Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications

Well-Defined, Air- and Moisture-Stable Palladium-Imidazo[1,5-a]pyridin-3-ylidene Complexes: A Versatile Catalyst Platform for Cross-Coupling Reactions by L-Shaped NHC Ligands

We describe the development of [(NHC)Pd(cinnamyl)Cl] complexes of ImPy (ImPy = imidazo[1,5-a]pyridin-3-ylidene) as a versatile class of precatalysts for cross-coupling reactions. These precatalysts feature fast activation to monoligated Pd(0) with 1:1 Pd to ligand ratio in a rigid imidazo[1,5-a]pyridin-3-ylidene template. Steric matching of the C5-substituent and N2-wingtip in the catalytic pocket of the catalyst framework led to the discovery of ImPyMesDipp as a highly reactive imidazo[1,5-a]pyridin-3-ylidene ligand for Pd-catalyzed cross-coupling of nitroarenes by challenging C-NO2 activation. Kinetic studies demonstrate fast activation and high reactivity of this class of well-defined ImPy-Pd catalysts. Structural studies provide full characteristics of this new class of imidazo[1,5-a]pyridin-3-ylidene ligands. Computational studies establish electronic properties of sterically-restricted imidazo[1,5-a]pyridin-3-ylidene ligands. Finally, a scalable synthesis of C5-substituted imidazo[1,5-a]pyridin-3-ylidene ligands through Ni-catalyzed Kumada cross-coupling is disclosed. The method obviates chromatographic purification at any of the steps, resulting in a facile and modular access to ImPy ligands. We anticipate that well-defined [Pd-ImPy] complexes will find broad utility in organic synthesis and catalysis for activation of unreactive bonds.

Ligands modification strategies for mononuclear water splitting catalysts

Artificial photosynthesis (AP) has been proved to be a promising way of alleviating global climate change and energy crisis. Among various materials for AP, molecular complexes play an important role due to their favorable efficiency, stability, and activity. As a result of its importance, the topic has been extensively reviewed, however, most of them paid attention to the designs and preparations of complexes and their water splitting mechanisms. In fact, ligands design and preparation also play an important role in metal complexes’ properties and catalysis performance. In this review, we focus on the ligands that are suitable for designing mononuclear catalysts for water splitting, providing a coherent discussion at the strategic level because of the availability of various activity studies for the selected complexes. Two main designing strategies for ligands in molecular catalysts, substituents modification and backbone construction, are discussed in detail in terms of their potentials for water splitting catalysts.

The Core Difference between a Mesoionic and a Normal N-Heterocyclic Carbene

The properties of the abnormal N-heterocyclic carbene (NHC) 1,4-dimesityl-3-methyl-1,2,3-triazolin-5-ylidene were comprehensively compared to those of the related normal carbene 1,3-dimesitylimidazolin-2-ylidene using a range of steric and electronic probe techniques (% V _bur, steric maps, Tolman electronic parameter, alane, Huynh electronic parameter, selone, and pK _a values). The two NHCs were determined to be sterically equivalent (isostructural), while the triazolin-5-ylidene was found to be a stronger σ-electron donor and a much weaker π-electron acceptor. These results were used to demonstrate that the electronic properties of these NHCs could affect the stereochemical outcome of an NHC-catalyzed reaction.

Breaching Bacterial Biofilm Barriers: Efficient Combinatorial Theranostics for Multidrug-Resistant Bacterial Biofilms with a Novel Penetration-Enhanced AIEgen Probe

The formation of microbial biofilms is acknowledged as a major virulence factor in a range of persistent local infections. Failures to remove biofilms with antibiotics foster the emergence of antibiotic-resistant bacteria and result in chronic infections. As a result, the construction of effective biofilm-inhibiting and biofilm-eradicating chemicals is urgently required. Herein, we designed a water-soluble probe APDIS for membrane-active fluorescence and broad-spectrum antimicrobial actions, particularly against methicillin-resistant Staphylococcus aureus (MRSA), which shows multidrug resistance. In vitro and in vivo experiments demonstrate its high antibacterial effects comparable to vancomycin. Furthermore, it inhibits biofilm formation by effectively killing planktonic bacteria at low inhibitory concentrations, without toxicity to mammalian cells. More importantly, this probe can efficiently penetrate through biofilm barriers and exterminate bacteria that are enclosed within biofilms and startle existing biofilms. In the mouse model of implant-related biofilm infections, this probe exhibits strong antibiofilm activity against MRSA biofilms, thus providing a novel theranostic strategy to disrupt biofilms in vivo effectively. Our results indicate that this probe has the potential to be used for the development of a combinatorial theranostic platform with synergistic enhanced effects for the treatment of multidrug-resistant bacterial infections and antibiofilm medications.

