Carbene Transfer from Triazolylidene Gold Complexes as a Potent Strategy for Inducing High Catalytic Activity

Application of first-row transition metal complexes bearing 1,2,3-triazolylidene ligands in catalysis and beyond

Triazole-derived N-heterocyclic carbenes, triazolylidenes (trz) have become an interesting alternative to the ubiquitous Arduengo-type imidazole-derived carbenes, in part because they are stronger donors, and in other parts due to their versatile synthesis through different types of click reactions. While the use of trz ligands has initially focused on their coordination to precious metals for catalytic applications, the recent past has seen a growing interest in their impact on first-row transition metals. Coordination of trz ligands to such 3d metals is more challenging due to the orbital mismatch between the carbene and the 3d metal center, which also affects the stability of such complexes. Here we summarize the strategies that have been employed so far to overcome these challenges and to prepare first-row transition metal complexes containing at least one trz ligand. Both properties and reactivities of these trz complexes are comprehensively compiled, with a focus on photophysical properties and, in particular, on the application of these complexes in homogeneous catalysis. The diversity of catalytic transformations entailed with these trz 3d metal complexes as well as the record-high performance in some of the reactions underpins the benefits imparted by trz ligands.

Exploiting click-chemistry: backbone post-functionalisation of homoleptic gold(I) 1,2,3-triazole-5-ylidene complexes

The synthesis of a homoleptic azide-functionalised Au(I) bis-1,2,3-triazole-5-ylidene complex is reported, starting from a backbone-modified 1,2,3-triazolium salt ligand precursor. The incorporated azide handle allows for a straightforward modification of the complex according to click-chemistry protocols without impacting the steric shielding around the metal center, demonstrating the superiority of the presented triazole ligand framework over imidazole based systems. Employing the SPAAC and the CuAAC reactions, post-modification of the complex is facilitated with two model substrates, while retaining very high antiproliferative activity (nanomolar range IC50 values) in A2780 and MCF-7 human cancer cells.

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.

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.

Gold(I) Bis(1,2,3-triazol-5-ylidene) Complexes as Promising Selective Anticancer Compounds

The synthesis and antiproliferative activity of Mes- and iPr-substituted gold(I) bis(1,2,3-triazol-5-ylidene) complexes in various cancer cell lines are reported, showing nanomolar IC₅₀ values of 50 nM (lymphoma cells) and 500 nM (leukemia cells), respectively (Mes < iPr). The compounds exclusively induce apoptosis (50 nM to 5 μM) instead of necrosis in common malignant blood cells (leukemia cells) and do not affect non-malignant leucocytes. Remarkably, the complexes not only overcome resistances against the well-established cytostatic etoposide, cytarabine, daunorubicin, and cisplatin but also promote a synergistic effect of up to 182% when used with daunorubicin. The present results demonstrate that gold(I) bis(1,2,3-triazol-5-ylidene) complexes are highly promising and easily modifiable anticancer metallodrugs.

Experimental and Computational Study on Photophysical Properties of Mesoionic Chalcogenones

N-Heterocyclic carbene adducts with main group elements (NHC=E) have aroused great interest and have been widely investigated in coordination chemistry. Among them, N-heterocyclic carbene adducts with chalcogens (NHC=Ch) have been known for a long time. Their investigations mostly focused on synthesis, coordination chemistry and electrochemistry. Their photophysical properties still remain unexplored. In this work, the photophysical properties of mesoionic carbene adducts with sulfur and selenium have been investigated both in solution and solid state. These compounds showed blue fluorescence in dichloromethane. While in solid state, orange to red room-temperature phosphorescence can be observed, and dual emission was found in mesoionic thiones. Furthermore, time-dependent density functional theory (TD-DFT) calculations were used to obtain insights into the luminescent mechanism.

