Preventing NHC Dissociation

Dissociation of the NHC ligand from the metal is often driven by steric strain. Thus one strategy for preventing such dissociation is to decrease their overall steric impact on the coordination sphere of the metal. The use of ancillary Hgands that remove electron density from very electron rich systems may also prevent dissociation, although these two effects are sometimes difficult to separate. 

For example, the Crudden group showed that [(IMes)RhCl(PR3)2] complexes underwent dissociation of the carbene upon heating to 60-80 Treating this species with CO led to the less electron-rich and less sterically crowded complex [(IMes)RhCl(PR3)(CO)], which was stable for weeks at 80 °C. The latter complex also displayed significantly improved oxidative stability compared with the bis carbene analogue. 

Nickel complexes provide a valuable platform for the study of steric effects since shorter Ni-L bonds magnify these effects. When [Ni(CO)4] was treated with various carbenes, the expected [(NHC)Ni(CO)3] complexes were isolated for the less sterically hindered carbenes. However when carbenes featuring larger iV-substituents, such as adamantyl or t-Bu, were employed, an equilibrium was established that involved dissociation of the NHC from the coordination sphere of the metal. 

In accord with this, the attempted preparation of [(ItBu)2Ni] from ItBu and various Ni species gave different products depending on the reaction conditions, but not the expected [(ItBu)2Ni] complex. The synthesis of [(IPr)2Ni] via reaction of [Ni(COD)2] with the free IPr carbene was reported. However, in their attempted synthesis, Louie et al. reported that the reaction between [Ni(COD)2] and IPr actually resulted in the establishment of an equilibrium between this complex and free IPr with a = I.  

Similar dissociative equilibria were observed in NHC-Pd complexes, particularly bis-NHC complexes (see Section 3.3.5). Herrmann and co-workers showed that under the conditions of the Suzuki-Miyaura reaction, [(NHC)2Pd] complexes decomposed generating Pd black with the exception of mesityl-substituted complex 68 (Equation (3.12)). Interestingly, despite this observation, complex 71 was the most active of those examined. 

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