The first synthesis of an isocyanide-functionalized imidazolium salt and transition metal complexes thereof

An isocyanide-functionalized imidazole was obtained from 4-(1H-imidazol-1-yl)aniline by the Ugi method and subsequently transformed into the corresponding imidazolium salt by treatment with MeI. Coordination of the isocyanide residue allowed the synthesis of several transition metal complexes containing a peripheral imidazolium cation, which are suitable starting materials for the formation of mixed ligand isocyanide-NHC heterometallic complexes.

Synthesis and Properties of Pyridine-Fused Triazolylidene-Palladium: Catalyst for Cross-Coupling Using Chloroarenes and Nitroarenes

The synthesis and catalytic activity of pyridine-fused triazolylidene as a novel abnormal N-heterocyclic carbene (aNHC) ligand is described. The evaluation of physical properties using X-ray crystallographic analysis and infrared spectroscopy revealed that these triazolylidenes have a high electron-donating ability toward the metal center. The application of this triazolylidene to the palladium-catalyzed cross-coupling of chloroarenes and nitroarenes with arylboronic acids showcased its ability to activate C-Cl and C-NO₂ bonds.

A Mesoionic Carbene-Pyridine Bidentate Ligand That Improves Stability in Electrocatalytic CO2 Reduction by a Molecular Manganese Catalyst

Tricarbonyl Group 7 complexes have a longstanding history as efficacious CO₂ electroreduction catalysts. Typically, these complexes feature an auxiliary 2,2'-bipyridine ligand that assists in redox steps by delocalizing the electron density into the ligand orbitals. While this feature lends to an accessible redox potential for CO₂ electroreduction, it also presents challenges for electrocatalysis with Mn because the electron density is removed from metal-ligand bonding orbitals. The results presented here thus introduce a mesoionic carbene (MIC) as a potent ligand platform to promote Mn-based electrocatalysis. The strong σ donation of the N,C-bidentate MIC is shown to help centralize the electron density on the Mn center while also maintaining relevant redox potentials for CO₂ electroreduction. Mechanistic investigation supports catalytic turnover at two operative potentials separated by 400 mV. In the low operating potential regime at -1.54 V, Mn(0) species catalyze CO₂ to CO and CO₃^2-, which has a maximum rate of 7 ± 5 s^-1 and is stable for up to 30.7 h. At higher operating potential at -1.94 V, "Mn(-1)" catalyzes CO₂ to CO and H₂O with faster turnovers of 200 ± 100 s^-1, with the trade-off being less stability at 6.7 h. The relative stabilities of Mn complexes bearing MIC and 4,4'-di-tert-butyl-2,2'-bipyridine were compared by evaluation under the same electrolysis conditions and therefore elucidated that the MIC promotes longevity for CO evolution throughout a 5 h period.

Redox-Switchable Complexes Based on Nanographene-NHCs

A series of rhodium and iridium complexes with a N‐heterocyclic carbene (NHC) ligand decorated with a perylene‐diimide‐pyrene moiety are described. Electrochemical studies reveal that the complexes can undergo two successive one‐electron reduction events, associated to the reduction of the PDI moiety attached to the NHC ligand. The reduction of the ligand produces a significant increase on its electron‐donating character, as observed from the infrared spectroelectrochemical studies. The rhodium complex was tested in the [3+2] cycloaddition of diphenylcyclopropenone and methylphenylacetylene, where it displayed a redox‐switchable behavior. The neutral complex showed moderate activity, which was suppressed when the catalyst was reduced.

