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Metal–Organic Frameworks (MOFs) and Coordination Polymers

Metal–Organic Frameworks (MOFs) and Coordination Polymers

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236



Chapter 7 Metal –Organic Frameworks (MOFs) and Coordination Polymers



7.1



INTRODUCTION



Over the past decade, coordination compounds with infinite structures have been

studied intensively. In particular, compounds with frameworks constructed from

metal ions and organic bridging ligands form a family of polymers that are known

as coordination polymers or metal – organic frameworks (in this chapter, we will

use the term coordination polymers). The expression coordination polymer first

appeared early in the 1960s, and by the mid 1960s, this family of compounds had

already been reviewed.1 In an effort to open up this area of chemistry fully, versatile

synthetic approaches involving assemblies of molecular building blocks (metal ions,

bridging ligands, counter anions, and guest molecules) in solution have been pursued

to achieve the formation of target structures. The key to success is the design of

molecular building blocks that direct the desired architectural, chemical, and physical

properties to result in solid-state materials. Coordination polymers are highly

crystalline materials, and chemists need to obtain good quality single crystals to understand their structure and properties. However, in general, such polymers are insoluble

in solvents, with maintenance of their original framework. Therefore, the most

common crystallization method, recrystallization, is not available. In a surprisingly

short period, the structural chemistry of coordination polymers has reached a mature

level because of the application of useful crystallization methods, such as slow diffusion and hydrothermal and solvothermal methods. A survey of recent research work

shows an extraordinary increase in the number of published articles. The next challenge in this area is to develop coordination polymers with chemical and physical

functionalities by modifying the coordination framework. Unique functionalities

have already begun to be found in coordination polymers. In particular, the porous

properties of coordination polymers have been studied intensively.

Porous compounds have attracted the attention of chemists, physicists, and

materials scientists because of scientific interest in the creation of nanometer-sized

spaces and the discovery of novel phenomena in them through their characterization

and processing, as well as commercial interest in their application in separation,

storage, and heterogeneous catalysis. Until the mid-1990s, there were two types

of porous materials: inorganic and carbon-based materials. Zeolites, which are

among the best-known porous inorganic solids, are three-dimensional crystalline,

hydrated alkaline or alkaline earth aluminosilicates with the general formula

(MIMII1/2)m(AlmSinO2(mỵn)) . xH2O (n ! m) (M ¼ cations).2,3 Their frameworks,

built from corner-sharing TO4 tetrahedra (T ¼ Al, Si), delimit interconnected tunnels

or cages into which H2O molecules and cations are inserted. The porosity is provided

by elimination of the H2O molecules, with the framework usually remaining unaffected. The cavities, usually quantified structurally by the number of polyhedra surrounding the pore, are used for molecular sieve requirements in gas separation and

catalytic operations. Activated carbon has a high porosity and high specific surface

area, but has a disordered structure, the essential feature of which is a twisted network

of defective hexagonal carbon layers, cross-linked by aliphatic bridging groups.4 The

width of the layer planes varies, but is typically about 5 nm. Simple functional groups



7.1 Introduction



237



and heteroelements are incorporated into the network, and are bound to the periphery

of the carbon layer planes.

Recently, porous coordination polymers (PCPs) have been developed, which are

beyond the scope of the former two classes of porous materials.5–25 Werner complexes, [M(NCS)2(4-methylpyridine)4] (M ¼ NiII or CoII),26 Prussian blue compounds,27–29 and Hofmann clathrates and their derivatives, which are built of CN

linkages between square planar or tetrahedral tetracyanometallate(II) units and octahedral metal(II) units coordinated by complementary ligands,29–31 are known as

porous materials that can incorporate a variety of small molecules. Their main frameworks utilize small cyanide anions as bridging ligands. Since the early 1990s, research

on the structure of porous coordination polymers with longer bridging ligands than

the cyanide ligand has increased greatly, and some examples with functional pores

(anion-exchange, catalytic properties, and adsorption) have begun to appear.32–36 A

survey of the research published in recent years shows an extraordinary increase in

the number of articles. The following three features would provide pivotal advantages:

(1) designability, (2) regularity, and (3) flexibility.

