Protonolysis of a Ruthenium–Carbene Bond and Applications in Olefin Metathesis

Bouffard J., Keitz B.K., Tonner R., Guisado-Barrios G., Frenking G., Grubbs R.H., Bertrand G.

The formal cycloaddition between 1,3-diaza-2-azoniaallene salts and alkynes or alkyne equivalents provides an efficient synthesis of 1,3-diaryl-1H-1,2,3-triazolium salts, the direct precursors of 1,2,3-triazol-5-ylidenes. These N,N-diarylated mesoionic carbenes (MICs) exhibit enhanced stability in comparison to their alkylated counterparts. Experimental and computational results confirm that these MICs act as strongly electron-donating ligands. Their increased stability allows for the preparation of ruthenium olefin metathesis catalysts that are efficient in both ring-opening and ring-closing reactions.

Dunbar M.A., Balof S.L., Roberts A.N., Valente E.J., Schanz H.

The new hexacoordinate catalysts (PCy3)(DMAP)2Cl2Ru═CH(p-C6H4)CH2NMe2 (7) and (PCy3)(DMAP)2Cl2Ru═CH(p-C6H4)N(CH3)2 (8) have been synthesized via exchange of a PCy3 ligand for two DMAP ligands from their (PCy3)2Ru precursors 4 and 5 in a one-step reaction in high yield. The catalysts promoted controlled ROMP of two cationic exo-7-oxanorbornene derivatives under homogeneous conditions in various acidic protic media, including acidic aqueous media. Very low polydispersities were accomplished in TFE/0.1 M HClaq. (1:1 v/v) with PDIs as low as 1.05.

Melaimi M., Soleilhavoup M., Bertrand G.

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Guisado-Barrios G., Bouffard J., Donnadieu B., Bertrand G.

