Constraining Flexibility in the MIL-88 Topology through Integration of 3-Dimensional Linkers

ABSTRACT: Metal−organic frameworks (MOFs) make up a class of crystalline, nanoporous materials that are recognized for their tunability. While some MOFs demonstrate flexibility, this characteristic can pose challenges in achieving precise pore control or establishing permanent porosity. Specifically, MIL-88B is notable for its high flexibility, as it is constructed from metal trimer clusters and two- dimensional linkers (2DLs) featuring planar, aromatic cores, allowing significant structural changes. In this study, we synthesized two new MOFs, NU-2010 and NU-2011, which are structurally analogous to MIL-88B but incorporate ditopic three-dimensional linkers (3DLs) with sterically bulky cores and higher symmetry. Our aim was to investigate whether the introduction of 3DLs could mitigate the flexibility observed in MIL-88B. We employed a combination of single-crystal and powder X-ray diffraction techniques to assess the flexibility of MIL-88B, NU-2010, and NU-2011 under various conditions, including thermal activation, solvent exchange, and temperature changes. Our findings indicate that incorporating 3DLs significantly reduces the framework flexibility in NU-2010 and NU-2011 relative to MIL-88B.