1. High designability: the key to success in obtaining highly functional materials

is the design of the desired architectural, chemical, and physical properties of

the resulting solid-state compounds. One can take advantage of this in the

design of PCPs because the formation reactions mostly occur under mild conditions, and the choice of a certain combination of discrete building units leads

to a high probability of a desired extended network.

2. Regularity: regular pore distribution in a porous solid is important for adsorption because when the size of a pore is comparable to that of a guest molecule,

then the periodic potential from the pore wall can influence the form and orientation of the adsorbed guest molecule. A regular pore distribution can be

readily realized for coordination polymers as well as inorganic porous

materials. The pores of coordination polymers have a regular periodical structure because of their crystalline form, which affords a periodic potential on

their channel surface. The structural relationships between adsorbed guest

molecules and host frameworks [e.g., (a) position of the guest molecules in

the pores, (b) the assembled structure of the guest molecules in the pores,

and (c) the perturbation that the guest molecules receive from the pore

walls] are key subjects for understanding the adsorption behavior and the

physical or chemical properties of adsorbed guest molecules in the pores.

In addition, molecules confined in a uniform restricted nanospace form molecular assemblies and provide unique properties that are not realized in the

bulk state.

3. Flexibility: recent reports on the dynamic properties of PCPs show that they

are much more flexible than generally believed. Dynamic pores can form a

type of “soft” framework with bistability, whose two states oscillate back and

forth between one of two counterparts. A system can exist in either of two

states for different parameters of an external field. The structural rearrangement



238



Chapter 7 Metal –Organic Frameworks (MOFs) and Coordination Polymers



of molecules proceeds from the “closed” phase to the “open” phase in response

to a guest molecule. Some PCPs have such flexibility, and they can be developed

as a special class of materials, such as highly selective gas sensors or gas separation compounds, that cannot be obtained using a rigid porous material.

Dynamic structural transformations based on these flexible frameworks are

among the most interesting phenomena, and are presumably a characteristic

of coordination polymers, the so-called third generation PCPs.37



7.2



BUILDING BLOCKS



Basic building blocks to create PCPs are metal ions as connectors, bridging ligands as

linkers, and guest molecules as templates. In this section, we introduce well-designed

building blocks, as shown in Figure 7.1.



7.2.1



Secondary Building Units



Polynuclear clusters constructed from two or more metal ions and carboxylate units

[so-called secondary building units (SBUs)] can have special coordination numbers

and geometries. SBUs are conceptual units that are not employed in the synthesis

as a distinct molecular building block. However, specific SBUs can be generated

in situ under the correct reaction conditions.6 Because the metal ions are locked

into their positions by the carboxylate units, SBUs are sufficiently rigid to produce

extended frameworks with a high structural stability. In SBUs with terminal guests,

the reactivity of the metal site can be studied through the removal of these guests,

which creates coordination sites.



7.2.2



Metalloligands



Coordinatively unsaturated metal centers (UMCs) can provide functionalities as chromic sensors and flexible frameworks, because the ligation and release of the UMCs’

guests often influence the coordination geometry or ligand field. Moreover, it is

well known that a variety of metal ions can act as active centers in catalyzed reactions.

A combination of UMC characteristics and traditional porous properties (shape and

size selectivity) can be used to create desired, highly efficient, and tailor-made functional materials. Recently, a new synthetic method has been proposed for PCPs with

UMCs using metalloligands, that is, two-step self-assembly.38 First, a metalloligand is

synthesized that acts as a framework linker and a source of coordinatively unsaturated

metal centers. Second, the metalloligand is added to another metal ion, which acts as

a nodal unit in a framework. Consequently, two types of metal center coexist in the

framework, and a larger space around the metal ion in the channel wall can be

obtained, which is significant for an attack by a guest molecule. The key point of

this method is a partial separation of the metal functionalities: the framework node

and the coordinatively unsaturated metal centers.



