CONSTRUCTION OF MULTIDIMENSIONAL METAL-ORGANIC FRAMEWORK

VIA SELF-ASSEMBLY APPROACH: THE HARVEST OF INTERESTING

MOLECULAR TEXTURES

By

Bich Tram Nguyen Pham

A thesis submitted in conformity with the requirements

for the degree of Master of Science

Graduate Department of Chemistry

University of Toronto

Copyright by Bich Tram Nguyen Pham, 2008

ii

Construction of multidimensional metal-organic framework via self-assembly approach:

the harvest of interesting molecular textures

Bich Tram Nguyen Pham

Master of Science, 2008

Department of Chemistry

University of Toronto

Abstract

Metal organic framework (MOF) has emerged as a new class of porous, thermally stable

material which has attracted great attention due to their wide applications in gas storage,

separation, catalysis etc. Self-assembly is the operative mechanism of MOFs syntheses;

however, the control of MOF self-assembly is still a challenge in the construction of

predetermined, structurally well-defined MOFs. The goal of the research is to arrive at

multidimensional, highly porous and functional MOFs via hierarchical assembly of smaller

molecular building blocks and, at the same time, to examine the possibilities for different

interesting molecular textures. This goal is to be accomplished by the knowledge of ligand

coordination mode, and geometry as well as logical choices of ligands and metals from which

the MOFs are to be constructed from. Preparations of novel frameworks as well as other

interesting molecular architectures are highlighted with their structures characterized.

iii

Acknowledgements

First and foremost, I’d like to express my gratitude to my supervisor, Datong for giving

me the opportunity to learn more about inorganic chemistry from my project. His guidance,

understanding and many nights running diffraction for my crystals are deeply appreciated.

I want to thank the Department of Chemistry for the vibrant research niche and financial

supports. I’d like to express my many thanks to funding agencies including NSERC, CFI and

the Connaught Foundation for their generosity in funding this research.

I also want to let my groups members, Yang (for guiding me the very first steps on how

to do research), our good old Alen (for his helpful discussions), Ali (what’s up?), Hualing (for

being so cute), Ping Pong Peng (for being a great friend), Liisa “Linda” Lund (for making me

nervous with your countdown), Runyu (for those late dinners and great sport), and Fiona, know

that I am grateful to have worked with them for they are so fun, welcoming and for their great

teamwork. Also, I want to thank Yen, Thi, Marc, Andy and many more friends in the

department who has made my time here filled with laughters and very memorable. I am also in

debt to other colleagues, Geogetta Masson, Wendong Wang, Joanna Poloczek, Bettina Lotsch in

the department for their assistance with many different instruments in characterizing my

compounds. I also want to thank Christoph Deckert and Richard Huang, the undergraduate

students who has tremendously expedited the ligand synthesis.

Finally, I’d like to thank my family so much for their endless supports, encouragement

and love during my graduate study.

iv

For my parents, Duy, Bao, di Tuyen, di Tu, and Lacey

v

Would not trade this experience for any other

vi

Table of Content

Page

Abstract ii

Acknowledgements iii

Table of Contents vi

List of Tables viii

List of Figures ix

List of Schemes xi

List of Abbreviations xii

Chapter I: Introduction 1

1.1 Supramolecular self-assembly (SA) 1

1.2 MOF composition and synthesis influenced by self-assembly process 2

1.2.1 Secondary building unit 3

1.2.2 Metallic nodes 4

1.2.3 Linkers 4

1.2.4 MOF synthesis 5

1.3 The significance of metal-organic frameworks 5

1.3.1 MOFs as attractive H2 storage system 6

1.3.2 MOFs as catalysts 9

1.3.3 MOFs functioning in chromatography 10

1.4 Luminescence in MOFs 10

1.5 Scope of the Thesis 11

vii

Chapter II: Experimental 14

2.1 Synthesis of 3,3'-(biphenyl-4,4'-diyl)dipentane-2,4-dione (LigAH2) 14

2.2 Synthesis of 4,4'-(ethyne-1,2-diyl)dibenzoic acid LigBH2 15

2.3 Synthesis of Zn2(LigA)(phen)2(OAc)2,, 1 15

2.4 Synthesis of [Zn(LigA)(CH3OH).CHCl3]n , 2 15

2.5 Synthesis [Zn2(LigB)2(DMSO)2]·2MeOH, 3 15

2.6 Synthesis [Eu2(LigB)3(DMSO)2(MeOH)2]·(DMSO)2(H2O)3, 4 16

2.7 Synthesis [Tb2(LigB)3(DMSO)2(MeOH)2]·(DMSO)2(H2O)3, 5 16

Chapter III: Results and Discussion

3.1 Construction of dinuclear Zn(II) complexes of LigA2- 28

3.2 Construction of 1D Zn-containing coordination polymer 30

3.3 Construction of 3, a 2D interpenetrating Zn-containing framework 33

3.4 Construction of 4 and 5, 3D open frameworks 37

Chapter IV: Conclusion and Future Outlook 44

References 47

Appendix 50

viii

Lists of Tables

Page

Table 1 Crystallographic data for compounds 1-5 17

Table 2: Selected bond lengths (Å) and angles (deg) of numbering atoms in 1 19

Table 3: Selected bond lengths (Å) and angles (deg) of numbering atoms in 2 19

Table 4: Selected bond lengths (Å) and angles (deg) of numbering atoms in 3 20

Table 5: Selected bond lengths (Å) and angles (deg) for 4 21

Table 6: Selected bond lengths (Å) and angles (deg) for 5 23

ix

List of Figures

Page

Chapter I: Introduction

Figure 1: Illustration of secondary building unit 3

Figure 2: Type I gas absorption isotherm 7

Figure 3: Interpenetration and interweaving MOFs 9

Figure 4: Antenna effect in lanthanide luminescence 11

Figure 5: Schematic representation of research objective 12

Chapter III: Discussion

Figure 6: Crystal structure of LigAH2 with only the alpha hydrogen shown (in pink) being

chelated by the acac moiety. 26

Figure 7: UV-vis absorption profile of LigAH2 (dash line) and sodium salt of LigA2-

(solid line) 27

Figure 8: ORTEP drawing of Zn2(LigA)(phen)2(OAc)2 (1) crystallized from

DCM/hexanes 28

Figure 9: Crystal structure of 1, Zn2(LigA)(phen)2(OAc)2 grown from CHCl3/hexanes 29

Figure 11: 1D coordination polymer 2, [Zn(LigA)(CH3OH).2CHCl3]n 31

Figure 12: ORTEP drawing of the building block of 2 31

Figure 13: Schematic description of the self-assembly of pillar supported structure 32

Figure 14: UV-vis absorption of free LigBH2 (bold solid line) and its salt in different

water and DMSO (dash and thin solid line) 33

Figure 15: (a) Structure of the paddle-wheel dinuclear Zn carboxylate units of

framework 3; (b) Structure of the 2D grid of framework 3. 34

Figure 16: 2D interpenetrating, sheet-like structure of 3 35

x

Figure 17: Emission spectra of MOF 3 under different excitation wavelengths 36

Figure 18: Normalized solid state emission and excitation profile of MOF 3 36

Figure 19: The asymmetric unit of 4 (left), and 5 (right) 38

Figure 20: Bimetallic Eu unit showing the versatile coordination modes of the carboxylate

groups of LigB2- 39

Figure 21: Space-filling diagram of the extended structure of 4 showing triangular channel

looking down the a axis. 39

Figure 22: Powder X-ray diffraction calculated and found pattern

for framework 5 40

Figure 23: Magnified region of 2θ = 3 to 9o in both calculated and found PXRD patterns

of 5 40

Figure 24: TGA of 4 and 5 under N2 atmosphere and heating rate of

5oC/min 42

Figure 25: Weight loss of 5 over time (min) 43

Figure 26: Solid state excitation-emission spectrum of MOF 4 44

xi

List of schemes

Page

Scheme 1: Synthesis of LigAH2 via Ullmann coupling 26

Scheme 2: Synthesis of dinuclear Zn complex 1, Zn2(LigA)(phen)2(OAc)2 28

Scheme 3: Synthesis of 2, [Zn(LigA)(CH3OH).2CHCl3]n 30

xii

List of Abbreviations

1 Zn2(LigA)(phen)2(OAc)2

2 [Zn(LigA)(CH3OH).2CHCl3]n

3 [Zn2(LigB)2(DMSO)2]·2MeOH

4 [Eu2(LigB)3(DMSO)2(MeOH)2]·(DMSO)2(H2O)3

5 [Tb2(LigB)3(DMSO)2(MeOH)2]·(DMSO)2(H2O)3

LigAH2 unprotonated ligand A (3,3'-(biphenyl-4,4'-diyl)dipentane-2,4-

dione)

LigBH2 unprotonated ligand B (4,4-(ethyne-1,2-diyl)dibenzoic acid)

acac Acetylacetone)

δ chemical shift

CHCl3 chloroform

Cont. continued

d day

deg degree

ρ density

CH2Cl2 dichloromethane

DCM dichloromethane

DMSO dimethyl sulfoxide

N2 dinitrogen

N/A not applicable

dpdo 4,4’-dipyridyldioxide

Et3N triethylamine

FW formula weight

GC gas chromatography

xiii

g gram

h hour

lig ligand

M metal

min minute

mmol milimole

MOFs metal-organic frameworks

mol mole

NMR nuclear magnetic resonance

Phen 1,10-phenanthroline

ppm parts per million

% percent

PXRD powder x-ray diffraction

1D one-dimension

rt room temperature

SXRD single crystal x-ray diffraction

THF tetrahydrofuran

3D three-dimension

2D two-dimension

λ wavelength

wt weight

0D zero-dimension

1

CHAPTER I

INTRODUCTION

1.1 Supramolecular self-assembly (SA)

The foundation of supramolecular chemistry was laid down less than fifty years ago and has

since developed rapidly and become increasingly interdisciplinary. Supramolecular chemistry

has diversified into host-guest chemistry and crystal engineering. Upon association of the host

and guest, the properties of either or both constituents can change. A more stable aggregate

often results. We have enjoyed many applications of inclusion complexes such as drugs

(Nitropen), catalysts, sensors for clinical measurements.1

Pre-organization, recognition, and self-assembly are central concepts of supramolecular

chemistry.2

Preorganization is the spontaneous arrangement of reagents into appropriate spatial

orientation to facilitate chemical reactions or self-assembly processes.1a The structure adopted

by a molecule in the crystalline state is determined by a balance of attractive and repulsive

forces, each possessing varying degree of strength, distance dependence and directionality.3

Hydrogen bonding exhibits the strongest directional properties and is often used in the field.