Exploring the stability of the NHC-metal bond using thiones as probes

The metal-carbon bond in N-heterocyclic carbene (NHC) metal complexes, which are ubiquitous in modern homogeneous catalysis, is often conjectured to be robust. Here, carbene dissociation was evaluated from a series of complexes with metals of relevance in catalysis containing either an Arduengo-type 2-imidazolylidene or a mesoionic 1,2,3-triazolylidene ligand through thione formation, revealing remarkable kinetic lability of the NHC-metal bond for, e.g. Ir^III, Rh^III, and Ni^II complexes.

Silver-Free Au(I) Catalysis Enabled by Bifunctional Urea- and Squaramide-Phosphine Ligands via H-Bonding

A library of gold(I) chloride complexes with phosphine ligands incorporating pendant (thio)urea and squaramide H-bond donors was prepared with the aim of promoting chloride abstraction from Au(I) via H-bonding. In the absence of silver additives, complexes bearing squaramides and trifluoromethylated aromatic ureas displayed good catalytic activity in the cyclization of N-propargyl benzamides, as well as in a 1,6-enyne cycloisomerization, a tandem cyclization-indole addition reaction and the hydrohydrazination of phenylacetylene. Kinetic studies and DFT calculations indicate that the energetic span of the reaction is accounted by both the chloride abstraction step, facilitated by the bidentate H-bond donor via an associative mechanism, and the subsequent cyclization step.

Optimizing Catalyst and Reaction Conditions in Gold(I) Catalysis-Ligand Development

This review considers phosphine and N-heterocyclic carbene complexes of gold(I) that are used as (pre)catalysts for a range of reactions in organic synthesis. These are divided according to the structure of the ligand, with the narrative focusing on studies that offer a quantitative comparison between the ligands and readily available or widely used existing systems.

Mechanochemical Synthesis of Tripodal Tris[4-(1,2,3-triazol-5-ylidene)methyl]amine Mesoionic Carbene Ligands and Their Complexation with Silver(I)

The conjugate acids of 1,2,3-triazolylidene mesoionic carbenes can be prepared in a straightforward fashion by alkylation of 1-substituted 1,2,3-triazoles. However, this becomes a much more challenging proposition when other nucleophilic centers are present, which has curtailed the development of ligands containing multiple 1,2,3-triazolylidene donors. Herein, methylation of a series of tris[(1-aryl-1,2,3-triazol-4-yl)methyl]amines possessing both electron-rich and electron-deficient aromatic substituents, using Me₃OBF₄, is shown to proceed with much higher chemoselectivity under mechanochemical conditions than when conducted in solution. This provides a means to reliably access a series of tricationic tris[4-(1,2,3-triazolium)methyl]amines in good yields. DFT calculations suggest that a potential reason for this change in regioselectivity is the difference between the background dielectric of the DCM solution versus the solid state, which is predicted to have a large effect on the relative thermodynamic driving force for alkylation of the tertiary amine center versus the triazole rings. Homoleptic silver complexes of the triazolylidene ligands derived therefrom, of formulas [Ag₃(1a-d)₂](X)₃ (X^- = BF₄^-, TfO^-), have been isolated and fully characterized. In the case of the ligand bearing the smallest aryl substituents, 1b, argentophilic interactions yield a triangular Ag₃ core. The [Ag₃(1a-d)₂](X)₃ silver salts are viable agents for transmetalation to other transition metals, demonstrated here for cobalt. In the case of 1a, the complex [Co^II(1a)(NCMe)](OTf)₂ was obtained. Therein, the bulky mesityl substituents enforce a tetrahedral geometry, in which only the triazolylidene donors of 1a coordinate (i.e., it acts as a tridentate ligand). Transmetalation of the less sterically encumbered ligand 1b yields six-coordinate cobalt(III) complexes, [Co^III(1b)(Cl)(NCMe)](OTf)₂ and [Co^III(1b)(NCMe)₂](OTf)₃, in which the ligand coordinates in a tetradentate fashion. These are the first examples of tris(1,2,3-triazolylidene) ligands containing an additional coordinating heteroatom and, more generally, of tetradentate 1,2,3-triazolylidene ligands. Crucially, we believe that the divergent chemoselectivity under mechanochemical conditions (vs conventional solution-based chemistry) demonstrated herein offers a pathway by which other challenging synthetic targets, including further multidentate carbene ligands, can be prepared in superior yields.

