The Design,Syntheses And Endohedral Functionalization Of Polyoxovanadate-based Metal–organic Cages

In nature,ferritin,virus particles,and plenty of enzymes have cage-shaped shells and highly functionalized interiors.These functional groups affect the interior chemical environment and participate in a myriad of chemical reactions inside organisms.Chemists have taken inspiration from nature and developed a diverse array of metal–organic cages（MOCs）,to mimick the behavior of biological shells.However,the exploration of their endohedral functionalization so far has been rather limited.Previous endohedral functionalization of such cage molecules has largely relied on modifying their organic linkers to covalently append targeted functional groups to the interior surface.We herein introduce an effective coordination method to bring in functionalities.3 polyoxovanadate MOCs and 7 endohedrally decorated MOCs have been obtained.Furthermore,we investigate the van der Waals volume of functional groups,π-πstacking interaction,and pre-assembly functionalization/post-assembly functionalization on the self-assembly of MOCs,laying the foundation for further design,syntheses,and endohedral functionalization of polyoxovanadate-based metal–organic cages.The main contents of this dissertation are listed as below:By making use of the strong coordination ability of phosphonates,we introduce functional groups into the vertices([V₆^IVO₆（OCH₃）₉（SO₄）]⁺,hereinafter referred to as{V₆S})of cages,and obtain 3 endo functional MOCs（complexes 1–3）and 1channel type functional MOC（complex 4）.Then two conclusions are obtained.First,the outcome of endohedral functionalization strongly depends on the size/shape of interior cavities.Phenylphosphonic acid and biphenyl-4-phosphonic acid are both successfully introduced into the cubic cage（VMOC-2）with the large cavity,forming functional cages（1–2）,respectively.As expected,the tetrahedral cage（VMOP-11）with a smaller void can only accommodate phenylphosphonic acid.In such manner,an endohedrally functionalized cage（3）is obtained.Second,when introducing a large group into a small cage,the steric effect of functional groups will hinder the formation of original cages.When the functional groups（phenylphosphonic acid）are introduced into the cage（VMOP-14）with the smallest internal cavity,due to the steric effect of adjacent phenyl groups,the self-assembly process of the original cage could be changed and a channel type functional cage is formed（4）,which is similar to the chloride channel protein.To bring more insight into the influence of counter cations and functional groups on the self-assembly of MOCs and cage structure conversion,we use azobenzene dicarboxylic acid（H₂ABDC）as the organic linker,and{V₆S}as the vertex of the cage.First,in DMF/CH₃OH mixed solvent,H₂ABDC and{V₆S}form the tetrahedral cage（complex 5）.In addition,changing the DMF to DEF constructs complex 6,whose structure is formed by two tetrahedra that are nested with each other,which is the first triply interlocked[2]catenane based on metal clusters.Further experiments prove that the main reason of complex 6’s formation is the dimethylammonium.Lastly,functional group（phenylphosphonic acid）is introduced to the above experimental system to form the functional cage（complex7）.During its formation,the self-assembly of MOC is guided by theπ-πstacking interaction between functional groups and organic linkers.It is proved that functional groups can change the coordination angle of a hexavanadate cluster and thus affect the self-assembly of MOCs.Finally,switching the organic linker from H₂ABDC to H₄QPTC,we obtain a cuboid cage（complex 8）and explore the influence of functional groups on the self-assembly of the cage in the pre-and post-assembly functionalization.By using post-assembly functionalization,a functional cage（complex 9）is obtained,which has similar structure to complex 8,yet with a larger interior cavity volume.However,using pre-assembly functionalization,complex 10 is obtained,which features a twisted cuboid shape and a much smaller interior void space.Therefore,using identical materials and different orders of introducing functional groups,the thermodynamic product（10）and kinetic product（9）can be obtained,respectively.Furthermore,the exceptional stability of these polyoxometalate–organic systems facilitates mass spectrometric characterization,which confirms the composition of the complexes,and indicates the methoxide groups on the vanadium cluster nodes are labile.