The field of coordination polymers and metal–organic frameworks (MOFs) has evolved over a period of approximately 25 years to a stage where it is one of the most widely investigated areas of materials chemistry. The field has impacted many areas of science including commercial applications from gas storage agents to new investigations as drug-delivery vehicles . Coordination polymers and MOFs are prepared from the combination of metal cations, commonly d- or f-block metals, and ligands capable of bridging metal centres to create polymeric structures which extend in one, two or three dimensions. The key features of coordination networks and MOFs that make them so attractive to researchers from many fields are their regular structure and the positioning of components in three-dimensional space. The beauty of MOFs lies in the fact that through a basic understanding of coordination chemistry, tracing all the way back to the original discoveries of Werner, the careful chemist can use relatively robust and predictable coordination bonds to position different components of a polymeric framework structure with respect to one another. This important feature of MOFs allows researchers to manipulate structures at the molecular level within polymeric framework structures and hence to modify the properties of the resulting materials. It is this basic premise which underscores the contribution and promise of MOFs in so many fields.
The first use of the term coordination polymer can be traced back to 1916 , and many even consider Prussian blue (first reported in the eighteenth century) the first member of this now vast field of study. However, the field began to grow following seminal papers by Robson and co-workers in the early 1990s [3,4], followed by significant developments by Kitagawa and co-workers  and Yaghi and co-workers , who demonstrated for the first time that it was possible to adapt the approach put forward by Robson to make extensive families of compounds with specific structural topologies. This major advance illustrated that coordination framework structures could be readily adapted and modified in a systematic manner, allowing detailed understanding of the effects of structure, porosity and chemical functionality. Yaghi and co-workers  proposed the term ‘reticular chemistry’ to describe the systematic synthesis of families of frameworks and reported the idea in detail.
This ability to design and control, to varying extents, the three-dimensional arrangement of the metals and ligands has led to a new era of materials design. Materials with pores of specific size, shape and chemical environment can be designed for applications such as molecular sorption, storage and separations, and when combined with metal atoms new types of heterogeneous catalysts become possible. The extended network structures of these materials also lead to cooperative properties, such as electrical conductance, magnetic ordering or switching of unusual optical or thermal expansion properties. The modular design of the synthesis allows for the further tuning of properties across related materials, or even the combination of multiple properties into a single material.
The use of MOFs for gas adsorption is one of the primary interests in developing such materials. The high porosity and surface areas of these materials can be exploited for entrapment of guest species. Such systems have been exploited over a number of years with particular emphasis on the storage of H2 , CH4  and CO2 . The range of gases being studied has expanded over recent years with CO2 now receiving extensive attention. Articles in this theme issue develop these topics and report elegant examples of gas adsorption.
Zaworotko and co-workers  report an extensive study of the adsorption of CO2 by a range of MOFs, drawing comparisons between new MOFs and literature examples. Importantly, the authors conclude that those materials that are constructed using inorganic building blocks, such as (SiF6)2− anions, demonstrate improved CO2 adsorption properties in comparison with other MOF systems. Indeed, this improved performance is observed even in the presence of water, which is always present in flue gases from industrial sources. The study is an excellent example of the drive to move MOFs from the laboratory to an industrial setting.
The stability of MOF materials is a continuing thread in a number of the studies reported in this theme issue and demonstrates the importance of taking what were once aesthetically pleasing laboratory curiosities to real-world applications. Bu and co-workers  report the synthesis of a strontium-based MOF that exhibits excellent stability even in the presence of boiling water, acids and bases, representing a further step forward towards making materials that can be used in a variety of chemically harsh reaction conditions. Kaskel and co-workers  pick up this theme in their study of a zirconium MOF, a member of a family that has been demonstrated to exhibit significantly enhanced stability compared with many other MOFs. The study also touches on a common theme in the field, the existence of polymorphs or supramolecular isomers of frameworks constructed from the same building blocks. Schröder and co-workers  also identify this issue in a series of MOFs built from copper(II) and ligands that contain both pyridyl and carboxylate donors. Indeed, in one example, three distinct supramolecular isomers of the MOF are isolated and characterized by single-crystal X-ray diffraction. The complexity exhibited by framework structures is an important issue when trying to prepare large amounts of phase-pure material for subsequent application.
The use of MOFs as selective adsorption agents is clearly important for gas storage studies but perhaps has wider application for other small molecules. Brammer and co-workers  pick up this theme in a study of porous silver(I) MOFs that selectively adsorb arene and fluoroarene molecules in contrast to alkanes and perfluoroalkanes. Such selectivity is highly important in the development of materials that can ultimately be used in the purification of small organic molecules. This is a growth area in the field of MOFs, and it is anticipated that many further developments in this area will emerge over the coming years.
Another theme that has emerged in the field of MOFs over the last decade is the idea of modifying the framework material following the initial synthetic procedures, so-called post-synthetic modification. In this issue, Sumby and co-workers  report a relatively new development in post-synthetic modification, namely post-synthetic metalation. They achieve this through the synthesis of MOFs that contain vacant coordination sites, in their case a vacant bidentate bis-pyrazolyl ligand, which can be accessed via the channels that run through the framework structure. Treatment of the MOF with transition metal salts allows the introduction of metal complexes to the bidentate coordination site. This synthetic procedure opens pathways to a vast range of new materials and possible application in catalysis and selective adsorption and reactivity. Champness and co-workers  also report a MOF that supports a transition metal complex on the struts of the framework, in their case rhenium or manganese diiimine--tricarbonyl--halide complexes. Such species are known to exhibit interesting photochemistry and in the study reported herein the effect of variation of cations that propagate the MOF structure is investigated.
The studies reported in this theme issue reveal the most topical and emerging subjects in the field of MOFs. From investigations of selective gas/guest adsorption to new photochemical properties, it is abundantly clear that MOFs continue to be highly important for a wide variety of applications. The successful use of MOFs on a larger scale is highly dependent on the development of stable and pure materials, again a theme picked up by a number of the studies reported in this theme issue. Although there have been major advances in the logical and designed synthesis of MOFs since Robson's original studies, it is clear that challenges remain in the field. New directions for the application of MOFs emerge on a regular basis and new topics, for example the exploitation of the framework as a matrix for the support of active molecular species, are covered in this theme issue. It is clear that the attractive ‘design-ability’ of MOFs is a fertile playground for the chemist and the impact of MOFs continues to spread across the sciences.
We declare we have no competing interests.
No funding has been received for this article.
One contribution of 8 to a theme issue ‘Coordination polymers and metal–organic frameworks: materials by design’.
- Accepted October 14, 2016.
- © 2016 The Author(s)
Published by the Royal Society. All rights reserved.