Part One
Design of Multifunctional Porous MOFs
Chapter 1
Design of Porous Coordination Polymers/MetalāOrganic Frameworks: Past, Present and Future
Satoshi Horike and Susumu Kitagawa
1.1 Introduction
At the end of the 1990s, a new porous compound with an inorganicāorganic hybrid framework had an impact on the field of porous materials and represented a new family for porous chemistry. Porous coordination polymers (PCPs), also known as metalāorganic frameworks (MOFs), have regular pores ranging from micro- to mesopores, resulting in a large pore surface area, and a highly designable framework, pore shape, pore size, and surface functionality. Their structures are based on organic ligands as linkers and metal centers as the connectors. The rich functionality and designability of the organic ligands and the directability and physical properties of the metal ions are fascinating for the design of various functions, not only conventional adsorptive functions such as storage, separation, and catalysis, but also other physical/chemical functions that can be integrated in the frameworks. Whereas the components of PCPs are connected by coordination bonds and other weak interactions or noncovalent bonds (H-bonds, Ļ-electron stacking, or van der Waals interactions), the interactions lead to structural flexibility and dynamics in the crystalline state, which also promotes the unique character of PCPs in the field of porous materials. As synthetic techniques and knowledge have increased in the last decade, we are now ready to design advanced porous functions by making full use of the chemical components and structural topologies. In this chapter, we introduce the background of PCPs/MOFs with some of the main framework designs and describe the unconventional porous properties of multifunctional porous materials based on ligandāmetal networks.
1.2 Background and Ongoing Chemistry of Porous Coordination Polymers
Coordination polymers (CPs) are a family of compounds with extended structures formed by metal ions and organic and/or inorganic ligands with coordination bonds. They can provide various frameworks constructed from one-, two-, and three-dimensional networks. The late transition metal elements (Cu, Ag, Zn, and Cd) tend to provide this type of framework and the chemistry of CPs has been elucidated with the development of single-crystal X-ray crystallography. The term ācoordination polymerā was used in a paper in 1916 [1], but there was no means of demonstrating infinite frameworks without single-crystal X-ray crystallography. A three-dimensional coordination framework connected by a CN bridge was realized in 1936 [2], namely the well-known Prussian Blue compounds. Currently, coordination polymers having porous properties are termed PCPs or porous MOFs, and therefore we suggest ācoordination frameworkā as an all-inclusive term because the chemistry of the background is defined as āchemistry of coordination space.ā To understand the background of this chemistry, there are three important concepts: (1) framework, (2) molecular metalāorganic hybrid, and (3) porosity.
1. Concept of Framework
It is well known that CPs provide us with one-, two-, and three-dimensional motifs. In particular, the structural concept of a framework was demonstrated by Hofmann and KĆ¼spert [3], whose compounds are known as the family of Hofmann compounds having a two-dimensional layer-based architecture. The first X-ray crystallographic structure was obtained in 1949 [4]. The complete three-dimensional framework, the so-called Prussian Blue complex, appeared in 1936 and a comprehensive study was performed by Iwamoto et al. in 1967 [2, 5].
2. Molecular MetalāOrganic Hybrid
Hofmann and Prussian Blue compounds have structures bridged by the inorganic ion CNā, and therefore have a restricted variety of structures. On the other hand, frameworks having organic linkers afford not only designability but also functionality of frameworks. The X-ray crystal structure of the metalāorganic coordination framework of [Cu(adiponitrile)2]Ā·NO3 appeared in 1959 [6]. Since then, many compounds in this category have been synthesized and characterized crystallographically. Yaghi et al. termed these compounds āmetalāorganic frameworks (MOFs)ā in 1995 [7]. [Cu(adiponitrile)2]Ā·NO3 contains the NO3ā anion in the voids. Such compounds are regarded as clathrate-type CPs, however, which are not categorized as āporousā compounds. By the late 1990s, many clathrate-type CPs/MOFs had been synthesized.
3. Porosity
Porosity means āthe quality or state of being a porous entity, which has many small holes that allow water, air, and so on, to pass through.ā The porosity is antithesis to Aristotle's proposition, āNature abhors a vacuum.ā Indeed, closely packed solid structures formed by molecules and ions can easily form. Researchers have often misunderstood that the crystallographic structure of MOFs having guest species in their voids is a porous material. In 1997, āporosityā was demonstrated to give a compound that maintains a porous structure without guests in the pores; gas sorption experiments under ambient conditions were carried out for stable apohosts [8, 9]. Reversible gas storage properties were identified and the PCPs have attracted wide attention as new porous materials. Since that point, the number of reports on PCPs has been increasing rapidly, and many researchers have been developing strategies for the design of porosity, some of which are intrinsically unique to PCP materials.
1.2.1 Frameworks with High Surface Area
One of the great advantages of PCPs/MOFs is their high surface area, attributable to the low density of the porous structure. An MOF composed of Zn4O clusters connected by benzenedicarboxylate (bdc), [Zn4O(bdc)3] (MOF-5), was synthesized in 1999 and possesses a cubic structure with an ordered three-dimensional (3D) porous system (Figure 1.1a) [10]. This compound has a BET surface area of 3800 m2 gā1 [11]. Many porous compounds have been synthesized on the basis of this structural motif, and this approach has been intensively developed to design important porous frameworks. Some related frameworks, [Zn4O(btb)2] (MOF-177) and [Zn4O(bbc)2] (MOF-200) {btb = 1,3,5-benzenetribenzoate; bbc = 4,4ā²,4ā³-[benzene-1,3,5-triyltris(benzene-4,1-diyl)]tribenzoate)} also possess high porosity; the reported BET surface areas for these compounds are 4746 and 6260 m2 gā1, respectively [12, 13]. The self-assembly process of structure growth often faces network interpenetration, which precludes a high surface area, but further improvements in the design of pore network topologies could avoid interpenetration to achieve extremely high surface areas.
Porous frameworks constructed from two or more kinds of ligands are in some cases effective in the design of high surface area compounds. Zn4O(t2dc)(btb)4/3 (UMCM-2) (t2dc = thieno-3,2-bithiophene-2,5-dicarboxylate) (Figure 1.1b) is also made up of Zn4O clusters and two distinct ligands contribute to the construction of the porous framework [14]. There is a narrow distribution of micropores at 1.4ā1.6 and 1.6ā1.8 nm and a mesopore at 2.4ā3.0 nm and the calculated BET surface reaches 5200 m2 gā1.
Another framework, [Cr3F(H2O)O(bdc)3] (MIL-101), is made from the linkage of terephthalate and chromium trimer units that consist of three Cr cations and the Āµ3O oxygen anion [15]. The pore space is constructed from two cages with diameters of 2.9 and 3.4 nm which are connected with windows with diameters of 1.2 and 1.45 nm, respectively. The compound has a BET surface area of 4100 m2 gā1 and, compared with the Zn4O-type metal cluster, the framework is more stable against water and other chemical species and it has also been utilized as a porous matrix for post-synthesis or hybridization with other species such as metal particles [16].
A paddle-wheel-type dimetal cluster is a popular building unit to construct frameworks. Many transition metals can form this type of cluster and it affords square grid extended networks. [Cu(H2O)]3(ntei) (PCN-66) is prepared by the combination of 4,4ā²,4ā³-nitrilotris(benzene-4,1-diyl)tris(ethyne-2,1-diyl)triisophthalate (ntei) and a Cu2+ paddle-wheel cluster and the ...