Hydrogen bonding in some cases can help add a new dimension to a network even in the

absence of a covalent linker.

Molecular recognition requires shape and electronic complementarities between host and

guest molecules. “Key and lock” and “induced fit” concepts have been proposed to explain such

phenomena.

There are covalent and supramolecular self-assemblies; formation of the latter is driven by

weak, but numerous intermolecular forces including hydrogen bonding, and van der Waals

interactions (i.e. π-π stacking, dipole-dipole and ion-induced interactions),. The interactions that

bind molecules in crystals are also ones responsible for molecular assembly in solution. By the

2

virtue of having many weak, non-covalent forces, or for that matter metal-donor bonds, the final

products as a result of self-assembly is in thermodynamic equilibrium with its components. This

lead to unique and important properties of most supramolecular systems: they have an in-built

capacity for error correction that is normally unavailable in fully covalent system. In other

words, a system that self-assembles can do so reversibly. Irreversible aggregation without

adjustment of the positions of its components leads to glasses or amorphous materials.3

In summary, all the mentioned processes play a sequential role in complexation:

preorganization selects out appropriate spatial orientation of sub-components for their

subsequent recognition by one another followed by the spontaneous self-assembly of the

required supramolecular entity.2

1.2 MOF composition and synthesis influenced by self-assembly process

Coordination polymers and supramolecules are formed by self-assembly process — a facile

approach to materials of useful properties.4 Thus, the physical property of individual

constituents, such as luminescence, chemical functionality, and chirality, can be translated into

that of the assembled MOFs.11 In coordination polymers, metal-ligand modules are linked

infinitely into one, two or three dimensions via metal-ligand covalent bonds and the ligand must

contain bridging moieties. There are several ways to define MOFs. MOFs can also be thought as

coordination polymers.5 Some large, extended MOFs structures are indeed

metallosupramolecules.6 They are also assemblies of metal ions or clusters functioning as nodes

and organic ligands as the linkers.7 The cavities of as-synthesized MOFs are often filled with

solvent molecules or counter ions. MOFs are thus essentially host-guest systems.

The functions of materials are largely determined by their solid state structures.5 This

theorem is in turn the driving force for the desire to rationally design and control the assembly

of MOF building blocks. MOFs are highly crystalline, and their crystals can be regarded as a

manifestation of self-assembly and self-organization mentioned above.

3

1.2.1 Secondary building unit:

As pioneers in the field of MOF, Yaghi and coworkers have also coined the term secondary

building unit or SBU which refers to molecular complexes or clusters that can be extended into

porous networks using polytopic linkers.7 SBU is a useful tool for the overall topology

prediction for MOFs. As illustrated in Figure 1, the four carbon atoms of the paddle wheel form

square secondary building units (Figure 1a) which in turn can, by means of linkers, form

polyhedra, 2D sheets, and 3D networks.8 Factors such as solvent templating, metal ion

solvation, linkers’ geometries, temperature, pH, presence and nature of available anions may

play decisive roles in the formation of various structures.6,9

MM

O O

OO

O

O

O

O

MM

OO

OO

O

O

O

O

MM

OO

OO

O

O

O

O

MM

OO

OO

O

O

O

O

MM

OO

OO

O

O

O

O

Linker

paddle wheel unit

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

SBU

Figure 1: Illustration of secondary building unit.8

Researchers have been trying to find a way to control and predict the structural outcomes of

MOFs. However, there has not yet been a way to truly control a self-assembly process; rather,

what commonly so-called “rational” design can only help do so in a pseudo-controlled fashion.6

Utilizing SBUs and the linkers of well-known geometry and chemical properties can lead to

better predictions. For a given shape of the building blocks, only few simple, high symmetry

network of general importance are most likely to form from these subunits. 9

4

1.2.2 Metallic nodes:

The metal atoms in MOFs can be thought of as templating joints for bridging ligands.5 Late

transition metals are often used in MOF construction. Lanthanide metals are also used to impart

luminescence properties to the framework. Although frameworks made of light main group

metal cations such as Li+, Na+, and Al3+ would be lighter, they have not been observed to date.

Metal ions can be introduced to the nodal positions of MOFs as single metal centers or as metal

clusters. Group 2 MOF are rare, having few reports on Mg, Ca, Ba, Sr-containing structures.10, 11

Among metal clusters at nodes, Zn cluster are most often encountered.8, 12

Utilizing metal clusters as nodes of MOFs can be an effective approach to circumvent the

lack of coordination sites that is often experienced with transition metals (i.e. Cu, Zn, Cd etc.)

largely because metal clusters can accommodate sterically demanding organic ligands.12b This is

also one of the strategies to obtain rigid structure without a tendency to interpenetrate.13

1.2.3 Linkers:

The ability to derivatize and modify organic linkers gives chemists limitless access to

various linkers. Oxygen- and nitrogen-donor ligands are often used in MOF synthesis. Polytopic

ligands such as 4,4’-(ethyne-1,2-diyl)dibenzoic acid are widely used as ligands due to their

ability to bind to various metals with versatile binding modes.14

The use of molecular rather than monatomic bridges leads to the desirable outcome of

enlarging the void space of MOFs by means of further extending the distance between metal

nodes.11 Expansion of the network can be achieved by using long or polytopic ligands which

effectively enlarge the pore of the framework cavity.7 However, this strategy is not always

preferable, especially when MOFs are to be functioning in gas inclusion. This aspect will be

discussed later on.

5

1.2.4 MOF synthesis:

In MOF synthesis, what essentially happens is the assembly of discrete molecular units into

extended network.7 This synthetic approach allows reactions to take place at room temperature.

Generally, little is known about the M-L species preformed in solution before the assembling

process.5 MOFs are typically obtained by means of slow vapor diffusion and solvo-thermal

synthesis. In the former synthetic condition, the materials can be obtained in one-pot synthesis,

often under mild conditions. 9, 11 A solvo-thermal reaction follows a liquid nucleation model in

which a fast kinetics of nucleation and crystal growth or the reversible nature of the process can

lead to formation of good quality crystals.13,14 Heating helps lower barrier to rotation of the

linkers which can result in more stable MOF structures. Solvent switching in synthesis may

also produce novel structures.9,24

The often found solvent molecules or counter-anions in MOF cavities can act as templating

agents directing the formation of certain structural outcomes. Additionally, dimensionality of

MOFs can depend on the solvent or/and the strength of base employed to deprotonate the

organic ligands.15 For example, the use of triethylamine as the base is preferable over pyridine

in forming 3D MOF as the latter is a weaker base and a better ligand. Coordination of pyridine

to the metal centers will prevent the desired organic ligands from coordinating to the metal

centers, limiting the dimensionality of the framework.

In order for a pre-determined MOFs structure to be attained, MOFs constituents need to be

able to self-correct; that is, to assemble in a reversible manner. This is also an inherently

prominent feature of self-assembly process.

1.3 The significance of metal-organic frameworks:

As the unrenewable fossil fuel becomes increasingly depleted, hydrogen comes into play

as one of the best alternative fuels due to its highest energy density among all common fuels

(e.g. three times the energy density of gasoline).11 For hydrogen economy to be feasible, a light

6

material with high capacity is required for the safe storage of hydrogen. This remains

challenging in realizing a hydrogen economy.11

The US Department of Energy (DOE) has set a target for hydrogen storage system — 6

weight per cent and 45 kg H2 per m3 by 2010, and 9 weight per cent and 81 kg H2 per m3 by

2015. Thus far, compressed hydrogen gas and cryogenically stored liquid hydrogen are being

used by many automobile manufacturers. However, 90% of the system mass is contributed by

the containment vessels. Another significant drawback is the large amount of energy input for

condensation in the cryogenic storage. 11 Possibility of breakthrough in hydrogen storage

efficiency has therefore been pursued in chemical systems, examples of which include metal-

hydride complexes. Mg2NiH4, NaAlH4, and borohydrides of Group 1, 2, and 13 elements all

have large gravimetric capacities; however, they has all have their own drawbacks such as high

desorption temperature, requirements for catalyst for recycling, and unknown reversibility. 11

Hydrolysis of alkali metal-hydrides provides a convenient route to H2 release; however, the

decomposition products generated from the process need to be recycled off-board. Carbon

nanotubes while having low density, high surface area and good chemical stability can only

uptake a few weight per cent of H2 even under high pressure. Overall, research in H2 storage

still awaits a breakthrough material. To reduce mechanical requirements of pressurized vessels,

MOFs have been investigated for their ability to physically absorb H2 .11

Metal organic framework (MOF) is a class of microscale, crystalline inorganic- organic

hybrid that has first captured the research attention as a new candidate for hydrogen storage

materials. MOFs have many attributes, including high porosity and facile synthesis, making

them the focus of hydrogen storage research.11

1.3.1 MOFs as attractive H2 storage system:

The purpose of this section is to highlight MOFs’ applications as H2 storage systems and

through that many structural insights about MOFs will be better elucidated to readers. Many

7

MOFs have very high apparent surface area which makes them appealing H2 storage materials.

Among the reported structures, MOF-177 by Yaghi and coworker exhibits the highest uptake of

N2 among all materials to date. It has a surface area of 4500 m2/g and a pore volume of 0.69

cm3/cm3.16 This translates to an uptake of up to 7 weight per cent H2. However, measurements

for a large set of Zn4O-based MOFs revealed that only a small fraction of the surface is

occupied by the guests. This then necessitated a need for pore optimization.

