Deprotonation Property of Polyoxometalates with Different

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DEPROTONATION PROPERTY OF POLYOXOMETALATES WITH DIFFERENT

LACUNARY METAL IONS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirement for the Degree

Master of Science

Kexing Xiao

May 2019 DEPROTONATION PROPERTY OF POLYOXOMETALATES WITH DIFFERENT

LACUNARY METAL IONS

Kexing Xiao

Thesis

Approved: Accepted:

______Advisor Dean of the College Dr. Tianbo Liu Dr. Ali Dhinojwala

______Committee Member Dean of the Graduate School Dr. Toshikazu Miyoshi Dr. Chand Midha

______Department Chair Date Dr. Tianbo Liu

ii ABSTRACT

Some biomacromolecules own unique deprotonation property due to water ligands which coordinated with metals. Like biomacromolecules, several Keplerate-type polyoxometalates (POMs) were reported to behave as weak polyprotic acids in aqueous solutions due to partial deprotonation of water ligands that attached to the non-Mo metal centers. These macroions show connections to biomacromolecules in self-assembly, self- recognition, catalysis, etc. By changing deprotonation degree, the unique bio-mimic features can be accurately tuned. Therefore, studying the deprotonation property of molecules is important as it will provide a simple model for people to understand more complicated biomacromolecules.

In this work, a series of niobium/tungsten mixed-addendum POMs with different lacunary metal ions are used to investigate the deprotonation property of nanoscale molecules. Acid-base titration and isothermal titration calorimetry (ITC) were applied to characterize two POM clusters whose water ligands coordinated with two different lacunary metals, Europium and Copper. The results show that the deprotonation only occurs with the addition of base into the solutions, and the deprotonation capacity of POMs with different metals is different. The POM cluster whose water ligands attached to the Eu

iii metals releases more protons than the cluster whose water ligands attached to the non- lanthanide ions. In addition, there exist two deprotonation sites and two stages during the whole deprotonation process. It is assumed that protons are initially deprotonated from water ligands which are attached to lacunary metal ions with the addition of base, and then released from hydroxide groups linked with niobium. The deprotonation from the coordinated water ligands and the linker can be distinguished for the POMs with lacunary copper ions, which is attributed to the charges of each cluster currently. Overall, the deprotonation property varies for POMs with different metal ions. The acidity strength of these weak nanoacids also differs depending on disparate lacunary metal ions.

iv ACKNOWLEDGEMENT

First, I would like to express my sincere gratitude to my advisor, Dr. Tianbo Liu, for his guidance and support during my master study at the University of Akron. He led me into this area and provided me with opportunities to study the complex solution behavior of macromolecules. It is a great honor for me to work with him. Meanwhile, I want to address my great appreciation to Dr. Dongdi Zhang at Henan University for providing samples for us in this project.

Besides, I would like to appreciate my committee member, Dr. Toshikazu Miyoshi, for his helps and advice in my academic life. I would also like to thank Jiahui Chen, a senior PhD candidate in our group, for his helps and inspirations on my experiments and many fields.

Finally, I want to appreciate all the group members for their assistance during the two years. I want to thank my family and my friends for always supporting me in everything.

v LIST OF CONTENTS

LIST OF TABLES ...... viii

LIST OF FIGURES ...... ix

CHAPTER I. INTRODUCTION ...... 1

1.1 Introduction of polyoxometalates ...... 1

1.1.1 Polyoxometalates (POMs) ...... 1

1.1.2 Deprotonation property of POMs ...... 3

1.2 Solution behavior of POMs ...... 5

1.2.1 Self-assembly behavior of POMs ...... 7

1.2.2 Self-recognition behavior among POMs ...... 11

1.3 Connections to some biomacromolecules ...... 13

1.4 Study motivation ...... 15

II. EXPERIMENT ...... 18

2.1 Sample Preparation ...... 18

2.2 pH Meter ...... 19

2.3 Isothermal Titration Calorimetry ...... 20

2.4 Conductivity Meter ...... 21

vi III. RESULT AND DISCUSSION ...... 22

3.1 Dissociation and deprotonation of POMs in pure water ...... 22

3.2 Deprotonation of POMs with additional base ...... 24

3.2.1 pH measurement ...... 24

3.2.2 ITC measurement ...... 29

IV. CONCLUSION ...... 32

REFERENCES ...... 34

vii LIST OF TABLES

Table Page

1 Dissociation and deprotonation processes of POM in pure water……………..23

2 pH values of POM and KOH aqueous solutions at different molar ratio……...25

3 The number of released protons from one POM at different molar ratios……..26

viii LIST OF FIGURES

Figure Page

1 Classical POM structures in polyhedral representations…………………………..2

2 Some typical large polyoxometalate molecular clusters and their sizes…………..3

3 3.6 nm wheel-type POMs {Mo154} self-assemble into hollow, spherical

“blackberry” structures in aqueous solutions………………………………………….....8

4 Transition from discrete macroions to blackberries, then to macroions again due to

the change of solvent content for 1.0 mg/mL {Mo132} in water/acetone mixed solvents..9

