Coordination Polymers Versus Metal− Organic Frameworks

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 2969–2970

PerspectiVes Coordination Polymers Versus Metal-Organic Frameworks

Crystal Growth & Design 2009.9:2969-2970. Downloaded from pubs.acs.org by UNIV OF GOTHENBURG on 01/23/19. For personal use only.

Kumar Biradha,*,† Arunachalam Ramanan,*,‡ and Jagadese J. Vittal*,§ Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India, Department of Chemistry, Indian Institute of Technology, New Delhi-110016, India, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543, Singapore ReceiVed December 18, 2008; ReVised Manuscript ReceiVed May 23, 2009

ABSTRACT: Different terminologies such as coordination polymers, metal-organic frameworks, and hybrid inorganic and organic framework materials have been used to describe the nonmolecular or extended solid-state structures containing metal ions and organic spacer ligands. In this perspective, we discuss its origin as well as the pros and cons of using these terminologies in the literature. Crystal engineering deals with understanding the growth of crystalline solids in terms of recognizable supramolecular interactions. Analysis of such interactions between molecules (organic, inorganic, or metal complex) is more obvious as the solids are held together only by weak forces. However, influence of nonbonding interactions in the formation of nonmolecular or extended solids is less obvious due to the occurrence of ionic, covalent, or coordinate bonding in one or more dimensions. A significant example of the latter case is that of coordination polymers (CPs) which are also known as metal-organic frameworks (MOFs). The crystal engineering studies which deal exclusively with weak noncovalent interactions such as hydrogen bonds have been termed as organic crystal engineering, while those with metal-coordination bonds have been called inorganic crystal engineering. The majority of the latter studies deal with the engineering of CPs or MOFs. Yet another term hybrid inorganic and organic framework materials1,2 is also in use, but not so popular given the wider implication of the term. One might think that the usage of different terms for the same solidstate structure is confusing and often considered redundant. Furthermore, it appears to be relevant in view of the recent discussions on terminologies of polymorphism, pseudo-polymorphism, and co-crystals.3-9 In this perspective, we aim to analyze the pros and cons of using CP and MOF terminologies. The topic search using Sci-Finder Scholar for the term “coordination polymer or polymers” reveals that the term CP dates back to before 1950s, while the term MOFs dates to the late 1990s as shown in Table 1. From the statistics, one observes * Corresponding authors. Phone: 91-3222-283346; fax: 91-3222-255303; e-mail: [email protected] (K.B.); phone: 91-11-26591507; fax: 9111-26582277; e-mail: [email protected] (A.R.); phone: +65-65162975; fax: +65-6779-1691; e-mail: [email protected] (J.J.V.). † Indian Institute of Technology, Kharagpur. ‡ Indian Institute of Technology, New Delhi. § National University of Singapore.

that the usage of the two terms has increased tremendously during the period 2001-2008 along with the expansion of this new area of research that also led to the birth of two specialist journals CrysEngComm and Crystal Growth & Design. The term CP is derived from organic polymers and defined in Wikipedia10 as “the term given in inorganic chemistry to a metal coordination compound where a ligand bridges between metal centers, where each metal centre binds to more than one ligand to create an infinite array of metal centers”. A fundamental problem of comparing CPs with organic polymers is that the organic polymers by definition are macromolecules made up of monomers or oligomers associated through covalent bonds with defined molecular weights. In contrast, coordination polymers are comprised of metal-organic units linked together at least in one dimension to form an infinite array through extended covalent or coordinate interactions.11 A number of publications using the term CP ignores this fact and represents the formulas of CP with n where n is an integer. For example, when 4,4′-bpy forms CP with MX2 (X ) anion), it is represented as [M(4,4′-bpy)2X2]n; there are some reports where n is replaced by infinity. Since in a crystal which is finite, such extended interactions are infinite, the use of n or infinity to represent polymeric species appears to be redundant. Another problem associated with CP is it has a close resemblance, but is conceptually very different, to another term “coordination polymerization”, which has often been used out of context in the literature.12 Coordination polymerization deals with the addition of a monomer to a growing macromolecule through an organometallic active center. The term “coordination polymer” was defined by J. C. Bailar in 1964, when he compared organic polymers with inorganic compounds which can be considered as polymeric species.13 In comparison, he established rules for the building and the required properties of new species involving metal ions and

10.1021/cg801381p CCC: $40.75  2009 American Chemical Society Published on Web 06/08/2009

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Table 1. Details of the Hits for CPs and MOFs in the Literature using Sci-Finder Database Search years

coordination polymers

metal organic frameworks

up to 1950 1951-1960 1961-1970 1971-1980 1981-1990 1991-2000 2001-2008

1 12 175 130 154 531 3348

0 0 0 0 0 8 687

organic ligands. Of course, it is another issue that CPs are completely different from organic polymers; they are highly crystalline and cannot be processed like organic counterparts. The metal-ligand bonds can easily be broken due to solvation, and hence the species existing in solution need not be true monomers or oligomers. Since this class of solid is distinctly nonmolecular in nature, one might think that MOF will be a more desirable term than CP. Wikipedia defines MOF as “crystalline compound consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous”. This definition seems to be very specific for crystallinity and dimensionality; the latter is restricted to only extended interactions through covalent or coordinate bonds and not to other nonbonding interactions. Also, porosity is only an additional feature and is not particular to the definition. In fact, it was Yaghi who first introduced the term MOF for the newly synthesized copper 4,4′-bipyridyl complex that exhibited extended metal-organic interactions.14 According to him, “the term coordination polymer is undoubtedly the most nebulous, as it simply signifies the extended connection of metal and ligand monomers through coordination bonds with no regard towards the final structure or morphology”.15 In the literature, MOFs are quite often associated with hydrogen storage properties or in general, porosity; if the material does not exhibit this property, they are perceived to be coordination polymers! Inorganic chemists probably prefer to use the term CPs, while solid-state chemists tend to prefer the term MOF. Multiple terminologies do exist in science and as long as we know the meaning of them, there should not be any confusion. Further, unanimous agreement will certainly eliminate confusion and frustration among the research community.16 The term coordination polymer very broadly encompasses all the extended structures based on metal ions linked into an infinite chain, sheet, or three-dimensional architecture by bridging ligands, usually containing carbon atom.17 Whereas the term MOF is very much appropriate to use for three-dimensional networks, it is inappropriate to use for extended one-dimensional or two-dimensional networks. A framework structure has been defined in solid-state science as “a crystalline structure in which there are strong interatomic bonds which are not confined to a single

Perspectives

plane, in contrast to a layer structure.” Following this definition, even layered structures such as clays may not be qualified as “framework structures”. Hence, it is appropriate to describe onedimensional and two-dimensional extended structures as coordination polymers, while three-dimensional structures may be called as metal-organic frameworks17 (see Supporting Information). Of course, this demarcation is arbitrary and different from what Yaghi et al. proposed recently on the basis of the strength of the M-ligand bonds.18 The use of term MOF may not necessarily be restricted to the three-dimensional structures with porosity or gas storage properties; after all, the proof of porosity in crystals is considered to be a burden!19 Although the new terminologies such as porous coordination polymers (PCPs)20 and metal-organic rotaxane frameworks (MORFs)21 have been introduced, they are the extensions of the existing terminologies and add no confusion to the literature. Acknowledgment. We thank the referees for expressing their views and making us take a stand in this perspective. Supporting Information Available: Examples of CPs, MOFs and organic-inorganic hybrid materials. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG801381P