Chemistry Of N-heterocyclic Carbenes Research Articles

Manganese(III) complexes stabilized with N-heterocyclic carbene ligands for alcohol oxidation catalysis.

The chemistry of N-heterocyclic carbenes with Earth-abundant manganese has largely focused on low-valent systems for reductive catalysis. Here, we have decorated imidazole- and triazole-derived carbenes with phenol substituents to access higher-valent Mn(III) complexes [Mn(O,C,O)(acac)], where acac = acetylacetonato, and O,C,O = bis(phenolate)imidazolylidene (1) or bis(phenolate)triazolylidene (2). Both complexes catalyze the oxidation of alcohols in the presence of tBuOOH as terminal oxidant. Complex 2 is slightly more active than 1 (TOF up to 540 h-1vs. 500 h-1), yet significantly more robust towards deactivation. Secondary and primary alcohols are oxidized, the latter with high selectivity and essentially no overoxidation of the aldehyde product to carboxylic acids unless the reaction time is substantially extended. Mechanistic investigations using Hammett parameters, IR spectroscopy, isotope labelling experiments, and specific substrates and oxidants as probes support the formation of a manganese(V) oxo system as the active species and subsequent turnover-limiting hydrogen atom abstraction.

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.

Recent advances of N-heterocyclic carbenes in the applications of constructing carbo- and heterocyclic frameworks with potential biological activity.

In recent years, N-heterocyclic carbenes (NHCs) have established themselves as a masterful and promising type of organocatalyst for the speedy construction of medicinally and biologically significant molecules from common and accessible small molecules. In particular, various cyclic scaffolds, including carbocycles and heterocycles, have been synthesized using NHCs via cycloaddition reaction. An exhaustive review focused on the chemistry of NHCs in these cyclic molecules has yet to be reported. In this contribution, a general picture of the utilization of NHCs in constructing twelve kinds of bioactive cyclic skeletons is firstly presented. We provide a systematic and comprehensive overview from the perspective of cycloaddition reactions; moreover, the limitations, challenges, and future prospects were discussed.

Non-Classical Anionic Naked N-Heterocyclic Carbenes: Fundamental Properties and Emerging Applications in Synthesis and Catalysis

Ongoing research exploring the chemistry of N-heterocyclic carbenes (NHCs) has led to the development and discovery of new NHC subclasses that deviate beyond Arduengo’s prototypical N,N′-disubstituted imidazol-2-ylidene-based structures. These systems continue to enable and extend the fundamental role of NHC ligands in synthesis and catalysis. In this regard, the advent of protic NHCs has garnered particular interest. This derives in part from their applications to the selective preparation of unique molecular scaffolds and their unprecedented bifunctional reactivity, which can be exploited in transition metal-catalyzed processes. In comparison, the synthetic applications of closely related anionic naked NHCs remain rather underexplored. With this in mind, this review highlights the interesting fundamental properties of non-classical anionic naked NHCs, and focuses on their emerging applications in synthesis and catalysis.

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.

Preparation and Post-Assembly Modification of Metallosupramolecular Assemblies from Poly( N-Heterocyclic Carbene) Ligands.

Although the coordination chemistry of N-heterocyclic carbenes (NHCs) with transition metals has been explored for half a century, only in the past ten years has the chemistry of metallosupramolecular assemblies based on poly-NHC ligands been studied more extensively. Remarkable discrete assemblies featuring poly-NHC ligands including two-dimensional metallacycles and three-dimensional metallaprisms/cages have since emerged. These assemblies are mostly obtained starting from various imidazolium or benzimidazolium salts. Driven by the increasing interest in new supramolecular architectures from carbon donor ligands, design, and construction of poly-NHC metal assemblies has become a rapidly growing area of research. The metal-carbene bond length is fixed to approximately 2.0 Å in linear NHC-M-NHC complexes. This allows the use of such complexes bearing olefin-substituted NHC ligands as templates for subsequent photochemical [2 + 2] cycloaddition reactions. The postassembly modification of such assemblies has been actively explored in recent years. In this review, we focus on the synthetic methods, characterization, structural features, and postassembly modifications of metallosupramolecular assemblies obtained from poly-NHC ligands.

