Download - Counterion Interaction and Association in Metal-Oxide Cluster Macroanionic Solutions and the Consequent Self-Assembly

DOI: 10.1002/ijch.201000079

Counterion Interaction and Association in Metal-OxideCluster Macroanionic Solutions and the Consequent Self-AssemblyPanchao Yin,[a] Dong Li,[a] and Tianbo Liu*[a]

1. Introduction

Polyoxometalates (POMs) are a large class of metal-oxide cluster anions consisting of early transition metals(usually Mo, W, V, Nb, and Ta) in their highest oxidationstates and oxo ligands.[1] Due to the multiple valences andcoordination formats of the center metal ions and variouskinds of connections of polyhedron units, POMs demon-strate extremely rich topological structures, sizes, andcharges (see Figure 1[1,2]). On the other hand, the polarand hydrophilic surface of POMs is very similar to thebulk metal-oxide surface, which is responsible for numer-ous physicochemical properties like electric conductivity,redox properties, and electron transfer, as well as availa-bility for chemical and catalytic reactions.[3]

In POM solutions, the electrostatic interaction betweenPOMs! surfaces and their counterions is important for de-termining their surface properties and consequently theirapplications.[4] Meanwhile, the structures and propertiesof the POMs can be controllably modified by interactingwith different types of cations, including cationic surfac-tants and ionic liquids.[4]

Compared to simple ions, POM anions have muchlarger sizes and lower charge densities. They are highlysoluble and form real solutions, which is different fromthermodynamically unstable colloids. Thus, the solutionbehavior of POM anions, especially their interaction withcounterions, is different from these two types of systems.On the other hand, POM macroions can be used for un-derstanding polyelectrolyte solutions, which are very com-

plicated and still poorly understood because both inter-molecular and intramolecular interactions need to be con-sidered. POM macroions with well-defined molecularstructure, mass, shape, and charge density are ideal, sim-plified model systems where only intermolecular interac-tion exists.[2a,5] As a result, POM macroions are extremelyvaluable for understanding the transition from simpleions to colloids, and particularly important for exploringthe macroion–counterion interaction when the two par-ties have ineligible, but still not significant size differen-ces.

2. A Retrospective Summary on the InteractionBetween POMs and Their Counterions

2.1. Initial Studies on Ion Pairing

During the 1960s and 1970s, when single crystal X-raydiffraction technology had not been fully developed, elec-trical chemistry was utilized as the major technique tostudy POMs in solution. Pope started the research on the

[a] P. Yin, D. Li, T. LiuDepartment of Chemistry, Lehigh University6 E. Packer Ave., Bethlehem, PA 18015, USAphone: 1 610 758-6536fax: 1 610 758-2935e-mail: [email protected]

Abstract : The interaction between large metal-oxide polyan-ions and their counterions is unique. Owing to their sizedisparity, there is a moderate ion-pairing effect and loosedistribution of counterions around macroions, which leadsto the unique solution behavior and the self-assembly of themacroions in polar solvents, and the counterion exchangecapability around macroions. Furthermore, the macroion–counterion interaction also affects the catalytic behavior ofthe polyoxometalate (POM) clusters. Replacement of func-

tionalized cations helps to modify the POM anions throughstatic charge attraction. At the same time, the strong POM–counterion interaction can also lead to counterion-depen-dent synthesis. Recent developments on theoretical simula-tions help to understand this interaction at the molecularscale. This review summarizes the chronological progress ofthe exploration of macroion–counterion interaction (boththeoretically and experimentally) and its impact on relatedresearch fields.

Keywords: cluster · electrostatic interactions · ion-pairing · polyoxometalate · self-assembly

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Review

reduction process of heteropoly blue.[6] In his work onKeggin-type POMs, he suggested that the reduction of aKeggin anion was limited by the overall ionic charge andthat the maximum number of electrons which could beadded without accompanying protonation to an ion ofcharge –n was 8-n. Protonation, which was necessary toreduce the total charge density of the POMs, could helpto add more electrons to the POMs. Under this circum-stance, protonation can be considered as an ion pairingbetween protons and POMs anions.[6]

Following Pope!s initial discovery, Weinstock and Hillsucceeded in preparing nine 1 :1 association complexes

between alkali-metal cations M+ (Li+ , Na+ , and K+) andthree representative vanadium(V)-substituted Keggin het-eropolytungstates by carefully controlling the POM size,structure and charge, solvent, temperature, buffer, as wellas electrolyte compositions and concentrations (see Equa-tion 1).[7]

!Xn"VVW11O40#!9$n#$ "M" % f!M"#!Xn"VVW11O40#g!8$n#$;

X % P!V#; Si!IV#andAl!III#!1#

Obviously, adding more M+ ions in solution would shiftthe equilibrium to right side and the reduction potential,E, of the solution became more positive. Equation 2

E % !EPOM " EMPOMKMPOM&M"'#=!1"KMPOM&M"'# !2#

illustrates the relation between the reduction potentialand the concentration of metal cations, which is consis-tent with experimental results (Figure 2). For a specificPOM, the reduction potential for the ion pair varied withthe type of counterions and increased with counterionsize, with the order M+ = Li+ < Na+ < K+. At thesame time, the solvated ion pairs decreased in size in theorder M+ = Li+ > Na+ > K+ .[7] They further tested theeffect of cationic size on the rate and energy of electrontransfer to the ion pairs by assigning key physicochemicalproperties to the series of ion pairs, which would be re-duced by organic electron donor compound in the experi-ment.[8] Four different counterions (THAN+ (tetra-n-hex-ylammonium), Li+, Na+, and K+) were tested; THAN+

is one expected not to form ion pairing. The initial reac-tion rates were obtained for Keggins with four differentcounterions, and then the corresponding kobs (the reduc-tion reaction rate constant) values were calculated. Foreach counter-cation, kobs values increased with increasingcounterion concentration. If the solution had no ion pair-ing, kobs values were insensitive to the change in ionicstrength. Thus, changes in kobs values as a function of ionconcentration indicated ion pair formation between M+

and Keggin anions (Figure 3).[8]

