v me and other master students his selfless help in our daily lives. I gradually ...... Aqueous Solution, 2nd ed.; John Wiley & Sons Ltd.: Oxford, England, 2003. [3]. ... Deng, J.; Polidan, J. T.; Hottle, J. R.; Farmer-Creely, C. E.; Viers, B. D.; Esker, A.
GIANT SHAPE AMPHIPHILES BASED ON POLYOXOMETALATES (POMs)POLYHEDRA OLIGOMERIC SILSESQUIOXANE (POSS) HYBRIDS: SYNTHESIS AND CHARACTERIZATION
A Thesis Presented to The Graduate Faculty of the University of Akron
In Partial Fulfillment of the Requirements for the Degree Master of Science
Jing Jiang May, 2013
GIANT SHAPE AMPHIPHILES BASED ON POLYOXOMETALATES (POMs)POLYHEDRA OLIGOMERIC SILSESQUIOXANE (POSS) HYBRIDS: SYNTHESIS AND CHARACTERIZATION
Jing Jiang
Thesis
Approved:
Accepted:
Advisor Dr. Stephen Z. D. Cheng
Dean of the College Dr. Stephen Z. D. Cheng
Faculty Reader Dr. Steven S. C. Chuang
Dean of the Graduate School Dr. George R. Newkome
Department Chair Dr. Coleen Pugh
Date
ii
ABSTRACT The concept of shape amphiphiles was proposed about 10 years ago, describing particles that formed by building blocks with distinct shapes and interactions through chemical bonds. Among various classes of nano building blocks, Polyoxometalates (POMs) and Polyhedra Oligomeric Silsesquioxanes (POSS) are two classes that are ubiquitous in hybrid material research fields with potential applications ranging from biomaterials, catalyst engineering to photovoltaic applications. The shape amphiphiles formed by POMs and POSS through covalent bonds, thus, would be a class of promising hybrid material that possesses outstanding properties from both components.
Herein, a prototype of “giant shape amphiphile” with one hydrophobic isobutyl POSS (BPOSS) and one hydrophilic POM (Lindqvist-type hexamolybdate) is studied. We report the molecular design, synthesis strategy, characterization, and thermal analysis studies. Sonogashira coupling is utilized to synthesize the BPOSS-POM hybrids. The hybrids were fully characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy. Differential scanning calorimetry (DSC) is used to study the phase transition behavior of this material. And the structure information of single crystal formed by BPOSS-POM will be the focus of the future work. iii
ACKNOWLEDGEMENTS There are so many people I’d like to give my most sincere gratitude. My acknowledgment would firstly go to my advisor, Dr. Stephen Z. D. Cheng, for providing me a precious opportunity to work in his excellent group. He is a person full of vision and led me towards the field I’ve never reached. Thanks to his insightful guidance in my research and life, my academic career in Akron became meaningful and attractive. This project was completely under his supervision and firmly support. From the molecular design, sample preparation to data analysis, he gave me tremendous of advices and assistance. What I learn from him is that to be a professional researcher, one should never only focus on the basis of the problems but should also possess a keen insight into the frontier. I understand the significance of research work by his continuously pursuit of mystery in the field of polymer physics. In addition, the way how he conducts himself also gave me a lot of inspiration. All of these are lifelong benefits. My sincere gratitude is expressed to him. I’d also like to give my sincere thanks to Dr. Benard Lotz for his patient guidance on my research. His professional and tremendous knowledge impressed me and is invaluable in completing this thesis. His great passion on science had a huge influence on me and pushed me into the curiosity of my research work. The amazing power from him should be given much gratitude. After then the thanks is for Dr. Bojie Wang, who gave
iv
me and other master students his selfless help in our daily lives. I gradually adapted to American life and focused on learning and research with his enthusiastic and kind help. I am also grateful to Dr. Steven Chuang for helpful advises and providing his expertise through many helpful discussions. Dr. Zhengbiao Zhang from Soochow University is an exchange visitor to the University of Akron. Thanks to his kind-hearted care during the time he spent in Akron, my thesis was completed smoothly. I would also like to thank my group members, in particular, Mr. Kan Yue for teaching me pioneering synthetic skills, Mr. Mingjun Huang and Mr. Xueyan Feng for the guidance in fundamental physical analysis techniques. Special thanks are given to Mr. Hao Liu, the mentor of my master thesis. He gave detailed instructions on my research topic and without his advises on the tough problems, this thesis cannot be completed. I would also thank Mr. Xuihui Dong for his patient guidance and help during my research. His strict scientific style has influenced my research a lot. Sincere thanks were also expressed to Dr. Shun-Jun Ji, who was my advisor during the undergraduate school life. I started doing research in his group from my second year in Soochow University. During those two years, he gave me a lot of professional guidance and help. His strict training and patient instruction of the experimental skills offered me a solid foundation and right attitude to do research work. In addition, although he was very busy, he cared a lot about my school life and gave me a lot of inspiration when I was aboard and studying here. Sincere appreciation is for this benign person. Last but not least, I want to thank my family and friends. My parents are teachers and my family is not very wealthy. However, they gave me spiritual and financial support v
continuously during the two years when I study abroad. Their encouragements on my academic study and daily life were really appreciated. I am also grateful to my friends in U.S and China, who are always behind me and never can be replaced. They are always the most powerful backup force of me. Finally, I would like to thank all those people concerned about me and generously gave me their favor in my path of growth.