Surface area and pore volume can be determined by gas absorption isotherms. All porous

MOFs so far are microporous by IUPAC definition, having pore size of less than 2 nm and

displaying type I absorption isotherm (Figure 2). The assumption taken in such measurements

is that a homogeneous monolayer of the absorbate is formed on the walls of the absorbent. The

apparent surface area is then the product of the number of molecules uptaken and an accepted

value for the area occupied by an adsorbate molecule.11

Figure 2: Type I gas absorption isotherm .17

H2 uptake mechanism has been theoretically studied by means of computation. H2 can

bind to both the metallic nodes and the organic linkers.18,19 In the Zn-based MOF being

examined by Goddard and coworkers, the former is thought to be the more favored adsorption

sites at low pressure. However, at higher pressure, H2 are preferentially adsorbed onto the

organic linkers.18 Larger aromatic linkers are therefore recommended for improved designs of

8

MOFs in H2 storage. 18 Other strategies towards improving hydrogen uptake of MOFs include

linker modification, incorporation of coordinatively unsaturated metal and lighter metals

catenation, impregnation.11

Maximal absorption of guest molecules comes with maximizing the total van der Waals

forces on the adsorbates. It is worth noting that having a large pore size does not necessarily

mean maximal storage capacity for H2. In fact, many reported MOFs to date have cavities that

are too large for effective H2 storage. For example, MOF-5 structure has a pore diameter of 15.2

Å which is much larger than the 2.89 Å kinetic diameter of H2.11 If a shorter linker isn’t used to

reduce the pore size, MOF-5 is simply too leaky to store H2. MOFs with constricted pores might

appear non-porous with absorption isotherm measurements as their small pores could not admit

N2 but allow the passage of H2. Another case where this can happen is that the as-synthesized

MOFs contain solvent molecules in the cavities that occlude entrance of other targeted guest

molecules (i.e. H2, N2, CO2 etc.).20 Decreased pore size might translate to a decrease in

gravimetric density of hydrogen uptake; however, their volumetric capacities might be

enhanced. There will always be a compromise to make along the line of MOFs structural design.

Other alternatives in optimizing pore size for H2 uptake include impregnation and

catenation. In impregnation, a non-volatile, well-anchored guest is inserted to provide additional

H2 absorptive sites. Impregnated MOF-177 with C60 molecules encapsulated within its 11.8 Å

diameter pore can provide additional surface absorption sites.

Framework catenation has long been recognized in the MOF research and is often

encountered when long linkers are used.7,11 Catenation has also been interpreted as a means of

further stabilizing the framework that relies on the support of cavity solvent molecules. In some

cases catenation can lead to undesirably nonporous networks.14 Nevertheless, some materials

have been reported to display porosity while possessing interpenetrating networks.12a

9

Catenation takes the form of interpenetration and interweaving. The former occurs when two

or more frameworks have the maximal offset from each other (Figure 3b) while the latter occurs

when the frameworks have the minimal offset and exhibit many close contacts (Figure 3c).

Interpenetration is desirable as it maximizes the exposed surface of catenated MOFs while

interweaving results in blockage of potential absorption sites.11 Depending upon the degree of

penetration, the resulting framework still can exhibit void space for selectively trapping of small

molecules as demonstrated in this thesis. Still, there is not yet a control over formation of non-

penetrating over penetrating framework.

Figure 3: Interpenetration and interweaving MOFs a) repeating unit of MOFs with SBU shown

as cubes and linkers as rods; b) interpenetrating frameworks; c) interweaving framework.11

1.3.2 MOFs as catalysts:

Application of MOFs in this domain has been met with limited success, mainly because the

metals in MOFs are coordinatively saturated. Nevertheless, useful catalytic systems have been

achieved with MOFs in the catalysis of cyanosilylation of aldehydes20a, hydrogenation of

nitroaromatics, and oxidation of alkylphenylsulfides etc.20b Hill and coworkers have recently

reported a redox active vanadium- and terbium-containing MOF that can catalyze the oxidation

of PrSH to PrSSRPr using only ambient air under mild conditions. 21

10

1.3.3 MOFs functioning in chromatography:

The petroleum industry relies on the important process of separating linear from

branched alkanes in order to boost octane ratings of gasoline. MOFs can be useful for such

purposes due to their selective sorption and high thermal stability.15,22 Good GC separation of

mixture of linear and branched alkanes has been reported when MOF-508 was used as the

stationary phase.22

Overall, MOF applications originate from their abilities to function as host-guest

systems.

1.4 Luminescence in MOFs:

Luminescence is a light emitting phenomenon from any emissive substance. It involves

electronic transitions from excited states to the ground state. Luminescence can be classified

into fluorescence and phosphorescence depending on the nature of the excited state (i.e. singlet

vs. triplet excited state.23 Supramolecular luminescent materials are mostly small organic and

coordination compounds. The most important application of the latter is their use as sensors and

light-emitting diodes (LEDs). Luminescent materials then can be classified based on the origin

of emission; i.e. whether it is ligand-based or metal-based emission. Luminescent MOFs can

also be devised that way by incorporate the emissive lanthanide metal ions. For the scope of

this research, it is most relevant to discuss the single metal center-based emission. Many

lanthanide ions with a partially filled 4f shell are emissive due to f-f transitions. Depending on

the lanthanide ions, blue, red (Eu) or green (Tb) emission can be seen. f-f transitions are Laporte

forbidden rendering lanthanide luminescence rather weak (ε <10 M-1 cm-1). In order to enhance

luminescence, a suitable ligand can be chelated to the lanthanide ion. The role of the ligand is to

harvest photon energy and effectively transfer that energy to the emissive metal centers. Such

phenomenon is often termed the antenna effect and is describe in Figure 4.

11

Ligand based luminescence often occurs in organic molecules and coordination

compound. It involves the electronic transitions of the ligands.23 Many luminescent organic

molecules contain aromatic moieties. Both of the organic ligands employed in current study are

luminescent and are examples of such molecules. Many coordination compounds luminesce

because of the electronic transition of the ligands only. Often, formation of complex involving

luminescent ligand enhances the emission of the resulting complex as compared to that of free

ligands. The main reason rests in the ability of the ligands to make more rigid structures upon

chelating. Quite a number of Zn (II) complexes, including the Zn framework generated from this

study, are luminescent.5,23,24

The combination of organic ligands and metal center in MOF is seen as a method for

creating new types of electroluminescent materials for potential LEDs applications.5

The combination of organic ligands and metal center in MOF is seen as a method for creating

new types of electroluminescent materials for potential LEDs applications.5

1.5 Scope of the Thesis:

The goal of the research is to arrive at multidimensional, highly porous and functional

MOFs via hierarchical assembly of smaller molecular building blocks (Figure 5) and, at the

same time, to examine the possibilities for different interesting molecular textures. More

Figure 4: Antenna effect in lanthanide luminescence

Antenna

hv Luminescence

Excitation

Energy transfer

Eu (III)

12

specifically, a dinuclear Zn complex was created by using oxygen-donor, linearly bridging

ligand (i.e. LigAH2). Capping ligand such as phenanthroline was also purposely introduced in

creating such complex as it provides a mean for subsequent control of extra dimensional

synthesis using such dinuclear Zn as the building blocks. The attained Zn-based motifs can then

be polymerize into 1D coordination polymer by replacing the capping ligand with another

equivalent of the bridging ligand. Altering the choice of ligand as well as metal in the syntheses

can lead to the formation of new structures. Utilizing similarly linear but more rigid carboxylate

bridging ligand in combination with Zn whose maximal coordination number is well-known, a

novel 2D MOF structure was attained. This resulting 2D framework can then be further

extended into 3D by coordinating the newly chosen rigid ligand to metals having high

coordination number such as lanthanides. Overall, the strategy taken is very much a stepwise,

hierarchical assembly of small building blocks into larger, multidimensional frameworks. Such

strategy has been proven to be successful as different molecular architectures have been

obtained including luminescent 2D and 3D MOFs in the current research.

M =

n

Lig =

0-D 1-D 2-D 3-D

Figure 5: Schematic representation of research objective.

13

Luminescent MOFs are of important applications. Despite a great numbers of MOFs in

the literature, reports on luminescent MOFs are still scarce and their applications are still

extremely modest.5,24 In this class, MOFs exhibiting metal-based luminescence are outnumbered

those displaying ligand-based emission.24 This study will demonstrate the synthesis and

emissive properties of luminescent MOFs belonging to both categories. Porous MOFs such as

those obtained from current study can potentially be used as host-guest systems for detection of

pollutants using their inherent luminescent property as the trigger.

14

CHAPTER II

EXPERIMENTAL SECTION

General Remarks. Unless otherwise stated, all the chemicals were purchased and used without

further purification. Acetylacetone, DMSO, DBU, water, trimethylsilylacetylene were degassed

prior to reaction.

Physical characterization. Thermogravimetric analysis (TGA) was performed on a SDT Q600

unit under N2 at a heating rate of 3oC or 5oC/min (vide infra). Fluorescence spectra were

recorded on a Fluorolog-3 Jobin Yvon Horiba luminescence spectrometer equipped with a

450W xenon lamp as the excitation source. UV-vis absorption measurement was performed on

an Agilent 8453 single-beam diode-array spectrophotometer. Elemental analyses were

performed on a Perkin Elmer 2400 Series II CHN/S Elemental Analyzer at the Department of

Chemistry, University of Toronto. All samples were dried in vacuum prior to analysis. 1H-NMR

spectra were recorded on a Varian 300, 400 MHz and 13C-NMR spectra on 100 MHz NMR

spectrometer. Chemical shifts are reported in ppm relative to an internal standard of TMS.

2.1 Synthesis of 3,3'-(biphenyl-4,4'-diyl)dipentane-2,4-dione (LigAH2): followed modified

procedure reported by Jiang, Y. et al. 25

A mixture of K2CO3 (8 equiv, 13,6g), CuI (20mol%, 0.47g), and L-proline (40mol%,

0.5671g) in 50 ml DMSO were added to an argon-purged two-neck-Schlenk flask followed by

slow addition of 4,4’-diiodobenzene in 120 ml THF (5g, 0.0123mol). With good stirring, the

resulting mixture was refluxed at 90oC under N2 atmosphere for 60 hrs. The cooled solution was

acidified and extracted with DCM. The combined organic layer was washed with water, brine

(3x200ml); dried over Na2SO4 and concentrated. The residue was chromatographed (8:1

Hexane: EtOAc) to afford 3,3'-(biphenyl-4,4'-diyl)dipentane-2,4-dione (or LigAH2) in 38 %

(1.6 g) yield. 1H-NMR (400MHz, CDCl3) δ 16.7 (s, 2H), 7.66 (d, 4H), 7.27 (d, 4H), 1.94 (s,

15

12H); 13C-NMR (100 MHz, CDCl3) δ 191.1, 139.7, 136.4, 131.8, 127.5, 114.9, 24.4 (Appendix

for spectra). Anal. Calcd. for C22H22O4 (350.41): C, 75.41; H, 6.33. Found: C, 75.57; H, 6.14.

