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

Abstract

LRhCl(CO) 2 and LIrCl(CO) 2 have been used to quantify the electron donating properties of ligands via the respective ν (CO) frequencies (Rh- and Ir-scale). Following the synthesis of (NHC)RhCl(CO) 2 and (NHC)IrCl(CO) 2 complexes and the carbonyl frequency determination, it is now possible to convert the Rh- into the Ir-scale and vice versa. 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 ( R 2 = 0.993) of the Rh and the Ir redox potentials results in an equation for the redox potential transformation: E 1/2 (Ir) = 1.016 · E 1/2 (Rh) − 0.076 V. The linear regression ( R 2 = 0.97) of the Rh and Ir ν av (CO) results in an equation for the ν av (CO) transformation: ν av (CO)Ir = 0.8695 · ν av (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.