Synthetic Routes to Late Transition Metal–NHC Complexes

doi:10.1016/j.trechm.2020.06.001

Trends in Chemistry

Volume 2, Issue 8, August 2020, Pages 721-736

https://doi.org/10.1016/j.trechm.2020.06.001 Get rights and content

Highlights

The growing interest in the use of late transition metal–N-heterocyclic carbene ligand (NHC) complexes in catalysis, medicinal chemistry, and materials science has stimulated the development of various synthetic strategies, leading to the isolation of these compounds.
A recent procedure for the preparation of metal–NHC complexes consists of the use of weak bases and green solvents, operating in air and under generally mild conditions.
The weak base approach has allowed for synthesis in high yields and purity of many metal complexes in recent years, some of which have proven to be powerful synthons as well as efficient homogeneous catalysts and promising bioactive compounds.
Mechanistic studies have provided some important evidence on the role of both the weak base and the nature of the metal precursor used to obtain the target compounds.

Transition metal complexes bearing N-heterocyclic carbene ligands (NHCs) have gained a place of crucial importance in numerous areas of research such as medicinal chemistry, material sciences, and homogeneous/heterogeneous catalysis. In the present review, an updated overview of the main synthetic routes used for the preparation of this broad class of compounds along with their main applications are reported. Particular attention will be paid to the synthesis of late transition metal–NHC complexes using weak bases, eco-friendly solvents, and mild operating conditions. This simple synthetic approach, also known as the ‘weak base route’, represents a recent development with yet unrealized potential.

Section snippets

A Brief Overview of N-Heterocyclic Carbenes (NHCs)

Carbenes, in the broadest sense, are molecules containing a two-coordinate carbon with two unpaired electrons and, by virtue of their electronic deficiency, are traditionally considered highly reactive transient species involved in many organic transformations such as the cyclopropanation of alkenes. After the first pioneering studies published independently by Öfele [1] and Wanzlick [2] in 1968, in which they described the first metal–NHC complexes, Arduengo and coworkers obtained the first

Classical Synthetic Approach: Free Carbene and Transmetalation Routes

The two most widely used methods for the synthesis of transition metal–NHC complexes involve the use of free carbenes (isolated or generated in situ) or by carbene transfer reactions (transmetalation).

Use of Weak Bases as an Eco-Friendly Route

An alternative approach to the free carbene and transmetalation routes has been extensively investigated and involves the use of a weak base (e.g., K₂CO₃, NEt₃, or NaOAc) in reactions with the azolium salt and a metallic precursor of interest. Operating in air, under generally mild conditions and in green solvents, a wide range of carbene compounds with various transition metals are obtained in high yields and purity using this method. What follows is an overview of the use of the weak base

Concluding Remarks

As the enormous interest and activity in the field of transition metal–NHC chemistry and catalysis evolves, it is of interest to take a pause and evaluate the synthetic methods used to access these fascinating well-defined complexes. This evolution has taken us from the brute force approach of generating the free NHC at whatever the cost, to using transmetalation routes that use easily accessible agents, to now using weak bases and very mild conditions. Whether this approach is truly as general

Acknowledgments

The UGent BOF (starting and project grants to SPN) and the Moonshot project D2M are gratefully acknowledged for support.

Glossary

Azolium salts: salts obtained for the protonation or substitution at nitrogen of imidazoles and their derivatives.
Heteroleptic: a complex with two or more types of ligands.
Homoleptic: a complex with all ligands identical.
Ligand displacement: the exchange of one ligand for another, with no change in oxidation state at the metal center.
Synthon: a structural unit within a molecule, which, by means of a chemical reaction, can be converted into the desired product.
Transmetalation: a reaction that involves the

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Trends in Chemistry

ReviewSynthetic Routes to Late Transition Metal–NHC Complexes

Highlights

Section snippets

A Brief Overview of N-Heterocyclic Carbenes (NHCs)

Classical Synthetic Approach: Free Carbene and Transmetalation Routes

Use of Weak Bases as an Eco-Friendly Route

Concluding Remarks

Acknowledgments

Glossary

Adv. Inorg. Chem.

