Synthesis and Characterization of the κ2-acac-O,O Complex OsIV(acac)2PhCl and Study of CH Activation with Benzene

We have synthesized and fully characterized the air-stable complex (κ2-acac-O,O)2OsIV(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)2OsIII(C6H5)(C6D6) (cis-(C6D6)-2-Ph) found a mechanism involving CH activation by traces of Os(III) and Cl atom transfer between Cl-1-Ph and cis-(C6D6)-2-Ph. Experimental data showing that addition of reductants eliminates the induction periods suggest that CH activation occurs from an oxidation state lower than OsIV, 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 C6D6.