In Situ Formation of N-Heterocyclic Carbene-Bound Single-Molecule Junctions.

Self-assembled monolayers (SAMs) formed using N-heterocyclic carbenes (NHCs) have recently emerged as thermally and chemically ultrastable alternatives to those formed from thiols. The rich chemistry and strong σ-donating ability of NHCs offer unique prospects for applications in nanoelectronics, sensing, and electrochemistry. Although stable in SAMs, free carbenes are notoriously reactive, making their electronic characterization challenging. Here we report the first investigation of electron transport across single NHC-bound molecules using the scanning tunneling microscope-based break junction (STM-BJ) technique. We develop a series of air-stable metal NHC complexes that can be electrochemically reduced in situ to form NHC-electrode contacts, enabling reliable single-molecule conductance measurements of NHCs under ambient conditions. Using this approach, we show that the conductance of an NHC depends on the identity of the single metal atom to which it is coordinated in the junction. Our observations are supported by density functional theory (DFT) calculations, which also firmly establish the contributions of the NHC linker to the junction transport characteristics. Our work demonstrates a powerful method to probe electron transfer across NHC-electrode interfaces; more generally, it opens the door to the exploitation of surface-bound NHCs in constructing novel, functionalized electrodes and/or nanoelectronic devices.

Metallosupramolecular Architectures Obtained from Poly-N-heterocyclic Carbene Ligands.

Over the past two decades, self-assembly of supramolecular architectures has become a field of intensive research due to the wide range of applications for the resulting assemblies in various fields such as molecular encapsulation, supramolecular catalysis, drug delivery, metallopharmaceuticals, chemical and photochemical sensing, and light-emitting materials. For these purposes, a large number of coordination-driven metallacycles and metallacages featuring different sizes and shapes have been prepared and investigated. Almost all of these are Werner-type coordination compounds where metal centers are coordinated by nitrogen and/or oxygen donors of polydentate ligands. With the evolving interest in the coordination chemistry of N-heterocyclic carbenes (NHCs), discrete supramolecular complexes held together by M-CNHC bonds have recently become of interest. The construction of such metallosupramolecular assemblies requires the synthesis of suitable poly-NHC ligands where the NHC donors form labile bonds with metal centers thus enabling the formation of the thermodynamically most stable reaction product. In organometallic chemistry, these conditions are uniquely met by the combination of poly-NHCs and silver(I) ions where the resulting assemblies also offer the possibility to generate new structures by transmetalation of the poly-NHC ligands to additional metal centers forming more stable CNHC-M bonds. Stable metallosupramolecular assemblies obtained from poly-NHC ligands feature special properties such as good solubility in many less polar organic solvents and the presence of the often catalyticlly active {M(NHC)n} moiety as building block. In this Account, we review recent developments in organometallic supramolecular architectures derived from poly-NHC ligands. We describe dinuclear (M = AgI, AuI, CuI) tetracarbene complexes obtained from bis-NHC ligands with an internal olefin or two external coumarin pendants and their postsynthetic modification via a photochemically induced single or double [2 + 2] cycloaddition to form dinuclear tetracarbene complexes featuring cyclobutane units. Even three-dimensional cage-like structures can be prepared by this postsynthetic strategy. Cylinder-like trinuclear, tetranuclear, and hexanuclear (M = AgI, AuI, CuI, HgII, PdII) complexes have been obtained from benzene-bridged tris-, tetrakis-, or hexakis-NHC ligands. These complexes resemble polynuclear assemblies obtained from related polydentate Werner-type ligands. Contrary to the Werner-type complexes, cylinder-like assemblies with three, four, or six silver(I) ions sandwiched in between two tris-, tetrakis-, or hexakis-NHC ligands undergo a facile transmetalation reaction to give the complexes featuring more stable M-CNHC bonds, normally with retention of the metallosupramolecular structure. This unique behavior of NHC-Ag+ complexes allows the prepration of assemblies containing various metals from the poly-NHC silver(I) assemblies. Narcissistic self-sorting phenomena have also been observed for mixtures of selected poly-NHC ligands and silver(I) ions. Even a very early type of metallosupramolecular assembly, the tetranuclear molecular square, can be prepared from four bridging dicarbene ligands and four transition metal ions either by a stepwise assembly or by a single-step protocol. At this point, it appears that procedures for the synthesis of metallosupramolecular assemblies using polydentate Werner-type ligands and metal ions can be transferred to organometallic chemistry by using suitable poly-NHC ligands. The resulting structures feature stable M-CNHC bonds (with the exception of the labile CNHC-Ag+ bond) when compared to M-N/M-O bonds in classical Werner-type complexes. The generally good solubility of the compounds and the presence of the often catalytically active {M(NHC)n} moiety make organometallic supramolecular complexes a promising new class of molecular hosts for catalytic transformations and encapsulation of selected substrates.

