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Activated carbon, a representative porous material, was used for medical purposes in ancient Egypt (1550 BCE), as described in papyrus; it is still widely used today for water purification and other purposes. In 1753 CE, some three thousand years later, the first zeolites were discovered from natural ores as new porous materials and in the first half of the 20th century, zeolites were successfully synthesised artificially. Activated carbon, zeolites, and clay minerals are now well-known examples of porous materials, which are used in our daily lives and in industry. Activated carbon has an excellent adsorption function due to its various pore sizes, but it is not crystalline because it does not have regular pores. Conversely, zeolites have strongly bound inorganic elements such as Al3+ and silicate, giving them stable porous crystals with a regular structure. Guests, such as gas molecules, organic molecules, and ions, are incorporated into these pores. In other words, the stability of the framework is essential for the porous structure.

Thus, existing porous materials have been used for many years as an integral part of human life. If materials with functions such as storage and separation that surpass the functions of activated carbon and zeolites, or materials with completely new porous functions are discovered, and can be produced rationally using low-energy processes, they are expected to bring about revolutionary changes in human life. It is desirable to design nano spaces at will to realise such functions of porous materials, but it has been difficult to do so at the molecular level in zeolites and activated carbon. For this reason, no major scientific progress has been made in porous materials since the discovery of zeolites. In the late 1990s, however, the development of metal complex materials with stable pores based on a completely new concept, called porous coordination polymers (PCPs) or metal–organic frameworks (MOFs), made it possible to control pore size and shape at the molecular level.

CPs are infinite network solids prepared using coordination bonds from metal ions and organic or inorganic ligands. These contain structural motifs extremely important in CP design: (1) one-dimensional chains and ladders; (2) two-dimensional sheets such as square and honeycomb lattices; and (3) three-dimensional lattices such as diamond and rock salt. Especially in CPs with organic ligands, the combination of metal cations and ligands is almost unlimited, making it possible to construct extremely diverse network structures. There are about 90 000 structures of a vast number of CPs so far, among which more than 30 000 CPs with porous structures have been synthesised. 1  

In the late 1980s and early 1990s, structural chemistry studies were mainly focused on network structures themselves. CPs were synthesised using Cu(i) ions, which are prone to forming infinite network structures, and attention was paid to polymer-like structures (e.g., similarities to minerals and natural structures). The earliest examples were dinitrile CP [Cu{NC(C4H8)CN}2]NO3, 2 tris-nitrile CP, [Cu{C(C6H4CN)4}], 3 bipyridine CP [Cu2(4,4′-bpy)3(NO3)2](H2O)3, 4 and pyrazine CP, {[Cu2(pyz)3(MeCN)2](PF6)2(Me2CO)}. 5 It is no exaggeration to state that the chemistry of CP using organic ligands started with Cu(i) ions. Interestingly, the last CP contains organic acetone molecules in the pores of the honeycomb network, an early good example of CP crystals with potentially accessible porous structure. It was not clear at that time whether stable pores could be obtained. Crystals with permanent porosity, which are stable even in the absence of guests, have been long awaited.

In 1997, CPs of [M2(4,4′-bpy)3(NO3)4] (M = Co, Ni, Zn, 4,4′-bpy = 4,4′-bipyridine) were synthesised. 6 These showed a robust porous framework even after the removal of guests, and reversible gas (nitrogen, oxygen and methane) adsorption at room temperature between 1 and 35 atm. It is significant that the gas adsorption capacity at room temperature and high pressure is realised, and that it functions stably enough even under practical conditions. The following year, [Zn(bdc)(H2O)](DMF) was synthesised (bdc = benzene-1,4-dicarboxylate), 7 and its pore properties were investigated by low temperature and low-pressure carbon dioxide and nitrogen adsorption measurements. Subsequently, the stability of the crystal structure of [M2(4,4′-bpy)3(NO3)4] was reconfirmed by the detailed temperature dependence of the guest-free single crystal structure. 8 This clearly supported the presence of CP with robust pores. Since then, the following decades were marked by functional chemistry based on nanosized pores in CPs, resulting in those with various pores (sizes and shapes) and various network backbones. This chemistry has developed rapidly, involving many researchers around the world. Regarding the terminology of MOFs and PCPs, it is recommended 9 that MOFs are defined as ‘organometallic frameworks consisting of a coordination network of organic ligands containing potential voids’. This is synonymous with PCPs.

