From Micro to Nano: A Toolbox for Tuning Crystal Size and Morphology of Benzotriazolate-Based Metal–Organic Frameworks

Scaling down the size of metal–organic framework (MOF) particles is a challenging but important task for extending the spectrum of potential MOF applications. Herein, we describe downsizing particles of MFU-4, a benzobistriazolate-based Zn­(II)-MOF, which can be prepared by either solvothermal or microwave-assisted synthesis. We report on combining various synthetic strategies leading to a production of nanosized MOF crystals of well-defined sizes and shape. Our approaches include varying reaction time, temperature, solvent choice, and amount of additives (alkali hydroxides or selected amines) in order to gain control over the nucleation process. As a result, the crystal sizes can be tuned over a broad range, from 40 nm large globular particles up to 5 μm cubic crystals with narrow size distributions. Additionally, the article offers a comparison of different techniques used to determine the particle size of crystals. These include dynamic light scattering, X-ray powder diffraction (from Scherrer equation), and transmission electron microscopy

Single-Crystal to Single-Crystal Transformation of a Nonporous Fe(II) Metal–Organic Framework into a Porous Metal–Organic Framework via a Solid-State Reaction

We report the synthesis of an air-stable nonporous coordination compound based on iron­(II) centers, formate anions, and a 4,4′-bipyrazole (H₂BPZ) ligand. Upon thermal treatment, a porous metal–organic framework (MOF) formed due to decomposition of the incorporated formate anions. This decomposition step and the following structural changes constituted a single-crystal to single-crystal transformation. The resulting [Fe­(BPZ)] framework contained tetrahedrally coordinated iron­(II) metal centers. The framework was sensitive toward oxidation by molecular oxygen even at temperatures of 183 K, as followed by oxygen sorption measurements and a color change from colorless to metallic black. The semiconductor properties of the oxidized material were studied by diffuse reflectance UV/vis/NIR spectroscopy and dielectric spectroscopy