Insights into chemoselective fluorination reaction of alkynals via N-heterocyclic carbene and Brønsted base cooperative catalysis

A systematically theoretical study has been carried out to understand the mechanism and chemoselectivity of N-heterocyclic carbene (NHC)-catalyzed fluorination reaction of alkynals using density functional theory calculations. The calculated results reveal that the reaction contains several steps, i.e., formation of the actual catalyst NHC, the nucleophilic attack of NHC on the carbonyl carbon atom of a formyl group, the formation of Breslow intermediate, the removal of methyl carbonate group to afford cumulative allenol intermediate, C–F bond formation coupled with generation of (SO2Ph)2N− anion, esterification accompanied with formation of (SO2Ph)2NH, and dissociation of NHC from product. For the formation of Breslow intermediate via the [1,2]-proton transfer process, apart from the direct proton transfer mechanism, the H2O- and EtOH-mediated proton transfer mechanisms were also investigated, and the free energy barriers for the crucial proton transfer steps can be significantly lowered by explicit inclusion of the protic media EtOH. Furthermore, multiple analyses have also been performed to explore the roles of catalysts and origin of chemoselectivity. Noteworthily, the in situ formed Bronsted base (BB) (SO2Ph)2N− anion was found to play an indispensable role in the esterification process, indicating that the reaction undergoes NHC-BB cooperatively catalytic mechanism, which is remarkably different from the direct esterification pathway proposed in the experimental references. This theoretical work provides a case on the exploration of the dual catalysis in NHC chemistry, which is valuable for rational design on newly cooperative organocatalysis in future.

NHC‐stabilization of reactive units in main group and transition metal chemistry

A third decade with continuously growing attention for the chemistry of N-heterocyclic carbenes (NHCs) is nearing its completion and new applications for this compound class are still being discovered. For example, NHCs are employed as versatile ligands within novel transition metal and main group element compounds; even complexes with lanthanides and actinides are now known. Also, the application of NHCs in organocatalysis has been proven as an attractive metal-free method for accessing value added products in an efficient manner. Due to the strong σ-donor ability of NHCs, they are commonly employed as ligands in organometallic catalysts and often boost the catalytic activity above NHC-free analogues (e.g. Grubbs' 2nd generation and Organ's PEPPSI catalysts). Meanwhile, the utilization of NHCs to intercept otherwise elusive main group compounds has greatly influenced the direction of modern main group chemistry. More recently, the related cyclic(alkyl)(amino)carbenes (cAACs), with enhanced π-acceptor character, has provided exciting new vistas of reactivity including the formation and study of novel molecular inorganic radicals. Moreover, NHCs have spun off new ligand systems such as N-heterocyclic olefins (NHOs), as well as N-heterocyclic imines (NHIs). Although the chemistry of these ligand classes are still in their infancy in comparison to that of NHCs, they have already shown great potential for the stabilization of compounds with low-valent main group elements in addition to the implementation of NHOs and NHIs as supporting ligands within organometallic catalysis. This Special Issue of Zeitschrift für Anorganische und Allgemeine Chemie (ZAAC) entitled “NHC-stabilization of reactive units in main group and transition metal chemistry” aims to showcase the recently developed applications with regard to this topic. The 15 contributions to this issue are written by some of the international leading researchers in this field. These articles range from the synthesis, structural characterization and reactivity of novel main group and transition metal complexes to their use in catalysis. We were able to collect a wide range of papers and may indulge the readership with the studies on Group 2 (Mg; one contribution), Group 7 (Mn; one contribution), Group 11 (Au; one contribution), Group 12 (Zn, Hg; two contributions), Group 13 (B, Al, Ga; four contributions), Group 14 (Si, Ge, Sn; four contributions), and Group 15 (N, P; two contributions) compounds. We would like to thank all authors for their contributions to this special issue. Also, all referees who evaluated the submitted manuscripts are gratefully acknowledged for their kind and essential advice and suggestions. Shigeyoshi Inoue Eric Rivard