vi
TABLE OF CONTENTS Page LIST OF FIGURES ....................................................................................................... ix CHAPTER I. INTRODUCTION........................................................................................................1 II. BACKGROUND ........................................................................................................5 2.1 Shape Amphiphiles ................................................................................................5 2.1.1 Amphiphiles ....................................................................................................5 2.1.2 Nano-sized Building blocks .............................................................................7 2.1.3 Shape Amphiphiles .........................................................................................8 2.2 Recent Work in Our Group ....................................................................................8 2.3 Polyoxometalates (POMs) .................................................................................... 10 2.3.1 Definition ...................................................................................................... 10 2.3.2 Types of POMs ............................................................................................. 11 2.3.3 Surface Modification Methods ...................................................................... 12 2.4 Molecular Design ................................................................................................. 14 III. EXPERIMENTAL ................................................................................................... 15 3.1 Anhydrous and Oxygen Free Operation ............................................................... 15 3.2 Chemicals and Solvents ....................................................................................... 15 vii
3.2.1 Chemicals Used as Received ......................................................................... 16 3.2.2 Purification ................................................................................................... 16 3.3 Molecular Characterizations ................................................................................. 17 3.3.1 1H Nuclear Meagnatic Resonance (NMR) Spectra ......................................... 17 3.3.2 Fourier Transform Infrared Spectroscopy (FT-IR) ......................................... 17 3.3.3 Thin-layer Chromatographic Analyses (TLC) ................................................ 17 3.3.4 Differential Scanning Calorimetry (DSC) ...................................................... 17 3.4 Synthesis of Tetrabutylammonium Hexamolybdate (VI), [(n-Bu4N)2][Mo6O19] ... 18 3.5 Synthesis of 4-Ethynyl-2, 6-Dimethylaniline ........................................................ 19 3.5.1 Synthesis of 4-Iodo-2,6-Dimethylaniline ....................................................... 19 3.5.2 Synthesis of 4-Trimethylsilylacetylenyl-2, 6-Dimethylaniline ....................... 20 3.5.3 Synthesis of 4-Ethynyl-2, 6-Dimethylaniline ................................................. 20 3.6 4-Ethynyl-2, 6-Dimethylaniline-Functionalized Tetrabutylammonium Hexamolybdate (VI) .................................................................................................. 21 3.7 Synthesis of 4-Iodo-Benzoic Acid Monofunctionalized Isobutyl Polyhedral Oligomeric Silsesquioxane......................................................................................... 22 3.8 Synthesis of Isobutyl POSS Monofunctionalized Hexamolybdate(VI).................. 23 IV. RESULTS AND DISCUSSION .............................................................................. 25 V. SUMMARY ............................................................................................................. 32 REFERENCES .............................................................................................................. 34 APPENDIX ................................................................................................................... 38
viii
LIST OF FIGURES Figure
Page
1.1 BPOSS-Lindqvist based shape amphiphile ................................................................. 3 2.1 Definition of amphiphiles .......................................................................................... 5 2.2 Various self-assembly morphologies of block copolymers ......................................... 6 2.3 Molecular models of nano-building blocks used in our group .................................... 9 2 4 Molecular model of Lindqvist-type POMs ............................................................... 12 3.4 Synthetic route of Lindqvist POM............................................................................ 18 3.5 Synthetic route of 4-Ethynyl-2, 6-Dimethylaniline ................................................... 19 3.6 Synthetic route of Ethynyl - Functionalized Lindqvist .............................................. 21 3.7 Synthetic route of Iodo - Functionalized POSS ........................................................ 22 3.8 Synthetic route of POSS - Monofunctionalized Lindqvist ........................................ 23 4.1 1H NMR analysis of Isobutyl POSS Monofunctionalized Lindqvist ......................... 26 4.2 MALDI-TOF mass spectrum of Isobutyl-POSS Lindqvist ....................................... 28 4.3 TGA degradation of BPOSSLIND ........................................................................... 29 4.4 DSC thermograms of BPOSS-Alkyne, BPOSS-Lind and Lind-Iodide during heating. ...................................................................................................................................... 30 5.1 Summary ................................................................................................................. 32 A.1 1H NMR spectrum of Lindqvist POM ..................................................................... 38 A.2 MALDI -TOF spectrum of Lindqvist ...................................................................... 38 ix
A.3 1H NMR spectrums of 4-Iodo-2, 6-Dimethylaniline (a), 4-trimethyl silylacetylenyl-2, 6-Dimethylaniline (b) and 4-Ethynyl-2, 6-Dimethyl-aniline (c). ..................................... 39 A.5 MALDI-TOF mass spectrum of Ethynyl-Functionalized Lindqvist POM ................ 40 A.6 IR spectrum of Functionalized Lindqvist POM. ...................................................... 40 A.7 1H NMR spectrum of Iodo-Functionalized POSS .................................................... 41 A.8 MALDI-TOF mass spectrum of Iodo-Functionalized POSS .................................... 41 A.9 1H NMR spectrum of POSS –Monofunctionalized Lindqvist .................................. 42 A.10 MALDI-TOF mass spectrum of POSS –Monofunctionalized Lindqvist ................ 42
x
CHAPTER I
INTRODUCTION
Due to the formation of ordered structure through microscopic phase separation, the study of amphiphilic molecules with two distinct building blocks connected through covalent bonds have attracted tremendous of interests starting three decades ago.
[1]
Among them, block-copolymers are one of the most well-known conventional amphiphiles since they typically form ordered structures within nanometer scale.
[2-4]
A
lot of research works are focused on the self-assembly of block copolymers. [5-9] By tuning the volume fraction and immiscibility, ordered phases with thermodynamical equilibrium could be obtained, including lamellae, bicontinuous double gyroids, hexagonal packed cylinders, and body center cubic packed spheres.
[10]
Due to the
flexible chains with complex chain conformations, however, the structures of the blockcopolymers is difficult to be precisely controlled.
[11]
To this end, the incorporation of
nano-sized building blocks with fixed volume and shape into shape amphiphiles provides a new area for constructing ordered pattern through bottom up approach.
[12-14]
With intentional modification of the nano-sized building blocks, the amphiphilic nanosized giant molecules can be used to fabricate materials with specific physical properties through the structure formation. Our goal is to control the macroscopic 1
properties by tuning the microscopic functionality and thus adjust the structure formation. The concept of shape amphiphiles was first proposed in 2003 by Bruce et al that two building block with distinct shapes was connected through covalent bonds.
[1]
This
pioneer study aroused people’s great interest in the field. The definition of shape amphiphiles then be depicted as “amphiphilic molecules consists of two different parts which differ in both geometric and chemical symmetry” by Glotzer.
[15][16]
The
geometric symmetry difference can be generated by nano building blocks with different shapes, while the chemical symmetry difference can be given by modification on the nanoparticles’ surface. In our group, nano-sized building blocks with distinct shapes and modifiable surface (such as fullerene (C60), Polyhedral Oligomeric Silsesquioxane (POSS) and Polyoxometalates (POMs)) are employed as rigid scaffolds to load different surface functionalities and to synthesize giant molecules with precise structures by high efficient
chemistry.
