Using the synchrotron advanced techniques at the ALBA Synchrotron and Elettra Sincrotrone, a research team from ICMol visualized how the removal of volatile molecular linkers reshapes the local metal environment within the framework, revealing a clean and controllable route to create functional materials for sensing and molecular storage.

In nanoscience, the controlled generation of defects in crystalline materials is widely used to tune properties for improved performance. Structural vacancies and distortions can determine how materials absorb light and conduct electricity. In metal-organic frameworks (MOFs) – porous crystalline materials formed by metal nodes and organic linkers – the ability to fine-tune defects has become a powerful strategy to modify reactivity and stability. MOFs can be very useful to store gases, capture carbon dioxide, and catalyze chemical reactions. Proof of that is that the 2025 Chemistry Physics Awards has been for the creators of the first MOFs, Susumu Kitagawa, Richard Robson and Omar Yagh.

Traditionally, researchers have introduced defects in MOFs by working in solution, where reversible chemical bonds allow linkers and metal ions to rearrange dynamically. However, the flexibility of this method makes it difficult to control defect formation with precision. Solvent molecules can redistribute vacancies, which can lead to the formation of unpredictable architectures. Moreover, removing linkers in solution often causes unwanted side reactions – such as oxidation or charge imbalance – that alter the material’s chemistry.

In this new study, published in Advanced Materials, researchers from ICMol overcome these long-standing challenges with an innovative solvent-free approach that effectively “freezes” the structure in place and prevents self-healing.

Their method, called solvent-free thermal defect engineering, involves gently heating the MOF so that only its volatile linkers evaporate. Using a standard thermogravimetric analyser — a tool that precisely measures mass changes during heating — the team selectively removed neutral linkers from the crystalline lattice, leaving behind stable open metal sites.

As a proof of concept, they applied this strategy to the Hofmann-type MOF [Fe(pz){Pt(CN)4}], a standard coordination framework where iron atoms connect to platinum-cyanide layers through pyrazine linkers. The team managed to achieve a series of samples with increasing defect concentrations by carefully controlling the temperature. Remarkably, X-ray powder diffraction measurements conducted at the MSPD beamline of the ALBA Synchrotron confirmed that MOFs remained crystalline even after the thermal treatment and linker removal, and that the defective materials were stable after prolonged exposure to air. This technique was also used to monitor the structural evolution of the MOFs during the in situ thermal treatment, providing insights into how the atomic structure changes as linkers are removed.

To confirm how linker removal affects the crystal morphology and metal coordination, the researchers turned to electron microscopy and synchrotron-based spectroscopy at the Elettra Sincrotrone in Italy.

Spectroscopic and structural analyses revealed a progressive transformation of the local iron environment, from octahedral FeN6 geometries to tetrahedral FeN4 centres. These unsaturated Fe(II) sites retained their oxidation state and could readily bind small polar molecules such as water and acetonitrile. The degree of defectivity directly influenced the material’s reactivity: the more linkers were removed, the more active the MOF became in catalytic tests. In particular, the team demonstrated efficient catalysis of the cyanosilylation of benzaldehyde.

This solvent-free approach thus offers a direct way to “program” the number and type of functional sites in a MOF, opening a path to design materials with tailored chemical reactivity and adsorption properties. Beyond catalysis, such control could also be valuable for molecular sensing, energy conversion and gas storage applications.

This collaborative study introduces a simple yet powerful method of using heat instead of solvents to sculpt the internal architecture of molecular frameworks. The ability to selectively remove neutral linkers while maintaining charge balance and crystallinity makes this strategy broadly applicable across different classes of MOFs.

Structural effects of compositional defectivity in [Fe(pz)x{Pt(CN)4}]. a) Ex situ PXRD patterns of samples with varying degrees of defectivity compared to the simulated diffractogram of [Fe(pz){Pt(CN)4}] (CCDC code: IBUTEE). The inset shows the unit cell with the relevant (001) and (110) crystallographic planes, “*” denotes the position of new peaks after pz is completely removed. b) N2 adsorption/desorption isotherms. c) FT-IR spectra of the C≡N stretching region.