Carboxylate Ligand-Induced Intramolecular C−H Bond Activation of Iridium Complexes with<i>N</i>-Phenylperimidine-Based Carbene Ligands

Abstract

We report the synthesis and structure of iridium(I) complexes with N,N′-disubstituted perimidine carbene ligands and halides or carboxylate ligands. Iodo-, chloro-, and acetate-Ir(carbene)(cod) complexes were selectively prepared by changing the bases and silver salts. Intramolecular C−H bond activation to prepare a cyclometalated Ir(III) complex, (C ∧ C:) 2 Ir(OAc) ( 6 ), where C ∧ C: is a cyclometalated perimidine carbene ligand, was achieved using a carboxylate-ligated iridium complex; otherwise, C−H activation did not proceed. Acetate-Ir(carbene)(cod) ( 3c ) reacted with benzoic anhydride at room temperature to afford benzoate-Ir(carbene)(cod) ( 3d ). A perimidine-carbene- and phenylpyridine-ligated Ir(III) complex, (C ∧ C:)(C ∧ N)Ir(OCOR) ( 8 ), where C ∧ N is a cyclometalated phenylpyridine ligand, was isolated in good yield by mixing 3c or 3d and phenylpyridine under reflux in toluene. On the basis of the formation of 1,3-cyclooctadiene from the reaction mixture, we propose the following reaction mechanism for the intramolecular C−H activation: carboxylate-induced C−H bond activation and subsequent pyridine- or carbene-directed C−H bond oxidative addition, followed by the isomerization of 1,5-cyclooctadiene and reaction with carboxylic acid produces (C ∧ C:)(N ∧ C:)Ir(OCOPh). The cyclometalated complex 6 reacts with sodium acetylacetonate to form (C ∧ C:) 2 Ir(acac) ( 11 ), which exhibits phosphorescent emission at λ max = 555 nm (τ = 0.65 μs, Φ PL = 0.0018) in CH 2 Cl 2 solution at room temperature.