Among
these
building
blocks,
Polyhedral
Oligomeric
Silsesquioxane (POSS) is of particular interests. It is an inorganic rigid cage with the size of about 1 nm, and can be easily modified with a variety of side groups on its surface to introduce versatile surface functionality.
[17]
Due to high symmetry, however,
POSS cannot be selectively functionalized. For instance, the pure bi-functionalized POSS without isomers cannot be achieved because the reactivity of the eight reactive sites on the POSS cage is almost identical. On the other hand, this molecular design can be easily realized by the selective bifunctionalization by another nano-building block, the Lindqvist-type Polyoxometalate (POM). Lindqvist-type POM, with nano-sized 2
octahedral structure and none heteroatom, is the simplest type of POMs. POMs are the discrete anionic clusters consist of oxygen, sometimes also heteroatoms, and early transition metals in their high oxidation state. The diversity of modification on POMs enables them to be a class of nano building blocks that are ubiquitous in hybrid material researches with potential applications ranging from biomaterials, catalyst engineering to photovoltaic applications. Conjugation of POMs and POSS with covalent bond is expected to result in hybrid materials that possess outstanding properties from both components. As a prototype, the conjugation of octahedral Lindqvist POM and sphere-like isobutyl-POSS (BPOSS) was studied first.
Figure 1.1 BPOSS-Lindqvist based shape amphiphile
Herein, as shown in Figure 1.1, we report the design, synthesis, characterization, and thermal analysis of a prototype “giant shape amphiphile” with a hydrophobic isobutyl POSS (BPOSS) and hydrophilic POM (Lindqvist-type hexamolybdate). Sonogashira coupling is utilized to synthesize the BPOSS-Lindqvist hybrid. It was fully characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared 3
spectroscopy (FT-IR), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy. Differential scanning calorimetry (DSC) is used to study the phase transition of this material. The investigation of the crystal structure of this shape amphiphile will be studied in the future.
4
CHAPTER II
BACKGROUND
2.1 Shape Amphiphiles 2.1.1 Amphiphiles Just like water and oil, it is a common sense that two components with greatly different properties such as polarity, chemical compositions and so on generally will be different in their solubility. In our daily life, surfactants are widely used for cleaning. They are usually small molecules with a polar hydrophilic head and a hydrophobic alkane tail that have the special phase-transfer effect.
Figure 2.1 Definition of amphiphiles The word “amphiphile” contains the root “amphi”, another expression of “both” originates from “amphisbaena”, a two-headed snake which is an invincible patron saint 5
of Hera in the ancient Greek myths, and the other root “philia”, which means love and friendship. This word was created to illustrate a hybrid with two totally different moieties that were forced together. The most acceptable definition of amphiphile is “one molecule that consists of both hydrophobic and hydrophilic parts”. [1] The hydrophilic parts can be generated from hydrophilic functional groups such as hydroxyl group, carboxyl group and some other ionic groups. And hydrophobic parts are mostly given from alkyl groups, esters, or long hydrocarbon chains (polymer without hydrophilic groups). Generally, molecules that consist of two different parts with distinct solubility can be defined as amphiphiles,
such
as
cholesterol, Laurylbenzenesulfonic
Acid
(LABSA)
and
phospholipids. Among various amphiphiles, block-copolymers with distinct blocks have attracted tremendous of attention, mainly due to their ability to form ordered structures within nanometer scale through self-assembly process in both condensed state and solution, as shown in Figure 2.2. Various of ordered phase with thermodynamic equilibrium could be achieved depends on volume fraction and immiscibility of the block copolymers [2]
Figure 2.2 Various self-assembly morphologies of block copolymers 6
With development of synthetic methodologies, more and more attention nowadays has been shifted to the block polymers with versatile architectures, including triblock polymers, star polymer, and cyclic polymer and so on. The influence of geometry has been proved to be much more profound than prediction. However, traditional polymer with random coil configuration cannot provide fixed shape. A Nano building block with precisely defined structures as well as fixed volume and shape is greatly desired.
2.1.2 Nano-sized Building blocks As the basic unit of self-assembly process, building blocks are essential to successfully construct hierarchical structures, which can transfer and amplify the atomic and molecular functionality to macroscopic properties. In the previous work, we employed various nano-sized building blocks with distinct shapes and versatile surface functionalities to synthesize giant nano-sized molecules with precise chemical structures by highly efficient chemistry. Fullerene (C60), Polyhedral Oligomeric Silsesquioxane (POSS) and Polyoxometalates (POMs) are representative examples. Taking POSS for instance, it is a promising nano building block used to engineer various giant molecules in recent years.
[4-10]
POSS is an
inorganic rigid cage with the size about 1nm, which is suitable for preparing the nanosized giant molecules. In addition, it can be easily functionalized by a variety of functional groups on its surface so that it can be used as a scaffold to carry out surface modification. The most interesting thing is that the overall shape of POSS cage can be 7
modified by changing the functional groups on the surface. To make it clear, the nonfunctionalized T8 POSS is kind of sphere-like but the eight-site functionalized isobutyl POSS is more like a cubic.
2.1.3 Shape Amphiphiles Richard W. Date and Duncan W. Bruce introduced the concept of shape amphiphiles 10 years ago
[11]
. In their work, they studied a compound with a rod-like
part and a disk-like part linked covalently. The results showed that by changing the shape biaxiality parameter they can obtain the angle-dependent biaxial nematic phase instead of angel-independent uniaxial nematic phase, which indicating that the so-called “shape amphiphiles” would have unique properties. Based on Glotzer’s studies, shape amphiphiles can be defined as “amphiphilic molecules consists of two different parts which differ in both geometric and chemical symmetry”.
[12][13]
In other words, on one
hand, the geometric symmetry difference can be generated from different shapes of the nano building blocks, for instance, rod-like nanoparticles, disc-like nanoparticles, triangle nanoparticles, and cubic-like nanoparticles can be used to restrain their shape effect. On the other hand, chemical symmetry difference can be given by the various modification methods of the functional groups on the surface of the nanoparticles.
