Palladium(ii) complexes bearing mesoionic carbene ligands: catalytic application in domino Sonogashira coupling/cyclization reactions for one-pot synthesis of benzofuran and indole derivatives

Bioactive heterocycles such as benzofuran and indole derivatives were synthesized from commercially available 2-iodoarenes and alkynes via domino Sonogashira coupling followed by cyclization reaction using well-defined palladium PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) complexes (2a and 2b). These reactions tolerate a variety of 2-iodoarenes and diversely substituted terminal alkynes, resulting in the corresponding product in moderate to good yields in an open-air atmosphere. In particular, two palladium(ii) PEPPSI complexes 2a and 2b were synthesized in good yields from the reaction of corresponding 1,2,3-triazol-5-ylidene (MIC: mesoionic carbene), PdCl2, KI, and K2CO3 in pyridine at 110 °C and structurally characterized by various spectroscopic techniques including NMR spectroscopy, IR spectroscopy, HRMS and elemental analysis studies. Complex 2b is also characterized by X-ray crystallography.

Understanding Cooperativity in Homo- and Heterometallic Complexes: From Basic Concepts to Design

Cooperative effects have attracted considerable attention in recent years. These effects are ubiquitous in chemistry and biology and can govern interactions of proteins with other biomolecules, mechanisms of supramolecular recognition and polymerization, catalysis, assembly of compounds on surfaces, and physical properties such as magnetic, electronic or optical properties, e. g. Consequently, the understanding of cooperative effects can lead to a structure-property relation that can pave the way to future applications in various research areas; however, with regard to cooperative effects in homo- and heterometallic complexes, we still are at the beginning of understanding. Nevertheless, concepts to describe cooperativity of metal centers as well as methodologies to investigate and model these effects have emerged over the last years. This concept article gives an overview of these existing concepts, approaches, and strategies to understand cooperative effects in homo- and heterometallic complexes. Special emphasis is put on concepts to define cooperative effects, their quantification, as well as methods to investigate cooperative effects.

Isolation, Characterization and Reactivity of Key Intermediates Relevant to Reductive (Electro)catalysis with Cp*Rh Complexes Containing Pyridyl-MIC (MIC=Mesoionic Carbene) Ligands

In recent years, metal complexes of pyridyl-mesoionic carbene (MIC) ligands have been reported as excellent homogeneous and molecular electrocatalysts. In combination with group 9 metals, such ligands form highly active catalysts for hydrogenation/transfer hydrogenation/hydrosilylation catalysis and electrocatalysts for dihydrogen production. Despite such progress, very little is known about the structural/electrochemical/spectroscopic properties of crucial intermediates for such catalytic reactions with these ligands: solvato complexes, reduced complexes and hydridic species. We present here a comprehensive study involving the isolation, crystallographic characterization, electrochemical/spectroelectrochemical/theoretical investigations, and in-situ reactivity studies of all the aforementioned crucial intermediates involving Cp*Rh and pyridyl-MIC ligands. A detailed mechanistic study of the precatalytic activation of [RhCp*] complexes with pyridyl-MIC ligands is presented. Intriguingly, amphiphilicity of the [RhCp*]-hydride complexes was observed, displaying the substrate dependent transfer of H+ , H or H- . To the best of our knowledge, this study is the first of its kind targeting intermediates and reactive species involving metal complexes of pyridyl-MIC ligands and investigating the interconversion amongst them.