In 2001, Crabtree and co-workers first reported complex A, which features an imidazole ring bound at the C5 position (III), and not at C2 as commonly observed.[7] More recently, Huynh and co-workers[8] and Albrecht and co-workers[9a] showed that pyrazolium and 1,2,3-triazolium salts can serve as precursors to metal complexes of type B and C, which feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V as the ligand, respectively. As a consequence of their lineage, these have also been referred to as N-heterocyclic carbenes (NHCs). However, as no reasonable canonical resonance forms containing a carbene can be drawn for free ligands III–V without additional charges (see V′), these ligands have been described as abnormal or remote carbenes (aNHCs or rNHCs, respectively).[10] As they are, in fact, mesoionic compounds,[11] we suggest naming this family of compounds mesoionic carbenes (MICs). There have been no reported dimerizations of MICs III and IV, which suggests that the Wanzlick equilibrium pathway for classical carbenes is disfavored;[12] this observation should lead to relaxed steric requirements for their isolation. Moreover, experimental and theoretical data suggest that MICs III–V are even stronger electron-donating species than NHCs I and II, which opens up interesting perspectives for their applications.[10] Our recent success in the isolation of a free imidazol-5-ylidene III[13] and pyrazolin-4-ylidenes IV (cyclic bent allenes),[14,15] prompted us to investigate the possibility of preparing new types of stable neutral compounds that feature a lone pair of electrons on the carbon atom.[16] Preliminary calculations (B3LYP, 6–311G(d,p); for details, see the Supporting Information) predicted that the parent MIC V is located at an energy minimum, about 32 kcalmol−1 above the regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore, parent V is predicted to exhibit an appreciably large singlet–triplet band gap (56 kcalmol−1), which is a good predictor of carbene stability and thus of possible isolation. Herein, we report the preparation, isolation, and characterization of two free 1,2,3-triazol-5-ylidenes of type V. By analogy with the synthetic route used for preparing NHCs and the related species III and IV, 1,2,3-triazolium salts (2a,b) were targeted as precursors for the desired 1,2,3-triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl substituent (2,6-diisopropylphenyl, Dipp) was selected to provide kinetic stabilization to the ensuing free ligand. 1,2,3-Triazole 1 was obtained in 83% yield from the copper-catalyzed azide–alkyne cycloaddition (CuAAC, click chemistry) of 2,6-diisopropylphenyl azide and phenylacetylene.[17] The one-pot conversion of aniline into the desired aryl azide, followed in situ by CuAAC as reported by Moses and co-workers[18] was found to be especially convenient for the synthesis of 1. Alkylation of 1 with methyl or isopropyl trifluoromethanesulfonate afforded the corresponding tri-azolium salts in moderate to excellent yields (2a and 2b, respectively; Scheme 2). Scheme 2 Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b. Potassium bases have been identified as the reagents of choice for the depronation of carbene precursors, as they avoid the formation of stable carbene–alkali-metal adducts that are commonly encountered when lithium bases are used.[12,13,14a,19] Gratifyingly, triazolium salts 2a,b were cleanly deprotonated with either potassium bis(trimethylsilyl)amide or potassium tert-butoxide in ethereal solvents to afford the corresponding MICs Va and Vb in 55 and 39% yield, respectively. Deprotonation was evidenced by the disappearance of the triazolium CH signal in their 1H NMR spectra (2a: δ =8.62 ppm; 2b: δ =8.85 ppm) and the appearance of a signal at low field in the 13C NMR spectrum (Va: δ = 202.1 ppm; Vb: δ =198.3 ppm). The structure of Va was unambiguously confirmed by X-ray crystallography (Figure 1).[20] In the solid state, Va contains a planar heterocycle, characterized by bond lengths that are intermediate between those of single and double bonds; both of these features are indicative of electronic delocalization. Upon deprotonation, the C5 carbon bond angle becomes more acute (2a: 106°; Va: 100°), which is consistent with an increased s character in the σ lone pair orbital of Va compared to the C–H bonding orbital of the precursor 2a. This is in agreement with the generally observed trend for carbenes and their conjugate acids.[5] Figure 1 Molecular views (thermal ellipsoids set at 50 % probability) of 2a (top) and Va (bottom) in the solid state. For clarity, counter ions, solvent molecules, and H atoms are omitted, except for the ring hydrogen of 2 a. Selected bond lengths [A] ... In the solid state, with the exclusion of oxygen and moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–52°C decomp.) remained stable for several days at −30°C and for a few hours at room temperature. By contrast, Vb (m.p. 110–112°C) was significantly more stable, showing no sign of decomposition after three days at room temperature in the solid state. Upon heating in a benzene solution for 12 hours at 50°C, Va decomposed to give, among other products, triazole 3 (Scheme 3; for details, see the Supporting Information). We surmise that the latter product results from a nucleophilic attack of the carbon lone pair of Va on the methyl group of a second molecule of Va, giving rise to heterocycles 4 and 5, which react together to afford the observed product 3. This apparent rearrangement is reminiscent of that recently observed in the formation of imidazol-2-ylidenes of type I from imidazol-5-ylidenes of type III that contain an electro-philic Y group.[21] In agreement with this hypothesis, MIC Vb, which contains the less-electrophilic isopropyl group at the N3 position, appears much more robust with respect to this decomposition pathway. Scheme 3 Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy with the rearrangement of III into I. To evaluate the donor properties of 1,2,3-triazol-5-yli-denes, the [(Va)Ir(CO)2Cl] complex was prepared by addition of Va to [{Ir(cod)Cl}2] (cod = 1,5-cyclooctadiene), followed by treatment with an excess of carbon monoxide. The CO vibration frequencies (ν =2061 and 1977 cm−1; νavg = 2019 cm−1) are in line with those of the analogous iridium complex, previously reported by Albrecht and co-workers (νavg = 2021 cm−1),[9a] and are indicative of donor properties that are superior to those of NHCs I and II (νavg = 2022–2031 cm−1),[22] but inferior to those of MICs III (νavg = 2003–2006 cm−1)[23] and IV (νavg = 2002 cm−1).[14b] Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by compounds Va,b, possess an ensemble of properties that portend to their utility. The synthesis of their precursors is short and efficient, from readily available starting materials, yet is modular and thus amenable to a wide variety of potential analogues. As with other mesoionic carbenes III and IV, the dimerization of MICs of type V has not been observed; therefore, the preparation of comparatively unhindered MICs is predicted to be viable. Their donor properties are greater than those of NHCs of type I and II, but they are nonetheless available by deprotonation using mild bases (e.g. alkoxides), thus signaling their potential for applications, such as nucleophilic organocatalysis. Free triazolylidenes V complement the rapidly growing numbers of neutral carbon-based κ1C ligands that are now available. We predict that many other classes of MICs, that are derived from a variety of heteroaromatic scaffolds, can be isolated. This endeavor is currently the object of ongoing efforts in our laboratory.

Fu C., Lee C., Liu Y., Peng S., Warsink S., Elsevier C.J., Chen J., Liu S.