2.2 Recent Work in Our Group In recent years, our group is focusing on the system of nano-sized shape amphiphiles. For example, Haojan Sun’s work published in 2011 showed a giant shape 8
amphiphile
conjugating
fullerene
(C60)
and
isobutyl Polyhedral
Oligomeric
Silsesquioxane (BPOSS) by short covalent bonding. The fullerene is sphere-like and the isobutyl POSS (BPOSS) is cubic-like. And this molecule exhibited two different crystal structures in two different temperature windows.
[14]
The paper published in the same
year by Yiwen Li studied a series of new shape amphiphiles built up by two different kinds of POSS with distinct surface functionality.
Figure 2.3 Molecular models of nano-building blocks used in our group One of them is hydrophilic and the other one is hydrophobic. By self-assembly of these symmetry breaking nanoparticles, the hierarchical supramolecular structure can be obtained in bulk state.
[15]
Another example is the work done by Xinfei Yu in 2010,
which was about a giant shape amphiphile consisted of a hydrophilic head of carboxyl POSS and a hydrophobic polystyrene (PS) random coil tail. This so-called “giant surfactant” had a similar shape as the small molecular surfactant but a comparable size as block copolymers. It could form tunable interesting micellar morphologies in selected solutions and the PS tail was founded being highly stretched. [16] In conclusion, a lot of works related to shape amphiphiles have been done in our group, and most of which were based on the POSS building block. However, there are 9
also some intrinsic shortcomings of POSS as a building block. For instance, the pure bifunctionalized isomers cannot be achieved because the reactivity of the eight reactive sites on the POSS cage is almost the same. Actually, three bi-functional isomers obtained by bi-functionalization of POSS can hardly be purified. Howver, this molecular design can be easily realized by the selective bi-functionalization of the Lindqvist-type polyoxometalate (POM).
2.3 Polyoxometalates (POMs) 2.3.1 Definition Polyoxometalates, which can be also named as "polyoxoanions", are discrete inorganic metal-oxygen cluster anions. A plurality of metal-oxygen acid molecules, such as molybdic acid and vanadic acid, are condensed by dehydration synthesis to afford the clusters. They are formed by early transition metals at their high oxidation state and some heteroatoms. These early transition metals, such as Molybdenum (VI), Tungsten (VI), Vanadium (V, VI), are bridged with oxygen atoms by coordination bond and they are usually at their high oxidation state in order to utilize their high oxygen affinity. The species of metals result in various applications of POMs because of their unique properties such as nucleophilicity, redox properties, large sizes, acidity, large amount of negative charges, and high catalytic activity etc.
[17]
And their unique quasi-liquid
behavior, multi-functionalities (acidic, oxidizing, photoelectric catalytic functionalities) have drawn extensive attention of researchers in the field of catalysis research.
10
The applications, ranging from biomedical to material science, have been found in analytical and clinical chemistry, catalysis (including photocatalysis), biochemistry, medicine, and solid-state devices because of their structural, magnetic, electronic and catalytic properties. For instance, in both heterogeneous and homogeneous conditions, it can be used as acid and catalysts because of their specific structures, size and solubility.[18-21] What’s more, POMs with unique electron-acceptor properties can form mixed-valence and reduced compounds by accepting multiple electrons without major change in their molecular structure. [22] These compounds can be vital materials with particular electronic and magnetic properties in tremendous chemical and biological applications. [23]
2.3.2 Types of POMs Thousands types of polyoxometalates have been found, for example, the Lindqvist type polyoxometalates with no heteroatoms, the heteropolyoxometalates of Keggin, Dawson, Anderson and so on. In this thesis, the Anderson type POMs was tried first but failed because of its low yield and challenging purification. However, what we were interested in is its assembly structure, and for this reason Lindqvist type POM was chosen as our starting point.
11
Figure 2. 4 Molecular Model of Lindqvist-type POMs Lindqvist-type POMs, having nano-sized octahedral structure (about 1 nm of [Mo6O19]2-． 2 [TBA]+) and no hetero-atom, are the simplest kind of POMs. They are hexahedral polyoxometalate clusters, such as the negative divalent hexamolybdate cluster [Mo6O19]2-, the negative divalent hexatungstate cluster [W6O19]2- and the negative divalent pentatungstenmolybdate cluster [MoW5O19]2- . They commonly consist of twelve bridge oxygen atoms and six terminal oxygen atoms. All of the six terminal atoms could serve as a reactive site. So the Lindqvist-type polyoxometalates could serve as an octahedral template to constitute nanoparticle networks.
2.3.3 Surface Modification Methods Currently, modification of the Polyoxometalates by substituting the terminal oxygen atoms of M=O group with active functional groups is very popular. This method can be utilized to synthesize organic-inorganic hybrids with special properties generated 12
from both parts. According to the work of Maatta [24-27], Proust
[28]
, Errington [29] and
Wei[30-32], the major functionalization routes of Polyoxometalates can be concluded as preparing organoimido derivatives of them. To state in detail, for the hexamolybdate ion [Mo 6O19] 2- (Lindqvist type POMs cluster, which has been introduced above), the common functionalization methods are reacting with phosphinimines [24, 28], isocyanates [25, 26] or aromatic amines [29] to afford the organoimido derivative. By these methods, active functional groups can be linked to the polyoxometalates covalently so that further modification can be conducted. For example, giant
surfactant
can be formed
by conjugation with polymer chains, and
polyoxometalates network can be formed if conjugated with one or more other functionalized Polyoxometalates. [32] However, the three approaches listed above have their limitations. Reaction of phosphinimines or isocyanates with [Mo6O19] 2- requires strict condition and reaction of aromatic amines with [Mo6O19]
2-
has low yield even with anhydrous oxygen free
operation. In this case, in 2001, Wei put forward an improved method using dehydratingfunctional diimides (such as DCC or DIPC) under reflux acetonitrile. [31] The diimides served as dehydrating agents to ameliorate this functionalization method. This improvement is proved conducive to the reaction and this reaction could be completed in less than twelve hours with relatively high yield. What’s more, this improvement is not only applicable to monofunctionalizing [Mo6O19]2-, but also could be used to obtain the selective bifunctionalized derivatives of it. [21]
There are two isomers of bifunctionalized [Mo6O19]2-. One is cis-isomer and the other
is trans-isomer. This bifunctionalization reaction is selectively since the cis-isomer of 13
bifunctionlized [Mo6O19]2- was proved thermodynamically favored and it was found by Xia that increasing reaction temperature will lead to the increased yield of cis-isomer while decreased yield of trans-isomer.[33] Therefore by controlling the reaction temperature and time, we can get the selective bifunctionalized product of Lindqvist-type POMs.