2.2 Synthesis of 4,4'-(ethyne-1,2-diyl)dibenzoic acid (LigBH2): according to previously

reported procedure 4, 26

A 5-g reaction gave 76.6% LigBH2. The product is insoluble in common organic

solvents except DMSO. 1H NMR (300MHz, DMSO-d6) δ 13.19 (s, 2H), 7.99 (d, J = 8.4Hz,

4H), 7.71 (d, J = 8.4Hz, 4H); 13C-NMR (100MHz, CDCl3) δ 166.7, 131.8, 131.0, 129.6, 126.1,

91.1. Anal. Calcd. for C16H10O4·0.25H2O: C, 70.91; H, 3.89. Found: C, 71.11; H, 3.91.

2.3 Synthesis of Zn2(LigA)(phen)2(OAc)2, 1

A mixture of Zn(OAc)2.2H2O (0.0251g, 0.144 mmol), LigAH2 (0.0401g, 0.144mmol)

and phenanthroline (0.0206g, 0.144mmol) was dissolved in 6 mL of CH2Cl2/MeOH (1:1 by

volume) and stirred at ambient temperature for 2 hrs. After the solvents were removed in vacuo,

the residual solid was recrystallized by diffusing hexanes into a CH2Cl2 solution to afford the

title compound as colorless crystals.

2.4 Synthesis of [Zn(LigA)(DMSO)2(CH3OH)]n , 2

To a solution of Zn(OAc)2.2H2O (0.0210g, 0.0958mmol) in 1.6 mL of methanol was

added a solution of LigAH2 (0.0504g, 0.1438mmol) in 2.4 mL CHCl3. The resulting mixture

was filtered through Celite into a small vial which was then inserted into a bigger vial

containing a 0.28 M solution of Et3N in MeOH/CHCl3 (1:1 by volume). After several days, X-

ray quality crystals can be collected. The reaction is reproducible; however, every time an

unidentified insoluble solid formed together with the crystals.

2.5 Synthesis [Zn2(LigB)2(DMSO)2]·2MeOH, 3

A solution of Zn(NO3)2·6H2O (0.0168g, 0.0563mmol) in 3 mL of MeOH was mixed

with a solution of LigBH2 (0.0151g, 0.0564mmol) in 2 mL of DMSO. (Note: the DMSO

16

solution of LigBH2 was sonicated and heated slightly to improve the solubility of LigBH2.) The

resulting mixture was filtered through Celite into a small vial which was then inserted into a

bigger vial containing a 0.015 M solution of Et3N in MeOH/DMSO (1:1 by volume).The title

compound was collected as stable yellow, opaque crystals (16% yield) after several days.

Unidentified white needles were formed as a bi-product. Anal. Calcd. for

[Zn2C36H28O10S2]·2CH3OH: C, 51.88; H, 4.13. Found: C, 51.37; H, 3.46.

2.6 Synthesis of [Eu2(LigB)3(DMSO)2(MeOH)2]·(DMSO)2(H2O)3, 4

A solution of EuCl3.6H2O (0.055g, 0.150mmol) in 4 mL of MeOH was mixed with a

solution of LigBH2 (0.08g, 0.3mmol) in 12 mL of DMSO. The resulting mixture was filtered

through Celite. Vapor diffusion (similar to Section 2.5) using 0.015M Et3N in MeOH/DMSO

(1:1 by volume) was then performed. After 17d, single crystals of 4 were collected (0.055 g,

48%). The title compound is fully reproducible. However, an unidentified white, opaque solid

always formed as a bi-product. Anal. Calcd. for C58H60O20S4Eu2 (formulated as 4-H2O due to

partial solvent loss under vacuum): C, 46.16; H, 4.01. Found: C, 46.0; H, 3.70.

2.7 Synthesis of [Tb2(LigB)3(DMSO)2(MeOH)2].(DMSO)2(H2O)3, 5

Procedure for synthesis of 5 is similar to that of 4 except that EuCl3.6H2O was replaced

by TbCl3.6H2O. After 2 weeks, single crystals of 5 were be collected in a 42% yield (80 mg).

This synthesis showed good reproducibility. However, an unidentified white, opaque solid was

always formed as a bi-product. Anal. Calcd. [Tb2C58H60O20S4] (formulated as 5-H2O, due to

partial solvent loss under vacuum): C,45.74 ;H, 3.97. Found: C, 45.61; H, 3.51

17

X-ray crystallography.

Dif

frac

tion

da

ta

for

crys

tals

of

co

mpl

exes

1-5

wer

e co

llec

ted

at

150

K

on

a N

oniu

s K

appa

C

CD

diff

ract

omet

er w

ith

Mo

Kα

rad

iati

on (λ

= 0

.710

73 Å

), o

pera

ting

at

50kV

and

30

mA

.. T

he d

ata

wer

e in

tegr

ated

and

sca

led

usin

g th

e D

enso

-

SM

N p

acka

ge.

All

str

uctu

res

wer

e so

lved

usi

ng t

he d

irec

t m

etho

ds a

nd r

efin

ed b

y fu

ll-m

atri

x le

ast-

squa

res

proc

edur

es o

n F

2 usi

ng

SH

EL

XT

L 6

.10.

The

cry

stal

logr

aphi

c da

ta o

f 3-5

are

sum

mar

ized

in

Tab

le 1

. Sel

ecte

d bo

nd l

engt

hs a

nd a

ngle

s of

1-5

are

pre

sent

ed i

n T

able

2 to

Tab

le 6

, res

pect

ivel

y

Table 1: Crystallographic data for compounds 1-5

1

2 3

4 5

For

mul

a C

51H

43C

l 3N

4O8Z

n 2

C24

H25

Cl 3

O5Z

n C

19H

18O

6SZ

n C

58H

62O

21S

4Eu 2

C

58 H

62O

21S

4Tb 2

F

W

1076

.98

565.

16

439.

76

1525

.26

1541

.16

Tem

p (K

) 15

0(2)

K

150(

2)

150(

2) K

15

0(2)

15

0(2)

K

λ (Å

) 0.

7107

3 0.

7107

3 0.

7107

3 0.

7107

3

0.71

073

Cry

st. S

ys.

Mon

ocli

nic

Ort

horh

ombi

c O

rtho

rhom

bic

Mon

ocli

nic

Mon

ocli

nic

Spa

ce G

rp.

C2/

c P

na2 1

P

bca

P2 1

/c

P2 1

/c

a 27

.567

3(15

) 18

.010

(4)

15.8

816(

4) Å

10

.429

9(2)

10

.445

6(3)

b

12.5

710(

7)

16.7

93 (

3)

9.29

74(2

) Å

13

.521

9(3)

13

.531

2(4)

c

16.5

027(

10)

16.6

28 (

3)

25.2

870(

7) Å

31

.664

2(6)

31

.575

1(10

) α

(de

g.)

90

90

90

90

90

β (

deg.

) 10

4.38

3(3)

90

90

97

.620

8(13

) 97

.641

5(15

) γ

(deg

.)

90

90

90

90

90

Z

4 8

8 2

2

V(Å

) 55

39.7

(5)

5029

.0(1

7)

3733

.82(

16)

4426

.22(

15)

44

23.2

(2)

ρ(ca

lc.)

Mg/

m3

1.29

1 1.

493

1.56

5

1.14

4 1.

157

Abs

Coe

ff. (

mm

-1)

1.06

2 1.

328

1.46

1

1.55

1 1.

732

F(0

00)

2208

23

20

1808

15

32

1544

θ

ran

ge (

deg.

) 2.

96 t

o 25

.00

2.94

to

27.0

0 3.

03 t

o 27

.46

2.92

to

25.0

0 2.

99 t

o 25

.00

18

Table 1: Crystallographic data for compounds 1-5 (Cont.)

1

2 3

4 5

For

mul

a C

51H

43C

l 3N

4O8Z

n 2

C24

H25

Cl 3

O5Z

n C

19H

18O

6SZ

n C

58H

62O

21S

4Eu 2

C

58 H

62O

21S

4Tb 2

Ref

lns

coll

ecte

d 13

070

1997

2 28

751

3018

3 17

823

No.

of

uniq

ue r

efln

s 48

55

8392

42

53

7787

75

19

GO

F o

f F

2

1.01

9 1.

022

1.06

7 1.

044

1.06

9 R

1,w

R2 [I

>2σ

(I)]

0.

0928

, 0.2

560

0.09

40, 0

.228

2 0.

0545

, 0.1

373

0.07

76, 0

.229

9 0.

0767

, 0.

2245

R

1, w

R2

all

data

0.

1831

, 0.3

284

0.19

01, 0

.298

7 0.

0908

, 0.1

557

0.10

45, 0

.258

9 0.

1016

, 0.2

507

19

Table 2: Selected bond lengths (Å) and angles (deg) of numbering atoms in 1

Zn(1)-O(1) 1.981(5)

Zn(1)-O(2) 2.017(6)

Zn(1)-O(3) 2.058(8)

Zn(1)-N(2) 2.103(7)

Zn(1)-N(1) 2.162(6)

O(1)-C(16) 1.275(8)

O(2)-C(14) 1.262(10)

O(3)-C(24) 1.213(11)

O(4)-C(24) 1.191(11)

N(1)-C(1) 1.259(10)

N(1)-C(12) 1.355(9)

N(2)-C(10) 1.344(10)

N(2)-C(11) 1.363(10)

C(1)-C(2) 1.480(12)

O(1)-Zn(1)-O(2) 87.7(2)

O(1)-Zn(1)-O(3) 145.2(3)

O(2)-Zn(1)-O(3) 94.7(3)

O(1)-Zn(1)-N(2) 120.6(2)

O(2)-Zn(1)-N(2) 91.2(3)

O(3)-Zn(1)-N(2) 94.1(3)

O(3)-Zn(1)-N(2) 94.1(3)

O(1)-Zn(1)-N(1) 89.5(2)

O(2)-Zn(1)-N(1) 165.6(3)

O(3)-Zn(1)-N(1) 95.7(3)

N(2)-Zn(1)-N(1) 78.1(3)

C(16)-O(1)-Zn(1) 128.7(5)

C(14)-O(2)-Zn(1) 127.8(6)

C(24)-O(3)-Zn(1) 108.4(8)