The ion pairing effect in POM solutions also plays animportant role in photochemistry and catalytic chemistry.For example, in the photo excitation process of[NBu4]4[W10O32] in solution, Yamase found that[W10O32]5– was produced by electron transfer from[NBu4]+ to [W10O32]4– within the lifetime of the photo-flash (t1=2 = 50 ms), and upon protonation, [W10O32]5– un-derwent disproportionation to yield the two-electron re-duced and diprotonated species [H2W10O32]4–.[9] Giannottiobserved that the aerobic photocatalyzed oxidation of al-kanes in the presence of decatungstates Q4W10O32 (Q =Na, Me4N, Pr4N, Bu4N, Hex4N) led to hydroperoxide asthe primary product. In the catalytic reaction, nitrile sol-

Panchao Yin was born in Hubei, China,in 1987 and currently is a Ph.D. stu-dent in the research group of Prof.Tianbo Liu, at the Chemistry Depart-ment of Lehigh University, studying theself-assembly of macro-ions in solu-tion. In 2009, he obtained his B.Sc.degree in Polymer Science and Engi-neering from the Department of Chem-ical Engineering in Tsinghua University.

Dong Li was born in Jinan, China, in1983. He received his B.Sc. from Shan-dong University in 2006 and M.Sc.from Lehigh University in 2009. He iscurrently pursuing a Ph.D. in PhysicalChemistry under the supervision ofProf. Tianbo Liu. His research focuseson the solution behaviors of macro-cat-ions in solutions and the self-assemblyof viral capsids. He is a Constance N.Busch Fellowship recipient.

Tianbo Liu was born in Beijing, Chinain 1971, and received his B.S. degree inChemistry from Peking University in1994. He received his Ph.D. in Chemis-try from SUNY at Stony Brook in 1999,under the guidance of Professor Benja-min Chu. During this time he studiedblock copolymer solutions by usingscattering techniques. After spendingtwo more years in the same group as apostdoctoral associate, he started hisindependent research career at thePhysics Department of Brookhaven Na-tional Laboratory. In January 2005 he moved to Department ofChemistry, Lehigh University, where he is currently an associate pro-fessor of Chemistry. His laboratory focuses on understand the fun-damental behavior of complex solutions, especially hydrophilic mac-roions, inorganic–organic hybrid surfactants, and other colloidaland biological systems.

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Review P. Yin et al.

http://www.ijc.wiley-vch.de

vent and organic counterions of the catalyst competed inthe photo oxidation with alkane substrate and had signifi-cant influence on the quantum yield, rate, and selectivityfor the oxidation of adamantane.[10]

In other research, Antonio et al. applied small-angle X-ray scattering (SAXS) to study the Lindqvist type[Nb6O19]8– and provide an example of direct measurementof ion pair formation in aqueous solution.[11] By plottingthe SAXS intensity Ia(Q) versus the scattering vectorQ("–1), the radius of gyration (Rg) of the complexes canbe determined via the well-known Guinier Plot. SAXSresults were combined with crystallographic informationto show that the A+ (A = K, Rb, Cs) ions associatedwith the Nb6O19 anions observed in solid-state salts alsoprevailed under selected solution conditions (seeFigure 4), and that contact ion pairing even persistedwhen A was not in great excess.[11]

2.2. Theoretical Study on POM Ion Pairing in Solution

The theoretical simulation work on POMs started in1980s and most work focused on the structure, electronicproperties, and basicity of the oxygen sites. In 2005, mo-lecular dynamics (MD) was applied to the Keggin anionsin solution for the first time by Poblet et al.[12] Throughdynamic analysis, the results showed that the terminaloxygen atoms of the cluster were invariably most effec-tively solvated by water because of their prominent posi-

Figure 1. POM anions with different topologies and sizes.[2]

Figure 2. Plot of E1/2 vs [M+]: M+ =Li+ (&), Na+ (D), and K+ (O);[M+]= total cation concentration from MCl, POM counter-cations,and buffer. Reprinted with permission from ref. [7] . Copyright 2000,American Chemical Society.

Figure 3. Plot of initial rate (d[1red]/dt) with different counterionsin different concentrations. Reprinted with permission from ref. [8].Copyright 2001, American Chemical Society.

Figure 4. a) The neutral A8[Nb6O19] species (A = Rb, Cs). Largespheres are face-bonded Rb/Cs+ . b) K+–[Nb6O19]

8– association ob-served in the crystal structure. Reprinted with permission fromref. [11].