7.2 Building Blocks



239



Figure 7.1 A library of building blocks.



7.2.3



Redox-Active Ligands



Charge transfer (CT) interactions play a significant role in developing novel photoactive materials, although only a few examples of this type of interaction in PCPs

have been seen to date.39–42 One available method to engender CT interactions

between a host framework and a guest molecule is to use redox-active ligands, because

such ligands forming the pore walls of PCPs have enough space to attract a guest

molecule at a nearby site. For example, f[Zn(TCNQ)(4,40 -bpy)] . 6MeOHgn



240



Chapter 7 Metal –Organic Frameworks (MOFs) and Coordination Polymers



(TCNQ22 ¼ 7,7,8,8-tetracyano-p-quinodimethane dianion, 4,40 -bpy ¼ 4,40 -bipyridine) has a three-dimensional pillared layer framework with redox-active ligands,

TCNQ22.41 The ZnII ions are linked by TCNQ22 dianions to give a two-dimensional

corrugated layer. The 4,40 -bpy ligands act as pillars, with the ZnII ions in the adjacent

layers linked to form a three-dimensional pillared layer structure. This compound pos˚ 2 between the

sesses two-dimensional channels with cross sections of 3.4 Â 5.9 A

22

layers. As a result, a dense array of strong donor sites (TCNQ ) is formed on the

pore surface of the framework, providing a highly electron-rich surface for the guest

molecules. When f[Zn(TCNQ)(4,40 -bpy)] . 6MeOHgn is immersed in benzene, the

guest MeOH can be exchanged with benzene to form f[Zn(TCNQ)(4,40 -bpy)] .

2benzenegn within a period of 10 s, producing a compound that is red in color

versus the yellow color of the parent compound. The crystal structure of

f[Zn(TCNQ)(4,40 -bpy)] . 2benzenegn indicates that the benzene guests interact with

the most negatively charged carbon atom of the TCNQ22 dianions via C– H . . . Ctype hydrogen bonds, representing the existence of an electrostatic and/or CT interaction between the TCNQ22 dianions and the benzene guests. For other guests

(e.g., toluene, ethylbenzene, anisole, benzonitrile, and nitrobenzene), changes in

color different from that of f[Zn(TCNQ)(4,40 -bpy)] . 6MeOHgn, similar to that

observed for benzene, were seen, except for anisole and nitrobenzene, where the

occlusion of anisole and nitrobenzene forms light yellow and dark brown crystals,

respectively. Such a difference in color is associated with the electron-accepting

characteristics of the guest molecules.



7.2.4



Ligands Capable of Postmodification



Postmodification of the pore surface without destruction of the framework structure

after formation of PCPs is the next challenging study for obtaining comfortable

space for specific guests.43–53 Recently, novel strategies have been developed to introduce a catalytic metal center by postmodification of the porous framework.43–48 A

chiral bridging ligand, L1, with orthogonal functional groups, as shown in

Figure 7.1d, bridges the CdII centers of the one-dimensional [Cd(m-Cl)2]n units to

form an extended porous network in f[Cd3Cl6(L1)8] . 4DMF . 6MeOH . 3H2Ogn

with very large chiral channels with an area of approximately 1.6 Â 1.8 nm2.43

The channel walls of this porous compound have free – OH sites acting as secondary

functional groups that can further interact with other metal ions. Indeed, Ti(OiPr)4

can react with the chiral dihydroxy groups in the L1 parts of this CdII network, producing

an active heterogeneous catalyst for the addition of ZnEt2 to aromatic aldehydes to

produce chiral secondary alcohols. The performance of this heterogeneous catalyst

rivals that of the homogeneous analog under similar conditions. The three-dimensional

PCP [Zn4O(1,4-bdc)3]n (1,4-bdc22 ¼ 1,4-benzenedicarboxylate) can incorporate

photoactive Cr(CO)3 groups onto the benzene rings of 1,4-bdc22 ligands in an h6

fashion after construction of the framework.44 The PCP obtained, [Zn4O((h6-1,4bdc)Cr(CO)3)3]n, shows substitution of a single CO ligand per metal by either N2 or

H2 under gentle photolysis conditions, forming [Zn4O((h6-1,4-bdc)Cr(CO)2(X))3]n



7.2 Building Blocks



241



(X ¼ N2 or H2). These PCPs display a remarkable stability compared with their analogs

[(C6H5Me)Cr(CO)2(X)] (X ¼ N2 or H2) because the Cr0 centers are pinned to the walls

of the framework, which prevents reactions involving two or more Cr0 atoms and

enables the irreversible evacuation of CO.



