Aliphatic versus Aromatic C−H Activation in the Formation of Abnormal Carbenes with Iridium:  A Combined Experimental and Theoretical Study

The metalation of a series of C2-Me-substituted monoimidazolium and bisimidazolium salts to [Cp*IrCl₂]₂ is described. The reaction of the monoimidazolium salt provides the species Cp*Ir(aNHC)Cl₂, in which the NHC shows an abnormal coordination mode. The use of the bisimidazolium salt provides different reaction patterns depending on the linker length between the two azolium rings. For the methylene-linked bisimidazolium salt, the only compound obtained shows an unusual type of coordination in which the chelating ligand is coordinated through an abnormal NHC and a methylene group resulting from the CH activation of the C2-Me group. For the ethylene-linked bisimidazolium salt, a similar product is obtained, together with the chelating bis-abnormal-NHC species. All compounds have been fully characterized by usual spectroscopic techniques, and X-ray molecular structures are described. The formation of the reaction products, in the case of the methylene linker, has been rationalized by means of DFT calculations with inclusion of solvent effects (PCM). The calculations could not discriminate the nature of the first metalation between direct deprotonation of the ligand by the base and metalation through C−H activation at Ir. However both cases point to a kinetic preference for first metalation at the C2-Me group. The second metalation process is the result of kinetically preferred C−H activation at the C5 position.