2.4 Molecular Design Conjugation of POM and POSS with covalent bond is expected to result in hybrid materials that possess outstanding properties from both components. Firstly, we tried to monofunctionalize the octahedron-like Lindqvist by sphere-like isobutyl-POSS (BPOSS) to form a giant molecule that can be called "Shape Amphiphile". Sonogashira coupling is utilized to synthesize the BPOSS-Lindqvist hybrids. Therefore the isobutylPOSS (BPOSS) functionalized with an iodo end group and Lindqvist with an alkyl end group will be prepared as precursor in advance. The obtained hybrids will be fully characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and matrixassisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy. TGA and DSC will be used to study the phase transition of this material. The investigation of the supramolecular structure will be studied in the future.
14
CHAPTER III
EXPERIMENTAL
3.1 Anhydrous and Oxygen Free Operation The Schlenk line was used to provide anhydrous and oxygen free techniques. The highly reactive organometallic compounds and the catalyst involved in the Sonogashira coupling needed to avoid the oxygen and moisture during the reaction. A Schlenk line consisted of a glass manifold containing two-way taps. One way was for vacuuming and the other one was for providing insert gases. A pressure release and oil bubbler was attached to the end of the line to control the flow of the gas. The Schlenk line could be used for removing air and moisture before reaction and providing inert atmosphere when adding reactive reagent. The solvents were fresh distilled for degassing.
3.2 Chemicals and Solvents Most of the chemicals and solvents were ACS certified that could be used directly as received. However, some of them were purified before adding to the reaction in order to eliminate potential side reactions when necessary.
15
3.2.1 Chemicals Used as Received The following chemicals are used as received: anhydrous sodium molybdate (Strem Chemicals, 98+%), sodium molybdate dihydrate (Na2MoO4·2H2O, Sigma, 99.5+%), tetrabutylammonium bromide (Acros Organics, 99+%), 2,6-dimethylaniline (Acros Organics, 99%), potassium bicarbonate (KHCO3, Certified ACS), potassium carbonate (K2CO3, Certified ACS), ICl (Alfa Aesar), sodium thiosulphate (Na2SO3, Certified ACS), sodium chloride (NaCl, Certified ACS), trimethylsilylacetylene (Oakwood Products, 98%), N, N’-diisopropylcarbodiimide (DIPC, Aldrich, 99 %), 4iodobenzoic acid (Lancaster Synthesis), aminopropylisobutyl POSS (BPOSS-NH2, Hybrid Platics), anhydrous sodium sulfate (Na2SO4, Certified ACS), methanol (MeOH, Fisher Scientific, reagent grade), hexanes (Certified ACS), acetone(Certified ACS), diethyl ether(Et2O, Certified ACS), anhydrous alcohol(Certified ACS, 99+%).
3.2.2 Purification Most purification was associated with removing oxygen or moisture or other impurities. The purification methods mainly include recrystallization, distillation and fractionation. In the case of sonogashira coupling, it was crucial to remove all moisture and oxygen from the reaction system. Triethylamine (TEA, Et3N, Aldrich, 99%), dichloromethane (CH2Cl2, Aldrich, 99.5+%) and acetonitrile (CH3CN, Aldrich, 99%) were purified by distillation under nitrogen after being stirred over calcium hydride for 24 h. It was then stored over
16
activated 4 Å molecular sieves. Cuprous iodide (CuI, Aldrich, 98%) was freshly purified by stirring in acetic acid overnight, washed with acetone, and dried in vacuum.
3.3 Molecular Characterizations 3.3.1 1H Nuclear Meagnatic Resonance (NMR) Spectra All 1H spectra were acquired in CDCl3 (Aldrich, 99.8% D) or CD3CN (Aldrich, 99.8% D) or Acetone-D6 (Aldrich, 99.9%) using a Varian Mercury 300 NMR spectrometer. The 1H NMR spectra were referenced to the residual proton impurities in the CDCl3 at δ 7.27 ppm. 3.3.2 Fourier Transform Infrared Spectroscopy (FT-IR) Infrared spectra were recorded on an Excalibur Series FT-IR spectrometer (DIGILAB, Randolph, MA) by casting polymer films onto KBr plates from material solutions. The data were processed using Win-IR software. 3.3.3 Thin-layer Chromatographic Analyses (TLC) Thin-layer chromatographic analyses (TLC) of the functionalized materials were carried out on by spotting and developing samples on flexible silica gel plates (Selecto Scientific, Silica Gel 60, F-254 with fluorescent indicator) using mixture of different polar solvents as eluents. 3.3.4 Differential Scanning Calorimetry (DSC) The thermal characteristics of the bulk and crystal mat samples were obtained using a Perkin-Elmer PYRIS Diamond DSC coupled with an Intracooler 2P apparatus. A 17
typical sample weight was 1.0 mg for bulk samples and 0.3 mg for crystal mat samples. The difference in pan weights between the reference and sample was kept less than 0.005 mg. The heating and cooling rates were varied depending on the sample.
3.4 Synthesis of Tetrabutylammonium Hexamolybdate (VI), [(n-Bu4N)2][Mo6O19]
Figure 3.4 Synthetic Route of Lindqvist POM
A solution of commercial, ACS reagent grade, sodium molybdate dihydrate (Na2MoO4·2H2O) (10 g, 41.2 mmol) in 40 mL of water was acidified with 6.5 mL of aqueous HCI (12M, 78 mmol) in a 250 mL flask under vigorous stirring over a 1 min at room temperature. A solution of commercial, 99% pure tetrabutylammonium bromide (4.84 g, 15 mmol) in 8mL of water was then added by injection under vigorous stirring to cause immediate formation of a white precipitate. The resulting slurry was then heated up to 75-80 ºC with stirring for about 45 min. The white solid gradually changed to yellow. This crude product was collected by filtration and washed with 20 mL water for three times. Crystallization was accomplished by dissolving the air-dried crude product (8.68 g) in 320 mL of hot acetone (60°C). After hot filtration the filtrate was cooled to - 20 ºC. After 24 h, the yellow crystalline product was collected on a filter with suction, washed twice with 20 mL portions of diethyl ether, and dried for 12 h in vacuo (0.1 torr). Yield: 8 g, 84%. IR (KBr pellet, major absorbances): 742 (m), 800 (s), 880 (w), 890 (w, sh), 18
956(s), and 988 (w). 1H NMR (300MHz, CD3CN, ppm): 3.16 (t, +N-CH2-CH2, 16H), 1.65 (m, CH2, 16H), 1.41 (m, CH2, 16H), 1.00 (t, CH3, 24H). MS (MALDI-TOF): Calcd: 1606.2, Found: 1604.4 [M+TBA]+.
