Two Polymorphic Polyoxometalate-Based Metal–Organic Frameworks for the Efficient Synthesis of Functionalized Sulfoxides and Detoxification of Mustard Gas Simulants

The development of high-efficient
catalysts for the oxidation reaction
of sulfides received much attention in recent years due to the extensive
applications of such reactions from the utility in pharmaceutical
chemistry and biology to the detoxification of chemical warfare agents
in war. Herein, we report two innovative polyoxometalate-based metal–organic
frameworks (POMOFs) {[ε-PMo8VMo4VIO37(OH)3]­[Zn2(C10N2H8)­(H2O)2]2}2·8H2O (1-α and 1-β), which were characterized by elemental
analysis, UV–vis diffuse reflectance spectroscopy, X-ray single-crystal
and powder diffraction, X-ray photoelectron spectroscopy, and Fourier-transform
infrared spectroscopy techniques in detail. Single-crystal X-ray diffraction
analysis indicates that 1-α and 1-β are polymorphic with the identical compositions and analogical structures.
Compounds 1-α and 1-β are both
connected by Zn4-ε-Keggin polyoxoanions and 4,4′-bipyridine
(bpy) ligands to form twofold interpenetrated three-dimensional POMOF
structures. 1-β can be seen as a rotational polymorph
of 1-α, that is, 1-α takes the b axis as the rotation axis and rotates ∼39°
clockwise to obtain 1-β. Both compounds 1-α and 1-β can be used as heterogeneous catalysts
to catalyze the selective oxidation of multitudinous sulfides with
H2O2 as an oxidant. When the oxidation of methyl
phenyl sulfide was used as the template reaction, the yield of methyl
phenyl sulfoxide is close to 100% within 30 min by 1-α and 1-β and the oxidant utilization efficiency
is more than 93%. Gratifying catalytic effects have also been achieved
in the selective oxidation of phenyl and aliphatic sulfide derivatives.
Simultaneously, both 1-α and 1-β showed an extraordinary degradation efficiency of 2-chloroethyl
ethyl sulfide and have a half lifetime (t1/2) approximately 2.5 or 3 min, respectively, with 100% selectivity
toward the nontoxic product 2-chloroethyl ethyl sulfoxide and an abnormally
high oxidant utilization efficiency (94.5% for 1-α and 94.3% for 1-β) at room temperature. In addition,
after seven cycles and continuous cycle catalytic experiments, their
structures remained and catalytic activities did not decrease obviously,
revealing their outstanding recyclability and structural stability.