Self-Assembly And Gel Network Construction Of Organically Grafted Polyoxometalate Complexes

Molecular self-assembly, is the spontaneous organization of small molecules into high order organizations via weak intermolecular interactions. These intermolecular interactions are low bonding energy and non-directional non-covalent bonding, such as, van der Waals force, hydrogen bonding,π–πstacking, and hydrophilic–hydrophobic interaction. In the molecular self-assemble process, there is not always only one interaction, but cooperated with more interactions. Therefore, through rational molecular-structure design and adjustment of the chemical microenvironment to manipulate the equilibrium of these weak interactions, various self-assembled structures have been obtained.Polyoxometalates (POMs) are an intriguing class of inorganic cluster that show features as functional materials for catalysis, biomedicine, and magnetism. Form the view of molecular component, some functional atoms and groups had been evolved into the molecular structure to expand the functions of POMs. But the functions of POMs are not only related to the molecular structure and composing, but also related to the assemblies on higher scales. Therefore, constructing various assemblies of POMs has been attracted increasing interest recently. To overcome the low assemble ability of POMs, encapsulating POMs into the organic matrix with excellent assemble ability has become the research hot spot. One strategy is that forming an organic layer around the anionic POMs using cationic organic molecules via electrostatic interaction, and then using the assembly of the organic molecules to manipulate the organization of POMs in different assemblies. However, due to the non-directional nature of the binding sites of electrostatic interactions, it is difficult to control the arrangement of POMs on the nano-scale. To achieve this goal, we employ the pyridyl grafted Anderson-type POMs to form assemblies. By the directional organic grafted, the POMs arrangement can be control. Because pyridyl can form hydrogen bonding with carboxylic acid, we employ the organic dicarboxylic acid to connect POMs forming supramolecular polymer and gel, and discuss the influence factor of the gelation.Firstly, we employed the pyridyl-grafted Anderson-type POMs and long alkyl chain surfactants forming a supramolecular complex via electrostatic interaction, which assemble into"ribbon-like"assemblies. By the co-precipitated method, we get the surfactant encapsulated organic grafted Anderson-type POM complex. In a weakly polar solvent, the complex aggregated into fibers with a lamellar structure driven by van der Waals force among alky chains. After methanol was added gradually, van der Waals force and the order degree of alkyl chains increase and the distance among alkyl chains become smaller, which weaken the hydrogen bonding between amide and pyridyl groups and promote hydrogen bonding between pyridyl and terminated Mo=O formed gradually. The hydrogen bonding between pyridyl and the terminal O=Mo make organically grafted POM arranging into a tetragonal structure.Secondly, we employed the dicarboxylic acid to connect the POMs via hydrogen bonding between pyridyl and carboxylic acid to form supramolecular polymer. When different dicarboxylic acid was used, the distance between two POMs is different and the interface energy is different. When there is no dicarboxylic acid, the POMs are connected by hydrogen bonding between pyridyl and the terminated Mo=O. The density of the counterions is high and the interface energy is low, which is favored for forming crystal. When the distance between the POMs elongated by dicarboxylic acid, the interface energy of the polymer will increase, making the organically grafted POM change from crystal to gel. Therefore, through manipulating the distance between organically grafted POMs, we can control the existing state of POMs from crystal to gel.In conclusion, we discussed the self-assemble of organic-grafted POM with the different additions. Firstly, long alkyl chain surfactant was encapsulated around the POM via electrostatic interaction, forming the supramolecular complex, which self-assembles to ordered aggregation. And we can manipulate the arrangement of POMs directly. Secondly, dicarboxylic acid was used to connect the organically grafted POM via hydrogen bonding forming the supramolecular polymer. When the interface energy of polymer and solvent is proper, the polymer can reticulate into three-dimensional network to fix the solvent, forming gel.