Tandem-like vanadium cluster chains in a polyoxovanadate-based metal–organic framework for efficient catalytic oxidation of sulfides

Abstract

A novel polyoxovanadate-based metal–organic framework (POV-MOF), [Ni(bix)₂]{V₄O₁₁} (V-Ni-MOF, bix = 1,4-bis(imidazol-1-ylmethyl)benzene), was successfully synthesized and structurally characterized. Single crystal X-ray diffraction analysis shows that the {V₄} clusters in V-Ni-MOF are composed of four VO₄ tetrahedra and further connected into a tandem-like chain. The unique vanadium chains with high-density coordinatively unsaturated vanadium centers in V-Ni-MOF can efficiently catalyze the oxidation of sulfides, where sulfides were completely converted to sulfones with the mild oxidant hydrogen peroxide within 1 hour at 40 °C. To our knowledge, most POV-MOFs could efficiently catalyze the oxidation of sulfides only when a stronger oxidant such as tert-butyl hydroperoxide was used. We attempted to utilize density functional theory (DFT) calculations to explore the reason for the high activity of V-Ni-MOF under mild hydrogen peroxide. DFT calculations indicated that the tandem-like vanadium cluster chain has a lower lowest unoccupied molecular orbital (LUMO) energy level compared to a discrete vanadium cluster, which matches better with the highest occupied molecular orbital (HOMO) energy level of hydrogen peroxide to generate active peroxo-POV species. In addition, as a stable heterogeneous catalyst, V-Ni-MOF sustainably maintains an excellent catalytic activity after multiple cycles.