A naphthalene-based heterobimetallic triazolylidene IrIII/PdII complex: regioselective to regiospecific C-H activation, tandem catalysis and a copper-free Sonogashira reaction

Heterobimetallic complexes featuring mesoionic carbene (MIC) donor ligands are gaining enormous popularity in tandem catalysis owing to the combined action of two different metal centers during catalysis. A rare version of the heterobimetallic Pd^II/Ir^III complex possessing a cyclometalated mesoionic carbene (MIC) ligand is presented along with the analogous homodinuclear Pd^II complex. A sterically controlled regiospecific cyclometalation towards the formation of a six-membered ring complex over a five-membered ring complex has been performed using a naphthalene-based bis-MIC ligand platform. The interplay between regioselective vs. regiospecific C-H bond activation for the synthesis of cyclometalated Ir^III complexes has also been demonstrated using the corresponding naphthyl-derived mono-imidazolylidene ligand. Both homodinuclear Pd^II and heterobimetallic Pd^II/Ir^III complexes have been characterized using standard spectroscopic techniques including ¹H, ¹³C{¹H}, 2D correlation NMR spectroscopy and ESI mass spectrometry. The structure of the cyclometalated heterobimetallic complex has been established by single crystal XRD. The heterobimetallic complex has been employed as a pre-catalyst in the tandem Suzuki-Miyaura/transfer hydrogenation reaction and the homobimetallic Pd^II complex has been successfully employed as a catalyst in both the Sonogashira coupling and α-arylation of 1-methyl-2-oxindole.

Towards new coordination modes of 1,2,3-triazolylidene: controlled by the nature of the 1st metalation in a heteroditopic bis-NHC ligand

An unusual effect of the nature of the first metal coordination of a heteroditopic N-heterocyclic carbene ligand (L2) towards the coordination behavior of 1,2,3-tzNHC is explored. The first metal coordination at the ImNHC site (complexes 3 and 4) was noted to substantially influence the electronics of the 1,2,3-triazolium moiety leading to an unprecedented chemistry of this MIC donor. Along this line, the Rh^III/Ir^III-orthometalation in complexes 4 makes the triazolium C₄-H more downfield shifted than C₅-H, whereas a reverse trend, although to a lesser extent, is observed in the case of the non-chelated Pd^II-coordination. This difference in behavior assisted us to achieve the selective activation of triazole C₄/C₅ positions, not observed before, as supported by the isolation of the homo- and hetero-bimetallic complexes, 5, 6 and 7-9via C₅- and C₄-metalation, respectively. Furthermore, the %V _bur calculations eliminate any considerable steric influence and the DFT studies strongly support the selectivity observed during bimetalation.

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.

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.

Recent Advances in the Syntheses and Catalytic Applications of Homonuclear Ru-, Rh-, and Ir-Complexes of CNHC ^C Cyclometalated Ligands

In the past few decades, chemistry of cyclometalated species has gained momentum with increased applications in several areas of scientific developments. Cyclometalation reactions result in the formation of stable metallacycles through the generation of metal-carbon covalent bonds by activating the unreactive Csp² -H or Csp³ -H bonds. The extra stability gained by the formation of metallacycles enhances their applicability scopes especially in the area of homogeneous catalysis. In the recent research development in this area, NHC ligands (strong σ-donor and generally, weak π-acceptor) have been found to be one of the most suitable candidates for the intramolecular C-H activation process which leads to the cyclometalated species. The growth in the area of cyclometalation chemistry that started in the late 20^th century is still continuing and in the past few decades, various examples of NHC derived transition metal-based cyclometalated complexes came into the picture. As covering all the reported literatures in this area (includes mainly late transition metals) will exceed the limits of minireview, we restricted ourselves to the recent (2015 - May 2021) examples of the most common Ru-, Rh-, and Ir-based C_NHC ^C cyclometalated complexes and their applications in various homogeneous catalytic conversions such as transfer hydrogenation, amidation, oxidation of alcohols, annulations, and so forth.

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.

The transformations of a methylene-bridged bis-triazolium salt: a mesoionic carbene based metallocage and analogues of TCNE and NacNac

Unusual and unexpected chemical transformations often provide access to completely new types of functional molecules. We report here the synthesis of a methylene-bridged bis-triazolium salt designed as a precursor for a new bis-mesoionic carbene (MIC) ligand. The direct metalation with silver oxide led to the isolation and crystallographic characterization of a cationic tetranuclear octacarbene-silver(i) complex. During metalation the formal bis-MIC precursor undergoes significant structural changes and chemical transformations. A combined synthetic, crystallographic and (spectro-)electrochemical approach is used to elucidate the mechanistic pathway: starting from the methylene-bridged bis-triazolium salt a single deprotonation leads to a NacNac analogue, which is followed by a redox-induced radical dimerization reaction, generating a new tetra-MIC ligand coordinated to silver(i) central atoms. Decomplexation led to the isolation of the corresponding tetratriazoliumethylene, a profoundly electron-poor alkene, which is an analogue of TCNE.