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Counterion Interaction and Association in Metal-Oxide Cluster Macroanionic Solutions and the Consequent Self-Assembly


tions within the framework. On the other hand, bridgingoxo ligands, although located at positions less accessibleto solvent, had stronger interaction with the solvent mole-cules. In this work, hydrogen bonding formation was si-mulated to confirm that terminal positions had more con-tacts with water molecules than any other site of the clus-ter. Also, the authors indicated that the lifetime of suchcontacts was longer with bridging oxo ligands, probablydue to their higher atomic charge. The authors calculatedthe radial distribution functions of Li+, Na+, and K+ withthree Keggins bearing 3-, 4-, and 5- charges, respectively,and their diffusion coefficients. The effect of the micro-scopic molecular details of the solvent was a key aspectto interpret the simulation results due to a competitionbetween electrostatic interaction among the ions and thestability of the solvation shell. Further analysis showedthat the solvent-shared structures weakly bound to thePOM anions played a crucial role in the determination ofthe dynamic properties of the anions. The authors alsosuggested that the image of the ion pair as a well-definedmolecular conformation had to be substituted by a moredynamic picture in which the paired ions moved insidethe region bounded by Bjerrum!s length, with some pre-ferred locations that were the result of the more stablebalance between electrostatic interactions, entropic ef-fects, as well as the solvent effect (Figure 5).[12]

Wipff et al. performed theoretical research on the ag-gregation of Keggin anions in aqueous and methanol sol-utions.[13] Aqueous solutions of Keggin anions were simu-lated at two anionic concentrations with Cs+ , NBu4

+ ,UO2

2+ , Eu3+ , H3O+ , and H5O2+ as counterions. They re-

vealed significant counterion effect related to the degreeof salt dilution, as well as the cation–anion and anion–anion interactions. The hydrophobic NBu4

+ cationstended to surround POMs via loose contacts and createda “phase separation” between water and a humid, salty,

overall neutral domain where all ions were concentrated.The more hydrophilic cations were generally separatedfrom the POMs anions. The most important finding wasthe aggregation of POMs, mostly into dimers with shortinter-POM distances (P…P < 12 "), but also into oligo-mers in concentrated solutions where ca. 9 to 46% of thePOMs formed aggregates, depending on the type of coun-terions. While Eu3+ and UO2

2+ were fully hydrated andinteracted at short distances with POMs anions as sol-vent-separated ion pairs, Cs+ could form contact ionpairs, as well as solvent-separated ions. Among the mono-valent counterions, H5O2

+ led to the most serious aggre-gation, due to the influence of the protons. The POMs!dynamic properties were also dependent on the counter-ions: their diffusion coefficients were the lowest withNBu4

+ and highest with Cs+ , reflecting the degree of ioncondensation in water. The role of water on the solutionstate of the POM salts was further demonstrated by simu-lating the most concentrated systems in methanol solu-tion. Methanol solvated the counter-cations poorer thanwater did and could not form bridges between POMs,therefore a higher portion of anion-cation contacts, andno oligomers with short contacts could be found in meth-anol (Figure 6).[13]

Extending the molecular dynamic simulation to giantPOM macroanions would be very complicated and attrac-tive.[14] [H3Mo57V6(NO)6O183(H2O)18]21– is a nano-sizedanionic cluster and when the counterions are NH4

+ andK+ , the compound, (NH4)11K10[H3Mo57V6(NO)6O183-(H2O)18]·65H2O, has cavities that can accommodate a fewcounterions (see Figure 7). Experimental and theoreticalstudies have shown the distributions of different cations

Figure 5. Simplified two-dimensional illustration of a solvent-shared ion pairing between the PW12 anion and Na+ . The meanvalue of the angle Oterm–Owater–Na

+ is 1278. The mean value of thedistance between Oterm and Na+ is 4.75 !. The mean value of theangle between Owater and Oterm and the dipolar moment of thewater is 438. Reprinted with permission from ref. [12]. Copyright2005, American Chemical Society.

Figure 6. Snapshots of the different simulation boxes after 10 ns:a row, diluted aqueous solution; b row, concentrated aqueous so-lution; c row, concentrated methanol solution. Reprinted with per-mission from ref. [13]. Reproduced by permission of the PCCPOwner Societies.




http://pubs.rsc.org/en/Content/ArticleLanding/2008/CP/b810440a






on the cluster surface and in the cavities. The crystallo-graphic studies suggested that there were two differenttypes of cavities in this cluster, with one NH4

+ locating atthe center of each cavity due to strong hydrogen bonds.The first type of cavities were located in the upper andlower sides of the cluster (Figure 7a and Figure 8a, c).There were two such cavities in each cluster, which werecomposed of {Mo3V3O6} and parallel to each other (see

Figure 8a). The second type of cavities (six in total), com-posed of {Mo4VKO6}, were located around the sides ofthe cluster. The cavities on the upper side shared the po-tassium ions with the ones oin the lower side (seeFigure 7 and 8b, c). Assuming all the counter-cationsbeing Li+, MD simulation showed that all the Li+ werefound to stay preferably above the two first-type poresand in the area of the four terminal oxygen atoms(Figure 9).[14]

By using DFT calculations, Bo et al. studied the inter-action of different cations (Li+, Na+ , K+, Rb+, and Cs+)with [UO2(O2)(H2O)]n (n =4, 5, and 6) macrocyclicanions in order to uncover the mechanism of differentbuilding units (square (S), pentagonal (P), and hexagonal(H)) of uranyl-peroxide nanoclusters (see Figure 10).[15]

Structural geometries of different cations interacting withS, P, and H building units were optimized and used forenergy analysis. The complexation energies for all the in-teractions were calculated and indicated that the buildingunits selectively capture certain cations, and the strong af-finity of cations to the building units may support bendingenergies for the formation of different building units. Sand P building units had strong affinity to sodium ions. Ifno sodium was present in solution, S preferred to bind toLi+ while P preferred coordination to K+ . For the largestcations, Rb+ and Cs+ bound preferentially to hexa-gons.[15]

Figure 7. Polyhedron view of [H3Mo57V6(NO)6O183(H2O)18]21– with

NH4+ (individual gray spheres) and K+ (gray spheres linked to

POMs) as counter-cations. The small black dots are oxygen atoms.Reprinted with permission from ref. [14].

Figure 8. Two different type of pores in [H3Mo57V6(NO)6O183-(H2O)18]

21–. Reprinted with permission from ref. [14].

