel between the three-dimensional structures of these catalysts and their reported asymmetric induction in β-C(sp3)-H bond arylation reactions. Finally, cooperative noncovalent interactions present between the APAQ ligand and the substrate were identified as a crucial factor for imparting selectivity between chemically equivalent methylenic C(sp3)-H bonds prior to concerted metalation deprotonation activation.

Summary H/D exchange at formyl groups is the most direct approach for the synthesis of deuterated aldehydes. Platinum-group metal complexes have been employed to catalyze this transformation, with significant substrate scope limitations. Although N-heterocyclic carbenes can also activate the C–H bond of aldehydes through the formation of Breslow intermediates, benzoin condensation and other C–C-bond-forming pathways have so far outpaced synthetically useful H/D exchange. Investigation of the reaction of aldehydes with 1,2,3-triazolylidenes has revealed the reversible formation of Breslow intermediates and the inhibition of the condensation steps in methanol solvent. 1,2,3-Triazolylidenes catalyze the H/D exchange of aryl, alkenyl, and alkyl aldehydes in high yields and deuterium incorporation levels using deuterated methanol as an affordable D source. The unique properties of these mesoionic carbenes (MICs) enable a streamlined preparation of deuterated synthetic intermediates and pharmacophores that are highly valuable as mechanistic and metabolic probes.

It has been previously demonstrated that stable singlet electrophilic carbenes can behave as metal surrogates in the activation of strong E-H bonds (E = H, B, N, Si, P), but it was believed that these activations only proceed through an irreversible activation barrier. Herein we show that, as is the case with transition metals, the steric environment can be used to promote reductive elimination at carbon centers.

The coordination of Ge9 Zintl clusters at (carbene)CuI moieties is explored, and the complexes [(CAAC)Cu]2[η3-Ge9{Si(TMS)3}2] (1), (CAAC)Cu[η3-Ge9{Si(TMS)3}3] (2), and (MIC)Cu[η3-Ge9{Si(TMS)3}3] (3) are compared with their known N-heterocyclic carbene (NHC) derivatives (A and B), where CAAC = cyclic (alkyl)amino carbene, MIC = mesoionic carbene, and TMS = trimethylsilane. In analogy to the NHC derivatives, the synthesis of 1-3 proceeds by single-step reactions of (CAAC)CuCl or (MIC)CuCl with the [Ge9R2]2- and [Ge9R3]- [R = Si(TMS)3] cluster ligands, respectively, and yields complexes of (carbene)CuI (carbene = CAAC, MIC) moieties exhibiting η3-coordination modes of the Ge9 deltahedron to the Cu atom. In 1, [Ge9R2]2- acts as a η3-bridging unit for two (CAAC)CuI moieties, and 2 and 3 feature single (carbene)CuI (CAAC and MIC) fragments η3-connected to [Ge9R3]- units. Analysis of the bond lengths in comparison with known examples shows a bond expansion within the coordinated Ge3 triangular faces for all (carbene)CuIGe9 complexes (carbene = NHC, MIC, CAAC). All compounds are characterized by single-crystal X-ray diffractometry, NMR spectroscopy [1H, 13C{1H}, and 29Si{1H}], electrospray ionization mass spectometry, elemental analysis (C, H, and N), and for the first time also by IR and Raman investigations (for 2 and 3). The new complexes add to the known NHC derivatives and extend the exploration of Ge9 clusters with carbene ligands at CuI atoms.