C(1)-N(1)-C(12) 119.1(7)

C(1)-N(1)-Zn(1) 127.6(5)

C(12)-N(1)-Zn(1) 112.9(5)

C(10)-N(2)-C(11) 116.6(8)

C(10)-N(2)-Zn(1) 129.1(7)

C(11)-N(2)-Zn(1) 113.9(5)

N(1)-C(1)-C(2) 123.2(8)

C(3)-C(2)-C(1) 116.2(9)

Table 3: Selected bond lengths (Å) and angles (deg) of numbering atoms in 2

Zn(1)-O(4) 1.978(10)

Zn(1)-O(1) 2.014(10)

Zn(1)-O(3) 2.028(10)

Zn(1)-O(2) 2.029(9)

Zn(1)-O(5) 2.032(12)

Zn(2)-O(6) 1.949(10)

Zn(2)-O(7) 1.977(10)

Zn(2)-O(9) 1.991(10)

19

Zn(2)-O(8) 2.013(11)

Zn(2)-O(10) 2.045(13)

C(23)-O(5) 1.411(18)

O(7)-C(27) 1.249(16)

O(4)-C(15) 1.275(18)

O(1)-C(2) 1.280(17)

O(8)-C(36) 1.254(17)

O(6)-C(25) 1.304(17)

O(3)-C(13) 1.254(17)

C(46)-O(10) 1.36(3)

O(2)-C(4) 1.294(16)

C(26)-C(25) 1.409(18)

C(20)-C(9)#1 1.51(2)

O(9)-C(38) 1.282(17)

O(4)-Zn(1)-O(1) 152.4(5)

O(4)-Zn(1)-O(2) 86.5(4)

O(1)-Zn(1)-O(2) 88.3(4)

O(4)-Zn(1)-O(3) 88.5(4)

O(1)-Zn(1)-O(3) 88.6(4)

O(2)-Zn(1)-O(3) 162.9(5)

O(4)-Zn(1)-O(5) 108.9(5)

O(1)-Zn(1)-O(5) 98.7(5)

O(2)-Zn(1)-O(5) 101.1(5)

O(3)-Zn(1)-O(5) 95.9(4)

O(6)-Zn(2)-O(7) 86.6(4)

O(6)-Zn(2)-O(9) 150.4(5)

O(7)-Zn(2)-O(9) 86.0(5)

O(6)-Zn(2)-O(8) 90.5(5)

O(7)-Zn(2)-O(8) 159.6(5)

O(9)-Zn(2)-O(8) 86.6(4)

O(6)-Zn(2)-O(10) 104.9(5)

O(7)-Zn(2)-O(10) 105.2(5)

O(9)-Zn(2)-O(10) 104.7(5)

O(8)-Zn(2)-O(10) 95.1(5)

C(27)-O(7)-Zn(2) 130.3(10)

C(23)-O(5)-Zn(1) 129.9(9)

C(15)-O(4)-Zn(1) 126.0(8)

C(2)-O(1)-Zn(1) 125.7(10)

C(36)-O(8)-Zn(2) 127.0(9)

C(25)-O(6)-Zn(2) 131.9(9)

C(13)-O(3)-Zn(1) 128.7(10)

C(46)-O(10)-Zn(2) 123.8(13)

C(4)-O(2)-Zn(1) 129.2(9)

C(25)-C(26)-C(27) 121.9(14)

C(25)-C(26)-C(29) 114.3(12)

C(38)-O(9)-Zn(2) 128.6(9)

O(4)-C(15)-C(14) 127.0(12)

O(4)-C(15)-C(16) 111.4(13)

O(7)-C(27)-C(26) 125.7(13)

20

O(7)-C(27)-C(28) 118.7(13)

C(26)-C(27)-C(28) 115.4(14)

C(22)-C(21)-C(20) 121.4(18)

O(6)-C(25)-C(26) 122.0(11)

O(6)-C(25)-C(24) 117.3(13)

C(26)-C(25)-C(24) 120.7(14)

O(3)-C(13)-C(12) 117.5(14)

O(3)-C(13)-C(14) 121.3(15)

C(12)-C(13)-C(14) 121.2(14)

O(8)-C(36)-C(37) 126.0(13)

O(8)-C(36)-C(35) 115.1(13)

C(37)-C(36)-C(35) 118.9(14)

O(1)-C(2)-C(3) 127.7(15)

O(1)-C(2)-C(1) 110.7(14)

C(3)-C(2)-C(1) 121.6(14)

O(2)-C(4)-C(3) 123.0(14)

O(2)-C(4)-C(5) 115.4(12)

O(9)-C(38)-C(37) 123.3(13)

O(9)-C(38)-C(39) 115.7(13)

Table 4. Selected bond lengths (Å) and angles (deg) of numbering atoms in 3

Zn(1)-O(5) 1.984(3)

Zn(1)-O(3) 2.007(3)

Zn(1)-O(4)#1 2.046(3)

Zn(1)-O(2)#1 2.059(3)

Zn(1)-O(1) 2.073(3)

Zn(1)-Zn(1)#1 2.9786(9)

O(5)-Zn(1)-O(3) 103.57(13)

O(5)-Zn(1)-O(4)#1 97.98(12)

O(3)-Zn(1)-O(4)#1 158.35(13)

O(5)-Zn(1)-O(2)#1 99.23(13)

O(3)-Zn(1)-O(2)#1 90.70(13)

O(4)#1-Zn(1)-O(2)#1 87.92(12)

O(5)-Zn(1)-O(1) 100.95(13)

O(3)-Zn(1)-O(1) 89.86(13)

O(4)#1-Zn(1)-O(1) 83.93(13)

O(2)#1-Zn(1)-O(1) 159.09(13)

O(5)-Zn(1)-Zn(1)#1 170.28(9)

O(3)-Zn(1)-Zn(1)#1 86.15(9)

O(4)#1-Zn(1)-Zn(1)#1 72.31(9)

O(2)#1-Zn(1)-Zn(1)#1 80.30(9)

O(1)-Zn(1)-Zn(1)#1 78.88(9)

C(1)-O(1)-Zn(1) 127.9(3)

C(1)-O(2)-Zn(1)#1 127.2(3)

C(9)-O(3)-Zn(1) 119.4(3)

C(9)-O(4)-Zn(1)#1 136.3(3)

S(1)-O(5)-Zn(1) 121.99(17)

21

Table 5. Selected bond lengths (Å) and angles (deg) for 4

Eu(1)-O(5)#1 2.349(5)

Eu(1)-O(1) 2.391(4)

Eu(1)-O(8) 2.398(5)

Eu(1)-O(2)#1 2.403(4)

Eu(1)-O(6) 2.415(4)

Eu(1)-O(4) 2.422(4)

Eu(1)-O(7) 2.438(6)

Eu(1)-O(3) 2.582(4)

Eu(1)-O(5) 2.809(4)

Eu(1)-C(9) 2.843(6)

Eu(1)-C(17) 2.947(6)

Eu(1)-Eu(1)#1 4.1381(6)

S(1)-O(8) 1.513(6)

S(1)-C(26) 1.546(15)

S(1)-C(27A) 1.68(2)

S(1)-C(27B) 1.832(18)

O(1)-C(1) 1.260(8)

O(2)-C(1) 1.246(8)

O(2)-Eu(1)#1 2.403(4)

O(3)-C(9) 1.260(8)

O(4)-C(9) 1.257(8)

O(5)-C(17) 1.264(8)

O(5)-Eu(1)#1 2.349(5)

O(6)-C(17) 1.262(8)

O(7)-C(25) 1.369(9)

O(5)#1-Eu(1)-O(1) 73.37(15)

O(5)#1-Eu(1)-O(8) 148.09(16)

O(1)-Eu(1)-O(8) 138.46(16)

O(5)#1-Eu(1)-O(2)#1 77.95(16)

O(1)-Eu(1)-O(2)#1 130.56(16)

O(8)-Eu(1)-O(2)#1 78.88(16)

O(5)#1-Eu(1)-O(6) 123.21(15)

O(1)-Eu(1)-O(6) 81.86(15)

O(8)-Eu(1)-O(6) 74.01(17)

O(2)#1-Eu(1)-O(6) 81.35(15)

O(5)#1-Eu(1)-O(4) 89.34(16)

O(1)-Eu(1)-O(4) 75.47(16)

O(8)-Eu(1)-O(4) 96.58(18)

O(2)#1-Eu(1)-O(4) 143.66(16)

O(6)-Eu(1)-O(4) 132.53(15)

O(5)#1-Eu(1)-O(7) 78.13(18)

O(1)-Eu(1)-O(7) 134.79(18)

O(8)-Eu(1)-O(7) 74.7(2)

O(2)#1-Eu(1)-O(7) 74.29(18)

O(6)-Eu(1)-O(7) 143.33(18)

O(4)-Eu(1)-O(7) 69.79(16)

22

O(5)#1-Eu(1)-O(3) 131.35(15)

O(1)-Eu(1)-O(3) 69.46(15)

O(8)-Eu(1)-O(3) 73.76(15)

O(2)#1-Eu(1)-O(3) 150.69(17)

O(6)-Eu(1)-O(3) 81.24(15)

O(4)-Eu(1)-O(3) 51.90(15)

O(7)-Eu(1)-O(3) 107.64(19)

O(5)#1-Eu(1)-O(5) 73.65(14)

O(1)-Eu(1)-O(5) 66.78(14)

O(8)-Eu(1)-O(5) 116.17(15)

O(2)#1-Eu(1)-O(5) 66.93(14)

O(6)-Eu(1)-O(5) 49.59(15)

O(4)-Eu(1)-O(5) 141.59(16)

O(7)-Eu(1)-O(5) 135.68(17)

O(3)-Eu(1)-O(5) 116.67(14)

O(5)#1-Eu(1)-C(9) 109.00(18)

O(1)-Eu(1)-C(9) 66.90(17)

O(8)-Eu(1)-C(9) 87.99(18)

O(2)#1-Eu(1)-C(9) 162.35(18)

O(6)-Eu(1)-C(9) 106.48(18)

O(4)-Eu(1)-C(9) 26.07(17)

O(7)-Eu(1)-C(9) 91.0(2)

O(3)-Eu(1)-C(9) 26.29(17)

O(5)-Eu(1)-C(9) 130.23(16)

O(5)#1-Eu(1)-C(17) 98.77(19)

