Free N-heterocyclic carbenes (NHCs) are generally prepared by treatment of imidazolium precursors with strong alkali reagents, which usually produces inactive NHC dimers and would destroy the porous supports and thus make them to be difficult in recovery and reuse. Herein, we report the first stable CO2-masked N-heterocyclic carbenes (NHCs) grafted on a porous crystalline covalent organic framework (COF). The stable NHC-CO2 moieties in the COF-NHC-CO2 could be transformed in situ into isolated NHCs by heating, which exhibits superior catalytic performances in the hydrosilylation and N-formylation reactions with CO2. More interestingly, the NHC sites can reversibly form NHC-CO2 and thus can be easily recycled and reused while maintaining excellent catalytic activity. The density functional theory calculations revealed that NHC sites can be fully exposed after removal of CO2-masks and rapidly react with silanes, which endows COF-NHC with high catalytic activity.

A CO2-Masked Carbene Functionalized Covalent Organic Framework for Highly Efficient Carbon Dioxide Conversion.

Abstract N‐Heterocyclic carbenes (NHCs) are commonly prepared by deprotonation of azolium salts using strong anionic bases. This reaction is often unselective, yielding alkali metal NHC complexes or dimerized NHCs. Alternatively, free NHCs are obtained by the dechlorination of 2‐chloroazolium salts using electron‐rich phosphines. PPh3, PCy3, and PtBu3 are unsuitable for Cl+ abstraction, while the sterically encumbered tris(1,3‐tert‐butylimidazolidin‐2‐ylidenamino)phosphine 1 selectively removes Cl+ from 2‐chloroazolium salts. Since bulky 1 does not bind to metal complexes, it was used for the preparation of NHC complexes via in situ Cl+ abstraction from 2‐chloroazolium salts. The dechlorination was employed for the site‐selective monometallation with IrI, IrIII, RhI, RhIII, and RuII of a bis‐NHC precursor composed of a 2‐chlorobenzimidazolium and a 2‐chlorobenzimidazole group, followed by the preparation of the heterobimetallic IrIII/PdII complex [18](BF4)2 by a dechlorination/oxidative addition reaction sequence.

Synthesis of N‐Heterocyclic Carbenes and Their Complexes by Chloronium Ion Abstraction from 2‐Chloroazolium Salts Using Electron‐Rich Phosphines

Janus di-N-heterocyclic carbene (NHC) ligands are a subclass of poly-NHCs that feature coordination to two transition metals in a facially opposed manner. The combination of the structural features of Janus type ligands, with the properties conferred by the NHC ligands, has conferred Janus-di-NHCs with privileged attributes for their use in diverse areas of research, such as homogeneous catalysis, materials chemistry and supramolecular chemistry. In molecular chemistry, Janus di-NHCs constitute one of the most useful chemical platforms for constructing dimetallic structures, and this includes both homo- and hetero-dimetallic compounds. This review aims to cover the most relevant advances in the use of Janus-di-NHCs during the last 15 years, by classifying them according to their specific structural features.

https://pubs.rsc.org/en/content/articlepdf/2021/dt/d1dt02035h

Insights into the past and future of Janus-di-N-heterocyclic carbenes

Palladium(II) Complexes Bearing a Mixed Set of aNHC/Py/PR3/I2 Ligands: Applications in α-Arylation of Amide and Suzuki-Miyaura Coupling Reactions

Hetero- and Homobimetallic Complexes Bridged by a Bis(NHC) Ligand: Synthesis via Selective Sequential Metalation and Catalytic Applications in Tandem Organic Transformations

Bis-NHC precursors composed of an azolium and a 2-halogenoazole group connected by different linkers have been site-selectively metallated to give heterobimetallic complexes from a single-frame bis-NHC ligand. The azolium group reacts with a base and oxidized metal centers to give NHC complexes with no participation of the halogenoazole, while the halogenoazole reacts in an oxidative addition with low-valent transition metals to give azolato complexes with no participation of the azolium group.

Chemoselective synthesis of heterobimetallic bis-NHC complexes

Synthesis of Heterobimetallic Complexes through Chemoselective 2,4-Metalation of a Thiazolium Salt

The electronic properties of two indole-derived CAAC ligands, featuring N−Et (II) or N−H (IV) substituents, have been investigated by DFT calculations. The calculations revealed frontier orbitals (HOMO and LUMO) with energies closely related to those of the conventional N-DIPP substituted CAAC I. The selenium adduct of the N−H substituted CAAC 1 has been prepared and characterized by X-ray diffraction methods and 77Se NMR spectroscopy revealing π-acceptor properties of the CAAC similar to classical CAACs of type I. While the electronic properties of the indole-derived CAACs II and IV are similar to those of classical CAACs, their % Vbur is significantly smaller due to the smaller N-substituents falling in the range of % Vbur for sterically less demanding N-heterocyclic carbenes. 

Patrick D. Dutschke

Papers

Chemoselective synthesis of heterobimetallic bis-NHC complexes

Synthesis of N‐Heterocyclic Carbenes and Their Complexes by Chloronium Ion Abstraction from 2‐Chloroazolium Salts Using Electron‐Rich Phosphines

Synthesis of Heterobimetallic Complexes through Chemoselective 2,4-Metalation of a Thiazolium Salt

Steric and Electronic Properties of Indol‐Derived CAAC Ligands

Reactivity of Ruthenium(II) Complexes Bearing Bis-NHC Ligands