C−H Oxidative Addition of Bisimidazolium Salts to Iridium and Rhodium Complexes, and N-Heterocyclic Carbene Generation. A Combined Experimental and Theoretical Study

A series of bis-N-heterocyclic carbenes of rhodium and iridium have been obtained and characterized.
The formation of the M−C bond has been studied according to experimental and theoretical data, showing
that two different mechanisms are operative for the first (single proton deprotonation of the bisimidazolium
salt, or oxidative addition followed by deprotonation of the metal hydride) and second (oxidative addition
of the second bisimidazolium C−H bond, yielding a NHC-IrIII-H species) metalation processes. In the
case of complexes with long linkers between the imidazolium rings, reductive elimination of HCl affords
bisimidazolylidene complexes of IrI. According to the theoretical studies we concluded that thermodynamic
parameters would determine the formation of the NHC-IrIII-H species, while IrI-NHC species would be
kinetically favored in the case of complexes with long linkers between the azole rings. The crystal structures
of a series of Ir-bis(NHC) complexes are described.