We provide an overview on the state-of-the-art in transition-metal complexes formed with water-soluble NHC ligands. Paths to introducing water solubility by ligand design are elucidated and some general properties of water-soluble NHC complexes are highlighted. The enhanced hydrophilicity of water-soluble catalysts offers advantages in applications. While studies based on C-C coupling reactions still dominate the field, recent reports show water-soluble NHC complexes can be applied in metathesis and hydrogenation reactions and turn out to be among the best performing catalysts known. Nevertheless, wide areas of this young field remain to be investigated, offering great potential for future research.
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.
A new ammonia adduct of a N-heterocyclic carbene (NHC) has been isolated, which can be used as a reagent for the synthesis of transition metal carbene complexes. It represents the first example of a 1,2,3-triazolylidene with a 1,2,4-substitution pattern, thus opening a new subclass ('normal' 1,2,3-triazolylidenes) of sterically and electronically tunable NHCs.
1,2,3-Triazolylidenes as versatile, strong donor ligands have currently experienced a boost in complex synthesis as well as catalytic applications. Although many examples of "abnormal" 1,2,3-triazolylidenes have been described, their "normal" congeners have been barely examined to date (for abnormal carbenes the resonance structures of the carbenes cannot be drawn without adding additional charges, but this is possible for normal carbenes). Furthermore, no instance of utilization of this new ligand class in homogeneous catalysis can be found. Therefore, this work presents a variety of potential precatalysts descending from "normal" 1,2,3triazolylidene Au chloride complexes. Synthesis and thorough characterization of the new compounds are presented, together with special ligand features such as buried volume and suspected anagostic interactions. The activity of the isolated precatalysts is examined in the intramolecular hydroamination of alkynes and compared with that of a popular imidazolylidene system. It is found that the activity of the best-performing "normal" 1,2,3-triazolylidene systems is quite similar to that of the imidazolylidene systems. However, mercury drop poisoning experiments suggest that improvements in ligand design are required to enhance catalyst stability.
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