Multiunit Catalysts with Synergistic Reactivity: Three-Dimensional Polyoxometalate-Based Coordination Polymers for Highly Efficient Synthesis of Functionalized <i>p</i>‑Benzoquinones

The rational design
of highly efficient catalysts for the synthesis
of functionalized p-benzoquinones (p-BQs) is of great significance for the manufacture of bioactive compounds.
Herein, two 3D crystalline polyoxometalate-based coordination polymers
(POMCPs) are used as heterogeneous catalysts for the synthesis of p-BQs, which are H­[CuII(ttb)­(H2O)3]2[CuII(ttb)­Cl]2[PW12O40]·4H2O (1) (Httb = 1-(tetrazol-5-yl)-4-(triazol-1-yl)­benzene)
and [ClCu6I(trz)4]­[ClCu5I(trz)4]2[CuII(H2O)]­[PW12O40] (2) (trz =
1,2,4-triazole). Both compounds were characterized by elemental analysis,
IR, XPS, solid diffuse reflective spectroscopy, TG analysis, and single-crystal
X-ray diffraction. In 1, Keggin anions [PW12O40]3– locate in 1D square channels
constructed from wave-like Cu-ttb layers to form a 3D POMCP by coordinating
to Cu ions, and in 2, [PW12O40]3– anions situate in eight-membered Cu–trz channels
via Cu···O interactions to yield a 3D POMCP structure.
The catalytic activities of 1 and 2 have
been evaluated in the selective oxidation of alkylphenols/alkoxybenzenes/methylnaphthalene,
especially in the oxidation reaction of 2,3,6-trimethylphenol (TMP)
to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin
E key intermediate), with H2O2 as oxidant. By
using catalysts 1 and 2 under optimal conditions,
the yield of TMBQ can reach 99% and 96% within 10–20 min, respectively.
Both catalysts demonstrated high turnover frequencies (300 h–1 for 1 and 600 h–1 for 2) and the truly heterogeneous nature. 1 and 2 catalyzed the synthesis of p-BQs on the basis of
effective cooperative catalytic activities by POMs and metal nodes.