5 The average hydrodynamic radius (Rh) of the blackberry-type structures formed in

0.5 mg/mL aqueous solutions of {Mo72Fe30} at different pH, as measured by DLS at 90° scattering angle…………………………………………………………………………..10

6 In mixed dilute aqueous solutions, the clusters {Mo72Fe30} (top) and {Mo72Cr30}

(bottom) self-assemble into different blackberry structures of the Cr30 (yellow) and Fe30 type (blue)—with interfacial water between the macroions (right)—and do not form mixed species (such as the hypothetical structure shown on the left)…………………...12

7 The structure of KcsA……………………………………………………………..14

8 Polyhedral and ball-and-stick representation of different building blocks making up polyanion………………………………………………………………………………....16

ix 9 The number of released protons from POM/KOH aqueous solutions with different molar ratios………………………………………………………………………………26

10 ITC curves of 0.056 mM POM(Cu) and POM (Eu) aqueous solutions with the addition of 15.64 mM KOH……………………………………………………………..29

x CHAPTER I

INTRODUCTION

1.1 Introduction of polyoxometalates

1.1.1 Polyoxometalates (POMs)

The development in inorganic chemistry has led to the explorations of some large inorganic molecules represented by polyoxometalates (POMs). POMs are a large class of

metal-oxygen clusters with covalent linked polyhedrals as building-block units ((MOx unit, M is a transition metal atom). The metals are usually group VI (Mo, W) or group V

(V, Nb, Ta) transition metal atoms in their high oxidation states. POMs can be classified into two categories: one is isopolyoxometalate, composed of one kind of polyhedral, and the other is heteropolyoxometalate, composed of different kind of polyhedrals. For

2- example, the hexatungstate anion [W6O19] is an isopolyanion and the phosphotungstate

3- anion [PW12O40] is a heteropolyanion. The non-transition metal atom (P) in the latter one is called the heteroatom and the transition metal atom (W) is called the addenda atom. When a POM contains two or more addenda atoms, it is a mixed addenda cluster.

2- POMs span the molecular to nanoparticle size range, with the smallest [W6O19] to

1 the ultra-large cluster {Mo368} discovered by Müller et al. In addition, POMs can be

1 classified into several types based on their diverse structures. For instance, Lindqvist discovered the most symmetrical structure of an isopolyanion which was named

Lindqvist-type structure.2 Keggin firstly reported the structure of a heteropoly acid with the use of X-ray diffraction experimentally in 1934, and thus the structure was called

Keggin-type.3 Besides, various kinds of structures of POMs have been reported, such as

Anderson structure, Wells–Dawson structure, etc. 4 This class of compounds has attracted much attention owning to their various applications ranging from catalysis, medicine, supramolecular materials, materials science and nanotechnology. 5. Several examples of well-characterized structures of POM clusters are shown in Figure 1.6

2014 Elsevier

2 1.1.2 Deprotonation property of POMs

Most of large POM clusters, which represented by polyoxomolybdates, polyoxotungstates and their heteropolyoxometalate derivatives, show great hydrophilicity in nature and are very soluble in polar solvents. The molecular structures of POMs discussed in Chapter 1 are shown schematically in Figure 2. 7

Among various POM clusters, some of them contain water ligands that attached to their metals (non-Mo or non-W) and demonstrate deprotonation properties. They are neutral in single crystals and become negatively charged due to the partial deprotonation of their water ligands in aqueous solutions. As a result, the charge density of these macroanions is pH dependent.

Figure 2. Some typical large polyoxometalate molecular clusters and their sizes.

The water ligands of several POMs have been reported to deprotonate protons in aqueous solutions. For example, both the 2.5-nm-size, hollow, spherical “Keplerate”

8 VI III cluster {Mo72Fe30} ( [Mo 72Fe 30O252(CH3COO)12{Mo2O7(H2O)}2 {H2Mo2O8 -(H2O)}

3 9 (H2O)91] ·ca.150 H2O) and the structurally analogous {Mo72Cr30} ([{Na(H2O)12} ⊂

VI III {Mo 72Cr 30O252 (CH3COO)19 -(H2O)94}] ·ca.120H2O) have 30 potential deprotonation

III III sites due to the presence of 30 Fe (H2O) and Cr (H2O) groups on the surface.

