Reducing defect density in UiO-68–CHO is key for its efficient and reliable post-synthetic modification

Abstract

Post-synthetic modification (PSM) is a powerful tool for introducing complex functionalities into metal–organic frameworks (MOFs). Aldehyde-tagged MOFs are particularly appealing platforms for covalent PSM due to the high reactivity of aldehyde groups, but the same feature also makes their solvothermal synthesis challenging. In this work, we show that while lowering the temperature during the synthesis of aldehyde-tagged UiO-68 avoids aldehyde group degradation and yields a highly porous and crystalline material, the resulting UiO-68–CHO contains a large fraction of missing linker defects and, as a result, its PSM is both inefficient and non-repeatable. However, we also show that this problem could be solved by 1) using an excess of linker during the synthesis of the MOF and 2) by soaking the crude material in the solution of the linker, which together reduce the density of defects enough to yield an excellent substrate for PSM. Treatment of the ‘healed’ material with model amines gives nearly quantitative conversions of aldehydes into imines, even if no excess of reagents is used. Importantly, the PSM of the ‘healed’ UiO-68–CHO gives repeatable results over many days, unlike the PSM of the highly defective MOF. Owing to these developments, various functionalities, such as new coordination sites, drug cargo, chirality, and hydrophobicity, were successfully introduced into the UiO-68 framework. The deleterious influence of defects on the PSM of MOFs and the solution to this problem proposed herein are likely to be of general nature and hence might help in developing new and versatile platforms for covalent PSMs.