Towards organophosphonate-functionalised polyoxotungstate-based metal–organic frameworks

Abstract

Polyoxometalates (POMs) are a group of anionic soluble metal-oxide clusters containing d-block elements in high oxidation states. These compounds are highly valued for their wide range of electrochemical, photochemical, and catalytic properties, due to their remarkable flexibility in tuning both composition and structure. One common method of modifying POM is by connecting the electron-withdrawing organophosphonate groups to the active sites of metal oxides. When POMs are functionalised with organophosphorus compounds, it is possible to adjust their electronic structure and photochemical activity based on the electronic properties of the attached organic moieties. However, the high solubility of POMs restricts their application in sensors, electrodes, and catalysis regions. By linking POMs in a repeating pattern through bridging organic linkers, insoluble metal-organic frameworks (MOFs) can be formed. These MOFs retain the structural features of the parent molecular POMs and commonly enhance electrochemical activity. Herein, we report the synthesis of a new hybrid POM, K6[P2W17O57(PO5H9C13)2] using a biphenyl organophosphonate ligand. The hybrid POMs were characterised by FTIR, NMR and ESI-MS. A series of multi-phenyl organophosphonate functionalised POM were synthesised, and all hybrid POMs were applied to construct POM-based MOFs (POMOFs). The structures of resulting POMOFs were analysed by PXRD. Hybridisation with long-chain organophosphonate groups is expected to stabilise electron-conducting pathways of MOFs, potentially overcoming limitations such as its low conductivity and poor electrochemical durability. The design of these new POMOFs offers a promising strategy for the development of advanced materials for various battery systems