Oxidation of Styrene to Benzaldehyde Catalyzed by Schiff Base Functionalized Triazolylidene Ni(II) Complexes

Four new Schiff base functionalized 1,2,3-triazolylidene nickel complexes, [Ni-(L1NHC)2](PF6)2; 3, [Ni-(L2NHC)2](PF6)2; 4, [Ni-(L3NHC)](PF6)2; 7 and [Ni-(L4NHC)](PF6)2; 8, (where L1NHC = (E)-3-methyl-1-propyl-4-(2-(((2-(pyridin-2-yl)ethyl)imino)methyl)phenyl)-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 1, L2NHC = (E)-3-methyl-4-(2-((phenethylimino)methyl)phenyl)-1-propyl-1H-1,2,3-triazol-3-ium hexafluorophosphate(V), 2, L3NHC = 4,4′-(((1E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))bis(2,1-phenylene))bis(3-methyl-1-propyl-1H-1,2,3-triazol-3-ium) hexafluorophosphate(V), 5, and L4NHC = 4,4′-(((1E)-(butane-1,4-diylbis(azanylylidene))bis(methanylylidene))bis(2,1-phenylene))bis(3-methyl-1-propyl-1H-1,2,3-triazol-3-ium) hexafluorophosphate(V), 6), were synthesised and characterised by a variety of spectroscopic methods. Square planar geometry was proposed for all the nickel complexes. The catalytic potential of the complexes was explored in the oxidation of styrene to benzaldehyde, using hydrogen peroxide as a green oxidant in the presence of acetonitrile at 80 °C. All complexes showed good catalytic activity with high selectivity to benzaldehyde. Complex 3 gave a conversion of 88% and a selectivity of 70% to benzaldehyde in 6 h. However, complexes 4 and 7–8 gave lower conversions of 48–74% but with higher (up to 90%) selectivity to benzaldehyde. Results from kinetics studies determined the activation energy for the catalytic oxidation reaction as 65 ± 3 kJ/mol, first order in catalyst and fractional order in the oxidant. Results from UV-visible and CV studies of the catalytic activity of the Ni-triazolylidene complexes on styrene oxidation did not indicate any clear possibility of generation of a Ni(II) to Ni(III) catalytic cycle.

Insights into Triazolylidene Ligands Behaviour at a Di-Iron Site Related to [FeFe]-Hydrogenases

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).

Realizing 1,1‐Dehydration of Secondary Alcohols to Carbenes: Pyrrolidin‐2‐ols as a Source of Cyclic (Alkyl)(Amino)Carbenes

Herein we report secondary pyrrolidin‐2‐ols as a source of cyclic (alkyl)(amino)carbenes (CAAC) for the synthesis of CAAC‐Cu^I‐complexes and cyclic thiones when reacted with Cu^I‐salts and elemental sulfur, respectively, under reductive elimination of water from the carbon(IV)‐center. This result demonstrates a convenient and facile access to CAAC‐based Cu^I‐salts, which are well known catalysts for different organic transformations. It further establishes secondary alcohols to be a viable source of carbenes—realizing after 185 years Dumas’ dream who tried to prepare the parent carbene (CH₂) by 1,1‐dehydration of methanol. Addressed is also the reactivity of water towards CAACs, which proceeds through an oxidative addition of the O−H bond to the carbon(II)‐center. This emphasizes the ability of carbon‐compounds to mimic the reactivity of transition‐metal complexes: reversible oxidative addition and reductive elimination of the O−H bond to/from the C(II)/C(IV)‐centre.

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.

Mesoionic Imines (MIIs): Strong Donors and Versatile Ligands for Transition Metals and Main Group Substrates

We report the synthesis and the reactivity of 1,2,3‐triazolin‐5‐imine type mesoionic imines (MIIs). The MIIs are accessible by a base‐mediated cycloaddition between a substituted acetonitrile and an aromatic azide, methylation by established routes and subsequent deprotonation. C=O‐stretching frequencies in MII−CO₂ and −Rh(CO)₂Cl complexes were used to determine the overall donor strength. The MIIs are stronger donors than the N‐heterocyclic imines (NHIs). MIIs are excellent ligands for main group elements and transition metals in which they display substituent‐induced fluorine‐specific interactions and undergo C−H activation. DFT calculations gave insights into the frontier orbitals of the MIIs. The calculations predict a relatively small HOMO–LUMO gap compared to other related ligands. MIIs are potentially able to act as both π‐donor and π‐acceptor ligands. This report highlights the potential of MIIs to display exciting properties with a huge potential for future development.