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by (1)H and (13)C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd-C((saturated NHC)) is distinct from that of Pd-C((unsaturated NHC)). The Pd-C((saturated NHC)) bonds are fairly stable toward reagents such as CF(3)COOH, AgBF(4) and I(2), whereas Pd-C((unsaturated NHC)) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki-Miyaura coupling follows the order: (sat-NHC)(2)PdCl(2) > (sat-NHC)(unsat-NHC)PdCl(2 )> (unsat-NHC)(2)PdCl(2).

Leitao E.M., van der Eide E.F., Romero P.E., Piers W.E., McDonald R.

Initiation processes in a family of ruthenium phosphonium alkylidene catalysts, some of which are commercially available, are presented. Seven 16-electron zwitterionic catalyst precursors of general formula (H(2)IMes)(Cl)(3)Ru=C(H)P(R(1))(2)R(2) (R(1) = R(2) = C(6)H(11), C(5)H(9), i-C(3)H(7), 1-Cy(3)-Cl, 1-Cyp(3)-Cl, 1-(i)Pr(3)-Cl; R(1) = C(6)H(11), R(2) = CH(2)CH(3), 1-EtCy(2)-Cl; R(1) = C(6)H(11), R(2) = CH(3), 1-MeCy(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(2)CH(3), 1-Et(i)Pr(2)-Cl; R(1) = i-C(3)H(7), R(2) = CH(3), 1-Me(i)Pr(2)-Cl) were prepared. These compounds can be converted to the metathesis active 14-electron phosphonium alkylidenes by chloride abstraction with B(C(6)F(5))(3). The examples with symmetrically substituted phosphonium groups exist as monomers in solution and are rapid initiators of olefin metathesis reactions. The unsymmetrically substituted phosphonium alkylidenes are observed to undergo reversible dimerization, the extent of which is dependent on the steric bulk of the phosphonium group. Kinetic and thermodynamic parameters of these equilibria are presented, as well as experiments that show that metathesis is only initiated through the monomers; thus dedimerization is required for initiation. In another detailed study, the series of catalysts 1-R(3) were reacted with o-isopropoxystyrene under pseudo-first-order conditions to quantify second-order olefin binding rates. A more complex initiation process was observed in that the rates were accelerated by catalytic amounts of ethylene produced in the reaction with o-isopropoxystyrene. The ability of the catalyst to generate ethylene is related to the nature of the phosphonium group, and initiation rates can be dramatically increased by the intentional addition of a catalytic amount of ethylene.

Vougioukalakis G.C., Grubbs R.H.

The fascinating story of olefin (or alkene) metathesis (eq 1) began almost five decades ago, when Anderson and Merckling reported the first carbon-carbon double-bond rearrangement reaction in the titanium-catalyzed polymerization of norbornene. Nine years later, Banks and Bailey reported “a new disproportionation reaction . . . in which olefins are converted to homologues of shorter and longer carbon chains...”. In 1967, Calderon and co-workers named this metal-catalyzed redistribution of carbon-carbon double bonds olefin metathesis, from the Greek word “μeτάθeση”, which means change of position. These contributions have since served as the foundation for an amazing research field, and olefin metathesis currently represents a powerful transformation in chemical synthesis, attracting a vast amount of interest both in industry and academia.

Monsaert S., Lozano Vila A., Drozdzak R., Van Der Voort P., Verpoort F.

Olefin metathesis is a versatile synthetic tool for the redistribution of alkylidene fragments at carbon-carbon double bonds. This field, and more specifically the development of task-specific, latent catalysts, attracts emerging industrial and academic interest. This tutorial review aims to provide the reader with a concise overview of early breakthroughs and recent key developments in the endeavor to develop latent olefin metathesis catalysts, and to illustrate their use by prominent examples from the literature.

Vorfalt T., Leuthäußer S., Plenio H.

NHC with EWGs for RCM: Ruthenium complexes with two N-heterocyclic carbenes (NHCs), one of them substituted with electron-withdrawing groups (EWGs), are highly efficient (pre)catalysts for the synthesis of tetrasubstituted olefins and trisubstituted olefins by ring-closing metathesis reactions (RCM, see scheme).

Samojłowicz C., Bieniek M., Grela K.