Helping copper glow Copper's abundance makes the metal an appealing candidate for luminescence applications. However, many copper complexes tend to decay nonradiatively after photoexcitation. A recently described exception involves a two-coordinate complex that sandwiches the metal between an amide ligand and a carbene ligand. Hamze et al. thoroughly explored this motif and measured a nearly perfect luminescence efficiency. They used this property to produce a prototype blue organic light-emitting diode. The photodynamics appeared largely ligand-centered, with the excited state attributed to copper-facilitated charge transfer from amide to carbene. Science , this issue p. 601

International audience; The oxidation of the Breslow intermediate resulting from the addition of an N-heterocyclic carbene (NHC) to ben-zaldehyde triggers a fast deprotonation, followed by a second electron transfer, directly affording the corresponding acylium at E >-0.8 V (versus Fc/Fc+). Similarly, the oxidation of the cinnamaldehyde analogue occurs at an even higher potential and is not a reversible electrochemical process. As a whole, and contrary to previous beliefs, it is demonstrated that Breslow intermediates, which are the key intermediates in NHC-catalyzed transformations of aldehydes, cannot undergo a Single Electron Transfer (SET) with mild oxidants (E

Primary aminoboranes (RNHBR2 ), which are readily available by spontaneous dehydrocoupling of amines and boranes cleanly react at room temperature with aldehydes to give aldimines. The overall transformation from amines to aldimines can be conveniently performed by a sequential one-pot reaction. This synthetic strategy is especially useful for electron poor and bulky amines which are reluctant to react with aldehydes under dehydration conditions. Using a Glorius robustness screen, we show that this methodology is chemoselective, and functional group tolerant. Computational and experimental data support the irreversible formation of the aldimine product in marked contrast with traditional methods.

Classical carbenes are usually described as neutral compounds featuring a divalent carbon with only six electrons in their valence shell. It was only in 1988 that our group prepared the first isolable example, in which the carbene center was stabilized by a push pull effect, using a phosphino and a silyl substituent. In the last 30 years, a myriad of acyclic and cyclic push pull and push push carbenes, bearing different heteroatom substituents, have been isolated. Among them, the so-called N-heterocyclic carbenes (NHCs), which include cyclic (alkyl)(amino)carbenes (CAACs), are arguably the most popular. They have found a vast number of applications ranging from catalysis to material science, and even in medicine. In this Account, we focus on the synthesis, structure, electronic properties, coordination, and applications of a different class of stable cyclic carbenes, namely, 1H-1,2,3-triazol-5-ylidenes. In contrast with NHCs and CAACs, these compounds have no reasonable canonical resonance forms that can be drawn showing a carbene without additional charges. According to the IUPAC, they belong to the family of mesoionic compounds and thus they are named mesoionic carbenes (MICs). In 2010, we prepared the first stable 1,2,3-triazol-S-ylidene, via a CuAAC reaction, followed by alkylation of the resulting 1,2,3-triazole, and deprotonation. Later, we synthesized more robust N3-arylated counterparts from 1,3-diarylated-1H1,2,3-triazolium salts. Both synthetic routes can be carried out in multigram scales, making these MICS readily available. Importantly, MICs do not dimerize which contrasts with NHCs that can give the corresponding Wanzlick-type olefin. This property leads to relaxed steric requirements for their isolation; even C-unsubstituted MICs can be stored for months in the solid state at room temperature. The practicality and easily scalable syntheses of MICs allow for the preparation of polycarbenes, such as bis(1,2,3-triazol-5-ylidenes) (i-bitz), the analogues of the well-known 2,2'-bipyridines (bpy). MIC-transition metal complexes are excellent precatalysts for variety of chemical transformations, which include hydrohydrazination of alkynes, olefin metathesis, reductive formylation of amines with carbon dioxide and diphenylsilane, hydrogenation and dehydrogenation of N-heteroarenes in water, cycloisomerization of enynes, asymmetric Suzuki Miyaura cross-coupling, and water oxidation (WO) reactions. Besides their catalytic applications, MIC transition metal complexes have found applications in material sciences as exemplified by the preparation of the first iron(III) complex that is luminescent at room temperature. The peculiar properties of mesoionic triazolylidenes, combined with their enhanced stability, position them as excellent candidates to address some current challenges such as access to high-oxidation-state 3d metal complexes, the stabilization of highly reactive main group elements, the stabilization of nanoparticles, the preparation of efficient catalysts and photosensitizers based on earth-abundant transition metals, and the functionalization of self-assembled monolayers (SAMs) on gold.

