Synthesis of (NHC)Rh(cod)Cl and (NHC)RhCl(CO)2 complexes – Translation of the Rh- into the Ir-scale for the electronic properties of NHC ligands

Twenty-three different Rh complexes of the (NHC)RhCl(cod) and (NHC)RhCl(CO)2 type were synthesized from [RhCl(cod)]2. The electron donating nature of the NHC ligands was changed in a systematic manner. The redox potentials of the various (NHC)RhCl(cod) and the ν(CO) of the various (NHC)RhCl(CO)2 were determined. A correlation of the Rh redox potentials and the Rh ν(CO), respectively, with the related data from analogous (NHC)IrCl(cod) and (NHC)IrCl(CO)2 complexes established two linear relationships. The linear regression (R2 = 0.993) of the Rh and the Ir redox potentials results in an equation for the redox potential transformation: E1/2(Ir) = 1.016 · E1/2(Rh) − 0.076 V. The linear regression (R2 = 0.97) of the Rh and Ir νav(CO) results in an equation for the νav(CO) transformation: νav(CO)Ir = 0.8695 · νav(CO)Rh + 250.7 cm−1. In this manner the Rh and the Ir-scale for the determination of the electron donating properties of NHC ligands are unified.