O(1)-Eu(1)-C(17) 76.03(17)

O(8)-Eu(1)-C(17) 93.3(2)

O(2)#1-Eu(1)-C(17) 69.56(17)

O(6)-Eu(1)-C(17) 24.77(18)

O(4)-Eu(1)-C(17) 146.65(16)

O(7)-Eu(1)-C(17) 143.51(18)

O(3)-Eu(1)-C(17) 101.48(18)

O(5)-Eu(1)-C(17) 25.23(17)

C(9)-Eu(1)-C(17) 123.50(19)

O(5)#1-Eu(1)-Eu(1)#1 40.65(10)

O(1)-Eu(1)-Eu(1)#1 64.53(11)

O(8)-Eu(1)-Eu(1)#1 141.38(11)

O(2)#1-Eu(1)-Eu(1)#1 67.39(12)

O(6)-Eu(1)-Eu(1)#1 82.58(12)

O(4)-Eu(1)-Eu(1)#1 121.69(14)

O(7)-Eu(1)-Eu(1)#1 111.65(16)

O(3)-Eu(1)-Eu(1)#1 132.82(11)

O(5)-Eu(1)-Eu(1)#1 33.00(9)

C(9)-Eu(1)-Eu(1)#1 128.55(14)

C(17)-Eu(1)-Eu(1)#1 58.16(16)

O(8)-S(1)-C(26) 103.3(7)

O(8)-S(1)-C(27A) 103.9(9)

C(26)-S(1)-C(27A) 97.5(11)

O(8)-S(1)-C(27B) 101.7(6)

C(26)-S(1)-C(27B) 95.7(8)

23

C(27A)-S(1)-C(27B) 147.5(10)

C(1)-O(1)-Eu(1) 141.9(4)

C(1)-O(2)-Eu(1)#1 135.2(4)

C(9)-O(3)-Eu(1) 88.5(4)

C(9)-O(4)-Eu(1) 96.1(4)

C(17)-O(5)-Eu(1)#1 168.7(4)

C(17)-O(5)-Eu(1) 83.5(4)

Eu(1)#1-O(5)-Eu(1) 106.35(14)

C(17)-O(6)-Eu(1) 101.9(4)

C(25)-O(7)-Eu(1) 131.7(5)

S(1)-O(8)-Eu(1) 131.2(3)

O(2)-C(1)-O(1) 126.0(5)

O(2)-C(1)-C(2) 118.9(6)

O(1)-C(1)-C(2) 115.1(6)

O(4)-C(9)-O(3) 121.4(6)

O(4)-C(9)-C(10) 117.3(6)

O(3)-C(9)-C(10) 121.3(6)

O(4)-C(9)-Eu(1) 57.9(3)

O(3)-C(9)-Eu(1) 65.2(3)

C(10)-C(9)-Eu(1) 164.5(5)

O(6)-C(17)-O(5) 123.0(6)

O(6)-C(17)-C(18) 118.4(7)

O(5)-C(17)-C(18) 118.7(7)

O(6)-C(17)-Eu(1) 53.3(3)

O(5)-C(17)-Eu(1) 71.3(3)

C(18)-C(17)-Eu(1) 164.3(5)

Table 6. Selected bond lengths (Å) and angles (deg) for 5

Tb(1)-O(4)#1 2.303(5)

Tb(1)-O(5) 2.361(4)

Tb(1)-O(8) 2.364(5)

Tb(1)-O(6) 2.377(4)

Tb(1)-O(3) 2.382(4)

Tb(1)-O(1) 2.396(4)

Tb(1)-O(7) 2.422(6)

Tb(1)-O(2) 2.575(5)

Tb(1)-C(1) 2.823(7)

Tb(1)-O(4) 2.869(5)

Tb(1)-C(9) 2.957(7)

S(1)-C(26A) 1.43(3)

S(1)-O(8) 1.525(6)

S(1)-C(27) 1.713(12)

S(1)-C(26B) 1.850(18)

24

O(1)-C(1) 1.251(8)

O(2)-C(1) 1.256(8)

O(3)-C(9) 1.279(9)

O(4)-C(9) 1.250(9)

O(4)-Tb(1)#1 2.303(5)

O(5)-C(17) 1.246(8)

O(6)-C(17)#1 1.276(8)

O(7)-C(25B) 1.354(9)

O(4)#1-Tb(1)-O(5) 73.52(16)

O(4)#1-Tb(1)-O(8) 147.96(18)

O(5)-Tb(1)-O(8) 138.38(17)

O(4)#1-Tb(1)-O(6) 78.93(17)

O(5)-Tb(1)-O(6) 130.36(16)

O(8)-Tb(1)-O(6) 78.69(17)

O(4)#1-Tb(1)-O(3) 123.32(18)

O(5)-Tb(1)-O(3) 81.55(16)

O(8)-Tb(1)-O(3) 74.62(19)

O(6)-Tb(1)-O(3) 80.28(17)

O(4)#1-Tb(1)-O(1) 87.96(18)

O(5)-Tb(1)-O(1) 75.43(18)

O(8)-Tb(1)-O(1) 97.0(2)

O(6)-Tb(1)-O(1) 144.05(17)

O(3)-Tb(1)-O(1) 133.49(17)

O(4)#1-Tb(1)-O(7) 78.03(19)

O(5)-Tb(1)-O(7) 135.6(2)

O(8)-Tb(1)-O(7) 74.1(2)

O(6)-Tb(1)-O(7) 74.48(19)

O(3)-Tb(1)-O(7) 142.9(2)

O(1)-Tb(1)-O(7) 70.08(18)

O(4)#1-Tb(1)-O(2) 130.73(16)

O(5)-Tb(1)-O(2) 69.65(16)

O(8)-Tb(1)-O(2) 73.64(17)

O(6)-Tb(1)-O(2) 150.33(17)

O(3)-Tb(1)-O(2) 82.23(17)

O(1)-Tb(1)-O(2) 52.05(16)

O(7)-Tb(1)-O(2) 107.3(2)

O(4)#1-Tb(1)-C(1) 107.91(19)

O(5)-Tb(1)-C(1) 66.90(18)

O(8)-Tb(1)-C(1) 88.21(19)

O(6)-Tb(1)-C(1) 162.60(19)

O(3)-Tb(1)-C(1) 107.4(2)

O(1)-Tb(1)-C(1) 26.12(18)

O(7)-Tb(1)-C(1) 91.0(2)

O(2)-Tb(1)-C(1) 26.41(18)

O(4)#1-Tb(1)-O(4) 74.17(15)

O(5)-Tb(1)-O(4) 66.28(15)

O(8)-Tb(1)-O(4) 116.71(16)

O(6)-Tb(1)-O(4) 66.88(15)

O(3)-Tb(1)-O(4) 49.18(17)

25

O(1)-Tb(1)-O(4) 140.87(17)

O(7)-Tb(1)-O(4) 135.71(18)

O(2)-Tb(1)-O(4) 116.98(15)

C(1)-Tb(1)-O(4) 130.04(17)

O(4)#1-Tb(1)-C(9) 98.8(2)

O(5)-Tb(1)-C(9) 75.45(18)

O(8)-Tb(1)-C(9) 94.2(2)

O(6)-Tb(1)-C(9) 68.91(18)

O(3)-Tb(1)-C(9) 24.85(19)

O(1)-Tb(1)-C(9) 146.78(18)

O(7)-Tb(1)-C(9) 143.1(2)

O(2)-Tb(1)-C(9) 102.36(18)

C(1)-Tb(1)-C(9) 124.1(2)

O(4)-Tb(1)-C(9) 24.72(18)

C(26A)-S(1)-O(8) 91(2)

C(26A)-S(1)-C(27) 87.2(13)

O(8)-S(1)-C(27) 106.0(5)

C(26A)-S(1)-C(26B) 164(3)

O(8)-S(1)-C(26B) 101.7(6)

C(27)-S(1)-C(26B) 98.5(8)

C(1)-O(1)-Tb(1) 96.4(4)

C(1)-O(2)-Tb(1) 87.9(4)

C(9)-O(3)-Tb(1) 103.6(4)

C(9)-O(4)-Tb(1)#1 170.6(5)

C(9)-O(4)-Tb(1) 81.6(4)

Tb(1)#1-O(4)-Tb(1) 105.83(15)

C(17)-O(5)-Tb(1) 142.4(4)

C(17)#1-O(6)-Tb(1) 133.3(4)

C(25B)-O(7)-Tb(1) 132.3(6)

S(1)-O(8)-Tb(1) 130.7(3)

O(1)-C(1)-O(2) 121.4(6)

O(1)-C(1)-C(2) 117.6(6)

O(2)-C(1)-C(2) 120.9(7)

O(1)-C(1)-Tb(1) 57.5(3)

O(2)-C(1)-Tb(1) 65.7(4)

C(2)-C(1)-Tb(1) 165.7(5)

O(4)-C(9)-O(3) 123.6(6)

O(4)-C(9)-C(10) 118.7(7)

O(3)-C(9)-C(10) 117.6(6)

O(4)-C(9)-Tb(1) 73.7(4)

O(3)-C(9)-Tb(1) 51.5(3)

C(10)-C(9)-Tb(1) 163.7(5)

26

CHAPTER III

RESULTS AND DISCUSSION

Our interest in LigAH2 stemmed from its highly conjugated aromatic backbone which

can impart rigidity on resulting frameworks. Although LigAH2 was implied as an intermediate

in the literature,27,28 to the best of our knowledge it has not been documented in the design of

MOFs. Copper(I) catalyzed Ullmann coupling of 4,4’-diiodobiphenyl with excess amount of

acetylacetone yields the desired LigAH2 in 40% yield (Scheme 1). Synthetic protocol employed

in the current study was modified from previously reported procedure by Jiang et. al.25

O

O

O

O

H H

I

I

O O

+ K2CO3, 10% mol CuI, 20% mol L-proline

THF/DMSO, 90oC

Scheme 1: Synthesis of LigAH2 via Ullmann coupling

Figure 6: Crystal structure of LigAH2 with the acidic hydrogen shown (in pink) being chelated

by the acac moiety.

Crystal structure indicates that LigAH2 exists in the enol form in the solid states with the

exchangeable proton shared in between two oxygen atoms (Figure 6). The dihedral angle

between an acac plane and the adjacent phenyl plane is ~79º.

