Metal Carbene Complexes Research Articles

Synthesis and characterization of low-dimensional N-heterocyclic carbene lattices.

The covalent interaction of N-heterocyclic carbenes (NHCs) with transition metal atoms gives rise to distinctive frontier molecular orbitals (FMOs). These emergent electronic states have spurred the widespread adoption of NHC ligands in chemical catalysis and functional materials. Although formation of carbene-metal complexes in self-assembled monolayers on surfaces has been explored, design and electronic structure characterization of extended low-dimensional NHC-metal lattices remains elusive. Here we demonstrate a modular approach to engineering one-dimensional (1D) metal-organic chains and two-dimensional (2D) Kagome lattices using the FMOs of NHC-Au-NHC junctions to create low-dimensional molecular networks exhibiting intrinsic metallicity. Scanning tunneling spectroscopy and first-principles density functional theory reveal the contribution of C-Au-C π-bonding states to dispersive bands that imbue 1D- and 2D-NHC lattices with exceptionally small work functions.

DFT Insight into a Strain-Release Mechanism in Bicyclo[1.1.0]butanes via Concerted Activation of Central and Lateral C-C Bonds with Rh(III) Catalysis.

Transition-metal-catalyzed, strain-release-driven transformations of "spring-loaded" bicyclo[1.1.0]butanes (BCBs) are considered potent tools in synthetic organic chemistry. Previously proposed strain-release mechanisms involve either the insertion of the central C-C bond of BCBs into a metal-carbon bond, followed by β-C elimination, or the oxidative addition of the central or lateral C-C bond on the transition metal center, followed by reductive elimination. This study, employing DFT calculations on a Rh(III)-catalyzed model system in a three-component protocol involving oxime ether, BCB ester, and ethyl glyoxylate for constructing diastereoselective quaternary carbon centers, introduces an unusual strain-release mechanism for BCBs. In this mechanism, the catalytic reaction is initiated by the simultaneous cleavage of two C-C bonds (the central and lateral C-C bonds), resulting in the formation of a Rh-carbene intermediate. The new mechanism exhibits a barrier of 21.0 kcal/mol, making it energetically more favorable by 11.1 kcal/mol compared to the previously suggested most favorable pathway. This unusual reaction mode rationalizes experimental observation of the construction of quaternary carbon centers, including the excellent E-selectivity and diastereoselectivity. The newly proposed strain-release mechanism holds promise in advancing our understanding of transition-metal-catalyzed C-C bond activation mechanisms and facilitating the synthesis of transition metal carbene complexes.

Formal Insertion of a Metal Carbene Complex into a σ‐Carbon‐Carbon Bond. Gold‐Catalyzed Synthesis of 3H‐Indoles

AbstractWe report here a formal insertion of a metal carbene complex in a single carbon‐carbon bond. This behavior occurs with the participation of an α‐imino gold carbene complex, generated from benzofused triazapentalenes or, in a one‐pot procedure, from their 1‐propargyl‐1H‐benzotriazole precursors, and gold‐catalytically activated ynamides. As the result, 3H‐indole derivatives were obtained, with formation of a quaternary center. A computational analysis carried out on the reaction mechanism indicates that the bulkiness of the gold ligand plays a key role forcing a conformation, which includes aromatic interactions, that favors the approximation of the carbenic carbon to the reactive site. In addition, a negative crossover experiment rules out a cationic arene migration pathway.

Investigations of the Influence of Two Pyridyl-Mesoionic Carbene Constitutional Isomers on the Electrochemical and Spectroelectrochemical Properties of Group 6 Metal Carbonyl Complexes