Figure 9. Isosurface of the space distribution function (SDF) forlithium counterions around [H3Mo57V6(NO)6O183(H2O)18]

21– computedaccording to a classical molecular dynamics simulation using ex-plicit water solvent molecules (not shown). The isosurface regionsenclose the regions of highest probability of finding lithium cationsaround the cluster surface. Reprinted with permission fromref. [14].




2.3. Counterion-Dependent Synthesis

Biological systems are known to be sensitive to theircounterions. For example, metal ions have a significantrole in controlling the conformation of proteins andRNAs.[16] Due to the strong interaction between POManions and counterions, the charge and size of the coun-terions might have an effect on the symmetry and molec-ular structure of their final products. Knoth synthesizedCs6W5P2O23 by adding phosphoric acid to the mixture ofCsOH and tungstic acid. When adding different counter-ions (Li+ , Na+, and K+) to the solution of Cs6W5P2O23,the structures of the final products were different. Thepossible mechanism was that the different structures per-sisting in solution are stabilized by ion pairing with cer-tain metal ions.[17]

By carefully controlling the reaction conditions, M#lleret al. discovered that counterions added to the solutionsof reduced molybdate play significant roles in the finalmolecular structure.[18] In the optimized reaction condi-tion, the hedgehog-shaped cluster {Mo368} could be ob-tained in good yield by adding NaCl to the reduced andacidified solution of molybdate. However, if K+ ions arepresent in the reaction mixture, a derivative of the{Mo176} wheel-type cluster was obtained in which a novel{KSO4}16 ring was integrated.[19] The K+ ions have adirect effect on the activation of the ring-type cluster!ssilent receptor sites and prevent formation of the {Mo368}-type cluster in the presence of SO4

2–.[18,19] On the otherhand, a neutral, spherical {MoVI

72MoV30} cluster is formed

in presence of Li+ .[18,20] With the help of structural analy-sis, the counterions might induce a template effect in theformation of large POMs clusters.[18]

2.4. Counterions Mediate Self-Assembly Behavior

2.4.1. Self-Assembly of Macroions to “Blackberries”

The unique interactions between the macroions and theircounterions, including their proper size disparity and hy-drophilic natures, are important for the unique solutionbehavior of macroions. Recent synthesis efforts, especiallythe extensive work from Achim M#ller!s group, provide alarge variety of POM macroions with different sizes,

shapes, and charges. These giant POMs are quite solublein water, due to their charge and hydrophilic surface.[21]

The direct result of macroion–counterion interaction isthe strong attraction among POM macroanions and theformation of stable supramolecular structures in solutio-n.[2a] The characterization of these large assemblies wasachieved with the help of static light scattering (SLS) anddynamic light scattering (DLS) techniques. By measuringthe scattered intensity from dilute solutions at differentscattering angles, SLS can determine the average molecu-lar weight (Mw) and radius of gyration (Rg) of the soluteparticles. DLS measures the intensity–intensity time cor-relation function, which is analyzed by the CONTINmethod to calculate the average hydrodynamic radius(Rh) of the particles and particle size distribution.Common sense tells us that soluble ions should exist asdiscrete ions in their dilute solution. However, largestructures were detected in various POM solutions. A de-tailed study of these solutions suggested that the supra-molecular formation took a long time to reach equilibri-um at room temperature (usually several weeks tomonths).[2a,22]

A typical CONTIN analysis from DLS study of {Mo154}aqueous solution at pH 3.0 shows that large assemblieshad an average Rh of 45 nm and a narrow size distribu-tion (see Figure 11).[23] SLS study analyzed by Zimm plotindicated that the assemblies had an average Mw of2.54 $ 107 gmol–1 (1,150 {Mo154} in each assembly) and anaverage Rg of 45 nm. The relation of Rh = Rg for spheri-cal objects (TEM image in Figure 11) and the low masssuggested a hollow, single-layered vesicular structure withan average inter-{Mo154} distance of <1 nm. Since {Mo154}was fully hydrophilic, the aggregate was formed due todifferent driving forces from the surfactant vesicles wherehydrophobic interaction was critical. The nickname“blackberry” was given to the new aggregates due to thesimilarities between the two species (Figure 12).[23]

Unlike {Mo154}, {Mo72Fe30} is a weak electrolyte and canbe dissolved in water very slowly due to the deprotona-tion process of the water ligands on the surface. DLSstudies on the aqueous solution of {Mo72Fe30} showed twospecies: single macroions (Rh = 1.3 nm) and large assem-blies (Rh =32 nm), suggesting the formation of “black-berries” in solution.[24] An AFM measurement of{Mo72Fe30} assemblies on a positively charged Si substrateconfirmed the hollow structure. Ring-shaped structureswere found when the AFM tip tapped the Si surface im-mersed in {Mo72Fe30} solution. The size of the rings wasabout the same as the size of aggregates determined fromDLS results. As time went by, more and more “rings”were detected. The AFM results indicated that “blackber-ries” were negatively charged hollow structures and theirshells were so fragile that they could be broken by anAFM tip (Figure 13).[24]

The spherical blackeberry structures were observed indifferent POM macroanions with various morphologies.[25]

Figure 10. Schematic representations of peroxide-bridged[(UVIO2)4(O2)4] (S), [(U

VIO2)5(O2)5] (P), and [(UVIO2)6(O2)6] (H). View fromthe concave side of the ring. Reprinted with permission fromref. [15]. Copyright 2010, American Chemical Society.