Carbene chemistry of arsenic, antimony, and bismuth: origin, evolution and future prospects

The discovery of the first isolable N-heterocyclic carbene in 1991 ushered in a new era in coordination chemistry. The remarkable bonding properties of carbenes have led to their rapid proliferation as auxiliary ligands for a wide range of transition metals and main group elements. In the case of group 15, while carbene-stabilized nitrogen and phosphorus compounds are extensively studied, the scope of research has shrunk significantly from arsenic to bismuth. This is essentially attributed to the decrease in stability of the C-E bond upon descending the group. Even so, modulating the carbene backbone or introducing alternative synthetic strategies not only alleviates the stability issues but also offers promising results in terms of the bonding and reactivities of these compounds. The purpose of the present perspective is to provide a comprehensive overview of the origins and development of carbene chemistry of arsenic, antimony, and bismuth, as well as to highlight the future prospects of this field.

Mesoionic Dithiolates (MIDts) Derived from 1,3-Imidazole-Based Anionic Dicarbenes (ADCs)

Mesoionic dithiolates [(MIDtAr )Li(LiBr)2 (THF)3 ] (MIDtAr ={SC(NDipp)}2 CAr; Dipp=2,6-iPr2 C6 H3 ; Ar=Ph 3 a, 3-MeC6 H4 (3-Tol) 3 b, 4-Me2 NC6 H4 (DMP) 3 c) and [(MIDtPh )Li(THF)2 ] (4) are readily accessible (in≥90 % yields) as crystalline solids on treatments of anionic dicarbenes Li(ADCAr ) (2 a-c) (ADCAr ={C(NDipp)2 }2 CAr) with elemental sulfur. 3 a-c and 4 are monoanionic ditopic ligands with both the sulfur atoms formally negatively charged, while the 1,3-imidazole unit bears a formal positive charge. Treatment of 4 with (L)GeCl2 (L=1,4-dioxane) affords the germylene (MIDtPh )GeCl (5) featuring a three-coordinated Ge atom. 5 reacts with (L)GeCl2 to give the Ge-Ge catenation product (MIDtPh )GeGeCl3 (6). KC8 reduction of 5 yields the homoleptic germylene (MIDtPh )2 Ge (7). Compounds 3 a-c and 4-7 have been characterized by spectroscopic studies and single-crystal X-ray diffraction. The electronic structures of 4-7 have been analyzed by DFT calculations.

Mesoionic Carbene Complexes of Uranium(IV) and Thorium(IV)

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe₃)₂}₂(CH₂SiMe₂NSiMe₃){MIC}] (An = U, 4U and Th, 4Th; MIC = {CN(Me)C(Me)N(Me)CH}), which represent rare examples of actinide mesoionic carbene linkages and the first example of a thorium mesoionic carbene complex. Complexes 4U and 4Th were prepared via a C–H activation intramolecular cyclometallation reaction of actinide halides, with concomitant formal 1,4-proton migration of an N-heterocyclic olefin (NHO). Quantum chemical calculations suggest that the An–carbene bond comprises only a σ-component, in contrast to the uranium(III) analogue [U{N(SiMe₃)₂}₃(MIC)] (1) where computational studies suggested that the 5f³ uranium(III) ion engages in a weak one-electron π-backbond to the MIC. This highlights the varying nature of actinide-MIC bonding as a function of actinide oxidation state. In solution, 4Th exists in equilibrium with the Th(IV) metallacycle [Th{N(SiMe₃)₂}₂(CH₂SiMe₂NSiMe₃)] (6Th) and free NHO (3). The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of ΔG_298.15K = 0.89 kcal mol^–1. Energy decomposition analysis (EDA-NOCV) of the actinide–carbon bonds in 4U and 4Th reveals that the former is enthalpically stronger and more covalent than the latter, which accounts for the respective stabilities of these two complexes.

