Abstract
Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon–carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.
Highlights
The current concept of coordination bond in the metal complexes defies the earlier definition of the covalent bond in molecular structures
A comparison with that of monodentate N-heterocyclic carbenes (NHCs) ligands was carried out, and the results showed these metal complexes formed metal nanoparticles which turned out to be highly active in levulinic acid conversion to γ-valerolactone [132], Scheme 20
The continued interest in the synthesis of novel NHC ligands and investigation into their applications is evident from an increase in the number of literature reports about this class of ligands in recent years [256,257]
Summary
The current concept of coordination bond in the metal complexes defies the earlier definition of the covalent bond in molecular structures. Pfaltz et al introduced a library of bidentate NHC-pyridine type chelating ligands and their iridium complexes (49a–c) for catalytic asymmetric hydrogenation of olefins [127], Figure 4. The resulting iridium complexes, 60a–c, with chelate NHC-pyridine ligands were used in catalytic asymmetric hydrogenation reactions of selected alkenes. A comparison with that of monodentate NHC ligands was carried out, and the results showed these metal complexes formed metal nanoparticles which turned out to be highly active in levulinic acid conversion to γ-valerolactone [132], Scheme 20.
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