Ligand Design for A-Heterocyclic Carbenes NHC

While the substituents at the nitrogen atoms influence the steric surroundings of the metal, the electronic interaction is dependent on the substitiuents at C4 and C5 and the type of heterocycle (imidazole, imidazoline, benzimidazole, etc.). 

The steric bulk of the substituents at the nitrogen atom ranges from the small methyl groups as in Ofele s first chromium NHC complex (2) [1] to the very bulky adamantyl groups that were introduced by Arduengo in the first isolated NHC (3) [3]. 

In their binding properties NHC resemble basic alkylphosphines. However, their o -donor ability is more pronounced than observed with phosphines, and they show almost no r-acceptor property. The basicity and o -donor ability of the metal-coordinating carbon atom is strongly dependent on the type of heterocycle on which the NHC is based. 

An even broader variety of electronic purposes is accessible, if the imidazole backbone is changed, for example, by introducing more heteroatoms like nitrogen or sulfur [7, 34, 35]. 

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