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

A series of new Rh I triazole‐based N‐heterocyclic carbene (NHC) complexes [(cod)Rh(NHC)Cl] 2a – c were obtained by transmetallation from Ag I complexes using n ‐butyl, benzyl and neopentyl NHC wing tip substituents. The corresponding new Rh I ionic complexes ( 3a‐c ) were synthesized bytreating compounds 2a – c with triphenylphosphane and AgBF 4 . 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 PPh 3 ligand.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)