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46294-G3
Study the State of Counter-ions in Polyoxometalate Solutions

Tianbo Liu, Lehigh University

<>     This project is aiming for understand the role of small counter-ions in the solutions of hydrophilic polyoxometalate (POM) macroions, which are widely used as catalysts in petroleum industry and as novel functional nanomaterials. We have made the following progresses in the past fiscal year:

1. Radial distribution of small cations around POM macroanions <> 

     We have successfully obtained beamtime as a general user at Advanced Photo Source, Argonne National Laboratory to perform synchrotron X-ray scattering experiments. For the first time, we obtained the radial distribution of small metal cations around POM macroions (2.5-nm-radius, spherical “Keplerate” POM {Mo₇₂V₃₀}, with -31 charges). The major results are:

(1)   At very low {Mo₇₂V₃₀} concentrations, almost all the cations behave like free cations in solution, i.e., no obvious ion pairing effect.

(2)   At higher {Mo₇₂V₃₀} concentrations (> 0.5 mol/L), ion pairing effect emerges, as a distribution of K⁺ ions around {Mo₇₂V₃₀} macorions.

(3)   At low {Mo₇₂V₃₀} concentrations, adding acetone into the solution can decrease the polarity of the solvent. Accordingly, counter-ion association around {Mo₇₂V₃₀} can be observed.

    The appearance of the counter-ion association around POM macroions is coincident with the self-assembly of the POM macroions into “blackberry” structures in solution. This is strong evidence that the blackberry formation, a type of quite universal self-assembled structures by hydrophilic macroions, is driven by the counter-ion-mediated attractions. The work will be submitted to Phys. Rev. Lett.

2. Counter-ion transport over the “blackberry” membrane

     The blackberry structure itself is a hollow, porous sphere because the macroions are not touching with each other on the blackberry surface. Consequently, the blackberry surface can be treated as a special membrane. By using fluorescence and light scattering techniques, we found that the blackberry “membrane” is very unique because it allows the slow transport of small cations (such as Ca²⁺ and Mg²⁺) from the bulk solution into the space inside the blackberries. This is completely different from bilayer cell membranes which are not permeable to cations and special carriers are needed. Also, we find that the water encapsulated into the blackberry shells has a higher viscosity than that in bulk solution, possibly due to the different hydrogen bondings. The results are published in J. Am. Chem. Soc. 2008, 130, 1548. <>

3. Self-assembly of metal-organic macrocations

     Additional to the polyoxometalate macroanionic systems we have extensively studied, we want to explore whether the unique, counter-ion-mediated self-assembly of macroions into blackberry-type structures is universal for other types of systems. We have studied the solution behavior of (Wako) Pd₆L₄ {Pd = Pd(ethylenediamine), L = 2,4,6-tris(4-pyridyl)-triazine} metal-organic nanocages in the mixed solvents of water and acetone. Interestingly, blackberry formation was observed when certain acetone was introduced. The blackberry size increases with increasing acetone content, same as that in POM macroanionic solutions, indicating a charge-regulated self-assembly process. The results confirm that the blackberry formation is universal. The unique structure of the nanocages enables us to expand our study of macroions by considering of loading signal-sensitive materials. The results are published in J. Am. Chem. Soc. 2008, 130, 4226. <> <>4. Self-assembly of POM-coated Pd nanoparticles

     In this work, we demonstrate that the self-assembly of macroions into blackberry-type structures can be achieved by hydrophilic nanoparticles. The hydrophobic Pd nanoparticles become hydrophilic when coated with Dawson-type V-substituted POM K₉[H₄PV^IVW₁₇O₆₂] (HPV^IV) clusters. This work is published in Langmuir 2008, 24, 5277.

5. Self-assembly of Yttrium-containing polyoxometalate (K₁₅Na₆(H₃O)₉[(PY₂W₁₀O₃₈)₄(W₃O₁₄)]×9H₂O, or {P₄Y₈W₄₃}) macroanions in aqueous solution

     {P₄Y₈W₄₃} is a type of large polyoxotungstate. We find that it is unique because they demonstrate the properties of both “strong electrolyte” and “weak electrolyte” types of POMs in solution. Consequently, they can form blackberries and the blackberry size can be adjusted by either changing pH or changing solvent content. Thus it can be used as a valuable system to directly the effects of the two processes. The work is published in Langmuir 2008, 24, 9308.

6. Comparison among three types of POM giant “weak acids”

     Three Keplerate types of structurally similar giant POM clusters, {Mo₇₂Fe₃₀}, {Mo₇₂V₃₀} and {Mo₇₂Cr₃₀}, are compared for their solution behavior. {Mo₇₂Fe₃₀} and {Mo₇₂Cr₃₀} are similar in solution, both showing weak-acid features due to the partial deprotonation of their water ligands. {Mo₇₂V₃₀} is also a weak acid, but more or less behave like a weak acid salt due to the large amount of inherent charges inside its skeleton. The work helps to summarize the self-assembly of such large, hollow, spherical POM macroions in solution. The manuscript is submitted.

7. POM-organic hybrid materials

     By chemically attaching organic ligands to the large POM clusters, novel inorganic-organic hybrids can be synthesized. Such hybrids could show amphiphilic properties due to the hydrophilic POM clusters and the hydrophobic alkyl chains. For the first time, we confirm such behavior by observing the formation of bilayer vesicles formed by [n-Bu₄N]₃[MnMo₆O₁₈{(OCH₂)₃CNHCO-(CH₂)₁₄CH₃}₂] (Mn-Anderson-C₁₆) hybrids in water/acetonitrile mixed solvents. The manuscript is submitted. <> 

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