Tunable Flexibility and Porosity of the Metal-Organic Framework DUT-49 Through Post-Synthetic Metal Exchange

As a prominent and representative example of flexible metal-organic frameworks (MOFs), DUT-49(Cu) has gained attention due to the unique phenomenon of Negative Gas Adsorption (NGA); originating from an unprecedented struc-tural contraction during the gas adsorption. Herein, post-synthetic metal exchange is demonstrated to afford DUT-49 frameworks with a wide variety of metal cations, e.g. Mn2+, Fe2+, Ni2+, Zn2+, Cu2+ and Cd2+. The single-crystal-to-single-crystal conversion allowed to characterize the new MOFs by single crystal X-ray diffraction, indicating identical struc-ture and topology, as that of previously explored DUT-49(Cu) framework. This approach is proven successful in achieving Mn-Mn and Cd-Cd dimers, which are rare examples of M-M paddle-wheel SBUs. The relative stability and flexibility of the resulted frameworks are observed to be highly sensitive to the metal ion of the framework, following the trends predicted by Irving-Williams series. DUT-49(Ni) was recognized as a second material from DUT-49 series showing adsorption-induced transitions. A sequential increase in framework flexibility from rigid to flexible and from flexible to NGA has been achieved through selective incorporation of metal centers into the structure. Finally, hetero-metallic structures are formed by selective and controlled exchange of metal ions to finely tune the flexibility and NGA phenomenon of the framework.