Deprotonations of these surface groups can be initiated when the two clusters dissolved in water while the degree of deprotonation is highly controlled by the pH based on the deprotonation/ protonation equilibrium (Scheme 1). The deprotonation becomes less favorable with increasing negative surface charges. It has also been reported that each

{Mo72Fe30} and {Mo72Cr30} cluster releases about seven and five protons respectively when they are dissolved in water, which illustrates that the two clusters act as weak

10 11 nanoacids in aqueous solutions. The acidity strength of {Mo72Cr30} is slightly

3+ weaker than {Mo72Fe30} which is highly acceptable since [Fe(H2O)6] is a stronger acid

3+ 12 than [Cr(H2O)6] .

Scheme 1 Deprotonation/protonation equilibrium of a water ligand attached to a MIII center in a POM cluster.

13 14 15 - Another cluster {Mo72V30} present in the compound Na8 K14 (VO)2 [{Mo

8 (Mo)5 O21 (H2O)3}10{Mo (Mo)5O21-(H2O)3 (SO4)}2 {VO (H2O)}20 10-

({KSO4}5)2] · ca.150H2O has identical structure to {Mo72Fe30}cluster (Figure 2) while

4 carries certain amount of charges in crystals. {Mo72V30} has only 20 water ligands coordinated to vanadium atoms in this case, with 10 pointing inside and 10 pointing

outside the skeleton. Like the coordinated water ligands of {Mo72Fe30} and {Mo72Cr30}

clusters, the 10 surface water ligands of each {Mo72V30} cluster get partially

deprotonated in solutions. Consequently, the charge density of {Mo72V30} can also be

affected by solution pH. However, the acidic property of {Mo72V30} is quite dissimilar as it shows a higher oxidation state of the metal atoms (VIV), a smaller number of surface

11 water ligands and a high negative charge. Overall, one {Mo72V30} cluster carries 26 negative charges in crystals with the counterions being 8 Na+, 14 K+ and 2 VO2+ and then

+ 16 releases 5 H when dissolved in aqueous solutions.

It is worth noting that there exists a Yittrium-containing lacunary tetramer

16 9- {P4Y8W43} (K15Na6(H3O) [(PY2W10O38)4(W3O14)]·9H2O) whose charge density is also pH-dependent though it is not related to the deprotonation of water ligands (Figure

2). Such cluster total carries 30 negative charges which balanced by 21 charges from the

+ + + small metal ions (Na , K ) and another 9 charges from the H3O counterions. The latter can be tuned by solution pH.

1.2 Solution behavior of POMs

A solution is a homogeneous mixture of at least one solute dissolved in a solvent and solution behavior varies among different kind of solutions. As one type of solution,

5 an electrolyte solution owns charged solutes which makes its solution behavior more complicated. There exist two famous theories which are used to describe this complex solution behavior. One that is used to describe simple ionic solutions (such as NaCl aqueous solution) is Debye–Hückel theory 17, which works well for very dilute ionic solutions and can also be applied for solutions with higher concentrations. When the size of solute is large and up to the colloidal region, the solution behavior varies. The colloidal particles are suspended in the liquid media temporarily with the help of electrostatic interaction and are thermodynamically unstable. 18 The theory that describes this short- time stability of colloids in suspension is called DLVO theory 19 (Derjaguin-Landau-

Verwey-Overbeek theory), which depicts the balance between the electrostatic repulsion and van der Waals attraction of colloid particles.

Macroions are a group of soluble species between simple ions and colloids in term of size, which have distinctive solution property that differs from the other two types. The macroions cannot be treated as small simple ions due to their large sizes. On the other hand, they are quite soluble and can form homogeneous “real solutions” which are also disguisable from colloids. The macroions indeed show some connections to polyelectrolytes as they carry charges and have comparable sizes. Among several macroions, polyoxometalates (POMs) can be considered as simplified polyelectrolytes due to their well-defined structures, charge density, shape and stability. They exclude the intramolecular and intermolecular interactions caused by chain entanglement, and thus

6 can serve as good models for people to understand the complex solution behaviors of polyelectrolytes, such as biomacromolecules.

1.2.1 Self-assembly behavior of POMs

Several POMs have been reported to demonstrate a unique self-assembly behavior.

Instead existing as single ions, the macroions slowly form large structures in aqueous solutions even though they are quite soluble. 20 Such large structure is proved to be a hollow, spherical “blackberry” structure using static light scattering (SLS) and dynamic

21 light scattering (DLS) (Rg/Rh ≈ 1) by Liu et al (Figure 3). It has been further concluded that the dominant driving force for the self-assembly formation is the macroion– counterion interaction and hydrogen bonding rather than van der Waals force, hydrophobic interaction and chemical reactions. 22 Therefore, such unique self-assembly process is a counterion-mediated process for which counterions play an important role in blackberry formation. Meanwhile, the self-assembly process is a charge-regulated process as the blackberry size can be tuned by the charge density of macroions. 23

7 Figure 3. 3.6 nm wheel-type POMs {Mo154} self-assemble into hollow, spherical

“blackberry” structures in aqueous solutions. Reprinted with permission from reference