N-Heterocyclic carbene (NHC) ligands, introduced as analogues to phosphines, are recently getting wide attention in the design of diverse homogeneous catalytic systems.1-6 During recent years, olefin metathesis has gained a position of increasing significance.7-9 The ruthenium complex (PCy3)2(Cl2)RudCHPh 1 (Cy ) cyclohexyl) developed by Grubbs et al.10 constitutes a highly efficient metathesis catalyst11 tolerating most functional groups. In spite of the generally superb application profile of 1, its limited stability and the low activity toward substituted double bonds are major drawbacks. The initial success of olefin metathesis has spurred the intense investigation of new catalysts for this transformation. Inter alia, the recent introduction of NHCs * To whom correspondence should be addressed. E-mail: klgrela@ gmail.com. † Polish Academy of Sciences. ‡ Warsaw University. Cezary Samojłowicz was born in 1983 in Sokołów Podlaski, Poland. He obtained his MSc Eng. degree in chemical technology from the Warsaw University of Technology, studying sigmatropic rearrangements of sulfur ylides under the supervision of Tadeusz Zdrojewski. Before moving to olefin metathesis, he conducted work on supramolecular chemistry with David Reinhoudt at Twente University. Since 2007, he is conducting his PhD study under the supervision of Karol Grela.

Balof S.L., Yu B., Lowe A.B., Ling Y., Zhang Y., Schanz H.

Olefin metathesis catalysts (H2ITap)(PCy3)Cl2Ru=CHPh (4) and (H2ITap)Cl2Ru=CH-(C6H4-O-iPr) (5) [H2ITap = 1,3-bis(2′,6′-dimethyl-4′-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene] were used for the ring-opening metathesis polymerization (ROMP) of exo-7-oxanorbornene derivative 7 in the presence of various amounts of acid. Upon gradual protonation of the NMe2 groups of the H2Tap ligand, the metathesis activity of both catalysts were gradually reduced due to electronic changes of the N-heterocyclic carbene (NHC) ligand donor capability. The investigation of the ROMP polymer 8, DFT calculations and measurements of the initiation kinetics prove that the reduced activity is solely due to reduced rates of propagation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Simonovic S., Whitwood A.C., Clegg W., Harrington R.W., Hursthouse M.B., Male L., Douthwaite R.E.

Mononuclear and mixed-valence CuI/CuII complexes of N-heterocyclic carbene–phenolimine ligands have been structurally characterised, including a unique Cu6 cluster. Additionally, analogous NHC–phenolamine complexes have been prepared and the catalytic activity of both complex classes studied for 1,4-conjugate addition to enones and aziridination ofalkenes.

Schuster O., Yang L., Raubenheimer H.G., Albrecht M.

N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

Díez-González S., Nolan S.

In 2001, Sharpless and co-workers defined the concept of ‘Click Chemistry’ and the criteria for a transformation to be considered as ‘Click’. Since then, the copper-catalyzed reaction of azide and alkyne to produce 1,2,3-triazoles regioselectively (1,3dipolar Huisgen cycloaddition) has become the best Click reaction to date. Thanks to its mild conditions and high efficiency, this reaction has found a myriad of applications in biology and material science. Less attention has been focused on the development of novel copper(I)-based well-defined systems and to the amount of copper used. This last point is extremely important for future industrial applications and it might be one of the last challenges to overcome for this transformation. We recently reported the remarkable activity of [(NHC)CuX] complexes (NHC = N-heterocyclic carbene; X = Cl, Br) in this cycloaddition reaction and this catalytic system has already been applied to the preparation of triazole-containing carbanucleosides, porphyrins or platinum-based anticancer drugs. On the other hand, we have also studied a family of cationic NHC-containing complexes of general formulae [(NHC)2Cu]X (X = PF6, BF4). Interestingly, during the examination of their activity in the hydrosilylation of ketones we observed an enhanced reactivity of these complexes when compared to their neutral analogues [(NHC)CuCl]. This improved activity was rationalized via a more efficient activation pathway of the cationic pre-catalyst. Additionally, the second NHC ligand was found to have an active role in the catalytic cycle. Since under hydrosilylation conditions one NHC ligand is displaced by the base in the reaction mixture, we wondered if an alkyne could play a similar role to produce the active copper acetylide species, from [(NHC)2Cu]X species (Figure 1). Figure 1. Catalyst design

P'Poo S.J., Schanz H.

