Robust Barium Phosphonate Metal–Organic Frameworks Synthesized under Aqueous Conditions

The design and discovery of three-dimensional crystalline metal–organic frameworks (MOFs) from linkers with phosphonate coordinating groups and even alkaline earth metals is largely undeveloped. Herein, we report a strategy for realizing new, stable, and robust barium phosphonate MOFs, termed Empa-1 and Empa-2. The two-dimensional (2D) Empa-1 or three-dimensional (3D) Empa-2 could be realized by way of systematically modulating the ratio of Ba2+ with a tetratopic phosphonate-based linker that was crafted to incorporate nitrogen-rich triazine units bridged by a fixed piperazine core. In addition to this synthetic approach, temperature-dependent synchrotron-radiation powder X-ray diffraction analysis demonstrated that the 2D Empa-1 undergoes an irreversible phase transition upon heating and subsequent dehydration to form the 3D Empa-2. Given the presence of uncoordinated phosphonic acid moieties within the structure of 3D Empa-2, the CO2 sorption capabilities are reported. We believe our ability to link the alkaline earth metal barium with a novel tetratopic phosphonate linker, as evidenced by the robust structures of Empa-1 and -2, paves the way for further exploration and discovery of new crystalline, porous frameworks with greater structural diversity, stability, and wide-scale practical applicability.