Mail-Order Metal−Organic Frameworks (MOFs): Designing Isoreticular MOF‐5 Analogues Comprising Commercially Available Organic Molecules

Metal−organic frameworks (MOFs), a class of porous materials, are of particular interest in gas storage and separation applications due largely to their high internal surface areas and tunable structures. MOF-5 is perhaps the archetypal MOF; in particular, many isoreticular analogues of MOF-5 have been synthesized, comprising alternative dicarboxylic acid ligands. In this contribution we introduce a new set of hypothesized MOF5 analogues, constructed from commercially available organic molecules. We describe our automated procedure for hypothetical MOF design, comprising selection of appropriate ligands, construction of 3D structure models, and structure relaxation methods. 116 MOF-5 analogues were designed and characterized in terms of geometric properties and simulated methane uptake at conditions relevant to vehicular storage applications. A strength of the presented approach is that all of the hypothesized MOFs are designed to be synthesizable utilizing ligands purchasable online. ■ INTRODUCTION Metal−organic frameworks (MOFs) are a class of porous materials comprised of metal or metal oxide vertices interconnected by organic bridging ligands. MOFs have enjoyed considerable research interest due largely to their open pore structures and very high internal surface areas. Accordingly, MOFs are regarded as promising candidates for a number of critical energy-related applications, such as gas separations including carbon capture, hydrogen and natural gas storage, and catalysis. In a paradigm known as reticular chemistry, the constituent metal or organic components of a MOF can be substituted for alternative chemical species. The properties of MOF materials can therefore be controlled by the selection of building blocks with the appropriate chemistry. The reticular approach illustrates that a significantly vast number of MOF materials are possible; however, this combinatorial complexity makes the discovery of MOFs with targeted properties a serious challenge. The chemical space of possible MOFs is typically explored through the enumeration of building blocks and the exhaustive design of possible structures with a specific topology prescribed by the connectivity of the components. The archetypal MOF, and one of the most studied, is isoreticular (IR) MOF-1, or MOF-5. Isoreticular structural analogues of MOF-5 have been explored both experimentally and computationally. The simple, three-dimensional lattice structure ofMOF-5 (pcu net) permits a high degree of structural variance through substitution of the constituent benzene-1,4-dicarboxylic acid for various alternative dicarboxylic acids. Typically, a single, symmetric acid molecule is used, to avoid disorder; however, recent work has explored the possibility ofMOF-5 analogues comprisingmultiple distinct ligands. In enumerating possible MOF materials, an important question is how to select an appropriate database of ligands. In the highlight study in this area, Wilmer et al. analyzed a space of possible MOF crystal structures achievable by combining metal and organic components previously utilized in the literature and permitting additional ligand functionalization; their approach has enumerated approximately 137 000 hypothetical MOF structures. In the current contribution, we present an alternative approach to MOF design, emphasizing the selection of appropriate ligands to achieve a database of readily synthesizable structures. By designing MOFs that incorporate commercially available molecules, we aim to achieve a select set of materials that can be realized without the need for synthesis of organic ligands. Focusing on isoreticular analogues of MOF-5, our approach has four main steps. (1) We explore databases of commercially available molecules to identify those that fulfill the most basic requirements of MOF building blocks. (2) We then select only Received: February 24, 2013 Revised: March 29, 2013 Article