Remote Heteroatom Containing Organo-polyoxometalates: Synthesis, Post-functionalization And Application In Li-ion Batteries

Polyoxometalates(POMs) due to their diverse structural range and associated assortment in physical and chemical properties are very intriguing inorganic materials. These metal-oxo clusters not only provide with basis to understand the formation and structural features of transition metal coordination clusters with ligands such as –oxo but also provide nano scale structural templates for efficient construction and development of fascinating functional nano materials via self-assembly. Among the available library of POMs structures, the Lindqvist(hexametalate) structure is especially interesting in terms of stability and available opportunities of direct covalent functionalization at surface of these metal-oxo clusters. The covalent grafting at Lindqvist POMs structures gives rise to the availability of remote functional groups to participate in further chemical functionalization and to aid self-assembly procedures to develop new functional organic-inorganic hybrid nano-cluster materials. Although large number of organo-imido hexamolybdates has been developed, however, the class affording remote –thio containing functional groups is not reported before. Similarly, the hexavanadates having extended conjugated systems and remote –diazo functionalization are also scarce. More importantly, the polyoxometalates in general and Lindqvist structured homo-hexametalates are in stage of being explored for functional application, such as electronic materials. In the current piece of work, interesting derivatives of hexamolybdates and hexavanadates have been synthesized and explored for fascinating functional properties.The well-known DCC(N,N’-dicyclohexylcarbodiimide) protocol has been used to develop new organo-imido product range having –SCN and –SO3-(–thiocyanide and –sulfonate) via organic synthesis of interesting ligands and then further organo-imido functionalization, moreover another series of organo-imido compounds has been developed by designing a organic ligand containing remote morpholine and further organo-imido functionalization of hexamolybdate. The organo-hexavanadate with extended conjugation system via diazo group and affording remote –NH2 also were prepared, for the purpose an organic ligand with required structure was synthesized and further attached with the hexavanadate via modified standard procedure. The remote morpholine containing organo-imido hexamolybdate was also used to develop an efficient and stoichiometric approach for post-functionalization of organo-imido hexamolybdates by alkylation reaction. The post-functionalized products were got in yields of 90-95% with in-situ crystallization.The synthesized hexametalates were studied for their structure using single crystal XRD analysis, along with ESI-MS, IR, UV-Vis, 1H-NMR, TGA. The synthesized products further checked for their application in functional applications as anode active materials.The organically-derivatized hexametalates were tested as anode material for Li-ion batteries, and important practical and theoretical investigation was recorded. It was shown that organo-imido products, in general, give much higher capacity performance as compared to hexamolybdate itself. This property directly relates to practical evidence of electronic mobilization through d-π interactions via Mo≡N. Further, it was noted that a remote electron withdrawing group(as –SCN) will be effective to obtain a higher anode performance while using organo-imido hexamolybdates. For instance different remote functional groups including –CH3, –H, and –SCN were observed for their anode performances and the results were compared to un-functionalized hexamolybdate itself. With the increase of electron withdrawing effect in order of –CH3<–H<–SCN, the anode performance was increasing in the order of Mo6<-CH3<-H<-SCN. During the testing, remote –SCN containing derivative of hexamolybdate, abbreviated as Mo6–SCN, shows initial discharge capacity of ~1678 mAhg-1 with ~85% capacity retention and coulombic efficiency ~100% after 100 cycles. Similarly the organo-hexavanadates showed much higher capacity performance as compared to decavanadate. In short, the organic-inorganic hybrid organo-POM materials emerged as efficient energy materials as compared to parent inorganic cluster POMs. In future it can be hoped that further developments regarding understanding of parameters in designing the more efficient organo-POMs can result high performance electronic materials regarding energy storage.