Metal complexes of mesoionic carbenes (MICs) of the triazolylidene type and their derivatives have gained increasing attention in the fields of electrocatalysis and photochemistry. The redox activity of these metal complexes is critical for their applications in both the aforementioned fields. Easy accessibility and modular synthesis open a wide field for the design of ligands, such as bidentate ligands. The combination of an MIC with a pyridyl unit in a bidentate ligand setup increases the π acceptor properties of the ligands while retaining their strong σ donor properties. The analogy with the well-established 2,2′-bipyridine ligand allows conclusions to be drawn about the influence of the mesoionic carbene (MIC) moiety in tetracarbonyl group 6 complexes in cyclic voltammetry and (spectro)electrochemistry (SEC). However, the effects of the different connectivity in pyridyl-MIC ligands remain underexplored. Based on our previous studies, we present a thorough investigation of the influence of the two different pyridyl-MIC constitutional isomers on the electrochemical and the UV-vis-NIR/IR/EPR spectroelectrochemical properties of group 6 carbonyl complexes. Moreover, the presented complexes were investigated for the electrochemical conversion of CO2 using two different working electrodes, providing a fundamental understanding of the influence of the electrode material in the precatalytic activation.

Noble gas dative bonding with coinage metal carbene complexes: A theoretical study.

The structure and stability of noble gas (Ng) bound [NHCM]+ complexes (M = Cu, Ag, and Au) were investigated using Quantum chemical calculations. Dissociation energies, enthalpy, and free energy changes were computed to comprehend the stability of these Ng-bonded complexes. The nature of interactions associated to the bonding between metal and noble gas atoms was studied through the computation of electron density-based descriptors. Detailed electronic structure study revealed electron donation from the noble gas atoms towards the metal center, resulting in the formation of dative bonds.

Construction and Hierarchical Self-Assembly of a Supramolecular Metal-Carbene Complex with Multifunctional Units.

Hierarchical combinations involving metal-ligand interactions and host-guest interactions can consolidate building blocks with unique functions into material properties. This study reports the construction and hierarchical self-assembly of multifunctional trinuclear AuI tricarbene complex containing three crown ether units and three ferrocene units. Host-guest interactions between the multifunctional trinuclear AuI tricarbene complex and organic ammonium salts were investigated, revealing that crown ether-based host-guest interactions can effectively regulate the electrochemical properties of the complex. Utilizing bisammonium salt as the cross-linker and multifunctional trinuclear AuI tricarbene complex as the core, a stimuli-responsive and self-healing supramolecular gel with different functional units was obtained.

Rigidification-Induced Emissive Metal-Carbene Complexes for Artificial Light Harvesting.

A tetraphenylethylene (TPE)-based flexible imidazolium (L) salt was used to develop a di-nuclear silver(I)-tetracarbene (1) complex. Coordination-induced rigidity upon formation of 1 exhibited a 6-fold increase in emission intensity in acetonitrile compared to starting L. Despite TPE being a well-known aggregation-induced emissive moiety, AgI-N-heterocyclic carbene (NHC) complex 1 had a remarkably higher fluorescence emission (4-fold) in dilute solution when compared with L in its aggregated state. Finally, this enhanced emission was used to institute a new platform for an artificial light-harvesting system. 1 acted as an energy donor and efficiently transferred energy to Eosin Y (ESY) with a high saturation at a 67:1 (1/ESY) molar ratio. Use of rigidification-induced emission of the AgI-NHC complex to fabricate a light-harvesting scaffold is a new approach and can greatly impact the generation of smart materials.

Cytotoxic potential of silver, palladium, rhodium, ruthenium, and iridium complexes of a cycloheptyl-substituted lipophilic N-heterocyclic carbene ligand

Lipophilicity is a crucial parameter for cytotoxicity of metal complexes, in many cases associated with increased activity. In the present study, we synthesized a series of N-heterocyclic carbene (NHC) complexes of different metals with a lipophilic benzimidazolium chloride to investigate and compare their cytotoxicity. Rh– (4), Ru– (5), and Ir–NHC (6) complexes of a cycloheptyl-substituted benzimidazole-based NHC ligand (1) have been prepared by transmetalation reaction between Ag–NHC and the corresponding metal compound. The newly synthesized complexes have been characterized by NMR, IR, LC-MS spectroscopy, and elemental analyses. The anti-growth effects of the newly synthesized Rh–, Ru–, and Ir–NHC complexes and previously reported NHC precursor (1), Ag– (2), and Pd–NHC (3) complexes against human breast (MCF-7), colorectal (Caco-2), ovarian (A2780), and prostate (LNCaP) cancer cell lines have been investigated by MTT assay. The results revealed that the compounds provide stronger anti-growth effects against all cell lines compared to standard drug cisplatin. Among the compounds, benzimidazolium chloride, and Ag–NHC complex showed promising activities.