27

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

190 240 290 340Wavelength(nm)

Absorb

ance

LigAH2

LigA2-

Figure 7: UV-vis absorption profile of LigAH2 (dash line) and sodium salt of LigA2- (solid line)

UV-vis measurement of LigAH2 (Figure 7) indicates the free ligand has a broad band

absorption with the maximum at 275nm in CHCl3. The free LigAH2 is poorly soluble in MeOH;

however, its sodium salt is. Deprotonation of LigAH2 shifts the absorption maximum at 280nm

towards longer wavelength and also lowers its intensity of absorption.

3.1 Construction of dinuclear Zn(II) complexes of LigA2-.

Slow diffusion of hexanes into DCM solution containing the mixture of Zn(OAc)2.

2H2O, LigAH2 and phenanthroline in a 2:1:2 ratio yielded dinuclear Zn complex 1. Scheme 2

28

illustrates the reaction leading to the formation of complex 1. Zinc (II) acetate was chosen as

the acetate ion can act as a base to deprotonate LigAH2.

N

N

O

O

O

O

H H

Zn(OAc)2.2H2O

+

+

1:1MeOH: DCM

2

O

O

N

N

Zn

O

O

CH3

2

2

Scheme 2: Synthesis of 1

Figure 8: ORTEP drawing of Zn2(LigA)(phen)2 (1) crystallized from DCM/hexanes

The formation of 1 demonstrates the ability of LigA2- to coordinate to Zn(II) centers as a

dianion. Phenanthroline (phen) acts as a capping ligand while acetate ions complete the

coordination sphere of the octahedral Zn centers. Crystals of 1 grown from CH2Cl2/hexanes lose

solvent rapidly. Regrowth of the crystals in a different solvent system (i.e. CHCl3/hexanes)

resulted in a more stable structure. Interestingly, the coordination environment around the Zn

centers has changed from octahedral (Figure 8) to pseudo-trigonal bipyramidal (Figure 9) as the

29

bidentate acetate ions become monodentate upon switching of solvent system. However, the

unbound oxygen atoms on the monodentate acetate are very close to the Zn centers (i.e. 2.646 Å

Zn to O distance).

Figure 9: Crystal structure of 1, Zn2(LigA)(phen2) grown from CHCl3/hexanes.

The structure was found in the centrosymmetric space group C2/c. Within the

asymmetric units, the two Zn centers are related to each other by an inversion center. The

aromatic rings of the ligand are not co-planar, with a dihedral angle of ~43o. The Zn-N bond

lengths are 2.103(7) and 2.162(6) Å which are similar to other Zn reported small molecule with

similar N,N bidentate ligand.30 The distance between Zn to one oxygen atom of the ligand is

shorter than the other (Zn(1)-O(1) 1.981(5) Å and Zn(1)-O(2) 2.017(6)Å. The bite angle of acac

moieties O(1)-Zn(1)-O(2) of the ligand on Zn is 87.7(2)o.

If two capping ligands in 1, either acetate or phen, can be replaced by additional

equivalents of LigAH2 during the self-assembly process, new molecular textures such as

molecular square or 1D zig-zag polymer could be formed. Wang and coworkers has reported

such molecular square complex.31 Using tetraacetylethane dianion as the ligand which is similar

30

to LigAH2, the authors created a distorted Co-containing molecular square structure. However,

attempts to make molecular square from our dinuclear Zn complex failed.

3.2 Construction of 1D Zn-containing coordination polymer

Slow diffusion of Et3N at room temperature generated the 1D coordination polymer

(Scheme 3). Preliminary syntheses indicated a suitable Et3N concentration window lies from

0.0535M to 0.428M Et3N. The structure (Figure 11) was analyzed and refined in the

orthorhombic space group Pna21. The structure cocrystallized with chloroform solvent. There is

unlikely any π-π stacking interactions in the crystal as the separation between two adjacent

aromatic rings is ~ 5.9Å. The repeating unit of the polymer is identified in Figure 12 where the

Zn center is five-coordinated by two halves of the bidentate LigA2- and a methanol molecule.

The geometry at the Zn center is approximate square pyramidal as the angles formed by the

oxygen of MeOH, Zn and acac oxygen range from 95-105o. The bite angles of the acac moieties

onto Zn range from 86.6(4) (i.e. O(8)-Zn(2)-O(9)) to 88.3 (4)o(i.e. O(1)-Zn(1)-O(2)) (Table 3) .

Compound 2 is insoluble in common organic solvents.

O

O

O

OZn(OAc)2.2H2O + 3/2

slow diffusion

CHCl3/MeOH

O

OO

O

Zn

OCH3

H

n

2

Scheme 3: Synthesis of 2, [Zn(LigA)(CH3OH).2CHCl3]n

31

Figure 11: 1D coordination polymer 2, [Zn(LigA)(CH3OH).2CHCl3]n

Figure 12: ORTEP drawing of the building block of 2

Attempt to extend this 1D polymer into a 2D ladder network by replacing the

coordinating methanol with a pillar ligand such as 4,4’-dipyridyldioxide (dpdo) or 4,4’-bipyridyl

was unsuccessful. Bridging ligand biphenyl-4,4'-diyldimethanol has also been attempted. The

produced crystals, however, are too small for single-crystal diffraction analysis. This self-

assembly strategy (Figure 13) has been reported by Hill et. al. and Kitagawa and coworkers.21, 32

32

Figure 13: Schematic description of the self-assembly of pillar supported structure

The Zn centers in the structure of 2 are clearly running out of coordination sites; adding

additional dimension to create 2D and 3D networks therefore seems a daunting task. As

mentioned previously, using metal clusters as building blocks in constructing metal-organic

framework may solve the problem of limited coordination numbers of metal centers.

Lanthanide metals can be used in place of Zn due to a number of reasons: 1) their flexible

coordination mode, 2) high coordination numbers to allow addition of extra dimensions, 3) their

ease being hydrolyzed even at neutral pH to facilitate formation of metal clusters, 4) their

luminescence properties which can be imparted onto the resulting framework. Compared to

carboxylate ligands, the acac moieties of LigAH2 is bulky and thereby preventing the access of

other ligands to the metal to form multidimensional network. As a result of the aforementioned

rationale, 4,4-(ethyne-1,2-diyl) dibenzoic acid (LigBH2) was chosen as a ligand substituting

LigAH2 in building multidimensional framework. LigBH2 was obtained in good yield (76.6%)

following the reported procedure.4,26 The UV-vis absorption spectra of LigBH2 and its sodium

salt are shown in Figure 14. Free LigBH2 in DMSO shows absorption maxima at 306 nm and

326 nm (bold solid line).

33

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

190 240 290 340 390 440 490

Wavelength(nm)

Absorb

ance

LigBNa2 in DMSO

LigBNa2 in water

Free LigBH2

Figure 14: UV-vis absorption of free LigBH2 (bold solid line) and its salt in different

water and DMSO (dash and thin solid line)

Coordination of LigBH2 to Zn and Eu and Tb under slow diffusion condition at room

temperature leads to the formation of 2D Zn-containing sheet and 3D lanthanide-containing

open frameworks as described in the next sections.

3.3 Construction of 3, a 2D interpenetrating Zn-containing framework

Structure of 3 is composed of paddle-wheel dinuclear Zn carboxylate units (Figure 15

(a)) bridged by LigB2- to form a 2D square grid (Figure 15 (b)). In addition, the dihedral angle

between the two interpenetrating frameworks is approximately 84° (Figure 16). Four one-half of

the LigBH2 bridge the two Zn centers of the paddle wheel unit in a bis-monodenate fashion,

totaling four oxygen atoms around each Zn centers. These fours oxygen atoms forming the base

and one disordered DMSO molecule occupying the apex position of the square pyramidal

complete the five-coordinated environment around each Zn center.

34

(a)

(b)

Figure 15: (a) Structure of the paddle-wheel dinuclear Zn carboxylate units of framework 3; (b)

Structure of the 2D grid of framework 3.

35

Figure 16: 2D interpenetrating, sheet-like structure of 3

The framework crystallizes in the space group Pbca. Each paddle wheel unit shows Zn-

Zn distance of 2.979 (9) Å which is consistent with those observed in reported analogues.9 Such

distance is significantly shorter than that (i.e. 3.515 Å) reported by Allendorf and coworkers but

similar to that (i.e. 2.955(10)) reported by Fang et. al. 24, 34 The distances from Zn to the

carboxylate oxygens are not quite uniform with Zn(1)-O(3) (2.007(3) Å) being the shortest

distance of the four Zn-O bonds. However, these are similar to those reported in Zn paddle-

wheel analogue.34 The axial DMSO molecules are held tightly to the Zn center (Zn(1)-O(5)

1.984(3) Å).

MOF 3 crystals display green luminescence when irradiated with UV radiation. Figure

17 depicts the emission spectra of 3 obtained with different excitation wavelengths. Excitation

spectrum was measured with the detector set to 495nm (Figure 18)

36

0.0E+00

1.0E+06

2.0E+06

3.0E+06

4.0E+06

5.0E+06

6.0E+06

7.0E+06

8.0E+06

9.0E+06

1.0E+07

355 405 455 505 555

Wavelength (nm)

Arb

itra

ry u

nits 345nm

365nm

370nm

372nm

375nm

Figure 17: Emission spectra of MOF 3 under different excitation wavelengths

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

300 350 400 450 500 550 600 650 700

Wavelength (nm)

Arb

itra

ry u

nit

Figure 18: Normalized solid state emission and excitation profile of MOF 3

Luminescence of MOF 3 is essentially ligand-based emission as Zn2+ (d10) is non-

emissive. One potential advantage of MOF possessing such property is that it should readily be

Emission

Excitation

37

tunable by varying the nature of the linker, perhaps by changing the degree of conjugation in the

ligands. 24

3.4 Construction of 4 and 5, 3D open frameworks

What is going to be discussed for 4 will also be applicable to 5 unless otherwise noted.

Solution assembly of EuCl2.6H2O and TbCl2.6H2O with LigBH2 gave rise to 4 and 5,

respectively. Again, slow diffusion with volatile Et3N as the base yielded highly desirable 3D

porous framework whereas hydrothermal attempts resulted in no crystal but fine powder.

Framework or MOF 4 and 5 are isostructural; however, only 4 is luminescent.