3.5 Synthesis of 4-Ethynyl-2, 6-Dimethylaniline
Figure 3.5 Synthetic Route of 4-Ethynyl-2, 6-Dimethylaniline
3.5.1 Synthesis of 4-Iodo-2,6-Dimethylaniline To the solution of 2,6-Dimethylaniline (8 g, 66 mmol) and KHCO3 (10 g, 198 mmol) in MeOH (40 mL), a solution of ICl (14 g, 86 mmol)/CH2Cl2 (40 mL) was dropwisely added over 1 hour in an ice bath. The mixture was stirred at room temperature overnight and solvent was removed by the rota-evaporator. The residue was dissolved in 100 mL of CH2Cl2 and filtered. The filtrate was extracted with 100 mL of Na2SO3 saturated aqueous solution twice, 100 mL of water twice, and 100 mL brine, respectively, and dried over anhydrous Na2SO4. The solvent was removed by the rota-evaporator. The residue was dissolved in 5 mL of CH2Cl2 and purified by silica column chromatography with CH2Cl2: hexane = 1:2 to afford the pure orange product. Yield: 13g, 80%. 1H NMR (300MHz, CDCl3, ppm): 7.24 (s, Ar-H, 2H), 3.57 (s, Ar-NH2, 2H), 2.13 (s, Ar-CH3, 6H).
19
3.5.2 Synthesis of 4-Trimethylsilylacetylenyl-2, 6-Dimethylaniline To a solution of 4-Iodo-2, 6-dimethylaniline (3.357 g, 13.587 mmol) in 10 mL Et3N in a Schlenk flask, 98% pure trimethylsilylacetylene (2.002 g, 20.380 mmol) and CuI (29.8 mg, 27.174 μmol) were added. The oxygen was removed by freeze-pump-thaw for three times. Pd (PPh3) 2Cl2 (95.4mg, 13.587μmol) was then added under the N2 atmosphere. An additional freeze-pump-thaw was applied to further get rid of residual oxygen. The result slurry was slowly warmed up to room temperature and stirred at room temperature overnight. After the formed precipitation was filtered off (rinsed by Et 2O), the filtrate was dried by the rota-evaporator. The residue was dissolved in 5 mL of CH2Cl2 and purified by silica column chromatography with CH2Cl2: hexane = 1:2 to afford the pure product. Yield: 2.8 g, 95%. 1H NMR (300MHz, CDCl3, ppm): 7.10 (s, ArH, 2H), 3.73 (s, Ar-NH2, 2H), 2.14 (s, Ar-CH3, 6H), 0.23 (s, Si-CH3, 9H).
3.5.3 Synthesis of 4-Ethynyl-2, 6-Dimethylaniline Under N2 atmosphere, the suspension of K2CO3 (3.178 g, 23.033 mmol) in 50 mL of MeOH was added into the round bottom flask. Then the solution of 4trimethylsilylacetylenyl-2, 6-Dimethylaniline (1 g, 4.607 mmol) in 50 mL of CH2Cl2 was added by injection. The resulted mixture was stirred at room temperature overnight. After removal of the solvent by the rota-evaporator, the residue was dissolved in 50 mL of CH2Cl2 and the filtrate was dried by the rota-evaporator. The resulted residue was dissolved in 50 mL of H2O and then extracted by two proportions of 50 mL Et2O and three proportions of brine. The organic layer was collected and dried with anhydrous Na2SO4. After 1 h, the dried slurry was filtered and the solvent was removed by the rota20
evaporator. The residue was dissolved in 5 mL of CH2Cl2 and purified by Al2O3 column chromatography with CH2Cl2: hexane = 3:1 to afford the pure light yellow product. Yield: 411 mg, 62%. 1H NMR (300MHz, CDCl3, ppm): 7.12 (s, Ar-H, 2H), 3.75 (s, Ar-NH2, 2H), 2.93 (s, Ar-Ethynyl-H, 1H), 2.16 (s, Ar-CH3, 6H).
3.6 4-Ethynyl-2, 6-Dimethylaniline-Functionalized Tetrabutylammonium Hexamolybdate (VI)
Figure 3.6 Synthetic Route of Ethynyl - Functionalized Lindqvist
Transfer 15 mL of pre-dried CH3CN into a Schlenk flask under vacuum. Under N2 atmosphere, [(n-Bu4N) 2][Mo6O19] (1 g, 0.73 mmol), DIPC (101 mg, 0.80 mmol) and 4-Ethynyl-2, 6-Dimethylaniline (106 mg, 0.73 mmol) were added. After warmed up by the hot water the suspension became transparent solution. Then the solution was stirred under room temperature in N2 atmosphere for 12 hours. The resultant red-orange solution was filtered with CH2Cl2 to remove the white precipitates (N, N'-dicyclohexylurea). The solvent of the filtrate was removed and purified by silica column chromatography with CH2Cl2: acetone = 6:1 to afford the pure dark-red product. The solvent was removed by rota-evaporator. The crude product was dissolved in 10 mL of acetone and then 10mL of alcohol was added. The resulted mixture was still standing for 3 days and as the solvent slowly evaporate, the product precipitated out of the solution as orange crystals. The product was collected by filtration, washed with EtOH and Et 2O several times, and dried 21
under vacuum to yield 984 mg, 91%. 1H NMR (300MHz, CD3CN, ppm): 7.22(s, Ar-H, 2H), 3.43 (s, Ethynyl-H, 1H), 3.16 (t, +N-CH2-CH2, 8H), 2.61 (s, Ar-CH3, 6H), 2.00(m, CH2CH2CH2, 16H), 1.59(m, CH2CH2CH3, 16H), 1.03(t, CH3, 24H). IR (KBr pellet, major absorbances): 742 (m), 800 (s), 880 (w), 890 (w, sh), 956(s), and 988 (s, sh). MS (MALDI-TOF): Calc: 1733.3, Found: 1735.5 [M+TBA] +.