Laser light scattering (LLS) revealed the presence of the{Mo132} blackberry structure in water/acetone mixed sol-vents containing 3 to 70 vol% acetone, with their averageRh increasing from 45 to 100 nm with increasing acetonecontent (Figure 14). Only discrete {Mo132} clusters werefound in solution containing <3 or >70 vol% acetone,under which circumstances the discrete macroanions wereof very high charge density and very low charge density,respectively.[26] The transition from single clusters toblackberries on the left side clearly indicated that the vander Waals forces were not the major driving forces forthe self-assembly.[26] The average blackberry size in-creased linearly with increasing 1/e, with e being the sol-vent!s dielectric constant. Similar trends have also beenidentified in other POMs in mixed solvents.[27,28] Acharge-regulated self-assembly process was used to ex-plain the formation of blackberries, with a general expres-sion for the blackberry radius R is expressed as[29]

R ( $48lBu=y2 !3#

with lB ~ 56/e. Consequently, the size of the blackberriesis determined by three parameters: the solvent content(in e), the effective charge on the blackberries (describedby the zeta potential y), and the magnitude of the attrac-tive force among the macroions (u).[29]

Another important experiment is pH-dependent black-berry formation in aqueous solution.[30] {Mo72Fe30} is aneutral molecule that can release protons when it is dis-solved in water, and therefore can be treated as a weaknano-acid. By changing the pH of {Mo72Fe30} solution, theequilibrium between {Mo72Fe30} and its conjugative basescan be tuned, which would affect the charge density of{Mo72Fe30}. {Mo72Fe30} clusters existed as discrete, almostneutral molecules in aqueous solution at pH < 2.9, andthey became deprotonated and self-associated into single-layered blackberry structures at higher pH, while the as-sembly size was controlled by the pH. The average Rh ofthe self-assembled structures decreased monotonicallywith the increasing number of charges on the {Mo72Fe30}macroanions (from ~45 nm at pH 3.0 to ~15 nm at pH 6.6for 1.0 mgmL–1).[30]

As the in situ AFM results proved that the membranesof blackberry were quite soft, permeability tests indicatedthat the membranes selectively allowed cationic speciesto pass through them.[31] The permeability tests werebased on the monitoring of fluorescence spectroscopy ofwater-soluble dyes that were specifically sensitive for one(or two) type of ions. Chlorotetracycline (CTC) (sensitiveto Ca2+ and Mg2+), 6-methoxyquinoline (6-MQ) (sensi-tive to Cl–), and Coumarin 1 (sensitive to Br–) wereadded to different freshly prepared {Mo72Fe30} aqueoussolutions. The fluorophores could be partially incorporat-ed into the inner areas of blackberries during the black-berry formation. The dyes in bulk solution were quicklysaturated when extra salts containing the target ions were

Figure 11. (left) TEM image on dilute aqueous solution of {Mo154} macroions showing the existence of spherical, ~45 nm radius assemblies.(right) Zimm plot based on the SLS study of the {Mo154} aqueous solutions at pH 3; (inset) CONTIN analysis on the DLS study of the samesolution. Reprinted with permission from ref. [23].

Figure 12. Schematic plot showing the supramolecular blackberrystructure formed by {Mo154} macroions in aqueous solution. Re-printed with permission from ref. [23].




introduced. An additional very slow, continuous incre-ment in the fluorescence signal after the saturation sug-gested that the target ions could slowly pass the blackber-ry membranes and bind with dye molecules inside. Thetransmembrane transportation could be achieved bysmall cations but not by anions such as such as Cl– andBr– (Figure 15).[31]

2.4.1. Counterion-Mediated Attraction

Macroanion–counterion interaction plays an importantrole in blackberry formation. In order to accurately de-scribe the counterion association around macroions andto clarify its relation to the self-assembly of hydrophilicmacroions, SAXS was used to monitor the counterion dis-tribution around 2.5 nm, hollow, spherical POM{Mo72V30} in water and mixed solvents (acetone/water).[32]

In very dilute solution of {Mo72V30}, each macroanion isassumed to carry 31 negative charges, with the counter-ions being 14 K+ , 8 Na+ , 2 VO2+, and 5 H+ .[32] TheSAXS curve from the solution containing individual{Mo72V30} could be fit by the form factor of {Mo72V30}cluster. The Rg value obtained from the Gunier plot(10.8)0.5 ") suggested that there was no counterion as-sociation around macroions. The distance pair distribu-tion p(r), the probability of finding the vector length r ina molecule that will become zero at the maximum vectorlength, could be generated from the Moore analysis toprovide a more physically meaningful description of theparticle morphology. For {Mo72V30} in dilute aqueous sol-utions, the p(r) curves shown in Figure 16 (top) corre-spond to discrete {Mo72V30} clusters (a core–shell spheri-cal particle with a maximum dimension of 26 "), with nocounterion association. However, when the concentrationwas increased to more than 0.052 mm or a certain amountof acetone was introduced into the solution, another newdistant peak appeared in the p(r) plot. The original distri-bution remained unchanged, suggesting that the macro-ions still existed as discrete ions. This additional peak sug-gested that some counterions were closely associated withthe macroions and distributed in the distance range of0.2–0.9 nm to the surface of macroions. The peak due tothe associated counterions became more and more signifi-cant with increasing POM concentration or acetone con-tent. Meanwhile, Guinier plots indicated that the Rg

value of the {Mo72V30} macroions also increased accord-ingly. The appearance of the peak due to associated coun-terions was consistent with the appearance of the black-berry structures, indicating the direct connection betweenthese two issues and the role of counterions in the black-berry formation. A simple mean-field model was devel-oped to qualitatively elucidate the role of bound counter-ions in the attraction between two like-charge macroan-ions, as well as the effect of the dielectric constant of themedium on the interplay between counterion binding andthe self-assembly of the like-charge macroions. The re-sults from experiment and calculation confirmed thatmacroion–counterion association is the pivotal contribu-ting source of the attractive forces between the macroan-ions and, in turn, of their self-assembly into blackberrystructures in solution.[32]