The reduction of CO2 into formic acid or its conjugate base, using dihydrogen, is an attractive process. While catalysts based on noble metals have shown high turnover numbers, the use of abundant first-row metals is underdeveloped. The key steps of the reaction are CO2 insertion into a metal hydride and regeneration of the metal hydride with H2, along with the concomitant production of formate. For the first step, copper is known as one of the most efficient metals, as shown by the numerous copper-catalysed carboxylation reactions, but this metal has difficulties activating H2 to achieve the second step. Here, we report a catalytic system involving a stable copper hydride that activates CO2, working in tandem with a Lewis pair that heterolytically splits H2. In this system, unprecedented turnover numbers for copper are obtained. Surprisingly, through a combination of stoichiometric and catalytic reactions, we show that classical Lewis pairs outperform frustrated Lewis pairs in this process. Due to its ready availability and low cost, copper is an attractive metal for the homogeneous reduction of CO2 to formate. However, although CO2 can readily insert into copper hydrides to produce metal-bound formate, subsequent regeneration of the catalytic species with H2 is more challenging. Here a dual strategy is used, whereby a copper hydride activates CO2 and a Lewis pair heterolytically splits H2, leading to dramatically improved performance.

Cyclic (alkyl)(amino)carbenes with a six-membered backbone were prepared. Compared to their five-membered analogues, they feature increased % Vbur and enhanced donor and acceptor properties, as evidenced by the observed n → π* transition trailing into the visible region. The high ambiphilic character even allows for the intramolecular insertion of the carbene into an unactivated C(sp3)-H bond. When used as ligands, they outcompete the five-membered analogues in the palladium-mediated α-arylation of ketones with aryl chlorides.

A phosphino-arsinidene derived from a diazaphospholidine scaffold (Ar**NCH2 )2 PAs (where Ar**=2,6-bis[(4-tert-butylphenyl)methyl]-4-methylphenyl) was generated in situ and trapped with a range of reagents. Metathesis reactions between (Ar**NCH2 )2 PCl and either [Na(18-crown-6)][AsCO] or [Na(dioxane)x ][AsCO] are believed to afford an arsaketene, (Ar**NCH2 )2 PAsCO, which readily loses carbon monoxide under ambient conditions. The resulting transient arsinidene was shown to undergo cycloaddition reactions with excess AsCO- , but can be stabilised in the presence of Lewis bases. It displays contrasting reactivity from a previously reported phosphino-phosphinidene. With isonitriles, rearrangements occur, however with PPh3 a phosphine-stabilised arsinidene could be isolated and used for subsequent reactivity studies. This species was found to be a suitable starting material for a transition metal complex of the phosphino-arsinidene, in which the P-As moiety coordinates side-on to the metal centre.

Readily available room temperature stable organic mixed valence compounds are prepared by one-electron reduction of cyclic bis(iminium) salts [derived from cyclic (alkyl)(amino)carbenes] bridged by various spacers. These compounds show characteristic intervalence charge transfer (IV-CT) bands in the near-infrared (NIR). Cyclic voltammetry, EPR, IR, UV-vis, and X-ray studies, as well as DFT calculations, show that, depending on the nature of the spacer, these mixed valence compounds range from class III to class II.

