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

Wu-Hua Chen, Lai Yu-Zhen, Zhibiao Hu, Jinxian Liu, Xiang-Guang Guo, Yuan Qiu-Lan, Jin-Huan Zhao, Jin-Xiao Mi, Jiangshui Luo

Index: 10.1039/C8NJ00714D

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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 (VIII–OFCs) is an important research area. Additionally, incorporation of VIII–OFCs into a polyoxometalate (POM) is challenging due to the high standard reduction potential of VIII 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 from a precursor in a pH-value controlling redox step. The V(III)-oxyanion is designed to coordinate with organic ligands for further forming a intermediate in which the metal node V(III) connects both organic units and oxygen-atoms. By employing proper stepwise synthesis strategy, we successfully obtained an unprecedented Keggin-type tungstovanadophosphate, (H3DETA)3(DETA){Na{[VIII(DETA)]2[WVI8VIV4O36(PO4)]}2}•2H2O (abbreviated to VIII-POWV) (DETA = diethylenetriamine, C4H13N3), bi-cappped by VIII–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 VIII in VIII–POWV is uncommon and interesting. The novel structural compound, which contains mixed-valence vanadium (VIII/VIV), has better thermostability than most of common organic POMs and shows the expected paramagnetic properties, accompanying antiferromagnetic exchange interactions under higher temperatures.