Two transition-metal-modified Nb/W mixed-addendum polyoxometalates for visible-light-mediated aerobic benzylic C–H oxidations

The visible-light-induced selective oxidation of ubiquitous C–H bonds into valuable C=O bonds under aerobic conditions is one of the most attractive approaches for the construction of carbonyl-containing molecules. In this work, two transition metal-containing Nb/W mixed-addendum POMs dimers with the formula of K2Na2H5[(Fe(H2O)4)3(P2W15Nb3O62)2]·24H2O (POM[Fe]) and K2Na3H4[(Cr(H2O)4)3(P2W15Nb3O62)2]·32H2O (POM[Cr]) have been synthesized and characterized by various analytical and spectral techniques. POM[Fe] was proved to be an efficient photocatalyst for benzylic C–H oxidation under visible light and using oxygen as an oxidant to produce the corresponding carbonyl complex in good yields. A plausible mechanism involving superoxide radical was proposed for the catalytic reaction. POM[Fe] showed good reusability in the recycling experiments. IR spectroscopy and XRD analysis indicate that POM[Fe] can retain its integrity after catalysis.