3.7 Synthesis of 4-Iodo-Benzoic Acid Monofunctionalized Isobutyl Polyhedral Oligomeric Silsesquioxane
Figure 3.7 Synthetic Route of Iodo - Functionalized POSS
Amino monofunctionalized isobutyl Polyhedral Oligomeric Silsesquioxane (500 mg, 0.572 mmol), 4-iodo-functionalized benzoic acid (212 mg, 0.855 mmol) was dissolved in 50 mL of fresh distilled CH2Cl2. The solution was stirred for 1 min and then DIPC (143.8 mg, 1.139 mmol) and HOBt (115.5 mg, 0.855 mmol) were added into it. The mixed solution was stirred for 12 hours at room temperature. The resulted suspension was filtered and the solvent was removed by the rota-evaporator. The residue was purified by the silica column chromatography with CH2Cl2: Hexane = 5:1 to afford the pure product. Yield: 531 mg, 84%. 1H NMR (300MHz, CDCl3, ppm): 7.73(d, Ar-H, 2H), 22
7.43(d, Ar-H, 2H), 3.39 (t, Carbonyl-NH-CH2, 1H), 1.80(m, Acryl-CH2-CH2, 2H), 1.66(m, H of Methyne, 7H), 1.51 (m, CH2CH2CH2, 2H), 0.91(d, CH3, 42H), 0.56 (d/t, CH2-Si, 16H). MS (MALDI-TOF): Calc: 1126.2, Found: 1126.3 [M+Na]+ .
3.8 Synthesis of Isobutyl POSS Monofunctionalized Hexamolybdate(VI)
+
Pd (PPh3) 2Cl2 CuI
Figure 3.8 Synthetic Route of POSS - Monofunctionalized Lindqvist
To the solution of 4-Ethynyl-2, 6-dimethylaniline-functionalized Lindqvist (100 mg, 0.067 mmol) in 10 mL of solvent (CH3CN: CH2Cl2: Et3N = 1:1:1) in a Schlenk flask, 4-iodo-benzoic acid monofunctionalized isobutyl POSS (74 mg, 0.067 mmol) and CuI (1 mg, 0.335 μmol) were added. The oxygen was removed by freeze-pump-thaw for three times. Pd (PPh3) 2Cl2 (1mg, 0.670μmol) was added under the N2 atmosphere. An additional freeze-pump-thaw was applied to remove residue oxygen. The result slurry was slowly warmed up to room temperature and stirred at room temperature for 1 hour. After the formed precipitation was filtered off (rinsed by Et 2O), the filtrate was dried by the rota-evaporator. The residue was dissolved by 5 mL of CH2Cl2 and purified by silica column chromatography with CH2Cl2: hexane = 1:2 to afford the pure product. Yield: 2.8 g, 95%. 1H NMR (300MHz, CDCl3, ppm): 7.74 (d, Ar-H, 2H), 7.56(d, Ar-H, 2H), 7.15(d, Ar-H, 2H), 6.20 (t, Carbonyl-NH-CH2, 1H), 3.48 (m, Acryl-CH2-CH2, 4H), 3.28(t, +N23
CH2-CH2, 8H), 2.65(s, Ar-CH3, 6H), 1.87(m, H of Methyne, 7H), 1.64(m, CH2CH2CH2, 18H), 1.48(m, CH2CH2CH3, 16H), 0.98(d/t, CH3, 68H), 0.61(d/t, CH2-Si, 16H). MS (MALDI-TOF): Calc: 2711.6, Found: 2711.1 [M+TBA]+.
24
CHAPTER IV
RESULTS AND DISCUSSION
The Sonogashira coupling was conducted under anhydrous and air free condition. The Schlenk line technique could be used to provide high vacuum condition and inert atmosphere. The Lindqvist-type POMs was freshly prepared for concern about the instability of them. Small molecule linkage was also self-prepared. There were some difficulties when preparing the functionalized aniline linkage. The chromatography column was supposed to be very short and fast because the alkalescence of the molecule would make it blocked in the acidulous silica column. Recrystallization was an important method to purify the POM related products. It was because the anionic POM clusters had strong ionic interaction with their counter anions so that they were easy to crystallize. Apart from FT-IR technique, crystal color could serve as an easy approach for roughly monitoring the reaction. Color of unreacted Lindqvist-type POMs was bright yellow and turned to maroon after reaction. The amidation of POSS was needed to under anhydrous condition. Dehydration agent DIPC was used and it could obtain higher yield when equivalent of it closer to 1.2eq. When iodo-functionalized Lindqvist was treated with ethynyl-functionalized isobutyl-POSS under the catalyst of Pd (PPh3) 2Cl2 and CuI in N2 atmosphere, the 25
corresponding Isobutyl POSS and Lindqvist based shape amphiphile is formed according to the Sonogashira mechanism and this reaction usually completes within about 1 hour. On the other hand, however, if the iodo-functionalized isobutyl-POSS and ethynylfunctionalized Lindqvist were reacted under the same condition, it took about 6 hours to complete. The difference may due to the special catalytic properties of Lindqvist type POMs that the iodo-group can be activated and then the reaction was accelerated. Both ways could be used to synthesize the Isobutyl POSS and Lindqvist based shape amphiphile. After the synthetic strategy was successfully used to synthesis the Isobutyl POSS and Lindqvist based shape amphiphile，the composition and structure information of the final product (isobutyl-POSS Monofunctionalized Lindqvist) can be obtained by the characterizations as follows:
Figure 4.1 1H NMR analysis of Isobutyl POSS Monofunctionalized Lindqvist
26
The first evidence comes from 1H NMR spectrum, as shown in Figure 4.1, in which both the signals from POSS moiety and POM moiety can be clearly identified. The protons of the common alkyl groups keep the similar chemical shifts after the reaction. The resonance between 7 ppm and 8 ppm can be attributed to the aromatic protons (ArH). In which the signals at 7.74 ppm and 7.56 ppm can be assigned to the protons on the phenyl group adjacent to POSS cage. These peaks shift from 7.73ppm to 7.74ppm, and 7.43ppm to 7.56ppm, respectively. The imino protons (Carbonyl-NH-CH2) shift from 3.39ppm to 3.48ppm after reaction. Peak at 7.15 ppm comes from proton on the phenyl adjacent to the functionalized POMs. It shifts from 7.22ppm to 7.15ppm after reaction. The protons of methylene beside the acryl group are shifted from 2.16 ppm to 2.61 ppm in first step and slightly increased from 2.61ppm to 2.65ppm in second step. In particular, the protons of ethynyl group from functionalized POMs shifts from 2.93ppm to 3.43ppm in the first step, and this peak disappears in the second step, which significantly proves the success of this reaction. The ethynyl group distinguishes two kinds of protons. These two signals are protons from POSS and POM part, respectively. It can be concluded from the 1H NMR spectrum that the chemical shifts of the protons from the POSS slightly increase and those from the POMs slightly decrease comparing to each unreacted component after reaction. It indicates that in this molecule, the POMs part serves as an electron accepter and POSS part serves as an electron donor. The decrease of the electron density causes the increase of chemical shift. This phenomenon is interesting because POMs is usually served as an electron accepter with its unique metal properties.