ChemInform Abstract: Anionic N‐Heterocyclic Carbenes: Synthesis, Coordination Chemistry and Applications in Homogeneous Catalysis

AbstractReview: 304 refs.

ChemInform Abstract: Coordination Chemistry of Ditopic Carbanionic N‐Heterocyclic Carbenes

AbstractReview: 132 refs.

N-heterocyclic carbene-main-group chemistry: a rapidly evolving field.

This Award Article targets the evolving, yet surprisingly fruitful, chemistry of N-heterocyclic carbenes with low-oxidation-state main-group elements. Specifically, the chemistry of carbene-stabilized diatomic allotropes, diborenes, gallium octahedra, beryllium borohydride, and a host of related compounds will be presented. Providing a valuable historical perspective, the foundational work concerning the organometallic chemistry of gallium with sterically demanding m-terphenyl ligands from this laboratory will also be discussed.

Oxidative Addition of 2‐Halogenoazoles—Direct Synthesis of Palladium(II) Complexes Bearing Protic NH,NH‐Functionalized NHC Ligands

The chemistry of N-heterocyclic carbenes (NHCs) is dominated by N,N'-dialkylated or -diarylated derivatives. Such NHC ligands are normally obtained by C2-deprotonation of azolium cations or by reductive elimination from azol-2-thiones. A simple one-step procedure is described that leads to complexes with NH,NH-functionalized NHC ligands by the oxidative addition of 2-halogenoazoles to complexes of zero-valent transition metals.

ChemInform Abstract: N‐Heterocyclic Carbenes (NHCs) as Organocatalysts and Structural Components in Metal‐Free Polymer Synthesis

AbstractReview: 261 refs.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.

Group 7 transition metal complexes with N-heterocyclic carbenes

This tutorial review summarizes all works and highlights recent advances published in the growing field of group 7 N-heterocyclic carbene (NHC) complexes. It provides a valuable source for all scientists that are interested in conducting research with new compounds bearing these metals. The article provides an overview of all manganese NHC complexes that are known to date. There are a lot of examples where manganese NHC complexes show unpredicted behaviour during synthesis, very different from other transition metal NHC complexes and their higher homologues rhenium and technetium. These differences are depicted and discussed. Furthermore, the chemistry of technetium NHC compounds and their chemical behaviour are discussed. To date published work on technetium NHC chemistry has been restricted to the oxidation state +v. It was found that such compounds are very reactive but show great stability in dry air. Their radioactivity makes such compounds interesting candidates for radiochemical applications. Since most group 7 NHC chemistry was conducted on rhenium NHC complexes, this tutorial review highlights the chemical behaviour of such compounds. Rhenium(i) complexes reveal luminescent properties, making them interesting candidates for applications ranging from biological markers to organic light-emitting diodes (OLEDs). Another interesting feature is the radioactivity of some compounds, which makes them excellent candidates for radiopharmaceutical research; hence their synthesis and reactivity are discussed.

ChemInform Abstract: The N‐Heterocyclic Carbene Chemistry of Transition‐Metal Carbonyl Clusters

AbstractReview: ca. 100 refs.

Recent Advances in the Chemistry of Pyrazoles. Part 2. Reactions and N-Heterocyclic Carbenes of Pyrazole

This review article summarizes progress in N-alkylations, N-alkenylations, N-and C-arylations and hetarylations, halogenations, as well as organometallic chemistry of pyrazoles mainly achieved during the last decade. In addition, the generation and chemistry of N-heterocyclic carbenes of pyrazole (pyrazol-3-ylidenes) and remote N-heterocyclic carbenes of pyrazole (pyrazol-4-ylidenes) are described.