These macroions are classified into “strong electrolyte” and “weak electrolyte” type, depending on the nature of their charges. Examples of strong electrolytes include

24 25 26 the wheel-shaped {Mo154} , the porous, capsule-like {Mo132} and the “hedgehog”-

1 type species {Mo368} . They carry inherent charges in crystals which are balanced out by their counterions (multiple small cations). Consequently, the majority part of these clusters exists as macroanions after the release of small cations in solutions and carry delocalized charges. Besides, they are highly soluble in polar solvents due to their negative charges and layers of water ligands present on their surfaces. For the strong- electrolyte type macroions, their charge density is greatly influenced by solvent polarity,

which will eventually affect the self-assembly formation and size. For example, {Mo132}

8 clusters are observed to self-assembly into blackberry structures in water/acetone mixed solvents that contains different contents of acetone . 22 The blackberry size has been proved to show a linear relationship with the inverse of dielectric constant of the solvent which demonstrate a charge-regulated self-assembly process. (Figure 4) 7

Figure 4. Transition from discrete macroions to blackberries, then to macroions again

Society.

On the other hand, “weak electrolyte”-type POMs contain water ligands that

connected to their central metals (non-Mo centers), which represented by {Mo72Fe30},

{Mo72Cr30}, etc. They behave neutral in single crystals and carry localized charges due to the partial deprotonation of their water ligands in solutions. The equilibrium between

9 deprotonation and protonation is regulated by solution pH, which will result in different charge density of this type of macroions and correspondingly in various blackberry sizes.

The sizes of self-assembly structures formed by {Mo72Fe30} macroions increase with

10 decreasing the solution pH in a certain range (Figure 5). For {Mo72Cr30} cluster, it is

slightly less acidic than the {Mo72Fe30} cluster leading to a lower charge density and a larger blackberry structures at a same pH. 11 The two clusters show a unique behavior in a homogeneous mixed solution which will be discussed in next section.

Figure 5. The average hydrodynamic radius (Rh) of the blackberry-type structures

formed in 0.5 mg/mL aqueous solutions of {Mo72Fe30} at different pH, as measured by

DLS at 90° scattering angle. For pH < 2.9, the clusters are almost neutral, and no supramolecular structures are formed whereas they are not stable at pH > 6.6. Reprinted with permission from reference 10. Copyright © 2006 American Chemical Society.

10 Additionally, there exists another group of clusters which carry some charges in crystals and still deprotonate certain protons in aqueous solutions. Therefore, they are

treated as weak acid salt type electrolytes which represented by {P4Y8W43} macroions.

Their self-assembly behaviors are controlled either by solvent polarity or solution pH, which facilities to establish a connection between the effect of charge density and that of

solvent content. For {P4Y8W43} macroions, they can self-assemble into blackberry structures either in water or acetone/water mixed solvents. 27 The self-assembly size is tunable by the amount of acetone which is similar to the case of strong-electrolyte type macroions. Equally, changing solution pH can also modify the blackberry size which is similar to the case of weak-electrolyte type macroions. Therefore, charge density of macroions determines the self-assembly sizes as it leads to different strength of electrostatic repulsions among macroions.

1.2.2 Self-recognition behavior among POMs

In the field of supramolecular chemistry, self-recognition refers to the behavior that mixtures of components yield different superstructures without interference or crossover during the self-assembly process. 28 A self-recognition phenomenon has been recently observed based on an assembly process in a homogeneous dilute aqueous solution of two

29 weak-electrolyte type POM clusters: {Mo72Fe30}and {Mo72Cr30}. The concrete

structures and deprotonation properties of {Mo72Fe30}and {Mo72Cr30} clusters have been

11 discussed in above-mentioned context. Instead of self-assembling into mixed species, the two macroions form individual and homogeneous blackberry structures with interfacial water between the macroions, though they have identical geometrical structures.

Figure 6. In mixed dilute aqueous solutions, the clusters {Mo72Fe30} (top) and

{Mo72Cr30} (bottom) self-assemble into different blackberry structures of the Cr30

(yellow) and Fe30 type (blue)—with interfacial water between the macroions (right)—and do not form mixed species (such as the hypothetical structure shown on the left).

Two factors related to the deprotonation properties of water ligands which connected with different metal ions (Fe3+, Cr3+) are responsible for this self-recognition

12 behavior. One is attributed to difference of the charge density of these two POMs. As

{Mo72Fe30}and {Mo72Cr30} have same structures, the charge density is highly related to

the charges they carry (~ 7 for a {Mo72Fe30} and ~ 5 for a {Mo72Cr30}). The other factor is the liability of the coordinated water ligands. The residence time of water ligands in

3+ 3+ complex [Cr (H2O)6] is much larger than that of complex [Fe (H2O)6] , and also the exchange reaction rate for the former complex is much slower. 30 Therefore, the

interracial water layers between {Mo72Cr30} macroions are more stable and denser in the

29 {Mo72Cr30} blackberry superstructures (Figure 6).

