Pushing Electrons—Which Carbene Ligand for Which Application?

Abstract

This tutorial explores the diversity and characteristics of C-donor ligands. Particular emphasis is put on the conceptual design of the electronic properties for applications in coordination chemistry. More specifically, the implications of both σ and π effects are discussed. Cyclic (alkyl)(amino)carbenes as well as methanediides and carbones are brought in perspective to "conventional" N-heterocylic carbenes, mesoionic carbenes, and Fischer as well as Schrock carbene ligands.

An Efficient A3 Coupling Catalyst Based on a Silver Complex Bearing N‐Heterocyclic Carbene and Homoscorpionate Bis(3‐methyl‐mercaptoimidazolyl)borate Ligands

“…For example, some of these reactions require high reaction temperatures or a high percentage of catalyst loading is required to achieve higher yields of products . Among carbene ligands, N‐heterocyclic carbenes, commonly known as NHCs, are excellent sigma donor ligands and their electronic and steric properties are adjustable by changing the nature of substituents on the nitrogen atoms . While different salts or nanostructures of coinage metals have been utilized as catalysts for A 3 coupling reactions, reports in regards to the N‐heterocyclic complexes of these metals are rare.…”

Borylated N‐Heterocyclic Carbenes: Rearrangement and Chemical Trapping

“…Singlet carbenes have attracted a huge amount of interest in the past three decades, following initial reports from Bertrand and Arduengo . This reflects not only their application as ancillary ligands in a range of noble metal catalysts (drawing on the strength of the associated M−C bonds), but also their use as bespoke donors for the stabilization of a range of compounds featuring p ‐ , d‐, and f‐block metals with unprecedented structural motifs . CR 2 species of this type feature a filled σ‐type orbital as the HOMO, with a formally vacant pπ‐orbital typically being the LUMO .…”

“…This reflects not only their application as ancillary ligands in a range of noble metal catalysts (drawing on the strength of the associated M−C bonds), but also their use as bespoke donors for the stabilization of a range of compounds featuring p ‐ , d‐, and f‐block metals with unprecedented structural motifs . CR 2 species of this type feature a filled σ‐type orbital as the HOMO, with a formally vacant pπ‐orbital typically being the LUMO . The HOMO–LUMO energy gap is critical in dictating patterns of reactivity and donor properties as ligands, and can be tuned, for example, by variation of the R substituents or by adjusting the angle at carbon.…”

Switching the nature of catalytic centers in Pd/NHC systems by solvent effect driven non‐classical R‐NHC Coupling

“…Their properties allow to obtain metal complexes by conventional laboratory methods. Tinkering with their structure is easy in comparison with several other classes of ligands and provides a key to control catalytic activity of NHC complexes . Optimization of chemically flexible NHC ligand‐based catalytic systems is convenient and represents a rewarding task.…”

“…Tinkering with their structure is easy in comparison with several other classes of ligands and provides a key to control catalytic activity of NHC complexes. [20] Optimization of chemically flexible NHC ligand-based catalytic systems is convenient and represents a rewarding task. In some cases, replacement of a phosphine ligand with NHC allows substantial enhancement of the catalyst activity.…”