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

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.