Carbon-Donor Ligands

Introduction

Carbon-donor ligands are a class of ligands that contribute carbon atoms in chemical reactions and they are usually referred to as carbene ligands. Carbon monoxide is the simplest carbon-donor ligands, and N-Heterocyclic carbenes are an important class of compounds that are indeed regarded as most versatile carbon-donor ligands in transition metal and organometallic catalysis. A N-Heterocyclic carbene contains a nonbonded singlet lone pair and is an electron-rich neutral donor ligand, these neutral two-electron donors show pronounced σ-donor ability with only little to no π-back-bonding. Due to their strong σ-electron-donating properties, N-Heterocyclic carbenes ligands usually form strong bonds with most metal centers and are therefore highly resistant toward decomposition. Beyond that, their facile availability, synthetic flexibility, and beneficial impact on (catalytic) reactivity of the metal center have bring many advantages in research of carbene metal complexes. Because of the advantages mentioned above, N-Heterocyclic carbenes has disclosed highly diverse fields of applications.

Applications

N-Heterocyclic carbenes have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, photophysical, and biochemistry^[1].

Fig.1 Some structure of N-Heterocyclic carbenes complexes

• Catalysis: The ability of N-Heterocyclic carbenes ligands as strong σ-donors is a big advantage to improve catalytic performance. N-Heterocyclic carbenes can combine with many metals such as rhodium, gold, silver, palladium, iridium, ruthenium, osmium, copper, Iron, cobalt, nickel, molybdenum and so on. The resulting coordination compounds can catalyze many organic reactions, the major types of reactions, viz, redox-neutral reactions (cross-coupling, metathesis, substitution), reductions (hydrogenation, transfer hydrogenation, hydroelement additions), and oxidations (including oxidative couplings). In which, the hydrogenation of organic substrates can be catalyzed by the Ir, Ru, Rh, Pd, and Os complexes based on N-Heterocyclic carbenes. The oxidation of alcohols and amines can be catalyzed by the Ru, Os, Rh, and Ir complexes based on N-Heterocyclic carbenes. Cross-coupling reactions, in particularly Suzuki-Miyaura-type aryl-aryl coupling can be catalyzed by N-Heterocyclic carbenes based Pd complexes and more recently also be catalyzed by N-Heterocyclic carbenes based Ni complexes. And cyclization reactions including the condensation of aldehydes and isonitrile esters to produce oxazolines can be catalyzed by N-Heterocyclic carbenes based Ag and Au complexes.
• Photophysical field: N-Heterocyclic carbenes complexes have attractive optical properties and demonstrate potential for the preparation of new optical devices and organic light emitting diodes (OLEDs). While this field of application is relatively unexplored with N-Heterocyclic carbenes ligands, various complexes of Fe, Ru, Ir, Pd, Pt, and Au based on N-Heterocyclic carbenes ligands have shown promising photophysical properties.
• Biological field: (1) Anticancer activity: as a representative ligand of N-Heterocyclic carbenes, the triazolylidene based ruthenium and osmium complexes (as shown in Fig.2) containing a variety of substituents on the triazole scaffold reach anticancer activities in the submicromolar range. With this ligand both the Ru and the Os complexes are equally active. (2) Methyl transferase mimicry: the triazolylidene iridium complex offers a platform for the intramolecular transfer of a methylene group. The process is related to the cobalamine-catalyzed methylation of cytosine and other substrates.

Fig.2 The structure of triazolylidene based ruthenium and osmium complexes

• Others: N-Heterocyclic carbenes complexes show excellent device performance with an external quantum efficiency and luminescence property, and also can act as antioxidants.

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Reference

• Vivancos A.; et al. Mesoionic and related less heteroatom-stabilized N heterocyclic carbene complexes: synthesis, catalysis, and other applications[J]. Chemical Reviews. 2018, 120, 9493-9586.