Design of MOFs with Absolute Structures: A Case Study

Metal-organic frameworks (MOFs) with absolute structures are of particular interest as the asymmetry of their crystal structures enables wide applications from asymmetric catalysis to enantioselective separation. Previous reports on the synthetic strategies toward MOFs with absolute structures always required the use of chiral precursors or chiral templates, however, it is not always easy to obtain enantiopure starting materials. Here in, we report a new non-centrosymmetric MOF, MOF-829, synthesized from aluminium salt and achiral organic linker. Further comparisons are made between MOF-829 and a reported Al-based chiral MOF (MOF-520) that is composed of the same metal ion and linker, and similarly synthesized without using chiral compounds. The configurations of the building units, the absolute structures of both MOFs, and their topologies were investigated in detail. We found that (i) topology is one of the determining factors in the formation of non-centrosymmetric MOFs; (ii) the formation of chiral MOFs further requires the directionality of chiral linkers but more importantly chiral secondary building units (SBUs), which can be realized by fine tuning of the synthetic conditions. We envision that both synthetic exploration of chiral SBUs and the design of non-centrosymmetric topologies will open a new direction in the design of MOFs with absolute structures.