The third development in CPs (Figure 1) is a demonstration of the flexibility and dynamic properties inherent in the MOF structure. Flexible MOFs were predicted just before the year 2000, 10 and shortly thereafter, in early 2000, several flexible MOFs were synthesised: [Cu(BF4)2(4,4′-bpy)] (ELM1), 11 [Cr(iii)(OH)·{bdc}] (MIL-53), 12,13 [Cu2(pzdc)2(dpyg)] (CPL7) (pzdc = pyrazine-2,3-dicarboxylate; dpyg = 1,2-di(4-pyridyl)glycol), 14 [Cu2(dhbc)2(4,4′-bpy)] (CID1) (dhbc = 2,5-dihydroxybenzoate), and [Cu(bdc)(4,4′-bpy)2]. 15 These gave sigmoidal adsorption isotherms, which show no adsorption at low pressure but a sudden gate-opening behaviour when sufficient pressure is applied. This type of isotherm does not belong to any of the seven classifications (six-type adsorption isotherms by IUPAC). These exhibit a structural change from crystal to crystal in response to the pressure of adsorbates, a feature that distinguishes them from other porous materials and is reminiscent of the induced fit mechanism of bioenzymes and the cooperative phenomenon of haemoglobin.

Figure 1

Discontinuous progress in MOF chemistry. Reproduced from ref. 17 with permission from John Wiley & Sons, Copyright 2020.

Figure 1

Discontinuous progress in MOF chemistry. Reproduced from ref. 17 with permission from John Wiley & Sons, Copyright 2020.

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CPL7 and CID1 showed a different gate-opening pressure of the sigmoid curve for the adsorption of nitrogen, oxygen, carbon dioxide, and methane at room temperature. The clear recognition of such a weakly interacting low-boiling gas, even in a supercritical state, was unprecedented and a big surprise at the time. In other words, at the beginning of the 21st century, a new material called the flexible MOF was born, with excellent properties that clearly set it apart from existing porous materials, and it was a milestone in MOF science. Later, it was comprehensively viewed as a crystal phase transition involving deformation of the entire crystal structure and local molecular motion within a crystal, caused not only by chemical stimuli (gas adsorption) but also by physical stimuli (temperature, pressure, light, electric field). The flexibility and dynamics of these structures and the multi-stability due to their porosity distinguish them from conventional porous solids such as carbon, zeolites, phosphates and mesoporous silica.

CPs are categorised into three generations (Figure 2). 10 (1) CPs that collapse by this operation as the first-generation type (1G). (2) Substances that have a stable spatial structure and can be used (called PCP or MOF) as the second-generation type (2G). (3) The flexibility of the metal–organic ligand backbone is a characteristic of the third-generation type (3G), which is expected to have a dynamic structure and function. Later, it became clear that this property encompassed not only MOFs but also a wide range of other organic crystals, and the term soft porous crystal (SPC) emerged as a term to broadly include materials with that attribute. 16  

Figure 2

Classification of coordination polymers. Reproduced from ref. 16 with permission from Springer Nature, Copyright 2009.

Figure 2

Classification of coordination polymers. Reproduced from ref. 16 with permission from Springer Nature, Copyright 2009.

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Three important developments in CP along the timeline of the last few decades have had a major impact on the direction of research trends (Figure 1). 17 The first development is the discovery of organic ligand-bridging structures (first generation, 1G). The application of organic ligands to CPs has expanded chemistry in terms of modularity and design and has given rise to numerous new networks. Single-crystal X-ray analysis was essential to characterise these structures. The first organic ligand-bridged coordination polymers characterised by X-ray crystallography were constructed from the d10 metal ion Cu+ and adiponitrile, as mentioned previously. 2  

The second noteworthy progression was the development of CPs with robust and permanent porosities by gas adsorption, which opened up a new direction for the chemistry of porous materials. In general, CPs are isolated as stable crystals with guest ions and/or solvents filling the pores. To utilise the pores, these guest molecules must be removed (by decompression and heating, etc.). Porous materials are characterised by their high density of introduced voids, in contrast to dense solid crystals. Crystal structures of organic molecule-based networks with pores are often disrupted or deformed by the removal of guest ions or molecules, and their porous function cannot be utilised. Regarding the mechanical and thermodynamic stability of the porous structure, it was common knowledge that porous materials using organic materials do not provide a robust porous framework. The demonstration of the strong and permanent porosity of CPs by gas adsorption in the late 1990s was an epoch-making event in CPs and opened a new direction in the chemistry of porous materials. Since then, many such porous crystals have been synthesised and are now also called MOFs and PCPs (2G).