NHC-heterocycle carbene gold catalyst intercalated in Layered Double Hydroxides as reusable hybrid catalyst in acidic medium

NHC-heterocyclic carbene metal complexes are efficient catalysts used for the oxidation, amination, cycloaddition or hydration of alkynes. Immobilization of such complex is a strategy to avoid its decomposition/deactivation and allow its recycling. One limitation of the immobilization of carbene metal complexes is the acidic pH required to perform the catalysis of hydration of alkynes. The support used must be resistant in such conditions. Layered Double Hydroxides (LDH) is a widely used material as catalyst support due to their tunable chemical composition and flexible open structure. However, the LDH phases usually found as catalyst support in the literature are chemically instable under severe conditions. In particular, when operating in extreme acidic pH. This study describes the development of a pH-resistant support by screening of LDH with various composition and formulation in order to obtain a stable material at pH down to 3.0, conditions required for a model reaction, the hydration of propargyl alcohol. Then, the intercalation of a NHC-heterocyclic carbene gold anionic complex (NHC-Au) was optimized and fully characterized. The newly heterogeneous catalysts were evaluated for the targeted hydration catalysis reaction. The selected support, a LiAl2 phase obtained by urea method and dried at 80 °C, allowed to immobilized up to 75% of the gold complex. The intercalated catalyst was finally used for the hydration of several alkynes with yields between 84% and 100% and was recycled up to 12 cycles without any activity loss.

Wittig and Wittig–Horner Reactions under Sonication Conditions

Carbonyl olefinations are among the most important organic syntheses that form C=C bonds, as they usually have high yields and in addition offer excellent stereoselectivity. Due to these advantages, carbonyl olefinations have important pharmaceutical and industrial applications. These reactions contain an additional step of an α-functionalized carbanion to an aldehyde or ketone to produce alkenes, but syntheses performed using metal carbene complexes are also known. The Wittig reaction is an example of carbonyl olefination, one of the best ways to synthesize alkenes. This involves the chemical reaction between an aldehyde or ketone with a so-called Wittig reagent, for instance phosphonium ylide. Triphenylphosphine-derived ylides and trialkylphosphine-derived ylides are the most common phosphorous compounds used as Wittig reagents. The Wittig reaction is commonly involved in the synthesis of novel anti-cancer and anti-viral compounds. In recent decades, the use of ultrasound on the Wittig reaction (and on different modified Wittig syntheses, such as the Wittig-Horner reaction or the aza-Wittig method) has been studied as a green synthesis. In addition to the advantage of green synthesis, the use of ultrasounds in general also improved the yield and reduced the reaction time. All of these chemical syntheses conducted under ultrasound will be described further in the present review.

USc2C2 and USc2NC Clusters with U-C Triple Bond Character Stabilized Inside Fullerene Cages.

The chemistry of f-block metal-carbon multiple bonds is underdeveloped compared to well-established carbene complexes of the d-block transition metals. Herein, we report two new actinide-rare earth mixed metal carbides and nitrogen carbide cluster fullerenes, USc2C2@D5h(6)-C80 and USc2NC@D5h(6)-C80, which contain U-C bonds with triple bond character and were successfully synthesized and characterized by mass spectrometry, UV-vis-NIR spectroscopy, Fourier transform infrared spectroscopy, single crystal X-ray diffraction, and DFT calculations. Crystallographic studies show that the two previously unreported clusters, USc2C2 and USc2NC, are stabilized in the D5h(6)-C80 carbon cage and adopt unique trifoliate configurations, in which C2/NC units are almost vertically inserted into the plane defined by the U and two Sc atoms. Combined experimental and theoretical studies further reveal the bonding structure of USc2C2 and USc2NC, which contain C═U(VI)═C and C═U(V)═N bonding motifs. The electronic structures of the two compounds are determined as U6+(Sc2)6+(C4-)2@D5h(6)-C804- and U5+(Sc2)6+(N)3-(C)4-@D5h(6)-C804-, respectively. Quantum-chemical studies confirm that the U-C bonds in both molecules show unprecedented multicenter triple-bond character. The discovery of this unique U-C multiple bond offers a deeper understanding of the fundamentals of uranium chemistry.

