Nickel(II) and Palladium(II) Complexes with Chelating N-Heterocyclic Carbene Amidate Ligands: Interplay between Normal and Abnormal Coordination Modes

Nickel(II) complexes containing tridentate ONCi ligands (i = abnormal N-heterocyclic carbene donors) and their catalytic application in Suzuki-Miyaura coupling reaction

New nickel complexes with tridentate phenoxy-amidate-aNHC donor groups were synthesized from the reactions between nickel acetate and imidazolium ligand precursors in net pyridine. An unusual degradation pathway was observed, leading to imidazole derivatives occupying the fourth coordination sites in these square planar complexes. A new imidazole-coordinated nickel complex was found to be efficient in catalyzing Suzuki-Miyaura cross-coupling with aryl chlorides under 3 mol% of catalyst loading. The catalytic activities were superior to those of its reported normal NHC counterpart. Instead of the common procedure of using additional phosphine, the addition of IMes·HCl significantly enhances the product yields of the catalytic reaction.

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.

A comparative study on the N-heterocyclic carbene adducts of Ih-C60, D5h-C70 and Sc3N@Ih-C80

The exclusive formations of the normal nNHC-C_60/70 and abnormal aNHC-Sc₃N@I_h-C₈₀ complexes in two recent experiments were suggested to be thermodynamically and kinetically controlled, respectively.

Rhenium complexes of bidentate, bis-bidentate and tridentate N-heterocyclic carbene ligands

Rhenium(i) tricarbonyl complexes of a range of bidentate, bis-bidentate and tridentate NHC ligands have been prepared. These NHC ligands are of interest for possible applications in the development of Tc-99m or Re-186/188 radiopharmaceuticals and the stability of two complexes were evaluated in ligand challenge experiments using the metal binding amino acids l-histidine or l-cysteine.

Ru–Ag and Ru–Au dicarbene complexes from an abnormal carbene ruthenium system

The first complexes containing an anionic dicarbene connected to two different d-block elements are reported. The synthetic pathway entails metalation and transmetalation reactions, starting from a cationic abnormal Ru-NHC complex.