Chelating N-heterocyclic carbene–carboranes offer flexible ligand coordination to IrIII, RhIII and RuII: effect of ligand cyclometallation in catalytic transfer hydrogenation

Abstract

Imidazolium salts linked by an ethyl tether to closo-dicarbadodecaboranes were reacted with [IrCp*Cl₂]₂, [RhCp*Cl₂]₂ or [Ru(p-cymene)Cl₂]₂ in the presence of Ag₂O to prepare complexes of the type [MCp*(NHC)Cl₂] (M = Ir, Rh; NHC = N-heterocyclic carbene) or [Ru(p-cymene)(NHC)Cl₂]. When the NHC contained an N-^tBu substituent, C–H activation of the ^tBu and subsequent alkyl coordination was observed at Ir. Coordination of the closo-dicarbadodecaborane moiety to Ir was possible to give 7-membered metallacycles, coordinated through the carbenic carbon of the NHC and either a carbon atom or a boron atom of the carborane. Examination of the Ir complexes in the transfer hydrogenation of acetophenone to 1-phenylethanol reveals that cyclometallation of the carborane moiety is important for catalytic efficacy, indicating a bifunctional mechanism and involvement of the dicarbadodecaborane anion.