Metal pyrazolate frameworks: crystal engineering access to stable functional materials

As the focus evolves from structure discovery/characterization (what it is) to property/performance exploration (what it is for), the pursuit of stable functional metal-organic frameworks (MOFs) has been ongoing in terms of both fundamental research and industrial implementation. Under the guidance of crystal engineering principles, a plethora of research has developed pyrazolate MOFs (metal pyrazaolate frameworks, MPFs) featuring strong coordination M-N bonding. This attribution helps them retain their structures and functions under the alkaline conditions required for practical use. Based on poly-topic pyrazolate ligands, various classic MOFs, such as Co(bdp), Fe2(BDP)3, Ni8L6, PCN-601, and BUT-55, to name a few, have revealed fascinating architectures, intriguing properties, and record-breaking performances in applications during the past decade. This review will present the full scope of MPFs to date: (1) the superiority and significance of constructing MPFs through the crystal engineering approach, (2) synthetic strategies adopted in building and/or modifying MPFs, (3) structural features and stability of the MPF community, and (4) potential applications in energy and environmental related fields. The future opportunities of MPFs are also discussed for designing the next-generation of smart materials. Overall, this review attempts to provide insights and guidelines for the customization of pyrazolate-based MOFs for specific purposes, which would also promote the development of stable functional porous materials for addressing societal challenges.