Self-assembly Of Organic Modified Anderson Polyoxometalate Controlled By Counter Cations And Solvent

Polyoxometalates(POMs)are a type of inorganic clusters with precise chemical composition and uniform size and topology.1 Due to the important physical and chemical properties sourcing from metal elements and oxidized state,heteroatom,and cluster framework,POMs exhibit diverse potential applications in catalysis,2 biomedicine,3 photoelectric materials,4 and energy materials,5 while the rich nanoscale morphology provides possibilities to serve as a building block for self-assemblies.6 However,in the case without assistant agents,the diversity and stability of POMs' assemblies are usually less controlled.7 To improve the capability in self-assembly,the polyanionic feature of POMs has been used to combine with cationic organic molecules through electrostatic interaction.8 The formed ionic organic-inorganic complexes bring up amphiphilicity and decrease the electrostatic repulsion,which direct to various self-assembly structures accompanying by synergistic functions in solutions,especially in organic media.9 One of the advantages in this route is that the complexes perform a whole unit whereas each of the components plays independent role in the formed self-assembled structures.10 The synergistic effect of organic component helps assemblies of POM clusters under a controlled state that the isolate one cannot conduct completely.Such an ionic interaction,however,yields excessive flexibility and consequently,some special assemblies with twisting packing structure along a fixed direction become difficult in several photo chromism and selective catalysis systems.For example,very few examples dealing with asymmetric self-assembly of ionic complexes were reported11 though the induced chirality of organic cationic components can be easily transferred to POMs via the electrostatic interaction.12 The reported results with chiral structures were mainly from the individual behavior of covalently grafted clusters.13 Beside lacunary POMs,14 the Anderson-type cluster that possesses a disk like architecture in diameter of ca.0.8 nm and thickness of 0.3 nm,has been demonstrated to have outstanding features differing from many other POMs,since this kind of clusters can also be conveniently modified with triol ligands covalently.15 The organically hybridized POMs display amphiphilic property in solutions deriving from both hydrophilic and hydrophobic interactions,depending on the media to be dissolved.As the chiral POMs are hard to synthesize based on their topologic structure meanwhile for those POMs possessing intrinsic chirality their asymmetry disappears quickly in aqueous solution due to the dynamic exchange of oxygens in cluster with those in water,it is a practicable approach to create chirality of POMs by induction of chiral countercations via ionic interaction while the cluster structure and other functional performance do not change.17 Following this idea,we herein would like to report our recent results by using simple counterions including inorganic and organic cations to modulate the self-assemblies of Anderson-type POMs that grafts a 1,3,5-triazine with two alkyl chains covalently in different solvent media.The spherical/planar self-assembled structure and helical packing of the POM as well as chiral amplification were built in solution system,which are very important for further potential applications in asymmetric catalysis in the ongoing studies.First,through covalent hybridization of polyoxometalate clusters,organic components with hydrophobic alkyl chains at the tail end are connected to the polyoxometalate clusters to form bilateral positive polyoxometalate supramolecular complexes.Four kinds of polyoxometalate clusters with different counterbalance ions have been successfully synthesized by using mature and simple electrostatic embedding method to replace counterbalance ions on Polyoxometalate clusters.The results of NMR,IR,ESI and elemental analysis showed that three kinds of polyoxometalate clusters with different counterbalance ions have been successfully synthesized.There are polyoxometalate clusters with TBA as cation,sodium as cation and chiral cation as counterion.Then I will summarize my work in two parts.Firstly,We formed a bilaterally positive supramolecular complex by connecting organic components with hydrophobic alkyl chains at the end to polyoxometalate.By using mature and simple electrostatic embedding method to replace the counterions on the polyoxometalate complex,it has been proved that four complexes with different counterions have been successfully synthesized by means of nuclear magnetism,infrared spectroscopy,mass spectrometry,element analysis and other test methods.There are counterions of tetrabutyl ammonium bromide,sodium and two corresponding chiral cations of polyoxometalate complexes.Second,the self-assembly of morphology and structure of anions can be changed by adjusting the environment and their countercations of complexes.Sodium ions are countercations with small size,high polarity and hydrophilicity.Polyoxometalate complex with sodium ions as countercations is dissolved in the system of methanol and water.As the polarity of mixed solution increases,the morphology of spherical,fibrous and flake structures are obtained successively.And when we replace the countercations with tetrabutyl ammonium bromide with low polarity and strong hydrophobicity,we get the structure of the sphere and fiber in methanol and water as the volume of water increases.When we changed the solvent system to chloroform and n-hexane,we got a flake-like structure.The morphology of the self-assembled by transmission electron microscopy(Tem)and X-ray diffraction showed that they are all layer structures,and with increase polarity of mixture solvent,the layered spacing will increase too and solvent system changing from strong polarity to weak polar solvent system layer spacing decreases,this is because the polyoxometalate complex itself amphiphilic and the polarity of the solvent system.Compete when we replace the cation from the achiral to chiral counterions,we put compounds self-assemble in pure chloroform organic solvent,observed CD signal but didn't get helical structure,and when the addition of n-hexane,we found that the chiral signal increases,when the ratio of mixed solvent of n-hexane to 10%,we got helical structure.Thus,it can be concluded that the addition of n-hexane amplifies the chirality and thus changes the self-assembled structure into a spiral shape.By combining ultraviolet spectroscopy and circular dichroism,we can analyze chiral signals from organic components of complex,while chiral signals of organic components are induced by chiral cations.We combine the ideal configuration of molecular length of polyoxometalate and the actual layer spacing,We believe that the accumulation of molecular self-assembly when mode is a state of bilayer folding arrange assembling,more to sodium ions in compounds and tetrabutyl ammonium bromide as cationic polymetallic oxygen cluster compounds based on the self-assembly of solvent polarity changes are part of the cross and the change of the whole cross,and more than two kinds of chiral metal oxygen clusters in spiral structure is reverse part accumulation model.Finally,we summarize and forecast the work content of this paper: we know that the self-assembly morphology of complexes can be changed by regulating the polarity of the solvent and counterions of the complexes.And through the changes between them,we try to explore the causes and rules of change.We expect that through the discussion of synthesis and self-assembly of this paper can be provides a new thinking of clusters,the future can try to replace the more functional clusters,also can replace with different properties of cationic to prepare more rich in nature and function of the bunch of supramolecular complex system and for more applications of chirality transfer,chiral amplification,asymmetric catalysis and chiral luminescence.