Baglioni et al. applied SAXS to monitor both freshlyprepared and several-months-old aqueous solutions of{Mo72Fe30} in order to study the transition from monomer

Figure 13. a) The preparation for the substrate. b) the tappingmode of AFM on the sample. c) AFM results on diluted solutions of{Mo72Fe30}.




to blackberry and the existence of oligomers in the pres-ence of monomers/vesicles.[33] Scattering curves of0.5 mgmL–1 {Mo72Fe30} aqueous solution were supportedin Figure 17a. The one-month-old solution showed higherlow-Q intensity when compared to the freshly preparedsolution, showing the evidence of presence of large aggre-gates. In Figure 17b, both SAXS curves display two linearregions, which, probably, could be explained by the co-ex-istence of monomers and aggregates in solutions. For thefresh solution, the Rg values associated with the two linesare 1.1 nm and 3.1 nm, respectively, corresponding to{Mo72Fe30} monomers (diameter 2.5 nm) and dimers orother oligomers.[33]

The counterion effect is important in the current casebecause the size disparity between the macroions andcounterions is obvious, but not significant (compare tothe case of colloids). In other words, in macroionic solu-tions, the counterions cannot be treated as point charges(in colloidal suspensions this is no problem) and conse-quently the hydrated size of the counterions becomes im-portant. This introduces more challenges to theoretical

approaches because the mean field approach cannot beapplied for the macroionic solutions quantitatively. Onthe other hand, the effect of counterions on blackberryformation is very complicated.The direct impact of the above discussions is that coun-

terions with the same valence but different hydrated sizeshave different interaction energies in relation to macro-ions. When different metal ions were gradually added to{Mo72Fe30} solution, blackberry size did not change at lowsalt concentrations, but when passing a critical salt con-centration (CSC), blackberry size would increase quickly(Figure 18) due to the decrease of the screening length ofthe macroions.[34] An obvious trend existed for the CSC:1) CSC rank for monovalent ions: Ru+ < K+ < H+ <Li+, Na+; 2) Divalent metal ions have a much lower CSCthan monovalent ions.

2.4.3. Counterion Exchange Around Macroions

The exchange of monovalent counterions around macro-ions has been confirmed by different approaches. A

Figure 14. a) Transition from discrete macroions (molecules) to blackberries, then to discrete macroions due to the change of solvent con-tent for 1.0 mgmL–1 {Mo132} in water/acetone mixed solvents. b) Plot of the average blackberry radius (in Rh) versus the inversed dielectricconstant (1/e) of the solvent for various POM macroions in water/acetone mixed solvents. Linear relationship roughly follows for these sys-tems. Reprinted with permission from ref. [26]. Copyright 2007, American Chemical Society.

Figure 15. Formation of fluorophore-containing {Mo72Fe30} blackberries in solution. The additional cations, once added into solution, in-stantly interact with fluorophores in bulk solution and on blackberry surfaces, subsequently entering into the blackberries, and interactingwith the fluorophores inside. The anions could not cross the membrane. (right) Change in fluorescence quantum yield of Coumarin 1, 6-MQ, and CTC with addition of KBr, KCl, and CaCl2, respectively. a) Instantaneous change occurs with the addition of salts. (b) Change influorescence quantum yield with time, once the addition of salt is stopped. Reprinted with permission from ref. [31]. Copyright 2008, Amer-ican Chemical Society.




simple observation is the change of blackberry size whena small amount of extra salts is added. For {Mo72Fe30}, theoriginal counterions are protons. Its blackberry size did

not change from that found in the salt-free solutionswhen 1–20 mm LiCl or NaCl was added. However, when0.1–10 mm KCl is added, blackberry size became consid-erably larger (~34.6 nm). The same result was observedwhen RbCl was introduced (Rh ~35.7 nm). A very possi-ble explanation is that K+ and Rb+ ions (smaller hydrat-ed sizes) could replace the protons around {Mo72Fe30}, in-creasing the attractive force between macroions and lead-ing to larger blackberries, while large hydrated ions suchas Li+ and Na+ could not replace the protons due totheir lower priority.[34]

This speculation was confirmed by isothermal titration(ITC) studies. For NaCl solution, there was no measura-ble binding of Na+ ions to macroions even with themolar ratio of Na+ to POM of 1000 :1. The titrationcurve for KCl indicated that binding between the K+ ionsand the {Mo72Fe30} macroions was present. 50–60 K+ wereneeded to completely saturate the binding sites (~6) on{Mo72Fe30}. However, <10 Rb+ ions were needed for sat-uration, suggesting that the binding to {Mo72Fe30} macro-ions was much stronger for Rb+ than for K+. The ITCstudies provide direct confirmation that the bindingstrength between {Mo72Fe30} and monovalent cations fol-lows the order of (Li+ ,Na+) < H3O+ < K+ < Rb+ <Cs+, which is completely consistent with the correspond-ing observed blackberry formation processes.[34]

Direct evidence for counter-cation exchange was ob-tained from Anomalous Small-Angle X-ray Scattering(ASAXS) studies. When Ru+ was present in {Mo72V30}solution, there was an obvious difference between theI(Q) responses obtained at different energies. The differ-ence indicated that Rb+ distributed homogeneously andisotropically around the macroions, which meant that the

Figure 16. Top: Distance distribution functions based on calculat-ed and experimental scattering data for {Mo72V30} obtained byusing an indirect Fourier transform of the primary SAXS data. (*):0.052 mm {Mo72V30}, (*): 0.013 mm {Mo72V30}, (—): {Mo72V30} calcu-lated. Bottom: Experimental distance distributions for 0.26 mm{Mo72V30} in water and acetone/water mixed solvents with variousacetone content (in vol%). (—): 75% acetone/water, (*): 65% ace-tone/water, (- - -): 45% acetone/water, (…): 10% acetone/water,(&): in pure water. Reprinted with permission from ref. [32].

