Synthesis and characterization of the κ²-acac- O, O complex Os^IV(acac)₂PhCl and study of CH activation with benzene

We have synthesized and fully characterized the air-stable complex (κ²-acac-O,O)₂Os^IV(Ph)Cl (Cl-1-Ph; acac-O,O = acetylacetonate), which reacts with C₆D₆ to generate Cl-1-Ph-d₅ in high yield and catalyzes the H/D exchange reaction between benzene and toluene-d₈ upon heating to 140 °C. To our knowledge, this is the first example of stoichiometric and catalytic, homogeneous, intermolecular CH activation of arenes by a discrete Os complex. The reactions show extended induction periods. DFT studies of Cl-1-Ph and cis-(κ²-acac-O,O)₂Os^III(C ₆H₅)(C₆D₆) (cis-(C₆D ₆)-2-Ph) found a mechanism involving CH activation by traces of Os(III) and Cl atom transfer between Cl-1-Ph and cis-(C₆D ₆)-2-Ph. Experimental data showing that addition of reductants eliminates the induction periods suggest that CH activation occurs from an oxidation state lower than Os^IV, consistent with the DFT predictions. Consistent with a Cl atom transfer mechanism, the triflate analogue of Cl-1-Ph, OTf-1-Ph, does not undergo a stoichiometric or catalytic reaction with C ₆D₆.