Inorganic-Organic Supramolecular Hydrogels Based On Covalently Modified Polyoxometalates

Polyoxometalates(POMs)are anionic metal-oxygen clusters of early transition metals(group 5 and 6).They show remarkable structural,size and charge diversity and have attracted much attention from different fields of sciences such as catalysis,material sciences,and medicine.Interestingly,in order to "add values",POMs can be modified with diverse organic molecules for the development of variety of functional materials via both covalent and non-covalent interactions.Modification of POMs in general and covalent functionalization in particular has attracted enormous attention because it is considered to be a distinct platform for the development of varieties of functional materials.The covalent functionalization of POMs involves grafting of organic molecules into lacunary Keggin and Dawson type POM clusters via organometallic compounds like organotin,and organosilicon or substituting the oxo ligands of Anderson and Dawson with alkoxo moieties.It generally follows two routes:pre-functionalization and post-functionalization.In the pre-functionalization approach the organic part is pre-modified with the desired functional and POM anchoring groups,which is then covalently attached to the POM cluster.In the second approach a POM hybrid with appending functional groups is coupled with organic compound of interest by making use of suitable grafting methods and reaction conditions.Over the last decades several POM hybrids have been synthesized and reported using one or the other pathway.Covalent functionalization of POMs facilitates the interactions between the organic component and the POM anions ultimately,leading to fabrication of stable hybrid materials with properties more than a simple combination of organic and inorganic parts.The particular importance of covalent functionalization is not only hinges on the formation of stable hybrids but also on providing different possibilities for further functionalizations.These can be considered as a vital platform for the construction of interesting supramolecular assemblies,such as supramolecular hydrogels,which can be defined as three-dimensionally networked polymer materials where the networking nature is based on different forms of non-covalent interactions such as hydrogen bonds,ionic bonds,van der Waals forces,etc.Particularly,supramolecular hydrogels that are constructed through host-guest interactions constitute a class of intriguing soft matter and have attracted significant attention due to their unique properties that include external stimuli responsive and self-healing properties.Due to these,supramolecular hydrogels are considered as materials of interest in different fields of studies such as drug delivery,sensors,and nano-devices.Nevertheless,there is considerable challenge pertaining to improving the mechanical strength of supramolecular hydrogels.In order to circumvent this problem,introduction of inorganic compounds is considered as a promising approach.In this regard POMs can be considered as promising candidates as inorganic constituent of organic/inorganic hybrid materials due to their unmatched physical and chemical properties.However,application of POM hybrids for construction of the hydrogels is still premature,probably due to the practical difficulty associated with fabrication of POM hybrids and also the supramolecular assembly.Particularly,the use of host-guest recognition approach has never been employed to prepare POM-based hydrogels.Therefore,to this end,a new POM-based supramolecular hydrogel is constructed based on host-guest interactions.To achieve this,firstly,an adamantane-modified Anderson cluster is synthesized and fully characterized by different techniques including single?crystal XRD,ESI-MS,1H NMR,13C NMR and IR.Secondly,the inclusion complex of Anderson-Adamantane with ?-CD is systematically studied by using 1H NMR and ESI-MS.After establishing the successful complexation of Anderson-Adamantane and ?-CD,the same approach is applied for the complex formation of Anderson-Adamantane with a polymerizable ?-CD form,namely,6-Acrylamido-?-CD.Thirdly,the Anderson-Adamantane-Acrylamido-?-CD complex is subjected to a condition of radical copolymerization in the presence of acrylamide to construct supramolecular hydrogels.The hydrogels are studied by using scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and rheological tests.Furthermore,construction of a new POM based supramolecular hydrogel is achieved.To begin with,a cyclodextrin-functionalized Anderson cluster(POM-CD)is synthesized by utilizing the post-functionalization protocol and fully characterized using different techniques such as NMR,IR and ESI-MS.The inclusion complex formation of the POM-CD with azobenzene-modified polyethylene glycol(PEG-AZO)is meticulously investigated with 1H NMR spectroscopy.The analysis of the 1H NMR spectrum of the complex(POM-CD-PEG-AZO)shows that the azobenzene moiety has undergone host-guest complexation with the ?-CD of the POM-CD hybrid.Subsequently,the hydrogel is obtained by copolymerizing the polymerizable POM-CD-PEG-AZO complex in the presence of acrylamide and ammonium persulfate(APS)at 60 ?.The hydrogels are characterized by using scanning electron microscopy(SEM)and rheological tests.These works demonstrate successful construction of new POM-based hydrogels based on host-guest recognition for the first time.These approaches of fabricating POM-based supramolecular hydrogels can be considered as a new pathway for the development of POM-based advanced functional materials.