Figure 17. Scattering curve of 0.5 mgmL–1 {Mo72Fe30} nanocapsules in water solution at 25 8C. Lower panel : (*) freshly prepared. Upperpanel: (D) 1 month old. b) Guinier plot of scattering curves reported in Figure 4. Lower panel: (*) freshly prepared. Upper Panel: (D) 1month old. Dashed line is the best fit according to the Guinier approximation for {Mo72Fe30} monomers. Dotted lines are the best fit accord-ing to the Guinier approximation for aggregates. Reprinted from ref. [33], with permission from Elsivier.




replacement of K+ by Rb+ in {Mo72V30} solution is possi-ble. Furthermore, pH value also decreased in the solutiondue to the release of more free protons as a result ofcounterion replacement.[34] The capability of distinguish-ing different monovalent cations in dilute solution isunique for these hydrophilic macroions.More importantly, research on the self-assembly of

POMs can provide inspiration for research on the self-as-sembly of biomolecules such as proteins, given that pro-teins and DNAs are also macroions. For example, viruscapsid proteins form nano-scaled hollow spheres withprotein subunits homogeneously dispersed on theirshells.[35] Besides the structural similarity between viruscapsids and the blackberries, both self-assembly behaviorsdemonstrate similar kinetic features by displaying sigmoi-dal kinetic curves in LLS studies. The lag period beforethe rapid growth of assemblies is largely dependent onionic strength and concentration of monomers.[35] The for-mation of dimers as a transition state can be used to ex-plain the appearance of a lag period. Ultracentrifugationstudies confirmed that slow dimer formation was respon-sible for the slow blackberry assembly process and, conse-quently, the sigmoidal curves.[35a] Based on the discussedsimilarities, the POM macroions might be useful assimple model systems for studying more complicated bio-macromolecular systems.[35]

2.4.4. Self-Assembly of Sn-12

The Sn-12 oxo cluster (Figure 19), {(RSn)12O14(OH)6}2+ ,first reported in 1989, is a positively charged organic–in-organic cluster.[36] The football-shaped cluster exhibitstwo positive charges that are located at the poles of the“football”, defined by the six-coordinated Sn ions and theu2 ligands. Because of the location of the two positivecharges, the cluster can interact with dianions to showsome interesting assembly behaviors.[36]

Very recently, Ribot et al. applied pulsed field gradientNMR spectroscopy to probe the anions mediated associa-tive behavior of {(RSn)12O14(OH)6}2+.[37] The chloroformsolutions of {(RSn)12O14(OH)6}(OH)2 with different con-

centrations were first investigated at different diffusiondelays. Experimental results indicated that the diffusioncoefficient is 5.2 $ 10–10 m2s–1 and independent of concen-tration and diffusion delay. The corresponding Rh was cal-culated as 0.83 nm, consistent with the size of the cluster.The experimental results of {(RSn)12O14(OH)6}(SO4) and{(RSn)12O14(OH)6}(C2O4) indicated that their diffusioncoefficients in chloroform and benzene were concentra-tion-dependent. In the chloroform solution of{(RSn)12O14(OH)6}(C2O4), the diffusion coefficient valuedecreased from 3.8 $ 10–10 to 1.7 $ 10–10 m2s–1 as the con-centration increased from 1 to 8 mm. The diffusion coeffi-cient obtained was always smaller than for{(RSn)12O14(OH)6}(OH)2, indicating that the macroca-tions formed aggregates in the presence of oxalate. Thedecrease in diffusion coefficient with concentration indi-

Figure 18. Change of blackberry size (in Rh) with added chloride salt concentration (A) and total ionic strength (B) for 0.5 mgmL–1

{Mo72Fe30} solutions. For each added cation salt there is a CSC (critical salt concentration), above which the blackberry size increases withincreasing salt concentration. Reprinted with permission from ref. [34]. Copyright 2010, American Chemical Society.

Figure 19. Molecular structure of {(RSn)12O14(OH)6}2+ . (six-coordi-

nated tin, green; five-coordinated tin, teal; u3-oxo, red; u2-oxo,purple; R, black; hydrogen, gray).




cated an increase in the degree of aggregation with con-centration.[37]

In chloroform, its diffusion coefficient decreased from3.2 $ 10–10 to 1.2 $ 10–10 m2 s–1 within the concentrationrange. Obviously, its diffusion coefficient was smallerthan that of {(RSn)12O14(OH)6}(C2O4) at any concentra-tion, indicating the formation of large aggregates, proba-bly due to the higher charge density of sulfate, and result-ed in stronger electrostatic interactions with Tin-12 mac-rocations. In addition to chloroform, {(RSn)12O14(OH)6}-(SO4) can be dissolved in benzene. The Rh in benzene,calculated from Stokes–Einstein equation, was largerthan that in chloroform at the same concentration. Ben-zene has a lower dielectric constant than chloroform, andtherefore a lower Debye length. The interaction betweensulfate and macrocation became stronger and at the sametime the repulsive force between two cations got muchweaker, leading to larger aggregates (Figure 20).[37]

2.4.5. Self-Assembly of Keggin POMs on Surfaces

Highly charged POMs can be adsorbed to form monolay-er on the surface of Au(0), Ag(0), Hg(0), and glasscarbon due to the strong electrostatic interactions be-tween the POMs! surface ligands and metal surfaces. Asthe result, POMs can be used for stabilizing nanoparti-cles.[38] Although POMs are highly negatively charged,studies revealed that POMs could almost fully coverwhole surfaces, with relatively small inter-POM distanceson the surface (1 to 2 nm).[38]

