Carboxylate Ligand-Induced Intramolecular C-H Bond Activation of Iridium Complexes with N-Phenylperimidine-Based Carbene Ligands

We report the synthesis and structure of iridium(1) 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 lambda(max) = 555 nm (tau = 0.65 mu s, Phi(PL) = 0.0018) in CH2Cl2 solution at room temperature.