7.2.5



Cartridges of Functional Groups



Tweaking the porous properties simply by replacing a molecular “cartridge” with a

different cartridge has been developed in a new class of PCPs.54,55 The threedimensional PCP, f[Zn3I6(tpt)2(triphenylene)] . x(nitrobenzene) . y(MeOH)gn (tpt ¼

2,4,6-tris(4-pyridyl)triazine), is composed of two interpenetrating networks, in

which the pores are surrounded by aromatic bricks.56 These bricks consist of alternatively layered tpt and triphenylene. The former forms an infinite three-dimensional

network via coordination to ZnI2, whereas the latter is involved in the threedimensional network without forming any covalent or coordination bonds with

other components. Therefore, the noncovalently intercalated triphenylene can be

replaced with functionalized triphenylenes (Fig. 7.1e) without causing any change

in the porous structure, and intercalated triphenylenes are regarded as being the cartridges of the functional groups (Fig. 7.2). For example, one of the triphenylenes

directs a phenol group towards the interior of the pores. The resulting PCP

selectively adsorbs alcohol guests such as propan-2-ol.



Figure 7.2 A schematic representation of a modular synthesis.54 (Reprinted in part with permission from

M. Kawano et al., J. Am. Chem. Soc. 2007, 129, 15418–15419. Copyright 2007 American Chemical

Society.)



242



Chapter 7 Metal –Organic Frameworks (MOFs) and Coordination Polymers



7.3 SYNTHESIS AND CHARACTERIZATION

METHODS

7.3.1



Synthetic Methods



The self-assembly process for coordination polymers is a useful method for the

following reasons: (1) a wide variety of frameworks can be realized just from

simple building blocks of metal ions, organic bridging ligands, and counter anions;

(2) easy and rational modification of organic bridging ligands is possible; (3) several

interactions such as coordination bonds, hydrogen bonds, aromatic interactions, M – M

bonds, and van der Waals interactions are available; and (4) the reaction can be controlled using the temperature, pH, solvent, and pressure. For coordination polymers,

recrystallization cannot be used to obtain high purity compounds or large single

crystals, because these compounds are insoluble in most solvents. Diffusion, hydrothermal, and solvothermal methods are common techniques used to produce pure

coordination polymers. A new synthetic approach has also been developed

(see below).



7.3.1.1



In Situ Synthesis of Ligands



The in situ slow hydrolysis of precursor ligands can result in less soluble phases that

are not accessible directly from their corresponding hydrolyzed ligands by virtue of the

presence of a large excess of metal ions under hydro(solvo)thermal conditions.57 For

example, CdII and ZnII coordination polymers that show second-order nonlinear optical properties can be obtained by reaction of the metal salts with cyanopyridine, and

pyridinecarboxaldehyde and its derivatives, whose cyano, carboxaldehyde, and ester

substituent groups slowly hydrolyze to form the corresponding carboxylic acid.58

The reaction of NaN3, 3-cyanopyridine, and ZnCl2 or CdCl2 under hydrothermal conditions yields [Zn(OH)(3-ptz)]n or [CdN3(3-ptz)]n (3-ptz2 ¼ 5-(3-pyridyl)tetrazolate).59 For each reaction, the IR spectrum indicates the absence of the cyano group,

which is consistent with a [2 ỵ 3] cycloaddition between the cyano group and the

azide anion.