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.

Pd(II)-Catalyzed Direct γ-C(sp3)-H Arylation between Free β2-Amino Esters and β3-Amino Esters and Aryl Iodides Using a Catalytic Transient Directing Group

Pd(II)-catalyzed direct γ-C(sp³)-H arylation coupling with free β²-amino esters and β³-amino esters using a commercially available catalytic transient directing group has been developed. This approach features high efficiency, broad substrate tolerance, easily accessible starting materials, and mild reaction conditions.

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO2 Reduction

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 Re^I 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 Re^I 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 Re^I complexes and for electrocatalytic CO₂ reduction.

Transition metal complexes of a bis(carbene) ligand featuring 1,2,4-triazolin-5-ylidene donors: structural diversity and catalytic applications

Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide results in the formation of [L-H2]Br2 which, upon salt metathesis with NH4PF6, readily yields the bis(triazolium) salt [L-H2](PF6)2 with non-coordinating counterions. [L-H2](PF6)2 and Ag2O react in a 1 : 1 ratio to yield a binuclear AgI-tetracarbene complex of the composition [(L)2Ag2](PF6)2 which undergoes a facile transmetalation reaction with [Cu(SMe2)Br] to deliver the corresponding CuI-NHC complex [(L)2Cu2](PF6)2. In contrast, the [L-H2]Br2 reacts with [Ir(Cp*)Cl2]2 to generate a doubly C-H activated IrIII-NHC complex 5. Similarly, the triazolinylidene donor supported diorthometalated RuII-complex 6 is also obtained. Complexes 5 and 6 represent the first examples of a stable diorthometalated binuclear IrIII/RuII-complex supported by 1,2,4-triazolin-5-ylidene donors. The synthesized IrIII-NHC complex 5 is found to be more effective than its RuII-analogue (6) for the reduction of a range of alkenes/alkynes via the transfer hydrogenation strategy. Conversely, RuII-complex 6 is identified as an efficient catalyst (0.01 mol% loading) for the β-alkylation of a wide range of secondary alcohols using primary alcohols as alkylating partners via a borrowing hydrogen strategy.

Hydroxyl-functionalized triazolylidene-based PEPPSI complexes: metallacycle formation effect on the Suzuki coupling reaction

We report the preparation and full characterization of a series of hydroxyl functionalized 1,2,3-triazolylidene-based PEPPSI complexes 2a-c and their catalytic application in the Suzuki cross coupling reaction of aryl chlorides/amides with boronic acids. Under basic reaction conditions, complexes 2a-c show a notable increase in their catalytic efficiency compared with two ether-wingtip functionalized PEPPSI analogues (3 and 4) and a commercially available NHC-Pd complex (5). The catalytic results suggest that deprotonation of the hydroxyl group in complexes 2a-c plays an important role in the overall process. Deprotonation of the alcohol moiety of complexes 2a-b with sodium tert-butoxide allows for the isolation of metallacycles 6a-b, which are proposed as the active species of cross coupling reactions.

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.

