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.