Unlocking porosity: structural tuning of urea-based MOFs

In the synthesis of metal-organic frameworks (MOFs), efforts to increase the pore sizes by elongating linkers often result in interpenetration, a challenge that is exacerbated when functional groups are added. Urea groups and their derivatives play an important role in supramolecular chemistry by providing directional hydrogen bonding donor sites and are often considered "privileged groups". Incorporating these moieties into MOFs allows for their spatial separation, a critical approach to enhancing their functional activity, simultaneously preventing interpenetration, and enlarging the pore size. In this study, we synthesized hydrophilic urea-functionalized MOFs using zinc ions and the ligands 4,4'-(carbonylbis(azanediyl))dibenzoic acid (L1) and 1,3-di(pyridin-4-yl)urea (L2). Different levels of interpenetration and pore size were produced by varying temperature and starting material concentrations. UA-1 and UA-2 displayed 4-fold interpenetration. Notably, UA-3 formed non-interpenetrated 1D hexagonal mesoporous channels with six urea groups around each hexagonal pore, making it the first example of a non-interpenetrated mesoporous urea MOF. This topology differs from existing RCSR representations. We also investigated the host-guest interactions when introducing various organic molecules into UA-3, using a combination of single crystal X-ray diffraction (SCXRD), thermogravimetric analysis (TGA) and solid-state nuclear magnetic resonance (ss-NMR) spectroscopy. SCXRD provided insight into the amount and position of solvent molecules within the channels of the framework.