Thiazol-2-ylidenes as N-Heterocyclic carbene ligands with enhanced electrophilicity for transition metal catalysis

Over the last 20 years, N-heterocyclic carbenes (NHCs) have emerged as a dominant direction in ligand development in transition metal catalysis. In particular, strong σ-donation in combination with tunable steric environment make NHCs to be among the most common ligands used for C-C and C-heteroatom bond formation. Herein, we report the study on steric and electronic properties of thiazol-2-ylidenes. We demonstrate that the thiazole heterocycle and enhanced π-electrophilicity result in a class of highly active carbene ligands for electrophilic cyclization reactions to form valuable oxazoline heterocycles. The evaluation of steric, electron-donating and π-accepting properties as well as structural characterization and coordination chemistry is presented. This mode of catalysis can be applied to late-stage drug functionalization to furnish attractive building blocks for medicinal chemistry. Considering the key role of N-heterocyclic ligands, we anticipate that N-aryl thiazol-2-ylidenes will be of broad interest as ligands in modern chemical synthesis.

Synthesis, Characterization and Photophysical Properties of Mesoionic N-Heterocyclic Imines

N -heterocyclic imines are widely used in transition-metal chemistry, main-group chemistry as well as catalysis, due to their enhanced basicity and nucleophilicity which benefit from their ylidic form. As their analogs, mesoionic N -heterocyclic imines, which feature more highly ylidic form, is still in its infancy though excellent works also achieved. Here we reported the synthesis, characterization and photophysical properties of mesoionic N -heterocyclic imines. TD-DFT are employed to get deeper insight into the mechanism of the photophysical behaviors. The unsubstituted mesoionic N-heterocyclic imines ( 1-3 ) displayed considerable quantum yields (QY: up to 43.8%) and could be potentially applied as luminescent materials.

β-Lactam and penicillin substituted mesoionic metal carbene complexes

1,2,3-Triazolylidene MIC M-complexes (M = Au, Pd, Pt) having 2-azetidinones and penicillin G substituents at the triazole ring were prepared by CuAAC on 2-azetidinones having a terminal alkyne tethered at N1, followed by alkylation of the 1,2,3-triazole ring and transmetallation [Au(I), Pd(II) and Pt(II)]. The Au-MIC complexes efficiently catalyze the regioselective cycloisomerization of enynes, while the Pt-MIC complexes were efficient catalysts in hydrosilylation reactions.

Recent advances in borenium catalysis

Borenium ions are strong Lewis acids because of the positive charge on boron. While their high reactivity had long restricted their role in organic synthesis to stoichiometric reagents, in the past ten years the introduction of suitable supporting ligands, such as N-heterocyclic carbenes, has enabled them to function as competent catalysts for various organic transformations involving the activation of strong covalent bonds, such as H-H, Si-H, B-H, C-H and C-C bonds. This review provides an overview of the recent advances in borenium-catalysed reactions with emphasis on catalyst synthesis, methodology development and mechanistic insight.

Multinuclear mesoionic 1,2,3-triazolylidene complexes: design, synthesis, and applications

The chemistry of multinuclear metal complexes bearing by N-heterocyclic carbene (NHC) ligands, is an area of fast growing interest in modern organometallic chemistry. In particular, complexes supported by mesoionic (MIC) 1,2,3-triazolylidenes are attracting a great deal attention due to their postulated superior donor capacity compared to classical NHC ligands. Despite the readily available synthetic routes to MIC-based ligand platforms featuring several substitution levels, most of the coordination chemistry of triazolylidenes is still dominated by mononuclear complexes. In this short review article, recent progress on the design and synthesis of multinuclear triazolylidene complexes (ranging from di- to tetranuclear species) is discussed. Special emphasis is placed on their structural features, electronic properties and catalytic applications.

Chemistry of Compounds Based on 1,2,3-Triazolylidene-Type Mesoionic Carbenes

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.

