N-Heterocyclic Carbene Complexes of Cobalt

Experimental and Theoretical Study of NiII‐ and PdII‐Promoted Double Geminal C(sp3)−H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

“…The relatively easy functionalization of pincer ligands and of NHC donors provides access to a great diversity of ligands and facilitates the finetuning of the properties of their metal complexes. [2][3][4][5]7,8,13,15,16,19,[41][42][43][44][45][46][47][48][49] The steric properties of NHCs can typically be modified by the choice of the N-substituents (or wingtip substituents) or the size of the heterocycle, whereas electronic properties are influenced by the nature and number of endocyclic heteroatoms, the site of attachment of the metal on the ring, the electronic saturation or unsaturation of the heterocycle, and a possible annellation resulting from fusing an additional ring on the basic framework. [50] Most synthetic methods leading to NHC complexes or to NHC-containing pincer complexes involve i) deprotonation of azolium salts to give a free or a coordinated NHC ligand (when a metal-bound ligand acts as an internal base), [7,8,41,[51][52][53] ii) transmetallation from silver, copper or mercury reagents, [7,8,23,[54][55][56][57] both approaches occur without modification of the metal oxidation state, [3,7,8,23,41] or iii) oxidative addition of an azolium C sp 2 (2)À H bond across a zerovalent metal precursor, which was introduced about 20 years ago.…”