Scope and Mechanistic Studies of Intramolecular Aliphatic C−H Bond Activation of N-Heterocyclic Carbene Iridium Complexes

Abstract

The series Cp*Ir(NHC) (Cp* = η 5 -pentamethylcyclopentadienyl; NHC = N-heterocyclic carbene) complexes, Cp*Ir(IEt)Cl 2 ( 1a ), Cp*Ir(IPr)Cl 2 ( 1b ), and Cp*Ir(IBu)Cl 2 ( 1c ) (IEt = 1,3-diethylimidazol-2-ylidene; IPr = 1,3-di- n -propylimidazol-2-ylidene; IBu = 1,3-di- n -butylimidazol-2-ylidene) have been prepared by the carbene-transfer method using silver salts. The reactions of 1a − c with 1 equiv of i -PrONa in isopropyl alcohol give chloro hydrido complexes Cp*Ir(NHC)(H)(Cl) ( 2a − c ). The reaction of 1a with 2 equiv of i -PrONa in isopropyl alcohol results in the intramolecular C−H activation of the ethyl group in the NHC ligand to give Cp*Ir(IEt‘)(H) ( 3 ), while the similar reactions of 1b and 1c give dihydrido complexes Cp*Ir(IPr)(H) 2 ( 4b ) and Cp*Ir(IBu)(H) 2 ( 4c ) as main products, respectively. These reactions proceed via alkoxo species, [Cp*Ir(NHC)(O i -Pr)]Cl and Cp*Ir(NHC)(O i -Pr)(H), as the key intermediates. Derivation of 3 into the chloro complex Cp*Ir(IEt‘)(Cl) ( 5 ) and cationic complexes [Cp*Ir(IEt‘)(L)]OTf [L = acetonitrile ( 6 ); pyridine ( 7 )] is also described.