27
Figure 4.2 MALDI-TOF mass spectrum of Isobutyl-POSS Lindqvist (m/z 2711.1, expected 2711.6) To further unambiguously confirm the molecular structure, MALDI-TOF mass spectra were utilized to characterize the products. Figure 4.2 shows MALDI-TOF mass spectra of product with TBA as contour ion with a single peak. The calculated molecular weight of the product is 2711.1 g/mol ([M+TBA]+), and the observed molecular weight is 2711.6 g/mol, very close to the proposed structure. The accordance of molecular weight to the proposed structure confirms the cleanliness of the reaction and the stability of the product.
28
Figure 4.3 TGA degradation of BPOSSLIND Thermal analysis of the hybrids provides us with the information about its thermal stability, as well as phase transition. Figure 4.3 is a thermal gravity analysis, in which the 5% weight loss temperature of the BPOSSLIND is about 262.2 °C. When the temperature is lower than the 200 °C, the TGA curve is steady and seems like a plateau, indicating that there is no degradation occurs. Only when the temperature increases beyond 250 °C, the curve shows a downward trend. Therefore, the product would be thermal stable below 200 °C.
29
Figure 4.4 DSC thermograms of BPOSS-Alkyne, BPOSS-Lind and Lind-Iodide during heating.
The crystallization behavior of BPOSSLIND was studied by the differential scanning calorimetry (DSC) measurements. To prevent thermal degradation, the sample 30
was only heated up below 200 °C.
It can be clearly observed that the melting
temperature of the BPOSSLIND crystal is about 175 °C, very close to that of the BPOSSAlkyne precursor. On the other hand, there is no melting peak corresponding to POM moiety (about 128°C), indicating that POM is not crystallized. Therefore it is speculated that the crystallization of BPOSS cage, rather than Lindqvist octahedron, dominates the crystallization of BPOSSLIND. This may be caused by the stronger interaction between the BPOSS cages based on the comparable sizes of the POSS and Lindqvist building blocks. The supermolecular structure will be studied in the future work.
31
CHAPTER V
SUMMARY
In conclusion, shape amphiphile based on Lindqvist POM and BPOSS (as shown in Figure 5.1) was successfully synthesized, and fully characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and matrix-assisted
laser
desorption
ionization
time-of-flight
(MALDI-TOF)
mass
spectroscopy. It provides us with a prototype to study nanoparticle based shape amphiphiles, giant molecules composed by POMs and organic-inorganic hybrids.
Figure 5.1 Summary The shape amphiphiles was studied by thermal analysis. The TGA measurement illustrated that it was relatively thermal stable less than 200 °C. Differential scanning 32
calorimetry (DSC) thermograms shows that the crystallization was dominated by the BPOSS cage, rather than that of Lindqvist octahedron, which can be inferred that only melting temperature of the BPOSS moiety was observed in DSC, while POM moiety is completely amorphous. In the future, the supramolecular structure Lindqvist-BPOSS based shape amphiphile in both solution and bulk state will be investigated. The selective POSS bifunctionalization of the Lindqvist type POMs to obtain the multifunctional POM-POSS based particles, especially the 2:1 adducts will be studied. Based on it, the thermodynamic stable and energetically most favored state and the supramolecular morphologies in the solid state and solution will be investigated. A lot of other problems, such as “Do they form hierarchical structures? Which factor is dominant? What is the role of topology? What are their potential applications?” will also be solved in later work.
33
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37
APPENDIX SPECTRUMS OF SELF-PREPARED SUBSTANCES
Figure A.1 1H NMR spectrum of Lindqvist POM
Figure A.2 MALDI -TOF spectrum of Lindqvist (m/z 1604.4, expected 1606.2) 38
Figure A.3 1H NMR spectrums of 4-Iodo-2, 6-Dimethylaniline (a), 4-trimethyl silylacetylenyl-2, 6-Dimethylaniline (b) and 4-Ethynyl-2, 6-Dimethyl-aniline (c).
Figure A.4 1H NMR spectrum of Ethynyl-Functionalized Lindqvist POM
39
Figure A.5 MALDI-TOF mass spectrum of Ethynyl-Functionalized Lindqvist POM (m/z 1735.5, expected 1733.3)
Figure A.6 IR spectrum of functionalized Lindqvist POM: Tetrabutylammonium Hexamolybdate(VI), [(n-Bu4N)2][Mo6O19] (a); 4-Ethynyl-2,6-DimethylanilineFunctionalized Hexamolybdate(VI) (b). The absorption peak in the red square dash box is the characteristic peak of Lindqvist POM(Mo=O).
40
Figure A.7 1H NMR spectrum of Iodo-Functionalized POSS
Figure A.4 MALDI-TOF mass spectrum of Iodo-Functionalized POSS (m/z 1126.3, expected 1126.2)
41
Figure A.9 1H NMR spectrum of POSS –Monofunctionalized Lindqvist
Figure A.5 MALDI-TOF mass spectrum of POSS –Monofunctionalized Lindqvist (m/z 2711.1, expected 2711.6)
42
