Asymmetric Synthesis And Covalent Modification Of Anderson-type Polyoxometalates Towards Cluster Oligomers

Polyoxometalates（POMs）represent an important group of metal-oxoclusters,usually composed of early transition metals（mainly V,Mo,and W）in high oxidation states.The structural diversity,and rich and stable physical/chemical properties of POMs make it widely used in many fields such as catalysis,medicine,and materials.Covalent modification of POMs is one of the main approaches to develop POM-based functional materials,and the vast majority of covalent modifications reported so far have focused on symmetric compounds with two identical ligands.Different ligands can also be used to generate asymmetric hybrid products;however,there are only a few POMs that can be asymmetrically modified.As a consequence,it is urgent to develop a versatile methodology for the asymmetic synthesis,and purification of organically modified POM hybrids.Therefore,in this thesis we propose a"modular"method for the asymmetric modification of Anderson-type POMs,and further integrate these asymmetric‘building blocks’into one system to construct POMs-oligomers.It is believed that this modular method provides a broader idea for the construction of organic-inorganic supramolecular materials.The main research contents of this thesis are as follows:1.We successfully prepare two asymmetrically modified POMs based on Al-templated Anderson cluster by using two different tripodal ligands in a stepwise modification manner.Though these POM hybrids are designed and synthesized in a stepwise manneer,a purification process,which is based on cation-exchange and solubility differences,is needed to remove the symmetrically modified by-products.The feasibility of this method has been proved by adopting different tripodal ligands,and the resulting asymmetric POMs have been systemmatically characterized using different techniques such as NMR,ESI-Tof-MS,and single-crystal XRD.Moreover,the introduction of different organic active sites on both sides of Anderson POM provides further possibilities for the subsequent development of more complex structures.2.By post-modification of the asymmetric POM hybrids with glutaric anhydride,and further activation thus obtained carboxylic acid group with DCC/NHS,and then modification with tris（hydroxymethylaminomethane）,a"modular"organic-inorganic hybrid platform is obtained,which can be further condensation with unilaterally-modified Al-Anderson POMs to construct Anderson-Anderson POMs-oligomers.At the same time,similar organic covalent modifications of the resulting POMs-oligomers,and Anderson-Anderson-Anderson POMs-oligomers can also be constructed.In addition,the NHS-activated POM hybrids have been grafted onto a Keggin POMs(Si W₁₁-NH₂)to construct a Andersson-Keggin-Anderson cluster-oligomers.The obtained compounds have been characterized by H-NMR and ESI-Tof-MS.The"modular"construction strategy provides strong support for the subsequent construction of POM oligomers and even more complex systems.