Activation of the Ground and Excited State of a Luminescent Osmium(VI) Dioxo Dicyano Complex with Lewis Acids.

The activation by Lewis acids (LAs) and visible-light excitation are two appealing strategies for generating highly oxidizing metal oxo species for catalytic oxidation reactions. Herein, we report the first systematic study on the activation of both the ground and excited states of a luminescent osmium(VI) dioxo dicyano complex (OsO2). Strong interactions of OsO2 with a variety of LAs were found via binding to the cyano ligands: LA-NC-Os-CN-LA, which allow the isolation and structural determination of the first examples of M=O/LA adducts. OsO2/2LA interactions also cause a large increase in the reduction potentials/oxidizing power of the adducts in both ground and excited states. Notably, the reduction potential (Epc) of OsO2 is shifted from -0.04 V (vs. NHE) to 1.21 V in the presence of Sc(OTf)3, which enables it to oxidize a variety of organic substrates. More significantly, the Epc for the excited state of OsO2 (OsO2*) is shifted from 2.21 to 3.50 V upon binding to Sc(OTf)3, which to our knowledge is the strongest metal oxo oxidant ever generated in solution. OsO2*/2Sc(OTf)3 readily catalyzes the oxidation of benzene and its derivatives by H2O2 at ambient conditions, with a TON up to 8660 and gram-scale production of phenol from benzene.