7.3.1.2



Microwave Methods



Microwave irradiation has been developed as a novel heating technique for the

hydro(solvo)thermal synthesis of organic or inorganic solid-state materials. In particular, this method has been shown to provide an efficient way to synthesize purely

inorganic porous materials with short crystallization times, narrow particle size distributions, facile morphology control, phase selectivity, and efficient evaluation of process parameters. Even under ambient conditions, microwave heating of a mixture of

starting materials results in an efficient reaction to provide interesting coordination

polymers. In this context, the first example discussed is [Cu2(oxalate)2(pyz)3]n

(pyz ¼ pyrazine), which has been synthesized by the author.60 The first successful



7.3 Synthesis and Characterization Methods



243



application of microwave synthesis for porous coordination polymers was achieved

in the case of f[Cr3FO(1,3,5-btc)2(H2O)3] . xH2Ogn (1,3,5-btc32 ¼ 1,3,5-benzenetricarboxylate, MIL-100), which has a hierarchical pore system (microporous ¼ 5 to

˚ and mesoporous ¼ 25 to 30 A

˚ ) with a very high specific surface area ($3100

9A

m2 . g – 1).61,62 The X-ray diffraction (XRD) pattern of a sample obtained after microwave irradiation (reaction temperature ¼ 493 K) for a period of 4 h agrees well with

the pattern of MIL-100 synthesized for a period of 4 d at 493 K using conventional

hydrothermal heating. The crystal yield from microwave synthesis is 44%, which

is comparable with the result of 45% from the conventional synthesis after 4 d. The

physical and textural properties are also very similar to those of the crystals synthesized using conventional hydrothermal heating. Other porous coordination polymers have been synthesized using microwave heating to shorten the reaction

time,63,64 and to discover new materials not yet obtained using conventional hydrothermal synthesis.65



7.3.1.3



Solvent-Free Methods



Solvent-free synthesis is of interest for several reasons. For example, it can give insight

into the roles of solvent molecules in templating porous structures, gives access to

large-scale green production processes, and even provides more convenient

laboratory-scale preparative methods. The three-dimensional coordination polymer

[Cu(isonicotinate)2]n is the first example that higher-dimensional connectivity,

which is generally required to support permanent open porosity, can be produced

by grinding.66 A ball mill was used to grind together Cu(O2CCH3)2 . H2O and isonicotinic acid. Typical conditions involved a 20 mL steel vessel containing a steel

ball bearing and approximately 0.5 g of reactants, at an oscillation rate of 25 Hz

maintained for 10 min. The compound obtained has the characteristic odor of acetic

acid, released as a by-product. The fully desolvated compound is obtained by heating

the mixture to 473 K for a period of 3 h, and shows a similar XRD pattern to that simulated from the single-crystal data for the empty framework host that was synthesized in

the solvothermal reaction between 4-cyanopyridine and CuCl2 in a mixture of water

and ethanol after heating to 423 K under autogenous pressure for a period of

48 h.67,68 This solvent-free method is quick, and gives a quantitative yield without

the need for solvents or external heating. Clearly, it can present higher efficiency in

terms of materials, energy, and time compared with solvothermal methods.



7.3.1.4



Addition of Organic Polymers



Precise control of crystal growth of coordination polymers is currently a key challenge

in their development as intelligent building blocks in optical, electronic, and catalytic

applications.69–72 There have been several advanced studies in which template molecules, such as surfactants, organic polymers, vesicles, Langmuir – Blodgett films,

and self-assembled organic monolayers serve as fields for nucleation and/or growth

of the crystallization of coordination polymers, while controlling the size,



244



Chapter 7 Metal –Organic Frameworks (MOFs) and Coordination Polymers



morphology, and orientation of the resulting crystals. However, to date, little attention

has been paid to the crystal growth control of coordination polymers with organic

ligands.

