Functionalised N-Heterocyclic Carbene Ligands in Bimetallic Architectures.

Kieren J Evans,Stephen M Mansell

doi:10.1002/chem.201905510

Kieren J Evans, Stephen M Mansell

Open Access

https://doi.org/10.1002/chem.201905510

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Abstract

N‐Heterocyclic carbenes (NHCs) have become immensely successful ligands in coordination chemistry and homogeneous catalysis due to their strong terminal σ‐donor properties. However, by targeting NHC ligands with additional functionalisation, a new area of NHC coordination chemistry has developed that has enabled NHCs to be used to build up bimetallic and multimetallic architectures. This minireview covers the development of functionalised NHC ligands that incorporate additional donor sites in order to coordinate two or more metal atoms. This can be through the N‐atom of the NHC ring, through a donor group attached to the N‐atom or the carbon backbone, coordination of the π‐bond or an annulated π‐donor on the backbone, or through direct metalation of the backbone.

Highlights

Bimetallic architectures combine two atoms of the same metal or two different metals in order to generate more diverse properties and chemical possibilities than from using one metal alone
Bimetallic compositions have demonstrated improved properties and reactivity in the solid state and in heterogeneous catalysis,[1] and the idea of using multiple metal atoms has been successfully exploited in coordination chemistry,[2] deprotonative metalation[3] and homogeneous catalysis.[4]
With the utility of N-heterocyclic carbenes (NHCs) well and truly established in coordination chemistry and catalysis,[5] more diverse designs of NHCs are being explored to expand upon this area.[6]

Summary

Introduction

Bimetallic architectures combine two atoms of the same metal (homobimetallic) or two different metals (heterobimetallic) in order to generate more diverse properties and chemical possibilities than from using one metal alone. Fusing the acetylacetonate (acac) moiety with an NHC backbone gives a bifunctional NHC with an X,L pocket in addition to the typical carbene L donor site (Scheme 12).[80] the RuÀRh bimetallic NHC complex 90 was synthesised by sequential metalation with [RuCl2(p-cym)]2 (yielding 89) and [Rh(cod)Cl]2, it was observed that reacting the dirhodium com-. 1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) was shown to react with nBuLi through deprotonation of the backbone forming a polymeric structure featuring coordination from both normal and abnormal NHC motifs (94, 95, Figure 15).[87] The potassium analogue has been described (96),[88] and was formed through transmetalation of LiIPr with KOtBu. Attempts to remove the K ion with the addition of 2,2,2-cryptand led to protonation of the carbene and degradation of the cryptand, suggesting the unstable nature of the free anionic NHC.

Summary and Outlook

Conflict of interest