Cyclooctadiene iridium complexes [Cp*Ir(COD)X]+ (X = Cl, Br, I): Synthesis and application for oxidative coupling of benzoic acid with alkynes

The cyclooctadiene iridium complexes [Cp*Ir(COD)X]PF6 ([1a−c]PF6; X = Cl, Br, I) were synthesized by reactions of Cp*Ir(COD) with halogens followed by a counterion exchange. The cyclooctadiene in these complexes is a thermally labile ligand. Complex [1b]PF6 reacts with trimethylphosphite to give [Cp*Ir{P(OMe)3}2Br]PF6 ([2]PF6) as a result of the cyclooctadiene replacement. The refluxing of [1b]PF6 in 1,2-dichloroethane affords the dimeric iodide [Cp*IrI2]2. The structures of [1b]PF6, [2]PF6 and [Cp*IrI2]2 were determined by X-ray diffraction. The Ir–COD bonding in [1a−c]+ and the related non-methylated complexes was analyzed by energy decomposition analysis. In the presence of silver salts, complexes [1a−c]PF6 (at 2.0 mol % loading) catalyze the oxidative coupling of benzoic acid with 1-phenyl-1-propyne in methanol at 60 °C to selectively give 4-methyl-3-phenylisocoumarin (3) or with diphenylacetylene in o-xylene at 160 °C to afford 1,2,3,4-tetraphenylnaphthalene (4).