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 nu(CO) of the various (NHC)RhCl(CO)(2) were determined. A correlation of the Rh redox potentials and the Rh nu(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 nu(av)(CO) results in an equation for the nu(av)(CO) transformation: nu(av)(CO) Ir = 0.8695.nu(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. (c) 2008 Elsevier B.V. All rights reserved.