Computational study of 1,2,3-triazol-5-ylidenes with p-block element substituents

1,2,3-Triazol-5-ylidenes with p-block element substituents have been investigated by DFT calculations, which show tunable electronic properties.

Planar-chiral ferrocene-based triazolylidene copper complexes: synthesis, characterization, and catalysis in asymmetric borylation of α,β-unsaturated ester

1,2,3-Triazol-5-ylidenes have recently attracted considerable attention as versatile ligands because of their strong electron-donating properties and structural diversities. While some efforts have been devoted to the development of chiral triazolylidene-metal complexes, there is no example achieving asymmetric induction by base-metal complexes with triazolylidene ligands. Herein, we synthesized planar-chiral ferrocene-based triazolylidene copper complexes, which enabled the asymmetric borylation of methyl cinnamate with bis(pinacolato)diboron with good enantioselectivity.

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

The modular syntheses of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes and their homotrinuclear gold(i) complexes are reported. Choosing the arene core allows fine-tuning of the exact oxidation potentials and thus tailoring of the electrochemical response. The tris[chloridogold(i)] complexes were investigated in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, showing cooperative behaviour vs. a mononuclear chloridogold(i) complex. Adding one, two, or three equivalents of 1,1′-diacetylferrocenium[tetrakis(perfluoro-tert-butoxy)aluminate] as an oxidant during the catalytic reaction (in situ) resulted in a distinct, stepwise influence on the resulting catalytic rates. Isolation of the oxidised species is possible, and using them as (pre-)catalysts (ex situ oxidation) confirmed the activity trend. Proving the intactness of the P–Au–Cl motif during oxidation, the tri-oxidised benzene-based complex has been structurally characterised.

Trinuclear gold(i) complexes of C₃-symmetric tris(ferrocenyl)arene-based tris-phosphanes with four accessible oxidation states catalyse the ring-closing isomerisation of N-(2-propyn-1-yl)benzamide with different rates depending on their redox state.

Ultrastable and Highly Catalytically Active N-Heterocyclic-Carbene-Stabilized Gold Nanoparticles in Confined Spaces

Controlling the size and surface functionalization of nanoparticles (NPs) can lead to improved properties and applicability. Herein, we demonstrate the efficiency of the metal-carbene template approach (MCTA) to synthesize highly robust and soluble three-dimensional polyimidazolium cages (PICs) of different sizes, each bearing numerous imidazolium groups, and use these as templates to synthesize and stabilize catalytically active, cavity-hosted, dispersed poly-N-heterocyclic carbene (NHC)-anchored gold NPs. Owing to the stabilization of the NHC ligands and the effective confinement of the cage cavities, the as-prepared poly-NHC-shell-encapsulated AuNPs displayed promising stability towards heat, pH, and chemical regents. Most notably, all the Au@PCCs (PCC=polycarbene cage) exhibited excellent catalytic activities in various chemical reactions, together with high stability and durability.

NHC-Stabilized Iridium Nanoparticles as Catalysts in Hydrogen Isotope Exchange Reactions of Anilines

The preparation of N-heterocyclic carbene-stabilized iridium nanoparticles and their application in hydrogen isotope exchange reactions is reported. These air-stable and easy-to-handle iridium nanoparticles showed a unique catalytic activity, allowing selective and efficient hydrogen isotope incorporation on anilines using D2 or T2 as isotopic source. The usefulness of this transformation has been demonstrated by the deuterium and tritium labeling of diverse complex pharmaceuticals.

