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Deprotonation Property of Polyoxometalates with Different 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
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