X-ray crystallography. Both 4 and 5 crystallize in the monoclinic space group P21/c.

The asymmetric units of 4 and 5 are shown in Figure 19. The bimetallic unit of structure 4 and 5

is shown in Figure 20. A center of inversion runs through the metal-metal bonds in both 4 and

5. Each Eu is nine-coordinated with two LigB2- contributing seven oxygen donor atoms and

MeOH and DMSO each contributes one oxygen donor atom. The coordination geometry around

Eu center is therefore tricapped trigonal prismatic.

There are three types of coordination modes adopted by LigB2-: truly chelating, truly

bridging and simultaneously bridging and chelating (Figure 20). Carboxylate exhibiting the first

coordination mode are O(4)-Eu(1)-O(3) in 4 and O(1)-Tb(1)-O(2) in 5 with the bite angle of

51.90(15)o and 52.05(16)o, respectively. The inter-metallic distances Eu-Eu and Tb-Tb are

comparable, being 4.1381(6) and 4.1386 (9) Å, respectively. For the carboxylate groups acting

intermediate between truly bridging and chelating, their oxygen atoms (i.e. O(5) in 4 and O(4) in

5) are being shared between two metal centers, displaying the longest among metal-carboxylate

oxygen bonds in 4 and 5 (Eu(1)-O(5) (2.809(4) Å) and Tb(1)-O(4) (2.869(5)Å)). In addition, as

observed, the coordination mode at one terminal of a LigB2- molecule is not necessarily the

same as the other terminal of the same ligand. One end of the carboxylate can adopt the syn-syn

bis-monodentate bridging mode while the other end acting as fully chelating. On the contrary, if

38

one carboxylate terminal acts intermediate between chelating and bridging, the other

carboxylate terminal of the same ligand will also adopt the same coordination mode. All of the

carboxylic groups in both 4 and 5 are coplanar with the aromatic rings to which they bound

except for the truly chelating carboxylate, small dihedral angles of 9.69o (in 4) and 6.18o (in 5)

are observed.

The oxygen atoms in the two truly bridging carboxylate groups are largely equidistant

from the metal centers with an average Tb-O distance of 2.3 Å and Eu-O of 2.4 Å. In both 4 and

5, the coordinating DMSO molecules are more tightly bound to the metal than MeOH.

Lanthanides are known to be oxophilic; as a result, DMSO is coordinating to lanthanide metals

via its O rather than S atom.

Figure 19: The asymmetric unit of 4 (left), and 5 (right)

39

Interpenetration observed in 3 has effectively decreased porosity of the framework. On

the contrary, due to the lack of structure interpenetration, 4 and 5 are highly porous with

triangular channels running along the a axis (Figure 21). Non-coordinating DMSO and water

molecules occupying the void of the channel help stabilize the framework.

Powder X-ray Diffraction. Powder X-ray diffraction pattern of 5 is presented in Figure

22. Several features can be noticed from the data. The simulated and found patterns are similar

with slight shift. This could be the result of expansion in the crystal lattice of the measured

sample. In fact, one reality must be keep in mind is that SXRD data are collected at 150 K

whereas PXRD measurements are performed at ambient temperature. Due to such a significant

difference in temperature, it’s hence likely that the pattern simulated from SXRD will be

slightly different from the experimentally obtained pattern. As a result the two most pronounced

Figure 21: Space-filling diagram of the extended structure of 4 showing triangular channel looking down the a axis.

Figure 20: Bimetallic Eu unit showing the versatile coordination modes of the carboxylate groups of LigB2-

40

peaks at 2θ =5.48 and 6.90oon the found patterns can be reasonably assigned as the ones at 2θ =

5.66 and 7.12o in the simulated pattern.

0

200

400

600

800

1000

1200

1400

1600

1800

3 6 9 12 15 18 21 24 27 30 332 theta

Inte

nsity Simulated

PXRD of 5

Observed

PRXD of 5

Figure 22: Powder X-ray diffraction calculated and found (bold) pattern for framework 5

0

200

400

600

800

1000

1200

1400

1600

1800

2000

3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9

2 theta

Inte

nstity

Calc. PXRD of 5

Found PXRD of 5

Figure 23: Magnified region of 2θ = 3 to 9o in both calculated and found PXRD patterns of 5

41

Figure 23 magnifies the region of 2θ = 3 to 9o for comparison. However, the quite

intense peak at 2θ = 7.62o of found pattern could not be accounted for on the simulated one

suggesting that the sample, while its crystallinity is proven, does contain a second phase. One of

the possibilities is that this peak arises from modified lattice due to the loss of non-coordinating

solvents in the cavities of the framework.

Thermogravimetric analysis. TGA was carried out under a nitrogen atmosphere with a

heating rate and a nitrogen flow rate of 5oC/min and 100 cm3/min, respectively.

Upon isolation, the as-synthesized crystals of 4 and 5 become opaque rapidly. TGA

diagrams of MOF 4 and 5 are shown in Figure 24.

55

60

65

70

75

80

85

90

95

100

105

0 100 200 300 400 500 600

Temperature (oC)

Weig

ht

(%)

3D Tb

3D Eu

Figure 24: TGA of 4 (bold) and under N2 atmosphere and heating rate of 5oC/min

A sample of single crystals of 4 (5.791 mg) shows gradual yet apparent three weight loss

stages in the TGA whose onset points are 80oC, 137oC and 319oC. At 44oC, two water

42

molecules in the cavity have been lost (calcd. 2.28% weight loss; found 2.36% weight loss). At

125oC, all cavity solvents (i.e. 2DMSO and 3H2O) have been lost in addition to the loss of two

coordinating MeOH and 1/2 of one coordinating DMSO (calcd. 20.02%; found 20.05%). From

137oC to 319oC, a loss of 41.67% was observed which corresponds to loss of the remaining 1.5

coordinating DMSO and a concomitant loss of 70% ligand (calcd. 39.8%; found 40.3%). The

framework 4 is thus said to be stable up to 137oC.

TGA of terbium-containing framework shows one fewer weight loss stages compared to

that of Eu framework. The onset points were determined as 61oC and 134oC. The run was

initiated with an isotherm segment for a period of 40 min at room temperature. This helps

confirm that the crystals indeed lose weight even without heating. However, plotting % weight

remained against time (Figure 25), one can notice a plateau from approximately 16.55 to 44.83

min which correspond to 20oC and 32oC. During this interval one can assume that the sample

weight has become more or less stable. At 32oC, a loss of 2.57% was already occurred

corresponding to loss of two water molecules in the cavity (Found 2.3%). Further loss of the

remaining water molecule in the cavity occurred from 32oC-61oC. From 61oC to 134oC, 24.02%

weight loss was observed which corresponds to the loss two DMSO in the cavity as well as two

coordinating MeOH and 0.5 coordinating DMSO. Framework 5 is therefore thought to be stable

up to 134oC.

43

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200

Time (min)

We

ight

(%)

Figure 25: Weight loss of 5 over time (min)

Luminescent properties of 4. The sensitized emission of lanthanide-aromatic

complexes is very common in rare earth chemistry.35 In contrast to the green luminescence

displayed by 3, single crystals of 4 produce red emission under UV irradiation. The solid state

excitation-emission of 4 has been studied at room temperature (Figure 26). The spectra exhibit

sharp emission at 615 nm arising from the 5D0 to 7F2 transition characteristic of Eu(III)

emission.36 As only 4 luminesces, LigB2- is a suitable antenna for Eu(III) ion but not for Tb(III)

ion. The established metal-based emission of MOF 4 might render it useful in the sensing of

small molecules. Aqueous solutions of Cu(NO3)2, MgSO4.7H2O, ZnSO4.H2O, CoCl2.6H2O,

Mn(OAc)2.4H2O, FeSO4.7H2O, Zn(OAc)2.2H2O in addition to common organic solvents and

pentafluorophenol have been tested for MOF 4 inclusion; however, none has shown evidence

for luminescence signal modulation thus far.

44

0

0.2

0.4

0.6

0.8

1

1.2

310 360 410 460 510 560 610 660 710

Wavelength (nm)

Arb

itra

ry u

nit

Emission

Excitation

Figure 26: Solid state excitation-emission spectrum of MOF 4

45

CHAPTER IV

CONCLUSION AND FUTURE OUTLOOK

It has been an ongoing challenge to absolutely predict the network topology of the

designed MOFs; nevertheless, various novel and interesting molecular textures have been

synthesized with the use of appropriate ligands and metals and characterized. The products

obtained represent both transition metal-based (i.e. 3) and lanthanide-based MOFs (i.e. 4, 5).

The results obtained demonstrated that building highly ordered and multidimensional networks

from discrete molecular units is indeed an attractive approach since it allows one-pot reactions

at room temperature.11 The constituents of a framework can be designed or chosen to transfer

desired physical properties onto the framework. The work in this thesis has provided an example

of, using emissive lanthanide metals to impart luminescence properties onto the framework. Of

the structures attained, 2D Zn-containing and 3D Eu-containing frameworks exhibit

luminescence properties and have solvents occupying their cavities. The ability for these

frameworks to function as useful host-guest systems is therefore very probable. Potentially, they

could selectively entrap deleterious compounds (pollutants, toxicants, ect.) and report the

presence of these compounds through luminescence signal modulation. For such useful

functions to become practical, further investigation is required.

Modification of the as-synthesized framework might lead to new properties. The existence

of a C≡C triple bond in LigB2- can serve as a starting point for further functionalizing of the

current framework to explore new reactivities. Furthermore, transition metal-carbonyl

complexes (e.g. Cr(CO)3) can be anchored at the benzene rings constituting most of the internal

surface in a η6 fashion. Due to rich photochemistry of these transition metal complexes, the CO

group can be replaced by N2 or H2. The numbers of aromatic rings incorporated in the attained

frameworks are twice as many as those in the system reported by Long et. al.. 37 While MOF 3-5

46

might be too heavy to function as practical hydrogen storage materials, they can serve as

potential catalysts and as rigid matrix for reactions otherwise can only be performed in gas

phase and supercritical fluids

47

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50

APPENDIX

SELECTED SPECTRA

51

O O

O O

HH

1 H-N

MR

52

O O

O O

HH

13C

-NM

R

53

CO

OC

H3

H3C

OO

C

1 H-N

MR

54

CO

OH

HO

OC

1 H-N

MR

55

CO

OH

HO

OC

13C

-NM

R