1.3 Connections to some biomacromolecules

Some biomacromolecules own unique properties due to water ligands which coordinated with metals, for example, the potassium channels. As one of the most widely distributed ion channels, potassium (K+) channels are found in virtually all organisms and perform vital functions. They locate at cell membranes that only allow selective efflux and influx of K+ ions across the membranes, in which the water molecules around K+ ions play an important role.

13 Figure 7. The structure of KcsA. a The atomic structure of KcsA viewed along the membrane plane. The pore-forming domain consists of the outer helix (magenta), loop regions (green), pore helix (blue), selectivity filter (yellow), and inner helix (orange). The

K+ ions are represented by purple balls with surrounding water molecules in red. EC is extracellular and IC is intracellular for short. b, c The enlarged view of the boxed area in

(a) containing the SF and the extracellular entryway. The K+ ions are in two configurations, either in S2 and S4 (b) or S1 and S3 (c) during conduction. The water molecules occupy the vacant ion positions in S1 and S3 (b) or in S2 and S4 (c). Other ions are located in the extracellular entryway (either S0 (b) or Sext (c)) and in the central cavity (Sc (a)). Copyright © 2015 Springer Nature.

14 K+ channel is a tetramer with each monomer containing one pore, and the total four pores comprise a pore for ions to move. The general structure of the porous domain can be described by the prokaryotic potassium channel (KcsA), whose structure is shown in

Figure 7.31The sequence of TVGYG behaves as the selectivity filter and the carbonyl oxygens of this sequence are functionalized as the K+ binding sites (S1-S4). After going from the intracellular region (IC), the K+ ions will become hydrated and enter the central cavity. When they go across the selectivity filter, they are dehydrated and only two K+ ions stay in the selectivity filter where each ion is in the middle of two water molecules.

The K+ ions will become hydrated again when they arrive at the extracellular side (SC).

1.4 Study motivation

Overall, water ligands are essential in biological systems. Investigation on the related deprotonation properties of POMs can provide a simplified model for us to understand the complicated behaviors of biomacromolecules. In addition, deprotonation property is very crucial for POM clusters not only because it determines the charges of the clusters but also because it affects their solution behaviors. As discussed in above- mentioned context, some research has already reported different acidities of these nanosized clusters owning to their coordinated water ligands. However, research that focuses on the effect of different metal ions to the deprotonation capacity and deprotonation sites has not yet been conducted.

15 Figure 8. Polyhedral and ball-and-stick representation of different building blocks making up polyanion. All cations and crystal water molecules are omitted for clarity.

Color code: WO6, blue polyhedra; NbO6, yellow polyhedra; Nb, yellow balls; P, purple balls; O, red balls

Herein, we will study the deprotonation property of a series of niobium/tungsten mixed-addendum POMs. The chemical formula of one type of them is (K22 -

[{Nb12P4W24O122}2 {X(H2O)4.5}4{Nb4O4(OH)6}] •18H2O), where X represents a lanthanide element. These POM clusters are Wells–Dawson-based tetramers (Figure 8)

with each polyanion consisting of a central {Nb4O6} core and two crossed

{Y2Nb12P4W24} units with a molecular diameter of 2.6 nm. The {Y2Nb12P4W24} unit is a

10- dimer formed by two [Nb6P2W12O61] anion subunits connected by two equatorial Nb −

O − Nb bridges and two polar YIII ions. 32 Another niobium/tungsten mixed-addendum

16 II POM cluster with Cu ions locating in the lacunars (K34 [{Nb12P4W24O122}2 {Cu -

(H2O)4}4 {Nb4O4(OH)6}] •36H2O) has identical structure and chemical formula with those of the above-mentioned POMs and only differs at the number of counterions (K+) and coordinated water ligands. In order to explore the effect of different lacunary metals on the deprotonation property of POMs, two POMs with different metal ions are chosen.

One is the POM cluster with EuIII+ (POM(Eu)) which is a lanthanide metal ion, and the other is the cluster with CuII+ (POM(Cu)) which is a non-lanthanide metal. Approaches including acid-base titration and isothermal titration calorimetry (ITC) have been applied to measure the deprotonation capability, deprotonation sites and acidity of the two POM clusters with disparate metal ions.

17 CHAPTER II

EXPERIMENT

2.1 Sample Preparation

A series of 1 mg/mL (5.6·10-5 mol/L) POM (Eu) and POM (Cu) aqueous solutions were prepared by dissolving single crystals in boiled water in the glove box in order to avoid the carbon dioxide effect. The solutions were tightly sealed in glass tubes for pH measurement, ITC measurement and conductivity measurement. To determine the number of deprotonated protons from each POM macroion, additional certain amounts of base were added to the solutions.

