(727c) Polymorphism in Metal-Organic Frameworks: Effect of Functionalization

Recently, considerable research has been focused towards development of novel Metal-Organic Frameworks (MOFs) that are targeted for specific applications. MOFs are 3-d nanoporous materials consisting of metal centers bonded to organic linkers, which offers ample opportunities for tuning fundamental properties such as diffusion and adsorption of small molecules. One commonly used approach involves the functionalization of the organic linker. This strategy is typically expected to yield a new iso-structural MOF with more favorable properties. Recently, alkyl substitution of BTC (BTC = 1,3,5-benzenetricarboxylate) was shown to form a topology that was completely different from the well-studied HKUST-1. Such substitution induced polymorphism has been previously shown to exist for Zeolitic Imidazole Frameworks (ZIFs).

In this talk, we report the results of Density Functional Theory (DFT) calculations that investigate the relative stability the two experimentally reported topologies of CuBTC. We extend these calculations to explore the hypothetical topologies for BTC MOFs. Our results show that the relative energies of the two experimental topologies for functionalized BTC MOFs are very similar, and that the difference is comparable to that found for the various ZIF polymorphs. In addition to methyl and ethyl, the effect of other substituents  will also be discussed. The relative stabilities of BTC polymorphs is also studied using a classical flexible force field which is then compared to the DFT results.