The metathesis activity of Grubbs' catalyst 1 was investigated in the presence of N-donor ligands (1-methylimidazole [MIM], 4-(N,N-dimethylamino)pyridine [DMAP], pyridine, and 1-octylimidazole [OIM]). Ring opening metathesis polymerization (ROMP) reactions of cyclooctene (COE), bulk-ROMP reactions of COE and norbornadiene (NBD), and ring closing metathesis (RCM) reactions of diethyl diallylmalonate (DEDAM) were conducted containing various equivalents of N-donor with respect to catalyst. ROMP reactions could be stopped using MIM (1-5 equiv) and DMAP (2-5 equiv), and slowed with pyridine (1-5 equiv) by factors >100, in benzene solution for 24 h. The stopped reactions could be initiated with excess phosphoric acid (H3PO4), and the reactions proceeded faster than with uninhibited Grubbs' catalyst in the first 4 min after reactivation. Thereafter, the reaction proceeded at the same rate as the reaction with the uninhibited catalyst. ROMP reactions in neat COE and NBD could be inhibited for 72 h using 2 equiv of MIM, DMAP, or OIM and activated with H3PO4 to give polymer gels within minutes or less. RCM reactions could be completely inhibited with MIM (1-5 equiv), but upon treatment with H3PO4, the reaction would proceed at a fraction of the initial rate accomplished by uninhibited Grubbs' catalyst 1. A structural investigation of the inhibited species showed that MIM and DMAP completely or partially transform catalyst 1 into the hexacoordinate species 5a or 5b producing free PCy3, which additionally acts as an inhibitor for the ROMP reaction. Upon reactivation, the PCy3 is protonated along the N-donor ligand; however, over the period of 5 min, the phosphine has been found to coordinate back to the ruthenium catalyst. Therefore, the reaction slows to the same polymerization rate as the reaction using the uninhibited catalyst at this point. Complexes 5a and 5b were isolated, characterized, and employed in ROMP and RCM experiments where they exhibited very low catalytic activity.

Mauduit M., Rouen M., Del Vecchio A., Kamal F.

AbstractOlefin metathesis is a fundamental transformation in organic chemist’s toolbox. Herein we report a methodological study on a set of bench stable, latent, ruthenium-indenylidene catalysts bearing two unsymmetrical unsaturated N-heterocyclic carbene (NHC) ligands. These complexes are successfully ‘activated’ and applied to ring-closing metathesis (RCM), cross-metathesis (CM), and ring-opening cross metathesis (ROCM) upon light irradiation in the presence of an organic photoactivator. The transformations could be performed at lower catalytic loading (down to 1 mol%) compared to the state-of-the-art photochemical activation methods, providing good yields in shorter reaction time

Saha T.N., Mondal B., Mahato R., Naskar R., Hazra C.K., Maity R.

AbstractCyclometalation offers a wide number of organometallic metallacycles showing diverse applications. However, such NHC complexes synthesized via an sp3 C−H bond activation are rare. An iridium(III) complex with a chiral mesoionic N‐heterocyclic carbene (MIC) ligand, where the IrIII forms an additional Ir−C bond via a regiospecific sp3 C−H bond activation at the N‐methylbenzyl wingtip, was synthesized and characterized. To our best knowledge, this represents the first example of cyclometalated iridium(III) complex possessing a chiral MIC donor ligand. The formation of the complex was followed by 2D correlation NMR spectroscopy and the molecular formula mass was evidenced by ESI‐HRMS mass spectrometry. The molecular structure of the IrIII‐MIC complex was unambiguously established by the single crystal XRD data. This cyclometalated IrIII complex was employed in the asymmetric transfer hydrogenation of 4‐bromoacetophenone, and the complex was successful to transfer chirality to the final alcohol molecules (up to 92 % ee).

Stroek W., Albrecht M.

Triazole-derived N-heterocyclic carbenes, available via click reactions, are versatile ligands for first-row transition metals, leading to complexes with attractive photochemical properties and catalytic activity, some defining the state-of-the-art.

Race J.J., Albrecht M.

Bezuidenhout D.I., Kleinhans G., Karhu A.J.

The introduction of monoheteroatom stabilized cyclic carbenes, less than a decade after the discovery of stable acyclic and N -heterocyclic carbenes (NHCs), soon led to the development of complex ligands fulfilling more than a simple spectator role. Particularly the imidazol-4-ylidenes (Im4), cyclic (alkyl)(amino)carbenes (CAACs) and 1,2,3-triazol-5-ylidenes (trz) have emerged as ligands for transition metal complexes that demonstrate a noninnocent role. The advancements of these nonclassical N-heterocyclic carbenes as noninnocent ligands are reviewed in this contribution. A short overview of the unique (stereo)electronic features of these classes of carbenes is provided. The focus of this study is however on the impact of Im4, CAACs and trz on the metal complex reactivity in catalytic applications where the ligands exhibit a chemically or a redox noninnocent role in cooperative, bifunctional or redox-switchable catalysis. The introduction of ligand-based chirality and the role played in chiral induction are included, as well as the effect of multinuclear cooperativity in polymetallic complexes of these nonclassical carbenes. The extension of these ligands to macromolecular systems, including surface functionalization for catalytic or material applications, are showcased. Finally, the potential for the nonclassical carbenes as noninnocent ligands in non-catalytic applications, such as light emitting and biological applications, are highlighted.