Functionalized organic polymers have been used widely as substances for the

preparation of metal-based nanoparticles. The selection of appropriate protective polymers leads to various colloid morphologies by controlling the growth and handling of

the agglomeration process, and also leads to attractive new properties based on

organic – inorganic hybridizations on the nanometer scale. Recently, the control of

crystal growth of the porous coordination polymer [Cu2(pzdc)2(pyz)]n (CPL-1,

CPL ¼ coordination pillared layer and pzdc22 ¼ pyrazine-2,3-dicarboxylate) has

been achieved by addition of the organic polymer poly(vinylsulfonic acid, sodium

salt) (PVSA), which has the ability to complex and stabilize CuII by electrostatic

interaction.73 The addition of PVSA to the reaction mixture significantly affects the

nucleation process, which determines the crystal size of this compound in the range

of 1 to 100 mm. PVSA controls the crystal size and shape but also controls the

preferential orientation of the plate crystals, which results in alignment of the channel direction in the bulk powder state. Nitrogen adsorption measurements show

typical characteristic micropore adsorption isotherms that are independent of the

crystal size. However, the adsorption speed of the samples shows a novel sizedependent feature because of changes in the diffusion length of N2 in the onedimensional channels.

Heterogeneous nucleation from insoluble functionalized organic polymers is

also a powerful, efficient technique for discovering and selecting among the different

polymorphic forms of coordination polymers. It has been proposed that the polymer

serves to direct the formation of one phase over others at the time of nucleation by

selectively stabilizing one mode of aggregation under conditions (e.g., solvent, temperature, and extent of supersaturation) that are otherwise identical. For example, the

three-dimensional coordination polymer f[Zn4O(2-atp)3] . xGgn (2-atp22 ¼ 2-aminoterephthalate, IRMOF-3, isoreticular metal –organic framework No. 3) with a

primitive-cubic framework structure74 is formed in the absence of insoluble

polymers. However, a new phase, PNMOF-3 (PNMOF ¼ polymer-nucleated

metal – organic framework) is additionally observed in crystallizations containing

cross-linked MAA/DVB copolymers (MAA ¼ methacrylic acid and DVB ¼ divinylbenzene), but is not observed when 4VP/DVB copolymers (4VP ¼ 4-vinylpyridine)

or polydivinylbenzene are used.75 The crystal structure of PNMOF-3 shows a framework with a composition of [Zn4(2-atp)3(NO3)2(H2O)2]n with hexagonal grid layers.

This result demonstrates the importance of acid functionality in the selection of this

framework.



7.3.2



Characterization Methods



In addition to routinely used methods, such as elemental analysis, IR and UV-vis-NIR

spectra, thermogravimetry-differential thermal analysis (TG-DTA), single-crystal

X-ray diffraction, and gas adsorption, there are some important characterization

methods for coordination polymers.



7.3 Synthesis and Characterization Methods



7.3.2.1



245



Synchrotron Powder XRD Measurements



The atomic-level crystal structures in coordination polymers, which are required information to fully understand the physical properties, are determined using XRD

measurements. Many coordination polymers form good quality single crystals that

are suitable for single-crystal XRD analysis because of their high degree of regularity.

Recent advances in collimation techniques and detector technology have enabled the

fast collection of highly redundant data which allow the measurement of unstable or

very small single crystals in a facile way. In the case of a powder (i.e., an aggregate

of microcrystals), which is unfavorable for single-crystal XRD measurements, the

crystal structure can be determined using synchrotron powder XRD measurements.

There are advantages to using a synchrotron light source: (1) the synchrotron radiation

light source provides a much higher resolution and higher counting statistics diffraction data compared with laboratory sources; and (2) using a high brilliance light

source, a diffraction experiment can be performed using a small mass of sample

over very short data collection times. Such advantages enable the collection of good

quality structural data that compares favorably with that obtained using single-crystal

XRD analysis. The powder diffraction method has the advantage of avoiding difficulties related to the collapse of a single crystal from the large volume changes during

guest adsorption and desorption.76

The determination of a one-dimensional ladder molecular array structure of O2

molecules was first performed using in situ synchrotron powder XRD measurements

on CPL-1 accommodating O2 molecules.77,78 The intermolecular distance of the

˚ ) is close to the nearest distance in the solid a-O2

adsorbed O2 molecules (3.28(4) A

phase, whose close-packed structure appears below 24 K. This result indicates that

O2 molecules adsorbed in the nanochannels form van der Waals dimers, (O2)2. The

X-ray structural analysis shows that O2 molecules are in the solid state rather than

the liquid state, even at 130 K and 80 kPa, which are much higher than the boiling

point of bulk O2 at atmospheric pressure, 54.4 K. This result is ascribed to the

strong confinement effect of CPL-1.

