Recently, we have reported on a dicopper system (CuIII(μ-O)2CuIII complex immobilized in mesoporous silica nanoparticles) that can mediate the catalytic conversion of toluene into benzaldehyde by O2, in which the oxidizing power of both O atoms is harnessed for catalytic turnover. This is the first example of a CuIII(μ-O)2CuIII complex capable of functioning like a "dioxygenase" in hydrocarbon oxidation. We have undertaken a mechanistic study to clarify how this catalytic conversion is accomplished without the input of sacrificial reductants. While the first O atom in the CuIII(μ-O)2CuIII complex can actively insert into a C-H bond, the second O atom left in the CuII(μ-O)CuII complex is inert. We show that a second molecule of O2 is involved in activating the dicopper catalyst, forming an O2 complex with the CuII(μ-O)CuII intermediate to give a species with the [Cu2O3]2+ core, which then mediates the transfer of the remaining O atom of the original O2 molecule to the organic substrate to complete the catalytic turnover. The study offers a mechanistically characterized analogue of the heterogeneous metal oxide catalyst that oxidizes organic substrates with the lattice oxygens by the Mars-van-Krevelen (MvK) mechanism at significantly higher temperatures.
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