Reversible C−H Bond Activation of a Bifunctional Phosphine Bridging Ligand across Two Unbonded Metal Centers

Reaction of [{Ir(μ-Cl)(cod)}₂] with the short-bite bifunctional N,P-donor ligand (1-benzyl-2-imidazolyl)diphenylphosphine (Ph₂PBzIm) gives the yellow complex [IrCl(Ph₂PBzIm)(cod)] (2). A further addition of [{Ir(μ-Cl)(cod)}₂] to 2 results in the reversible metalation of a phenyl ring across two unbonded iridium centers to give orange crystals of [IrCl(cod){μ-PPh(C₆H₄)BzIm}IrHCl(cod)] (1). Complex 1 is in equilibrium with the mononuclear complex [IrCl(Ph₂PBzIm)(cod)] and the active species undergoing the sp²-C−H activation, [{IrCl(cod)}₂(μ-Ph₂PBzIm)], in solution. Abstraction of one chloride ligand from 1 with AgBF₄ produces the deinsertion of the C−H bond yielding the cationic complex [{Ir(cod)}₂(μ-Ph₂PBzIm)(μ-Cl)]BF₄, which regenerates 1 upon addition of a chloride-soluble salt. The cationic complex [{Ir(cod)}₂(μ-Ph₂PBzIm)(μ-Cl)]BF₄ is inactive for the above-mentioned sp²-C−H bond activation and can be prepared alternatively from the reaction of 2 with [Ir(cod)(CH₃CN)₂]BF₄. A related binuclear sp²-C−H bond activation across two unbonded metals also occurs in the reaction of dppm with [{Ir(μ-Cl)(cod)}₂] in a 1:1 molar ratio. This reaction leads to a mixture in equilibrium of [{IrCl(cod)}₂(μ-dppm)] and the hydride complex [IrCl(cod){μ-PPh(C₆H₄)CH₂PPh₂)}IrHCl(cod)] in a 1.5:1 molar ratio, respectively, in dichloromethane at 20 °C. The structure of the mixed-valence complex 1 was solved by X-ray diffraction studies.