In addition, KOH aqueous solution was also prepared by using the boiled water in the glove box. The concrete concentration of KOH was determined by potassium hydrogen phthalate (KHP) aqueous solution, a primary standard solution, since the KOH can easily absorb water and carbon dioxide.

18 2.2 pH Meter

The SevenExcellenceTM bench meter was used in pH measurement. Before taking a measurement, the pH electrode must be calibrated initially by utilizing three buffer solutions with certain pH values at a given temperature. The corresponding calibration curve is then used to correlate the measured mV values of the electrode to the pH value of the test solution, so calibration is the most important step in the measurements. There are some factors which affects the accuracy of the calibration, such as temperature and buffer solutions. In order to solve these issues, we chose three standard solutions in order to cover the range of the sample solutions and we also set the pH values of these solutions at

20 which is the experimental temperature.

pH measurement is a very useful tool to directly help us determine the accurate deprotonation capacity, especially the number of released protons with a certain additional base. A series of 2 mL 1 mg/mL (5.6·10-5 mol/L) POM (Eu) and POM (Cu) solutions containing different KOH equivalence (10 eq., 20. eq., ···, 60. eq.) and KOH solutions without POM clusters (a blank controlled group) were prepared and measured by the pH meter.

19 2.3 Isothermal Titration Calorimetry

Isothermal titration calorimetry (ITC) is an effective technique to obtain the thermodynamic parameters of a reaction in solution quantitively. It detects the released

heat directly during the titration and thus can measure the binding constant (Ka), the change of enthalpy (ΔH) and the binding stoichiometry (n). Gibbs free energy (ΔG) and entropy change (ΔS) can then be determined after obtaining the initial parameters.

A commercial Isothermal Titration Calorimetry from TA Instrument was used to study the strength of the deprotonation and the acidity of the POMs. In the experiments,

250.0 μL 15.64 mmol/L KOH solution was titrated into 1 mL 1.0 mg/mL POM aqueous solution for 25 times with 10 μL each time. The temperature was set at 25 ℃, the interval between two injections was set as 600 s and the stirring speed was set at 250 rpm during the titration. In addition, 1 mL 1.0 mg/mL POM aqueous solution was titrated with 15.64 mmol/L KCl solution in order to exclude the existence of counterion (K+) dissociation.

After the titration, the data was analyzed by Nano Analyze Software from TA Instrument.

20 2.4 Conductivity Meter

4063 Traceable Portable Conductivity Meter was used in the conductivity measurements. The conductivity meter was initially calibrated by three standard solutions and then used to measure the conductance of 1 mg/mL POM aqueous solutions. The

conductance follows the equation κ = Λ푚 ∙ 푐 , where Λ푚 is the molar conductivity and c is the concentration of the electrolyte. Based on this relationship, the number of dissociated counterions from each POM molecule can be calculated when a POM cluster was dissolved in pure water. Meanwhile, the charges that each POM macroion carries can also be determined.

21 CHAPTER III

RESULT AND DISCUSSION

3.1 Dissociation and deprotonation of POMs in pure water

1 mg/mL POM(Eu) aqueous solution, POM(Cu) aqueous solution and pure water were measured by the pH meter and conductivity meter, respectively. The pH of pure water is 6.050. For the POM(Eu) aqueous solution, the conductivity is 71.9 μs/cm with a pH at 5.741. For the POM(Cu) aqueous solution, the conductivity is 97.0 μs/cm with a pH at 6.301. The conductivity of the macroions is generally negligible given their large sizes and relatively small charge density. Therefore, the conductivity of POM aqueous solutions is only contributed from different small ions (H+, OH-, K+) in this case. The

+ molar conductivity values are temperature-dependent and are calibrated with Λ푚(퐻 ) ≈

− + 31.084, Λ푚(푂퐻 ) ≈ 17.594 and Λ푚(퐾 ) ≈ 6.533 at 20 ℃. The concentrations of released small ions were thus calculated and the results are displayed in Table 1.

22 Table 1. Dissociation and deprotonation processes of POM in pure water

POM solution [K+] [H+]/[POM] [K+]/[POM] Charges Dissociation

(mM) degree (%)

POM (Eu) 1.10 0.03 19.66 ~20 90.9

POM (Cu) 1.49 0.01 26.31 ~26 76.5

There is no detectable proton released from both POM(Eu) and POM(Cu) clusters when they are dissolved in pure water. Such deprotonation behavior is completely

different from that of the typical Keplerate-type POMs (e.g. {Mo72Fe30}) which deprotonate certain protons from their coordinated water ligands when they are dissolved in pure water. The reason behind this phenomenon will be discussed in 3.2.2 based on further study. Additionally, the number of dissociated counterions (K+) for the two POM clusters varies with about 20 K+ per POM(Eu) and 26 K+ per POM(Cu) cluster in the aqueous solutions. As the charges of each macroion are attributed to such counterion dissociation, each POM(Eu) carries around 20 negative charges and each POM(Cu) carries around 26 negative charges. The dissociation degree (the ratio between dissociated counterions in aqueous solution and total counterions in the neutral crystal) is also different with 90.9% for POM(Eu)and 76.5% for POM(Cu).