A modular approach for the synthesis of Kekulé diradicaloids is reported. The key step is the insertion of a carbene, namely, a cyclic (alkyl)(amino)carbene (CAAC), into the C-H bonds of two terminal alkynes linked by a spacer. Subsequent hydride abstraction, followed by two-electron reduction of the corresponding bis(iminium) salts, affords the desired diradicaloids. This synthetic route readily allows for the installation of communicating spacers, featuring different degrees of aromaticity and lengths, and gives the possibility of generating unsymmetrical compounds with two different CAACs. Electron paramagnetic resonance (EPR), NMR, UV-vis, and X-ray studies in combination with quantum-chemical calculations give insight into the electronic nature of the deeply colored Kekulé diradicaloids. They feature a singlet ground state with varying degrees of diradical character in combination with small singlet/triplet gaps. Upon lengthening of the spacer, the properties of the compounds approach those of monoradicals in which steric protection of the propargyl radical moiety is necessary to inhibit decomposition pathways. Most of these diradicaloids are stable at room temperature, both in solution and in the solid state, but are highly oxygen-sensitive. They represent the first diradicaloids derived from iminium salts.

By flanking a carbene carbon with two substituents, it is possible to synthesize persistent triplet carbenes and isolable singlet carbenes. Isolable singlet carbenes are among the most powerful tools in chemistry, and they have even found medicinal and materials science applications. Between the rich chemistry of disubstituted carbenes and the transient parent carbene are the monosubstituted carbenes that, so far, have only been observed in matrices at very low temperatures of just a few K. Herein, we describe the synthesis of a crystalline monosubstituted carbene. The key for isolating such a species was to design the correct substituent, namely a benzo[c]pyrrolidino heterocycle, which can single-handedly tame the intrinsic tendency of carbenes towards dimerization. The π-donor ability of the nitrogen atom, coupled with the steric bulk of chemically inert substituents at the two adjacent quaternary carbons, make these scaffolds very attractive for the isolation of a variety of other hitherto elusive electron-deficient species.

Alkynes usually oligomerize to give rings with a conjugated π-electron system. In contrast, phosphaalkynes, R-C≡P, frequently give compounds with polycyclic structures, which are thermodynamically more stable than the corresponding π-conjugated isomers. The syntheses of the first CP tricyclic compounds are reported with either radical or cationic ground states stabilized by cyclic (alkyl)(amino)carbenes (CAACs). These compounds may be considered as examples of tricarbontriphosphide coordinated by carbenes and are likely formed via trimerization of the corresponding mono-radicals CAAC-CP. The mechanism for the formation of these tricarbontriphosphide radicals has been rationalized by a combination of experiments and DFT calculations.

International audience; Photolysis of [ bis(dicyclohexylamino)phosphanyl](trimethylsilyl)diazomethane leads to the corresponding stable carbene 1 b, which reacts with triethylborane affording a borane-carbene adduct 2b, characterized by spectroscopy in solution before rearrangement and fragmentation into f-dicyclohexylamino-C-(ethyl)(trimethylsilyl)phosphaalkene 3b; trimethylsilyltrifluoromethanesulfonate adds to l b giving the methylenephosphonium 4b. The formation of transient adducts of electrophilic carbenes with Lewis bases is well documented,' but there are only two reports of main group 3 adducts of carbene,2 including a stable alane-carbene complex.2a We have shown that the stable [bis(diisopropylamino)phosphanyl] (trimethylsily1)carbene la3 presents both carbene and phosphorus-carbon multiple bond reactivity, while calculations4 concluded that l a was best formulated as a phosphorus vinyl ylide. Since classical phosphorus ylides are known to react at carbon with Lewis acids giving stable adducts,S it was tempting to try to prepare a phosphanylcarbene-borane complex. Monitoring the reaction of triethylborane with [bis(diisopropylamino)phosphanyl](trimethylsilyl)carbene l a in toluene solution at-80°C by 31P NMR spectroscopy, two new signals were observed at 8 +116.0 (2a) and 249.4 (3a). After a few minutes, the high-field signal disappeared, and after work-up, phosphaalkene 3at and (diethyl)(diisopropylt Selected spectroscopic data for: lb, 2b, 3a, 3b and 4b. lb: 31P NMR (C6D6) 6-31.4; 29si NMR (c6D6) 6-19.7 (d, Jpsi 59.3HZ); l3C NMR (c6D6) 6 3.9 (d, J p c 10.