Structural Transformation of Porous Polyoxometalate Frameworks and Highly Efficient Biomimetic Aerobic Oxidation of Aliphatic Alcohols

Due to their inherent inert nature, it is difficult to oxidize unactivated aliphatic alcohols with molecular oxygen under mild conditions. Inspired by enzymatic catalysis, numerous biomimetic systems have been therefore established. However, low catalytic efficiency and easy auto-oxidative deactivation nature remain the problematic issues. To meet these challenges, we report herein a 2D porous polyoxometalate (POM) framework Gd4(H2O)26[WZn­{Cu­(H2O)}2(ZnW9O34)2]·24H2O (CZJ-11) and a 3D porous POM framework Gd4(H2O)24[WZn­{Cu­(H2O)}2(ZnW9O34)2]·11H2O (CZJ-12) transferred from CZJ-11 by partial dehydration, consisting of scaffolded redox-active Cu­(II) sites in the sandwich-type POM cluster [WZn­{Cu­(H2O)}2(ZnW9O34)2]12‑ (abbreviated as {Zn3Cu2W19}). To mimic the catalytic mechanism of enzymes, N-hydroxyphthalimide (NHPI) and tetramethylammonium bromide (TMAB) were introduced as cocatalysts, which performed as electron donor and electron-transfer mediator, respectively. The coupled catalyst systems demonstrate analogue properties with oxygenase enzymes in the aerobic oxidation of aliphatic alcohols under mild conditions. Compared with molecular POM counterpart and metalloporphyrins, the catalytic efficiency of these POM frameworks is predominant in aerobic oxidation of unactivated aliphatic alcohols by imitating the active sites and the catalytic mechanism of enzymes. Compared with metal–organic coordination complexes, such as metalloporphyrins, the pure inorganic frameworks offer significant superiority of robustness to auto-oxidation and simple recovery for recycling with retained high catalytic efficiency.