J. Organomet. Chem.

Tetrahedron

Organometallics

Organometallics

Organometallics

Organometallics

Polyhedron

Adv. Synth. Catal.

Curr. Opin. Chem. Biol.

J. Organomet. Chem.

Eur. J. Med. Chem.

Polyhedron

Org. Biomol. Chem.

Organometallics

Adv. Synth. Catal.

Polyhedron

Org. Lett.

Chem. Sci.

1,3-Dimethyl-4-imidazolinyliden-(2)-pentacarbonylchrom ein neuer übergangsmetall-carben-komplex

J. Organomet. Chem.

Direkt-synthese eines quecksilbersalz-carben-komplexes

Angew. Chem.

A stable crystalline carbene

J. Am. Chem. Soc.

N-heterocyclic carbenes in materials chemistry

Chem. Rev.

N-heterocyclic carbene adducts of main group elements and their use as ligands in transition metal chemistry

Chem. Rev.

Recent advances in rare earth complexes containing N-heterocyclic carbenes: synthesis, reactivity, and applications in polymerization

Catalysts

An overview of N-heterocyclic carbenes

Nature

Recent progress of N-heterocyclic carbenes in heterogeneous catalysis

Catal. Sci. Technol.

N-heterocyclic carbene complexes in C–H activation reactions

Chem. Rev.

N-Heterocyclic Carbenes in Organocatalysis

Imidazolium ionic liquids, imidazolylidene heterocyclic carbenes, and zeolitic imidazolate frameworks for CO2 capture and photochemical reduction

Angew. Chem. Int. Ed.

Quantifying and understanding the steric properties of N-heterocyclic carbenes

Chem. Commun.

Electronic properties of N-heterocyclic carbenes and their experimental determination

Chem. Rev.

The “Wanzlick equilibrium”

Angew. Chem. Int. Ed.

Stable abnormal N-heterocyclic carbenes and their applications

Chem. Soc. Rev.

N-Heterocyclic Carbenes. From Laboratory Curiosities to Efficient Synthetic Tools

A novel class of ruthenium catalysts for olefin metathesis

Angew. Chem. Int. Ed.

Olefin metathesis-active ruthenium complexes bearing a nucleophilic carbene ligand

J. Am. Chem. Soc.

Synthesis and activity of a new generation of ruthenium-based olefin metathesis catalysts coordinated with 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligands

Org. Lett.

Facile synthesis of silver(I)−carbene complexes. Useful carbene transfer agents

Organometallics

Copper N-heterocyclic carbene (NHC) complexes as carbene transfer reagents

Chem. Commun.

C−H oxidative addition of bisimidazolium salts to iridium and rhodium complexes, and N-heterocyclic carbene generation. A combined experimental and theoretical study

Organometallics

Oxidative addition of imidazolium salts to Ni0 and Pd0: synthesis and structural characterization of unusually stable metal–hydride complexes

Angew. Chem. Int. Ed.

Electron-rich olefin-derived neutral mono- and bis-(carbene) complexes of low-oxidation-state manganese, iron, cobalt, nickel, and ruthenium

J. Chem. Soc. Dalton Trans.

Synthesis and characterization of a series of new luminescent NHC-coordinated AuI –AgI tetra- and polymetallic complexes containing benzoate-bridged Ag2 dimers

Eur. J. Inorg. Chem.

Microwave-assisted synthesis of (N-heterocyclic carbene)MCl complexes of group 11 metals

Eur. J. Inorg. Chem.

Straightforward synthesis of [Au(NHC)X] (NHC = N-heterocyclic carbene, X = Cl, Br, I) complexes

Review
Synthetic Routes to Late Transition Metal–NHC Complexes

Imidazolium ionic liquids, imidazolylidene heterocyclic carbenes, and zeolitic imidazolate frameworks for CO₂ capture and photochemical reduction

Oxidative addition of imidazolium salts to Ni⁰ and Pd⁰: synthesis and structural characterization of unusually stable metal–hydride complexes

Synthesis and characterization of a series of new luminescent NHC-coordinated Au^I –Ag^I tetra- and polymetallic complexes containing benzoate-bridged Ag₂ dimers