Arene-polyfluoroarene π-π stacking between N-heterocyclic carbene ligands of pentamethylcyclopentadienyl group 9 metal complexes

(eta(5)-Pentamethylcyclopentadienyl)rhodium(kappa C-1-(2,3,4,5,6-pentafluorophenylmethyl)-3-benzylimidazol-2-ylidene) dichloride (la) and its iridium analogue (Ib) crystallize as dichloromethane solvates in the enantiomorphous space group P2(1)2(1)2(1). (eta(5)-Pentamethylcyclopentadienyl)rhodium(kappa C-1-(4-trifluoromethyl-2,3,5,6-tetrafluorophenylmethyl)-3-benzylimidazol-2-ylidene) dichloride (2a) and its iridium analogue (2b) crystallize as dichloromethane solvates in the polar space group Pca2(1). All the crystal structures possess columns of alternating complex and dichloromethane molecules which are linked by hydrogen bonding. Perpendicular to these is hydrogen bonding between the imidazol-2-ylidene hydrogen atoms and chloride ligands. The crystal structures of 2a and 2b also display intermolecular pi-pi stacking between the phenyl and trifluoromethyltetrafluorophenyl rings of molecules in a direction perpendicular to the columns and hydrogen bonding between the complex molecules. In contrast those of 1a and 1b display intermolecular lone pair-pi interactions in the same direction as the hydrogen bonding between the complex molecules. DFT calculations reveal that the pi-pi stacking and lone pair-pi interactions are of approximately the same energy. The difference in the structures arises from the steric requirements of the trifluoromethyl group.