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Control of Metal–Organic Framework Crystal Topology by Ligand Functionalization: Functionalized HKUST‑1 Derivatives

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posted on 05.11.2014, 00:00 by Yang Cai, Ambarish R. Kulkarni, You-Gui Huang, David S. Sholl, Krista S. Walton
Metal–organic framework (MOF) materials are nanoporous crystals that have attracted intense interest in the fields of adsorption and catalysis. MOFs are interesting in part because of the concept of reticular synthesis, which allows families of isostructural crystals to be developed by varying ligand length and functionality. The predictability associated with MOF crystal structures is complicated in situations where ligand functionalization leads to new crystal structures. In this work, we show experimentally how the crystal structures of derivatives of HKUST-1 (Cu-BTC) vary for a set of six functionalized ligands: methyl, ethyl, methoxy, bromo, nitro, and acetamide. Our experiments indicate that synthesizing MOFs with these functionalized ligands leads to multiple distinct crystal structures. We further show that these structures can be rationalized computationally using density functional theory (DFT) and electron localization function (ELF) calculations. This analysis led to a simple “design principle” for predicting the structure using the bonding characteristics of the functional groups to BTC linkers. This heuristic, in combination with the more detailed calculations of the kind we have presented, will be useful in future efforts to predictively control the crystal structure of similar MOF materials.