23 3.2 Deprotonation of POMs with additional base

This type of POM clusters was found to deprotonate protons when adding base into the aqueous solutions. To obtain more information about such unique deprotonation behavior, pH measurement and ITC measurement were conducted to qualitatively and quantitively study different deprotonation properties of the POM clusters under the effect of different metal ions. Europium (Eu) and Copper (Cu) are two distinct metals whose metal aquo complexes have disparate properties, which will eventually result in different deprotonation properties of POM clusters.

3.2.1 pH measurement

A series of freshly prepared 1 mg/mL POM aqueous solutions containing various

KOH equivalence and corresponding KOH aqueous solutions with same amount of KOH were measured after the calibration of pH meter. The concentration of KOH stock solution was standardized by KHP solution. All the experiments were conducted in the glove box with the protection of nitrogen gas as carbon dioxide will greatly influence the final results based on our previous experiments. The pH values of these solutions at different time are shown in Table 2.

24 Table 2. pH values of POM and KOH aqueous solutions at different molar ratios

Molar ratio of POM: POM (Eu) KOH POM (Cu) KOH KOH 1:10 8.625 10.781 8.058 10.777 1:20 9.263 11.100 9.774 11.124 1:30 9.874 11.289 10.681 11.306 1:40 10.547 11.415 10.987 11.428 1:50 10.921 11.502 11.166 11.534 1:60 11.127 11.584 11.288 11.612

The pH values of POM/KOH aqueous solutions and corresponding KOH aqueous solutions should be identical if there are no protons deprotonated from the macroion.

However, we can see that there is a significant decrease in the pH of POM /KOH aqueous solution from Table 2. This observation illustrates that some protons are deprotonated from both two POM clusters with additional base. The concrete number of released protons from each cluster were calculated based on the equation below. The dissociation constant of water (Kw) used to calculate the concentrations of hydroxide ions here is

14.16 as the temperature is 20 ℃.

− − [푂퐻 ]퐻2푂/퐾푂퐻− [푂퐻 ]푃푂푀/퐾푂퐻 푇ℎ푒 푛푢푚푏푒푟 표푓 푟푒푙푒푎푠푒푑 푝푟표푡표푛푠 = [푃푂푀]

The number of released protons from each cluster which stays in the environments containing different base equivalence were summarized in Table 3 and Figure 9 was also plotted showing a dynamic deprotonation process when adding different KOH into the solutions. There are two linear lines with dissimilar slopes for one POM after fitting the

25 data on Figure 9, which demonstrates that two stages exist in the whole deprotonation processes with additional base for both two POM clusters.

Table 3. The number of released protons from one POM at different molar ratios

Molar ratio of POM to 1:10 1:20 1:30 1:40 1:50 1:60 KOH

POM (Eu) 7.4 15.3 23.1 27.8 29.0 30.9 POM (Cu) 7.4 15.7 19.1 21.1 24.2 26.6

POM (Eu) 35 POM (Cu)

# released protons # released 5

0 0 10 20 30 40 50 60 KOH:POM

Figure 9. The number of released protons from POM/KOH aqueous solutions with different molar ratios.

When adding 10 equivalence KOH, each POM (Eu) and POM (Cu) cluster released

7.4 protons. The number of released protons is less than added KOH, which demonstrates

26 that they are both weak acids. The number of protons doubles with 20 equivalence KOH for both POMs. The values for one POM don’t follow such linear trend when adding much more bases. For two POMs, the number of released protons is not same any more with adding more bases, which shows that the deprotonation capacity of POMs with different metal ions varies in basic solutions. The coordination number of EuIII+ and CuII+ is 9 and 6, which results in different amount of water ligands coordinated with different lacunary metal ions in the compound and thus leads to a variation in the deprotonation capacity. The intersection point of the two lines represents the maximum number of released protons from a POM in the first stage. The maximum number of released protons in the first stage is about 26 and 17 for POM(Eu) and POM(Cu), respectively. In addition, as the maximum number of protons that can be deprotonated from the coordinated water ligands is 18 and 16 in a POM (Eu) and POM (Cu) cluster, two deprotonation sites exist for both POMs during the whole deprotonation process. In order to determine the order of deprotonation sites, we compared the pKa of the metal aquo

2+ 33 complexes. The pKa value of [Cu (H2O)6] is around 8 and is about 9 for [Eu

3+ 34 (H2O)9] . Though no accurate pKa value of Nb(OH)6 has been obtained so far, it is speculated to be higher than the two metal aquo complexes. Consequently, we speculate that the deprotonation occurs initially in the water ligands coordinated with lacunary metal ions and then in the linker containing niobium.

