A POM-based porous supramolecular framework for efficient sulfide–sulfoxide transformations with a low molar O/S ratio

Abstract

The selective oxidation of organic sulfides is a pivotal step in the preparation of sulfoxides that can act as synthetic intermediates when preparing fine chemicals, bioactive molecules, and asymmetric catalysis ligands. To construct high-performance heterogeneous catalysts for sulfide-sulfoxide transformations, herein, we designed and synthesized a supramolecular porous catalyst based on ε-Keggin polyoxometalates (POMs), TBA₂H₂[Zn₄(im)(Him)₂][ε-PMo₈^VMo₄^VIO₄₀]·3H₂O (1, Him = 1H-imidazole). Single-crystal X-ray diffraction analysis indicates that the porous framework of 1 can be obtained via the supramolecular stacking of one-dimensional helical chains alternately linked by Zn₄-ε-Keggin clusters and Him ligands. During the selective oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide (MPSO), compound 1 achieved a >99% yield toward MPSO and 90.3% oxidant utilization efficiency within 10 min. The corresponding turnover frequency (TOF), expressing catalytic activity, was up to 1200 h⁻¹. The catalyst also demonstrated extensive substrate tolerance during catalysis, and the corresponding yields of sulfoxides were satisfactory when only 1.2 equivalents of oxidant were used. Besides this, 1 can also effectively degrade a sulfur mustard simulant (2-chloroethyl ethyl sulfide) to the nontoxic product 2-chloroethyl ethyl sulfoxide within 10 min at room temperature, with an oxidant utilization efficiency of up to 94.5%. Importantly, the excellent catalytic activity of compound 1 was also proven via comparison with an analogous 3D compound 2 (TOF = 800 h⁻¹ at full MPS conversion), TBAH₂[K(Him)(im)][Zn₄(Him)(Hip)][ε-PMo₈^VMo₄^VIO₄₀] (Hip = 4-(1H-imidazol-2-yl)-pyridine), which was covalently assembled from one-dimensional Zn4-ε-Keggin POM chains and metal–organic units. Moreover, the truly heterogeneous nature of 1 and 2 was confirmed via cycling and hot-filtration experiments, and their structural stability was verified based on Fourier-transform infrared spectra and powder X-ray diffractometry (PXRD) patterns.