Combined XRD-paramagnetic 13C NMR spectroscopy of 1,2,3-triazoles for revealing copper traces in a Huisgen click-chemistry cycloaddition. A model case

Copper-catalyzed Alkyne-Azide Cycloaddition (CuAAC) click chemistry robustness has been demonstrated over recent years to produce 1,2,3-triazoles with excellent yields at mild conditions with simple purification methods. However, the consequences of having copper paramagnetic traces in final products, which complicate spectroscopic assignments and can produce inaccurate conclusions, has been scarcely discussed. Herein we present a strategy that combines X-Ray Diffraction (XRD) with 13C- paramagnetic Nuclear Magnetic Resonance spectroscopy, in order to demonstrate the presence of paramagnetic metal traces at standard Huisgen synthesis and purification conditions. We also demonstrate that the derivatization of 1,4-disubstituted-1,2,3-triazoles to produce 1,3,4,-trisubstituted-1,2,3.triazolium salts, promotes an efficient removal of Cu(II/I) moieties. Evidence of paramagnetic metal moieties is given using XRD structural analysis of abnormalities in torsional angles between substituents and the 1,2,3-triazole center, in parallel to 13C- paramagnetic NMR chemical shift and line width analysis. As model systems to demonstrate the importance of characterizing paramagnetic traces, we present the synthesis of novel 1-((3s,5s,7s)-adamantan-1-yl)-4-cyclopropyl-1H-1,2,3-triazole and its derivatized 1-((3s,5s,7s)-adamantan-1-yl)-4-cyclopropyl-3-methyl-1H-[1,2,3]-triazol-3-ium triflate salt.

Oxidative access via aqua regia to an electrophilic, mesoionic dicobaltoceniumyltriazolylidene gold(III) catalyst

A gold(III) complex with the hitherto most electron poor mesoionic carbene ligand is presented. Aqua regia was the oxidizing agent of choice for the synthesis of this unusual organometallic compound. The Au^III complex is redox-rich, and also acts as a catalyst for oxazole formation, delivering selectively a completely different isomer compared to its Au^I congener.

Synthesis of heterobimetallic gold(i) ferrocenyl-substituted 1,2,3-triazol-5-ylidene complexes as potential anticancer agents

1,2,3-Triazol-5-ylidene (trz) complexes of gold(i) containing a ferrocenyl substituent on the C4-position of the trz ring were synthesized to yield the neutral heterobimetallic gold(i) trz chlorido (2), gold(i) trz phenyl (3), and the cationic gold(i) trz triphenylphospine (5) complexes. In order to compare the effect of silver(i) as central metal vs. gold(i), [Ag(trz)2]+ (4) was also prepared, while variation of the C4-1,2,3-triazol-5-ylidene substituent from a ferrocenyl to a phenyl group was done to prepare the monometallic analogue of 5, namely the cationic Au(i) trz triphenylphosphine complex 6. The complexes were characterised with spectroscopic and electrochemical methods, and the single crystal X-ray structures of 2-6 were determined. NMR stability studies of 5 as a representative example of the series of complexes were performed to confirm the stability of the complexes in the solvent dimethylsulfoxide and in aqueous solution. The anti-cancer potential of 5 was evaluated against the lung cancer cell lines A549 and H1975, and the human embryonic kidney cell line (HEK-293) was used as a non-cancer model. IC50 values of 0.89, 0.23 and 5.43 μM, respectively, were obtained for A549, H1975 and HEK-293, respectively, indicating the activity and selectivity of 5 for cancer cells. Fluorescence microscopy experiments as a preliminary mode-of-action study evidenced an apoptotic cell death mechanism rather than necrotic cell death.

A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry

A new chelating mesoionic carbene ligand, derived from 1,2,3-triazoles, with two redox-active tert-butyl-phenolate linkers has been synthesized and explored towards its reactivity and electrochemical properties in early transition metal chemistry.