7.3.2.2



Inelastic Neutron Scattering



Inelastic neutron scattering studies have been used to investigate spectroscopically the

H2 – framework interaction because of the high potential of H2 as a clean energy

carrier. This technique provides a sensitive probe of the adsorptive sites for molecular

H2 because it monitors the hindered rotational transitions of bound molecules.

Promising observations were made for [Zn4O(1,4-bdc)3]n loaded with varying

amounts of H2.79 These spectra show that at least two distinct adsorption sites exist

on the framework, which are assigned to the inorganic and organic components.

Neutrons are ideal scattering probes for such experiments, because X-rays are insensitive to the low electron density of hydrogen atoms.

7.3.2.3



Computational Simulation



Based on the accumulated crystallographic and adsorption data of PCPs, computational modeling studies of small-molecule adsorption have been performed (an



246



Chapter 7 Metal –Organic Frameworks (MOFs) and Coordination Polymers



approach that is common in carbon and inorganic materials chemistry).80–82 PCPs

have an advantage for simulations, in that their well-characterized regular structure

eliminates the need to make assumptions about the host structures. Grand canonical

Monte Carlo simulations of H2, N2, Ar, CH4, CO2, and n-butane in PCPs have

been reported, and these show good agreement with measured isotherms.83–90

These calculations were also used for proposing new PCPs with improved performance.91–95 For example, the interaction energy between H2 and lithium alkoxide

groups in Li-modified PCPs is larger than in unmodified PCPs.91,92,94

7.3.2.4



Solid-State NMR Studies of Molecular Motion



NMR is an excellent method for studying the dynamics of molecules in solids. In general, all nuclear spin interactions are anisotropic, that is, they depend on the molecular

orientation within the applied magnetic field of the NMR experiment. In-depth information on the dynamic behavior of the guests and/or framework in PCPs is essential

for the evaluation and understanding of micropore filling and design of new functions.

In particular, 2H has been used extensively in molecular motion studies. This spin-1

nucleus gives rise to quadrupole coupling constants in the range 160 to 220 kHz in

most organic solids. Therefore, its static quadrupole-broadened powder patterns are

sensitive to motions with frequencies in the range 104 to 107 Hz, which makes

them ideal for studying a large range of motions in solids. Molecular motions of

MeOH guests adsorbed in the one-dimensional channels of CPL-1 have been

studied.96 The high degree of crystallinity of CPL-1 allows the MeOH guests to

undergo only two motions, rotation and wobbling, whose aspects have not been

reported in zeolites previously. The dynamic behavior of PCPs with rotational

groups has also been characterized.97,98



7.4



DESIGN OF FRAMEWORKS



7.4.1 Frameworks Suitable for Highly Porous, Robust,

Three-Dimensional PCPs

Three-dimensional PCPs tend to produce highly porous, robust frameworks because

coordination bonds are distributed in a three-dimensional manner in their polymers.

Three types of frameworks have been designed, systematically synthesized, and

characterized to obtain three-dimensional PCPs: (1) a-Po-type frameworks, (2) zeolitic frameworks, and (3) pillared layer frameworks.

The a-Po-type framework, whose appearance is like a jungle gym, is a candidate

for the construction of exceptionally rigid and highly porous structures with threedimensional intersecting channels. A systematic design of the pore size and functionality has been achieved in three-dimensional PCPs [Zn4O(L)]n (L ¼ a variety of

dicarboxylate ligands), as shown in Figure 7.3,74 in which an oxide-centered Zn4O

tetrahedron is edge-bridged by six carboxylates to give the octahedron-shaped SBU

that reticulates into a three-dimensional porous network. Other a-Po-type PCPs



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