Electron‐Rich Oxycarbenes: New Synthetic and Catalytic Applications beyond Group 6 Fischer Carbene Complexes

AbstractOxycarbenes have emerged as useful intermediates in synthetic chemistry. Compared to the widely studied oxycarbene metal complexes bearing Group 6 metals, the synthetic and catalytic applications of oxycarbenes beyond Group 6 Fischer carbene complexes are less explored because of the difficulty in controlling their reactivity and the need to use a stoichiometric amount of a presynthesized Group 6 metal carbene complex as the starting material. This Minireview summarizes early synthetic and catalytic applications of late‐transition‐metal oxycarbene complexes and highlights recent advances in free oxycarbene reactions and transition‐metal‐catalyzed reactions involving oxycarbenes. We hope this Minireview will inspire further developments in this emerging area.

Electron-Rich Oxycarbenes: New Synthetic and Catalytic Applications beyond Group 6 Fischer Carbene Complexes.

Oxycarbenes have emerged as useful intermediates in synthetic chemistry. Compared to the widely studied oxycarbene metal complexes bearing Group 6 metals, the synthetic and catalytic applications of oxycarbenes beyond Group 6 Fischer carbene complexes are less explored because of the difficulty in controlling their reactivity and the need to use a stoichiometric amount of a presynthesized Group 6 metal carbene complex as the starting material. This Minireview summarizes early synthetic and catalytic applications of late-transition-metal oxycarbene complexes and highlights recent advances in free oxycarbene reactions and transition-metal-catalyzed reactions involving oxycarbenes. We hope this Minireview will inspire further developments in this emerging area.

Carbohydrate-based N-heterocyclic carbene-metal complexes: a new avenue for sustainable catalysts in organic transformations

N-Heterocyclic carbenes (NHCs) and their metal complexes have been employed as catalysts in organic transformations owing to their high efficiency, cost-effectiveness, and extraordinary performance.

Introducing Ion Mobility Mass Spectrometry to Identify Site-Selective C-H Bond Activation in N-Heterocyclic Carbene Metal Complexes.

The activation of C-H bonds in a selective manner still constitutes a major challenge from a synthetic point of view; thus, it remains an active area of fundamental and applied research. Herein, we introduce ion mobility spectrometry mass spectrometry-based (IM-MS) approaches to uncover site-selective C-H bond activation in a series of metal complexes of general formula [(NHC)LMCl]+ (NHC = N-heterocyclic carbene; L = pentamethylcyclopentadiene (Cp*) or p-cymene; M = Pd, Ru, and Ir). The C-H bond activation at the N-bound groups of the NHC ligand is promoted upon collision induced dissociation (CID). The identification of the resulting [(NHC-H)LM]+ isomers relies on the distinctive topology that such cyclometalated isomers adopt upon site-selective C-H bond activation. Such topological differences can be reliably evidenced as different mobility peaks in their respective CID-IM mass spectra. Alternative isomers are also identified via dehydrogenation at the Cp*/p-cymene (L) ligands to afford [(NHC)(L-H)M]+. The fragmentation of the ion mobility-resolved peaks is also investigated by CID-IM-CID. It enables the assignment of mobility peaks to the specific isomers formed from C(sp2)-H or C(sp3)-H bond activation and distinguishes them from the Cp*/p-cymene (L) dehydrogenation isomers. The conformational change of the NHC ligands upon C-H bond activation, concomitant with cyclometalation, is also discussed on the basis of the estimated collision cross section (CCS). A unique conformation change of the pyrene-tagged NHC members is identified that involves the reorientation of the NHC ring accompanied by a folding of the pyrene moiety.