A leap forward in iridium-NHC catalysis: new horizons and mechanistic insights

This review summarises the most recent advances in Ir-NHC catalysis while revisiting all the classical reactions in which this type of catalyst has proved to be active. The influence of the ligand system and, in particular, the impact of the NHC ligand on the activity and selectivity of the reaction have been analysed, accompanied by an examination of the great variety of catalytic cycles hitherto reported. The reaction mechanisms so far proposed are described and commented on for each individual process. Moreover, some general considerations that attempt to explain the influence of the NHC from a mechanistic viewpoint are presented at the end of the review. The first sections are dedicated to the most widely explored reactions that use Ir-NHCs, i.e., hydrogenation and transfer hydrogenation, for which a general overview that tries to compile all the Ir-NHC catalysts hitherto reported for these processes is provided. The next sections deal with hydrogen borrowing, hydrosilylation, water splitting, dehydrogenation (of alcohols, alkanes, aminoboranes and formic acid), hydrogen isotope exchange (HIE), signal amplification by reversible exchange and C-H bond functionalisation (silylation and borylation). The last section compiles a series of reactions somewhat less explored for Ir-NHC catalysts that include the hydroalkynylation of imines, hydroamination, diboration of olefins, hydrolysis and methanolysis of silanes, arylation of aldehydes with boronic acids, addition of aroyl chlorides to alkynes, visible light driven reactions, isomerisation of alkenes, asymmetric intramolecular allylic amination and reactions that employ heterometallic catalysts containing at least one Ir-NHC unit.

Palladium(ii)-Acetylacetonato Complexes with Mesoionic Carbenes: Synthesis, Structures and Their Application in the Suzuki-Miyaura Cross Coupling Reaction

A series of novel palladium(ii) acetylacetonato complexes bearing mesoionic carbenes (MICs) have been synthesized and characterized. The synthesis of the complexes of type (MIC)Pd(acac)I (MIC = 1-mesityl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene (1), 1,4-(2,4,6-methyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (2), 1,4-(2,6-diisopropyl)-phenyl-3-methyl-1,2,3-triazol-5-ylidene (3); acac = acetylacetonato) via direct metalation starting from the corresponding triazolium iodides and palladium(ii) acetylacetonate is described herein. All complexes were characterized by ¹H- and ¹³C-NMR spectroscopy and high resolution mass spectrometry. Additionally, two of the complexes were characterized by single crystal X-ray crystallography confirming a square-planar coordination geometry of the palladium(ii) center. A delocalized bonding situation was observed within the triazolylidene rings as well as for the acac ligand respectively. Complex 2 was found to be an efficient pre-catalyst for the Suzuki-Miyaura cross coupling reaction between aryl-bromides or -chlorides with phenylboronic acid.

RuII, IrIII and OsII mesoionic carbene complexes: efficient catalysts for transfer hydrogenation of selected functionalities

Pyridine- and pyrimidine-appended triazolylidene donors are used as ligands for the transfer hydrogenation of a series of functionalities.

Rhodium, iridium and nickel complexes with a 1,3,5-triphenylbenzene tris-MIC ligand. Study of the electronic properties and catalytic activities

The coordination versatility of a 1,3,5-triphenylbenzene-tris-mesoionic carbene ligand is illustrated by the preparation of complexes with three different metals: rhodium, iridium and nickel. The rhodium and iridium complexes contained the [MCl(COD)] fragments, while the nickel compound contained [NiCpCl]. The preparation of the tris-MIC (MIC = mesoionic carbene) complex with three [IrCl(CO)2] fragments, allowed the estimation of the Tolman electronic parameter (TEP) for the ligand, which was compared with the TEP value for a related 1,3,5-triphenylbenzene-tris-NHC ligand. The electronic properties of the tris-MIC ligand were studied by cyclic voltammetry measurements. In all cases, the tris-MIC ligand showed a stronger electron-donating character than the corresponding NHC-based ligands. The catalytic activity of the tri-rhodium complex was tested in the addition reaction of arylboronic acids to α,β-unsaturated ketones.

C–H activation in IrIII and N-demethylation in PtII complexes with mesoionic carbene ligands: examples of monometallic, homobimetallic and heterobimetallic complexes

Mononuclear Pt^II and the first dinuclear Pt^II complexes along with a cyclometalated heterobimetallic Ir^III/Pd^II complex bearing mesoionic carbene donor ligands are presented starting from the same bis-triazolium salt.