Quinoline-tethered N-heterocyclic carbene complexes of rhodium and iridium: Synthesis, catalysis, and electrochemical properties

A series of rhodium and iridium complexes of quinoline-tethered hemilabile N-heterocyclic carbenes (NHC∧N) have been synthesized via either deprotonation of imidazolium salts or silver transmetalation. Deprotonation of imidazolium ions by ^tBuOK in the presence of [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene) afforded both chelating [Rh(COD)(NHC∧N)] ⁺ and monodentate [Rh(COD)(NHC)₂]⁺ complexes, while only the chelating carbene complexes were obtained for the iridium analogues. Silver transmetalation of this type of carbene to [M(COD)Cl] ₂ (M = Rh, Ir) consistently afforded M(NHC) (COD)Cl, maintaining the metal chloride and pendant quinoline entity. Carbene transmetalation to (Cp*IrCl₂)₂ gave an equilibrium mixture of neutral Cp*Ir(NHC)Cl₂ and ionic [Cp*Ir(NHC∧N)Cl]Cl. All these rhodium and iridium cyclooctadiene complexes can undergo one-electron oxidation in cyclic voltammetry. The variable-scan-rate cyclic voltammetry experiments indicate that these compounds undergo slow heterogeneous electron transfer and that the oxidized forms are relatively short-lived. A neutral Rh(COD)(NHC)Cl complex proved to be active in catalyzing the [3 + 2] cycloaddition reactions of diphenylcyclopropenone and internal alkynes. Crystal structures of metal complexes in each category have been reported.