Gauging Radical Stabilization with Carbenes.

Carbenes, including N‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence.

Gauging Radical Stabilization with Carbenes

AbstractCarbenes, includingN‐heterocyclic carbene (NHC) ligands, are used extensively to stabilize open‐shell transition metal complexes and organic radicals. Yet, it remains unknown, which carbene stabilizes a radical well and, thus, how to design radical‐stabilizing C‐donor ligands. With the large variety of C‐donor ligands experimentally investigated and their electronic properties established, we report herein their radical‐stabilizing effect. We show that radical stabilization can be understood by a captodative frontier orbital description involving π‐donation to‐ and π‐donation from the carbenes. This picture sheds a new perspective on NHC chemistry, where π‐donor effects usually are assumed to be negligible. Further, it allows for the intuitive prediction of the thermodynamic stability of covalent radicals of main group‐ and transition metal carbene complexes, and the quantification of redox non‐innocence.

Coordination chemistry of silver(I), gold(I) and nickel(II) with bis N-heterocyclic carbenes: applications in electrocatalytic hydrogen evolution reaction

The design and systematic investigation of an electrocatalyst is crucial in ensuring proper benchmarking of the catalyst developed. This perspective has led to molecular catalysts developed from N-heterocyclic carbene (NHC) metal complexes for electrocatalytic applications. Among the array of electrocatalytic applications available, hydrogen evolution reaction (HER) is at the forefront due to the ever-increasing demand for cleaner and sustainable energy. This work is focussed on the development of binuclear metal-NHC complexes from bis-{4-(2,6-diethylphenyl)-1,2,4-triazole} with a propyl spacer and hexafluorophosphate counterion. The NHC precursor and binuclear silver complexes prepared have been elucidated by single-crystal X-ray diffraction. An assessment of the binuclear complex bearing the same ligand with nickel(II) resulted in the formation of a counter held nickel derivative. The electrocatalytic performance of these metal organic derivatives in HER is evaluated along with their electrochemical impedance spectral discussion.

Schrock‐TypeSilylidenesandGermylidenesFound Among the Silylene and Germylene Complexes of the Early and Mid‐Transition Metals

AbstractThe chemistry of Schrock‐type silylidene/germylidene complexes is the subject of this review. This is preceded by the general consideration of the prototypical transition metal carbene complexes and their classification as either Fischer or Schrock carbene complexes. Following this background, silylene/germylene complexes of the early (groups 3–4) and mid‐ (groups 5–7) transition metals are briefly discussed in terms of their synthesis and nature of the transition metal‐silylene/germylene bonding interaction. Major attention will be paid for the preparation, bonding nature, and particular reactivity of those complexes, which, on the basis of their spectroscopic, structural, and computational studies, are reliably categorized as the Schrock‐type silylene/germylene transition metal complexes, that is silylidenes/germylidenes.

Carbene-Metal Complexes As Molecular Scaffolds for Construction of through-Space Thermally Activated Delayed Fluorescence Emitters.

The through-space charge transfer (CT) process is observed in Cu(I) carbene-metal-amide complexes, where conventional imidazole or imidazoline N-heterocyclic (NHC) carbene fragments act as inert linkers and CT proceeds between a metal-bound carbazole donor and a distantly situated carbene-bound phenylsulfonyl acceptor. The resulting electron transfer gives a rise to efficient thermally activated delayed fluorescence (TADF), characterized with high photoluminescence quantum yields (ΦPL up to 90%) and radiative rates (kr) up to 3.32 × 105 s-1. The TADF process is aided by fast reverse intersystem crossing (rISC) rates of up to 2.56 × 107 s-1. Such emitters can be considered as hybrids of two existing TADF emitter design strategies, combining low singlet-triplet energy gaps (ΔEST) met in all-organic exciplex-like emitters (0.0062-0.0075 eV) and small, but non-negligible spin-orbital coupling (SOC) provided by a Cu atom, like in TADF-active organometallic complexes.