Introduction to themed issue on N-heterocyclic carbenes

Few chemists would have predicted the scope and intensity of research in carbene chemistry initiated in 1991 by Arduengo's report on the isolation of the first stable “bottleable” N-heterocyclic carbene (NHC). Today we know a vast array of heterocyclic carbenes, differing in the size of the heterocycle, the type of heteroatoms within the ring and its substitution pattern.

As diverse as the topology of NHCs, are their applications. They are electron-rich nucleophiles which can act as organocatalysts with a wide scope of applications based on chiral modifications or a variation of their steric bulk. Possibly even more important, NHCs are superb ligands in organometallic chemistry. NHCs have been shown to coordinate to essentially all transition metals, the lanthanides and actinides and to numerous main group elements. In doing so, they often stabilize otherwise reactive organometallic species. Occasionally, the NHC acts as a non-innocent ligand in such complexes and a number of intramolecular C–H activation processes confirm this behaviour.

Transition metal complexes bearing NHC ligands have become valuable catalysts for selected transformations, most notable are the ruthenium catalyzed olefin metathesis and palladium catalyzed C–C cross coupling reactions. Additional applications like those recently described for gold–NHC complexes emerge regularly and interesting developments can be expected for this field in the future.

Shortly after A. J. Arduengo, III described the first stable NHC, he presented a lecture on his discovery in the Department of Chemistry of the Free University of Berlin where I served as faculty member at the time. The fascinating presentation enticed one of the graduate students in my laboratory to work as a postdoc in Arduengo's laboratory after graduation. He did so shortly thereafter which marked the starting point of his long lasting interest in NHC chemistry. It was the same graduate student whose group provided, 15 years later, the cover art for this themed issue on NHC chemistry. The cover picture shows a frustrated Lewis pair (FLP) made up from the Lewis acid (C₆F₅)₃B and an NHC as the Lewis base. The activation of unreactive substrates by FLPs constitutes one of the most modern developments in NHC chemistry and is highlighted in this themed issue with contributions by M. Tamm and D. Stephan.

For the themed issue we have tried to gather contributions from contemporary areas of research involving NHCs. Among these are novel annulated or C4/C5 alkylated as well as expanded-ring, fused polycyclic and chiral NHCs. The ever expanding field of silver and gold NHC complexes is represented by a number of contributions. NHC complexes of magnesium, zinc and uranium are described as well as manganese carbene transfer agents. A substantial number of contributions deal with complexes of donor-functionalized and pincer-type carbene ligands and their applications in selected catalytic reactions. Some years ago, Bielawski and coworkers developed rigid biscarbene ligands and the coordination chemistry of these ditopic ligands is highlighted in a number of manuscripts. The anti-cancer properties of NHCs are the topic of one contribution. Finally NHCs have found applications in bioorganometallic chemistry. Youngs et al. describe a theobromine derived silver complex with high antimicrobial activity and Metzler-Nolte et al. have studied the anti-proliferation activity of NHC gold peptide conjugates.

The themed issue contains a total of 49 contributions originating from four continents. It is hoped that it will serve as an overview on modern NHC chemistry and stimulate new research in this ever-burgeoning field.

F. Ekkehardt Hahn

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