The third development was the demonstration of the intrinsic flexibility and dynamic properties of the MOF structures. In condensed matter systems such as metal oxides, structural phase transitions of crystalline phases responsive to external physical stimuli are frequently observed and have been studied as a treasure trove of diverse solid–state properties such as conductivity, dielectricity, and magnetism. Structural phase transitions have also been well-studied in molecular organic crystals. When compared to external physical stimuli, the phenomenon of incorporation of chemical species into crystals is a kind of chemical reaction that also generates space in the solid for their diffusion and trapping. To incorporate guest molecules into a crystal, the crystal structure itself is broken (amorphous) or a structural phase transition occurs to give rise to a new crystalline phase. There is a competition between the interaction energy with guests and MOF that contributes to the stabilisation of the system and the energy of the structural change that contributes to its destabilisation. For the guests to penetrate the dense structure, and for a phase change to occur and a new crystalline phase to be formed, it is important that a new phase overcomes the energetically negative effect of the structural change, and that the interaction energy complements it and moves in the direction of the stabilisation. Molecular crystals of metal complexes are no exception and crystals in which metal complex-assembled crystals change colour (solvatochromism, vapochromism) on their exposure to moisture or organic (mainly solvent) vapour, were discovered early on.

Solid crystals are conventionally thought of as solid, but are there any that are soft enough to incorporate molecules? Since zeolites have a robust structure with pre-prepared pores, the introduction of guest molecules rarely changes the structure. It is common knowledge that materials with a porous structure have a mechanically and thermodynamically stable pore structure. The author studied the precedent of structural phase transitions induced by external stimuli in many solid-state materials and came up with the idea that MOFs have an intrinsically flexible framework that undergoes structural deformation in response to external stimuli, especially the concentration (pressure) of guest species, spreading cooperatively throughout a crystal. Subsequently, framework deformations of MOFs (flexible MOFs) were discovered due to the incorporation of guests, and it became clear that a phase transition occurs readily in MOF families, which is an outstanding feature of MOFs. This finding broke the common sense that porous materials have rigid pores and caused a paradigm shift in MOFs, where the materials are capable of framework deformation to suit the guest. This property has established a position as a completely new porous material (3G) that is distinct from conventional materials such as activated carbon and zeolites.

MOFs have opened up new horizons in practically all branches of engineering, physics, chemistry, biology and medicine (Figure 3). 18,19 They are increasingly being used for the storage, separation, and catalytic transformation of carbon dioxide, methane, and other gaseous and aromatic molecules, as well as drugs, with thousands of research reports being published every year. There are now many books and review articles on MOFs, but few have been written on flexible MOFs. Their cooperative phenomena lead to multiple chemical or physical changes upon stimulation of the materials, leading to their designation as multifunctional or ‘smart’ materials. Flexible MOFs are unique materials with structural flexibility not found in other porous materials. These structural features and functions produced by flexible MOFs are being developed for practical use because they are unique and cannot be found in other materials today.

Figure 3

Future prospects for MOFs contribution to social issues.

Figure 3

Future prospects for MOFs contribution to social issues.

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This book provides a basic and accessible overview of the historical background of the chemistry of flexible MOFs, their characteristics, especially their design and synthesis, dynamic structural analysis, functions, theoretical treatment and interpretation of mechanisms, and their applications. The chapters in this book are written by researchers who were involved in flexible MOF chemistry from an early stage and have developed the field. This book also presents many examples of flexible MOFs and summarises the research methods used to study them. The understanding of this chemistry will be of great help to young researchers working on porous materials, as well as to those who are already familiar with conventional porous materials, in creating new materials.

I would like to thank Professor Richard Walton, series editor, for his help and support in launching this project in May 2020. I would also like to thank Ms Katie Morrey (Commissioning Editor), Merlin Fox (Associate Publisher), Amina Headley (Editorial Assistant) and the Royal Society of Chemistry. I also thank Professor Shigeyoshi Sakaki, Assistant Professors Ken-ichi Otake and Shotaro Hiraide, Drs Ping Wang, Ziqian Xue, Maryam Nurhuda and Nathan C. Flanders, for their help in my peer review.

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