N‐Heterocyclic Carbene Complexes of Rhodium and Iridium: Steric Effects on Molecular Conformation

Abstract

AbstractA series of new RhI triazole‐based N‐heterocyclic carbene (NHC) complexes [(cod)Rh(NHC)Cl] 2a–c were obtained by transmetallation from AgI complexes using n‐butyl, benzyl and neopentyl NHC wing tip substituents. The corresponding new RhI ionic complexes (3a‐c) were synthesized bytreating compounds 2a–c with triphenylphosphane and AgBF4. These complexes were characterized by nmr spectroscopy and the structures of 2c, 3a, 3b, and 3c were determined by single‐crystal X‐ray diffraction. Compound 3b formed two polymorphs, the first reported incidence of polymorphism in this type of NHC. Ir analogues, 4, of complexes 3a–c were also synthesized; two complexes are isomorphous with the Rh analogues 4b and 4c while 4a has a different structure. Differences in syn and anti molecular conformation of the NHC ligands is rationalized in terms of stabilizing intramolecular C–H···π interactions and steric crowding of the PPh3 ligand.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)