Mesoionic Carbenes: Exceptionally Electron-Rich Carbon-Donor Ligands in Synthesis, Catalysis, and Materials Science

Dear Colleagues,

Ligands have the ability to control the electronic and structural properties of metal complexes and, therefore, to regulate the way they react. Consequently, major advances in molecular science in general are directly related to the emergence of new types of innovative ligands. 1,3-Imidazole derived N-heterocyclic carbenes (NHCs) (A) are a prominent example and ranked as very versatile carbon-donor ligands in organometallic chemistry and catalysis. NHCs typically bind to metals at the C2-carbon atom and are generally regarded as normal or classical NHCs. The strong σ-donor ability of NHCs leads to the formation of a rather robust M‒C_(NHC) bond, giving rise to very stable metal complexes. In addition, it also enhances the reactivity of metal complexes by making the metal atom more electron rich. NHCs have also been utilized in main-group chemistry and numerous molecules with fascinating structures and intriguing bonding situations have been stabilized by the use of NHCs. This is clearly because of the strong σ-donor strength, as well as auspicious steric features of NHCs. Over the past few years, a relatively new type of carbenes (B), in which the carbene carbon atom is located at the unusual C4-position, has received significant attention, as they are even stronger σ-donors than their classical (C2) analogs A. Similarly, 1,2,3-triazole derived C4-carbenes (D) have also been shown as very powerful ligands in catalysis and organometallic chemistry. These so-called abnormal-NHCs (aNHCs) are also referred to as mesoionic carbenes (MICs), as no neutral Lewis form without the introduction of formal charges can be written for B and D.

While D are stronger σ-donors than classical NHCs A and C, aNHCs (or MICs) B are exceptional. Whether described as aNHCs or MICs, these C4 carbenes B and D have already been recognized as very powerful ligands in synthesis and catalysis. Moreover, the scope of MICs is continuously expanding with the availability of their efficient synthetic methods. In addition, the possibility of incorporating new functionalities in MICs offers unique opportunities for controlling/modifying the stability, reactivity, and other properties of derived complexes.

The central aim of this Special Issue is to compile the original cutting-edge research in MIC chemistry that encompasses the structural and electronic investigations of unique molecular entities, catalysis, small molecular activation, organic synthesis, theoretical analysis, and molecular materials, among others.

Prof. Dr. Rajendra S. Ghadwal
Guest Editor

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