Strongly Electron-Donating Triazolylidene Ligands: Cationic Metal Carbonyl Complexes of 1-Methyl-1,2,3-triazole as Triazolium Surrogates

A new strategy for tuning the electronic properties of 1,2,3-triazol-5-ylidene metal complexes is reported using {Mo(η³-C₄H₇)(bipy)(CO)₂} or {Re(bipy)(CO)₃} fragments as substituents at the triazole N3 atom. The reaction of cationic molybdenum(II) and rhenium(I) 1-methyl-1,2,3-triazole compounds with the strong base KN(SiMe₃)₂ in the presence of electrophilic metal fragments, such as AgOTf (OTf = trifluoromethanesufonate) or [CuCl(IPr)] [IPr = 2,6-(diisopropyl)phenylimidazol-2-ylidene] affords a new type of 1,2,3-triazol-5-ylidene complexes. For silver(I) cationic bis(triazolylidene), [Ag(tzNHC^M)₂]OTf (M = [Mo], 2; [Re], 4), complexes are obtained, whereas in the case of Cu(I) mixed normal/mesoionic NHC, [Cu(IPr)(tzNHC^M)]OTf (M = [Mo], 7; [Re], 8) complexes are formed. This special type of mesoionic N-heterocyclic carbenes bear a metal fragment at the N3 atom of the 1,2,3-triazole moiety, showing notable enhancement of the carbene electron donor ability compared to conventional alkyl-substituted analogues. Transmetalation from cationic silver bis(triazolylidene) complexes 2 and 4, prepared using this methodology, has proven to be very efficient toward [M'Cl(cod)]₂ (M' = Rh, Ir; cod = 1,5-cyclooctadiene), affording the corresponding cationic bis(triazolylidene) [M'(cod)(tzNHC^M)₂]OTf (9-12) complexes. A subsequent reaction with CO(g) easily produces substitution of the diene ligand, affording the corresponding cis-dicarbonyl [M'(CO)₂(tzNHC^M)₂]OTf (13-16) compounds.

Forever young: the first seventy years of ferrocene

The discovery of ferrocene, [Fe(η5-C5H5)2], seventy years ago has significantly influenced chemical research and provided a key impetus for establishing and rapidly expanding organometallic chemistry, which has continued at a...

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...

Theoretical investigations in the reactions of group 15 analogues of the monocationic five-membered N-heterocyclic carbenes: interplay of electrophilicity, basicity, and aromaticity governing the reactivity

The nature of the group 15 analogs of the five-membered N-heterocyclic carbenes (G15-Rea; G15 = N, P, As, Sb, and Bi) was investigated by using various methodologies.

N-Heterocyclic carbene iron complexes catalyze the ring-opening polymerization of lactide

Poly(lactic acid), PLA, which holds great promise as a biodegradable substitute of fossil resource-derived polyolefins, is industrially produced by the ring-opening polymerization of lactide using a potentially harmful tin catalyst. Based on mechanistic insights into the reaction of N-heterocyclic carbene (NHC) iron complexes with carbonyl substrates, we surmised and demonstrate here that such complexes are excellent catalysts for the bulk polymerization of lactide. We show that an iron complex with a triazolylidene NHC ligand is active at lactide/catalyst ratios of up to 10 000 : 1, produces polylactide with relatively high number-average molecular weights (up to 50 kg mol⁻¹) and relatively narrow dispersity (Đ ∼ 1.6), and features an apparent polymerization rate constant k_app of up to 8.5 × 10⁻³ s⁻¹, which is more than an order of magnitude higher than that of the industrially used tin catalyst. Kinetic studies and end-group analyses support that the catalytically active species is well defined and that the polymerization proceeds via a coordination–insertion mechanism. The robustness of the catalyst allows technical grade lactide to be polymerized, thus offering ample potential for application on larger scale in an industrially relevant setting.

Iron(ii) complexes containing a mesoionic triazolylidene ligand are highly efficient catalyst precursors for the ring opening polymerization of lactide to poly(lactic acid), surpassing other iron complexes and also industrially utilized Sn(oct)₂.

Bimetallic PdII complexes with NHC/Py/PCy3 donor set ligands: applications in α-arylation, Suzuki–Miyaura and Sonogashira coupling reactions

Bimetallic PdII complexes bearing mixed NHC/Py/PCy3 donor set ligands are presented. A complex with a mixed NHC/PCy3 donor set ligands shows superior activity compared to the PEPPSI type complexes in α-arylation and Sonogashira coupling reactions.

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

The first cationic molybdenum oxo complexes were synthesized and immobilized on partially dehydroxylated silica. Vastly enhanced catalytic activity for terminal olefins was found compared to their neutral congeners.