In order to clarify the roles of counter-cations in theabove-described 2D order arrays of anionic clusters, Bar-teau studied the formation of POM monolayers on highlyoriented pyrolytic graphite (HOPG).[39] POMs with differ-ent compositions, sizes, and shapes in combination with aseries of counter-cations, such as H+ , Na+, K+ , and Cs+,were used to give a successful explanation of the shortcenter-to-center distances. Square and hexagonal packingand periodicities of from 11 to 14" were observedthrough STM images. Interestingly, as the counter-cationsizes increased, the nearest neighbor distance betweenPOM anions on the surface increased. Negative differen-

tial resistance current-voltage measurements indicatedcations existed in the area between POM anions adsorbedon the surface. When the counter-cations were protons,the assembly chose squared packing with lattice length ofca. 10 to 12". Interestingly, when the counter-cationsbeing potassium, the inter-POM length got larger and dis-torted square packing and hexagonal packing were ob-served. The same trend also appeared when H+ and Cs+

being counterions. With more Cs+ , the inter-POM dis-tance increased, together with the transition from squareto hexagonal packing.[39]

Based on these results, Weinstock summarized theeffect that might be important in the 2D self-assembly: 1)hydrogen bonding between protons and/or hydrated pro-tons and oxide ligands of the POM anions, 2) hydrogenbonding between metal-cation bound water molecules(aqua complexes) and oxide ligands of the POM anions,3) direct coordination between metal cations and oxide li-gands of the POM, and 4) electrostatic interactions be-tween positively charged cations (including their hydra-tion shells) and the negatively charged POM anionsthemselves.[38]

Through the reactions of metal salts with reducedPOMs and ligand-exchange reaction, nanoparticles canalso be stabilized by POM anions.[38,40] Monolayers com-posed of POM anions can be observed covering the sur-face of nanoparticles under cryo-TEM. The formation ofPOM anion monolayer around gold nanoparticles can beused as good example to explain counter-cation effect inthe resulting nano-structures. In Figure 21, the perimeterof the gold nanoparticles (R = 13.8 nm) contained a ringof 30)1 molecules of POM anions. The POMs were hex-agonal packed, with center-to-center distance being1.57)0.04 nm. Time-dependent cryo-TEM images re-vealed that monolayer growth occurred via “islands”, amechanism that pointed to cation-mediated attraction be-tween bound POMs. Zeta-potential results indicated thatthe POM-covered nanoparticles carried small net charges

Figure 20. The formation of large aggregate induced by dianions.Reprinted with permission from ref. [37]. Copyright 2010, AmericanChemical Society.

Figure 21. Cryo-TEM images of (A) citrate-protected and (B) 1-pro-tected gold nanoparticles. Panel C is a copy of the image in B, en-hanced (swolid blue circles to highlight (1) the “ring” of individual1 anions at the periphery of the gold particle, (2) coverage of theparticle’s surface indicated by the presence of the peripheral ring,and (3) “freely solvated” molecules of 1 “trapped” in the vicinity ofthe gold particle in the water–glass matrix. Scale bar = 10 nm. Re-printed with permission from ref. [40b]. Copyright 2009, AmericanChemical Society.




(29–). By knowing the ionic strength of the solution, theDebye length of the nanoparticles could be calculated as1.0 nm, suggesting that ca. 99% of counter-cations laywithin ca. 1.5 nm from the outer surface of the POMshell. On the other hand, by assuming hexagonal packingof the POMs, the repulsive energy from neighbor POMswas 80 kcal and the equilibrium constant was 10–60 byusing Gibbs equation. However, the adsorption Gibbsfree energy between alkanethiols and the gold surface arefavorable by up to 20 kcalmol–1. Therefore, it is quiteclear that many counter-cations were structurally integrat-ed into the POM shell, and without the counter-cations,monolayer formation would be energetically prohibit-ed.[38, 40]

3. Summary and Outlook

The electrostatic interactions between POM anions andcounter-cations have impact on the catalytic and redoxproperties, molecular conformation, and self-assembly be-havior. Different techniques, from early electrical chemis-try to modern gradient NMR and X-ray Scattering, havebeen employed for solving related fundamental problemsuch as the ion pairing and the counterion associationaround macroions. The size disparity between the macro-ions and the counterions makes their interaction compli-cated and extremely interesting. Particularly, the counter-ions are directly related to the blackberry formation pro-cess, and determine the properties of such blackberry as-semblies. At the same time, theoretical simulations havebeen quickly developed to explain the mechanism of ion-pairing. Though it is still difficult and time-consuming torun simulation on large POM macroions at atomic scale(especially counterion-mediated self-assembly), we expectthat in the near future this will be solved with more pow-erful computing tools.Furthermore, the study of countercation–macroion in-

teraction will provide the connection between POM mac-roions and biological macromolecules, e.g., the similari-ties between blackberry formation and virus capsid for-mation. It is well known that DNAs show very interestingsolution behavior based on DNA–counterion electrostaticinteractions. Very recently, scientists found that the intro-duction of metal ions could tune the conformation ofRNA. Therefore, the research on the electrostatic interac-tion in macroionic solutions can not only help the appli-cations of POMs as catalysts, but also provide simplemodels for understanding biological processes.

Acknowledgments

We gratefully acknowledge the helpful discussions withProf. Achim M#ller and the support of this work by theNSF (CHE-1026505), Alfred P. Sloan Foundation, andLehigh University.

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Received: December 11, 2010Accepted: January 5, 2011

Published online: February 16, 2011


