A new redox-based and stepwise synthetic strategy lead to an unprecedented mixed-valence Keggin-type tungstovanadophosphate (WVI/VIV) bi-capped by vanadium(VIII)-complexes

Abstract

To achieve improved properties such as lower biotoxicity and better biocatalytic performances, the preparation of low oxidation state (+3) vanadium (V)-organic framework compounds (V^III-OFCs) is an important research area. Additionally, the incorporation of V^III-OFCs into a polyoxometalate (POM) is challenging due to the high standard reduction potential of V^III ions and the POM's innate sensitivity to oxidation. A new targeted and redox-based synthetic strategy has been devised to generate a V(III)-oxyanion by reducing a precursor in a pH-value controlled redox step. The V(III)-oxyanion is designed to coordinate with organic ligands for further forming an intermediate in which the metal node V(III) connects both organic units and oxygen-atoms. By employing a suitable stepwise synthesis strategy, we successfully obtained an unprecedented Keggin-type tungstovanadophosphate, (H₃DETA)₃(DETA){Na{[V^III(DETA)]₂[W^VI₈V^IV₄O₃₆(PO₄)]}₂}·2H₂O (abbreviated as V^III-POWV) (DETA = diethylenetriamine, C₄H₁₃N₃), bi-capped by V^III-OFCs. Its structure has been further characterized by means of powder-XRD, EDS, elemental analysis, FT-IR, XPS, thermal analysis, and SQUID magnetic measurements, etc. The seven-coordination mode (VO₄N₃) of V^III in V^III-POWV is uncommon and interesting. The novel structural compound, which contains mixed-valence vanadium (V^III/V^IV), has better thermostability than most of the common organic POMs and shows the expected paramagnetic properties, accompanying antiferromagnetic exchange interactions under higher temperatures.