Au(III) and Pt(II) Complexes of a Novel and Versatile 1,4-Disubstituted 1,2,3-Triazole-Based Ligand Possessing Diverse Secondary and Tertiary Coordinating Groups

A novel 1,4-disubstituted 1,2,3-triazole-based ligand, 2-(4-(pyridin-2-yl)-1 H-1,2,3-triazol-1-yl)propane-1,3-diamine (ptpd), which possesses pyridyl and diamino secondary and tertiary coordinating groups was synthesized in excellent yield. The reactivity of 2-(1-phenyl-1 H-1,2,3-triazol-4-yl)pyridine (ptp), di- tert-butyl (2-azidopropane-1,3-diyl)dicarbamate (Boc₂-ptpd), and ptpd·3HCl was investigated with Au(III) and Pt(II) precursors. Analysis including X-ray crystal structures of [Au(III)Cl₃(ptp)] (1), [Au(III)Cl₂(ptpd)][Au(I)Cl₂][OH]{[NaAuCl₄·2H₂O]} n (3), and [Pt(II)Cl₂(ptpdH₂)][PtCl₄] (4) revealed that ptpd (i) serves as a monodentate ligand for Au(III) coordinating to the metal center via the pyridine nitrogen only, (ii) preferentially coordinates Au(III) via the bidentate diamino group over the monodentate pyridine group, (iii) can coordinate Pt(II) in a bidentate fashion via the pyridyl nitrogen and the triazole N-3, and (iv) can bridge two Pt(II) centers through bidentate chelation at the diamino group and bidentate chelation via the pyridyl nitrogen and the triazole N-3. ptpd represents a versatile ligand template for the development of mixed metal complexes.

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

Mesoionic carbenes are a subclass of the family of N-heterocyclic carbenes that generally feature less heteroatom stabilization of the carbenic carbon and hence impart specific donor properties and reactivity schemes when coordinated to a transition metal. Therefore, mesoionic carbenes and their complexes have attracted considerable attention both from a fundamental point of view as well as for application in catalysis and beyond. As a follow-up of an earlier Chemical Reviews overview from 2009, the organometallic chemistry of N-heterocyclic carbenes with reduced heteroatom stabilization is compiled for the 2008-2017 period, including specifically the chemistry of complexes containing 1,2,3-triazolylidenes, 4-imidazolylidenes, and related 5-membered N-heterocyclic carbenes with reduced heteratom stabilization such as (is)oxazolylidenes, pyrrazolylidenes, and thiazolylidenes, as well as pyridylidenes as 6-membered N-heterocyclic carbenes with reduced heteroatom stabilization. For each ligand subclass, metalation strategies, electronic and steric properties, and applications, in particular, in metal-mediated catalysis, are compiled. Mesoionic carbenes demonstrate particularly high activity in (water) oxidation, hydrogen transfer reactions, and cyclization reactions. Unique features of these ligands are identified such as their dipolar structure, their specific donor properties, as well as stability aspects of the ligand and the complexes, which provides opportunities for further research.

Phosphinines versus mesoionic carbenes: a comparison of structurally related ligands in Au(i)-catalysis

Gold(i) complexes based on a 2,4,6-triarylphosphinine and a mesoionic carbene derivative have been prepared and characterized crystallographically. Although structurally related, both heterocycles differ significantly in their donor/acceptor properties. These opposed electronic characteristics have been exploited in Au(i)-catalyzed cycloisomerization reactions. For the conversion of the standard substrate dimethyl 2-(3-methylbut-2-enyl)-2-(prop-2-ynyl)malonate the results obtained for both Au-catalysts were found to be very similar and comparable to the ones reported in the literature for other carbene- or phosphorus(iii)-based Au(i)-complexes. In contrast, a clear difference between the catalytic systems was found for the cycloisomerization of the more challenging substrate N-2-propyn-1-ylbenzamide. A combination of the phosphinine-based complex and [AgSbF₆] or [Cu(OTf)₂] leads to a catalytic species, which is more active than the mesoionic carbene-based coordination compound. We attribute these differences to the stronger π-accepting ability of phosphinines in comparison to mesoionic carbenes. The here presented results show for the first time that phosphinines can be used efficiently as π-accepting ligands in Au(i)-catalyzed cycloisomerization reactions.

Synthesis and characterization of a gold(i) bis(triazolylidene) complex featuring a large [(TpMe2)2K] anion

We report the synthesis, charaterization and catalytic performance of a unique bis(triazolylidene) gold(i) complex featuring a large [(Tp^Me2)₂K] anion.