Monitoring the Activation of Open Metal Sites in [FexM3–x(μ3-O)] Cluster-Based Metal–Organic Frameworks by Single-Crystal X-ray Diffraction

While trinuclear [Fe_xM_3-x(μ₃-O)] cluster-based metal-organic frameworks (MOFs) have found wide applications in gas storage and catalysis, it is still challenging to identify the structure of open metal sites obtained through proper activations and understand their influence on the adsorption and catalytic properties. Herein, we use in situ variable-temperature single-crystal X-ray diffraction to monitor the structural evolution of [Fe_xM_3-x(μ₃-O)]-based MOFs (PCN-250, M = Ni²⁺, Co²⁺, Zn²⁺, Mg²⁺) upon thermal activation and provide the snapshots of metal sites at different temperatures. The exposure of open Fe³⁺ sites was observed along with the transformation of Fe³⁺ coordination geometries from octahedron to square pyramid. Furthermore, the effect of divalent metals in heterometallic PCN-250 was studied for the purpose of reducing the activation temperature and increasing the number of open metal sites. The metal site structures were corroborated by X-ray absorption and infrared spectroscopy. These results will not only guide the pretreatment of [Fe_xM_3-x(μ₃-O)]-based MOFs but also corroborate spectral and computational studies on these materials.