Two Metal-Organic Frameworks with a Tetratopic Linker: Solvent- Dependent Polymorphism and Postsynthetic Bromination Daniela Frahm, Frank Hoffmann, and Michael Frö ba* Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany * S Supporting Information ABSTRACT: Synthesis of two polymorphous porous metal- organic frameworks (MOFs), both based on a linear tetracarboxylate linker containing ethinyl functionalities and copper ions, is presented. The resulting structure of the MOF is highly dependent on the solvent used during the synthesis: use of N,N-dimethylacetamide as a solvent results in a three- dimensional structure with fof topology (UHM-8)a wide- spread net regarding MOFs composed of linear tetracarbox- ylates and copper ionswhile a solvent mixture of N,N- dimethylformamide, dioxane, and water (2:1:1) gives rise to a three-dimensional structure based on the more rare stx net (UHM-9). Possible reasons why which topology is favored in each case is discussed in terms of the thermodynamic stability and solvent stabilizing effects. Furthermore, the first postsynthetic modification (PSM) of a nonterminal triple bond inside a MOF structure was accomplished by addition of bromine under mild conditions to UHM-8. Quantitative conversion rates and retention of the crystallinity of the brominated MOF could be confirmed by Raman and NMR spectroscopy and powder X-ray diffraction, respectively. ■ INTRODUCTION Although the term polymorphism is widely used in solid-state chemistry, an all-encompassing definition is elusive. The first modern definition, given by McCrone in 1965, reads as follows: “A polymorph is a solid crystalline phase of a given compound resulting f rom the possibility of at least two dif ferent arrangements of the molecules of that compound in the solid state.” 1 However, there are some cases, which are not unambiguously covered by this definition, arising mainly from the difficulty to decide always without remaining doubts if “a given compound” is “the same” or “not the same”. This applies, for instance, for tautomers. 2 Another famous example is the debate on the two proposed forms of aspirin crystals, which was recently fully resolved by Bond, Boese, and Desiraju, who could show that aspirin in some cases crystallizes as a mixture between the two slightly distinct forms even within one single crystal. 3 Due to such ambiguities, further terms such as “pseudopolymorphism” (covering, for example, hydrates and solvates) have been established. Unfortunately, this results in further discussions about this issue rather than creating distinctiveness. 4,5 In any case, polymorphism is a frequently occurring phenomenon in various areas of solid-state chemistry, particularly important in the pharmaceutical industry because polymorphs of the active compound are often known to have a decisive influence on its pharmaceutical properties. 6 Another large field of solid-state chemistry is given by metal-organic frameworks (MOFs), which constitute a class of hybrid materials composed of inorganic parts like metal ions or metal-oxygen clusters bridged by organic ligand molecules. 7-10 These crystalline coordination polymers exhibit manifold properties for a wide variety of applications like gas storage, 11-13 chemical separation, 14 sensing, 15,16 magnetism, 17 or catalysis. 18,19 Due to the enormous variety of building blocks and synthesis parameters influencing the crystal engineering process an almost infinite structural diversity is offered in which also the occurrence of polymorphism is no longer a rarity. 20-24 In the majority of cases, polymorphism in MOFs is manifested as constituting different topologies. A well-known example is given by the realization of the two different topologies nbo and pts of MOFs comprising tetracarboxylate linkers. Subtle changes of the synthesis conditions like temperature or duration can lead to the one or the other topology. 25,26 Furthermore, Chen et al. could show that the cobalt-based MOF-501 (nbo) can be postsynthetically converted into the slightly thermodynamically more stable polymorphic form MOF-502 (pts) simply by heating. 26 In addition to synthesis temperature and duration, often the solvent used plays a decisive and in many cases an unclear and extreme complex role. To date, several instances of solvent-dependent poly- morphisms in the area of MOFs are known (not to be confused with the “pseudopolymorphism” of solvates mentioned above). 27-31 Besides the cases in which different solvents lead Received: December 12, 2013 Revised: February 13, 2014 Published: February 19, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 1719 dx.doi.org/10.1021/cg4018536 | Cryst. Growth Des. 2014, 14, 1719-1725