Quantification and variation of characteristic properties of different ligand classes is an exciting and rewarding research field. N-Heterocyclic carbenes (NHCs) are of special interest since their electron richness and structure provide a unique class of ligands and organocatalysts. Consequently, they have found widespread application as ligands in transition-metal catalysis and organometallic chemistry, and as organocatalysts in their own right. Herein we provide an overview on physicochemical data (electronics, sterics, bond strength) of NHCs that are essential for the design, application, and mechanistic understanding of NHCs in catalysis.

The Measure of All Rings—N‐Heterocyclic Carbenes

N atom, thus providing carbenes derived from pyrazolium, isothiazolium, and even quinolinium salts that contain a stabilizing heteroatom in a remote position (G-J in Figure 1). Recently, carbenes such as K, which are comprised of only one heteroatom and lack delocalization through the heterocycle, have been discovered as versatile ligands, thus constituting another important class of carbenes with low heteroatom stabilization. Both the synthesis of the organometallic complexes of these ligands as well as the (catalytic) properties of the coordinated metal centers generally show distinct differences, compared to the more classical NHC complexes, such as C2-metallated imidazolylidenes. This review intends to describe such differences and highlights the chemical peculiarities of these types of N-heterocyclic carbene complexes. It introduces, in a qualitative manner, the synthetic routes that have been established for the preparation of such complexes, covering the literature from the very beginning of activities in this area up to 2008. While specialized reviews on some aspects of the present topic have recently appeared,7 a comprehensive overview of the subject has not been available thus far. Rather than just being descriptive, the present account is mainly directed toward the impact of these still unusual metal-carbene bonding modes on the electronic properties and on the new catalytic applications that have been realized by employing such new carbene complexes. As a consequence of our focus on complexes with less-stabilized heterocyclic ligands, systems comprising acyclic carbenes have not been included, and the interested reader is, instead, referred to the pioneering and

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Beyond Conventional N-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

The use of N-heterocyclic carbenes (NHCs) in chemistry has developed rapidly over the past twenty years. These interesting compounds are predominantly employed in organometallic chemistry as ligands for various metal centres, and as organocatalysts able to mediate an exciting range of reactions. However, the sheer number of NHCs known in the literature can make the appropriate choice of NHC for a given application difficult. A number of metrics have been explored that allow the electronic properties of NHCs to be quantified and compared. In this review, we discuss these various metrics and what they can teach about the electronic properties of NHCs. Data for approximately three hundred NHCs are presented, obtained from a detailed survey of the literature.

Quantifying and understanding the electronic properties of N-heterocyclic carbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

/pdf/stable-cyclic-carbenes-and-related-species-beyond-1d50zjyjzd.pdf

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes

Percent buried volume for phosphine and N-heterocyclic carbene ligands : steric properties in organometallic chemistry

Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

Katalytische Komplexe auf Rutheniumbasis und die Anwendung solcher Komplexe in der Olefinmetathese

L'invention concerne tout composes de formule (I) ou (II) dans laquelle: L est un ligand neutre; X, X' sont des ligands anioniques; R 1 et R 2 sont, independamment, un hydrogene, un alkyl en C 1 a C 6 , un perhalogenoalkyl en C 1 a C 6 , un aldehyde, une cetone, un ester, un nitrile, un aryl, un pyridinium-alkyl, un pyridinium-perhalogenoalkyl- ou un cyclohexyl en C 5 ou C 6 eventuellement substitue, un radical C n H 2n Y ou C n F 2n Y avec n compris entre 1 et 6 et Y un marqueur ionique, ou un radical de formule : R 1 pouvant etre un radical de formule (Ibis) quand le compose est de formule (I) ou de formule (IIbis) quand le compose est de formule (II) R 3 est un alkyl en C 1 a C 6 , ou un cycloalkyl en C 5 ou C 6 ou un aryl en C 5 ou C 6 ; R 0 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 , R 11 sont, independamment, un hydrogene, un alkyl en C 1 a C 6 , un perhalogenoalkyl en C 1 a C 6 , ou un aryl en C 5 ou C 6 ; R 9 , R 10 , R 11 pouvant former un heterocycle; X 1 est un anion. R 1 et R 2 pouvant former avec le N et le C auxquels ils sont rattaches un heterocycle.

Complexes catalytiques a base de ruthenium et utilisation de tels complexes pour la metathese d'olefines

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Determination of N-Heterocyclic Carbene (NHC) Steric and Electronic Parameters using the [(NHC)Ir(CO)2Cl] System

Katalytische Komplexe auf Rutheniumbasis und die Anwendung solcher Komplexe in der Olefinmetathese

Complexes catalytiques a base de ruthenium et utilisation de tels complexes pour la metathese d'olefines