27 We further came up with a speculation to explain these unique values. In the first stage, the maximum number of released protons for a POM (Eu) is 26, which we inferred that a POM (Eu) would release 18 H+ from the coordinated water ligands and 6 H+ from the hydroxide groups in the linker. On the other hand, a POM (Cu) only released 16 H+ from the coordinates water ligands in the first stage, which is extremely similar to the maximum number of protons (17 H+) obtained after fitting. Remaining 6 H+ from the linker will be deprotonated in the second stage for a POM (Cu) cluster. Such behavior was attributed to the charges of each POM cluster based on the current conjecture. POM

(Cu) carries more negative charges (~26) than POM (Eu) (~20), which make it harder for deprotonation from the linker. Therefore, deprotonation from coordinated water ligands and from hydroxide groups in the linker can be distinguished for POM (Cu), while cannot for POM (Eu). This phenomenon further confirms our speculation about the deprotonation order. Besides, it is also reasonable to suppose that pKa value of the hydroxide group which attached to the niobium ion should be equal to or a little bit higher than the pKa value of aqueous lanthanide ion. As for the released protons in the second stage, we just referred that they may come from the remaining hydroxide ions of the coordinated water ligands after these ligands released protons.

28 3.2.2 ITC measurement

ITC was conducted to measure the thermodynamic parameters of the interaction between the deprotonated protons from POM clusters and additional base in the solutions. The acid-base reaction should be strong enough for ITC to detect its released heat and help us further determine the binding constant, binding stoichiometry, etc. These parameters were all shown in Figure 10.

Figure 10. ITC curves of 0.056 mM POM(Cu) and POM (Eu) aqueous solutions with the addition of 15.64 mM KOH.

29 The binding stoichiometry (n) here represents the number of protons that reacted with the additional base during the whole titration. As the molar ratio between KOH and

POM is large enough, this value also represents the number of released protons from each cluster. Consequently, the number of released protons from one POM(Cu) and POM(Eu) cluster is 17.3 and 26.2, which is consistent with the values of the released protons in the first stage measured by the pH meter. ITC cannot measure the number of released protons in the second stage obtained in the pH measurement, which is because of the low strength of the interaction in the second stage. Overall, pH measurement is the most directed method to determine the accurate number of released protons from each POM cluster with adding base into the solutions, while ITC is still a useful method since it can provide the acidity of the weak acids. The Ka values on Figure 10 are the association constant of the interaction between POM clusters and additional hydroxide ions. The acidity

constants of the POMs were calculated based on 퐾푎푐푖푑 = 퐾푎푠푠⁄퐾푤. As the experimental

-14 temperature in the ITC measurement is 25 ℃, Kw used in the calculation is 10 . pKa of

POM (Eu) and POM (Cu) is 10.577 and 9.891, which further confirm that the two POM clusters are both weak acids. The acidity of POMs with different metal ions is completely different. POM(Cu) is stronger than POM(Eu), which corresponds with the above-

2+ 3+ mentioned discussion that [Cu (H2O)6] is more acidic than [Eu (H2O)9] . Meanwhile,

as both the concentration and Kacid value of the POM aqueous solution is small, the concentration of dissociated hydrogen ions of the solution is very small, even less than

30 10-7 at room temperature, which explains why deprotonation of POMs does not occur in pure water.

31 CHAPTER IV

CONCLUSION

In summary, the effect of different lacunary metal ions on deprotonation property of POMs was investigated. Results show that the dissociation and deprotonation properties of POM (Eu) and POM (Cu) clusters are truly different. The charges of the

POMs with different metal ions vary in aqueous solutions. Unlike common “weak electrolyte”-type POMs, no protons will be released from the two POM clusters in aqueous solutions. Deprotonation only becomes significant with adding additional base into the solutions. Based on the pH measurement, we determined that two deprotonation sites and two stages exist for both POMs in the whole deprotonation processes. However, the deprotonation capacity in the two stages is completely different for the two POMs.

For a POM (Cu) cluster, the released protons in the first stage are only deprotonated from its coordinated water ligands, while the protons in the first stage come from both the water ligands and the linkers for a POM (Eu) cluster. The factor that leads to a different deprotonation order between the two sites is attributed to the charges of each POM cluster. One POM (Cu) cluster carries more negative charges which makes the deprotonation from the linker harder. As for the overall deprotonation capacities, they are

32 significantly different for the POMs with different lacunary metal ions, which is because the two metals have different coordination number. In addition, we also determined that the acidity of the two POMs with different metal ions varies based on the ITC measurement. POM (Cu) is more acidic than POM (Eu), which corresponds with the acidity of their metal aquo complexes in solutions.

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