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 by treating 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. It 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 center dot center dot center dot pi interactions and steric crowding of the PPh3 ligand. ((C)Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)