Wu M., He Y., Zhang L., Wei R., Wang D., Liu J., Leo Liu L., Tan G.

Mukherjee N., Mondal B., Saha T.N., Maity R.

Maity R., Sarkar B.

Mesoionic carbenes (MICs) of the 1,2,3-triazolylidene type have established themselves as a popular class of compounds over the past decade. Primary reasons for this popularity are their modular synthesis and their strong donor properties. While such MICs have mostly been used in combination with transition metals, the past few years have also seen their utility together with main group elements. In this paper, we present an overview of the recent developments on this class of compounds that include, among others, (i) cationic and anionic MIC ligands, (ii) the donor/acceptor properties of these ligands with a focus on the several methods that are known for estimating such donor/acceptor properties, (iii) a detailed overview of 3d metal complexes and main group compounds with these MIC ligands, (iv) results on the redox and photophysical properties of compounds based on MIC ligands, and (v) an overview on electrocatalysis, redox-switchable catalysis, and small-molecule activation to highlight the applications of compounds based on MIC ligands in contemporary chemistry. By discussing several aspects from the synthetic, spectroscopic, and application point of view of these classes of compounds, we highlight the state of the art of compounds containing MICs and present a perspective for future research in this field.

Wei W., Jia G.

The coordination chemistry of ruthenium, osmium, technetium and rhenium with carbon-based ligands covering the literature from 2003 to 2018 is discussed. These metals form a vast number of coordination compounds with carbon-based ligands ranging from a single carbon atom (a carbido ligand) to polydentate hydrocarbon chains, and from monodentate organometallic ligands such as carbenes to multidentate ligands such as pincers and macrocycles. Particular focus of this review is given to syntheses, properties and applications of complexes with popular carbon-based ligands such as monodentate N-heterocyclic carbenes (NHCs), cyclometallated bidentate ligands, tridentate pincers and porphyrin-like macrocycles.

Morvan J., Mauduit M., Bertrand G., Jazzar R.

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) have led to numerous discoveries in the field of ruthenium olefin metathesis, until then largely dominated by the well-known N-heterocyclic...

Byun S., Park S., Choi Y., Ryu J.Y., Lee J., Choi J., Hong S.

Fluorinated imidazo[1,5-a]pyridine abnormal carbene ruthenium complexes (F-aImPy–Ru) with tricyclohexylphosphine copper chloride (Cy3P–CuCl) cocatalyst were developed for the ethenolysis/propenolys...

Mollar‐Cuni A., Byrne J.P., Borja P., Vicent C., Albrecht M., Mata J.A.

This is the pre-peer reviewed version of the following article: Selective conversion of various monosaccharaides into sugar acids by additive‐free dehydrogenation in water, which has been published in final form at https://doi.org/10.1002/cctc.202000544. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.

Suntrup L., Stein F., Klein J., Wilting A., Parlane F.G., Brown C.M., Fiedler J., Berlinguette C.P., Siewert I., Sarkar B.

Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three ReI complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The ReI compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO2 to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive ReI complexes and for electrocatalytic CO2 reduction.

Kamal F., Colombel-Rouen S., Dumas A., Guégan J., Roisnel T., Dorcet V., Baslé O., Rouen M., Mauduit M.

The activation of latent ruthenium complexes containing two unsymmetrical NHCs by copper and gold transmetalation leads to impressive initiation rates in olefin metathesis.

Malinowska M., Kozlowska M., Hryniewicka A., Morzycki J.W.

A new 18-electron ruthenium complex, where ruthenium catalytic center is coordinated with the N-mesitylimidazole and nitrate ligands, as well as o-isopropoxystyrene moiety, is reported. The synthesis and detailed characterization of the Ru complex, together with density functional theory calculations (DFT), are presented. The complex is air- and moisture-stable, although has weak catalytical activity in the model metathesis reactions. However, its activity increases upon the addition of an aqueous HCl 1 M solution. Activated Ru complex successfully promotes metathesis in organic solvents as well as in water, enabling efficient performance (even up to 100%) of the catalyst under environment-friendly conditions. The activation mechanism of the reported catalyst is supported by time-dependent DFT calculations and ab initio molecular dynamics simulations.