Solvent-Free Thermal Defect Engineering in Molecular Frameworks With Volatile Linkers.

Abstract

The controlled generation of defects in crystalline materials is widely used to tune properties for improved performance. This strategy is increasingly applied to metal-organic frameworks (MOFs), where coordination vacancies are commonly introduced in solution by exploiting the reversibility of metal-ligand bonds. Here, an innovative solvent-free approach for defect engineering in MOFs is reported based on the selective thermal removal of neutral volatile linkers. This method enables the generation of metal vacancies across a broad compositional space (0-100%) without requiring counterions, redox adjustments, or oxide formation to balance charge. Using a standard thermogravimetric analyser, the extent of linker sublimation is controlled with high precision and reproducibility. Key design criteria for applying this strategy are identified and validated with the Hofmann-type MOF [Fe(pz){Pt(CN)₄}] (pz = pyrazine). Structural and spectroscopic analyses reveal a local transformation from FeN₆ to FeN₄ environments, leading to redox-stable unsaturated Fe^II sites that remain chemically accessible. These open centres suppress spin crossover, coordinate to polar molecules such as water and acetonitrile, and catalyse Lewis acid-type reactions. The ability to generate functional open metal sites without solvents or charge-balancing agents offers an alternative route for designing defect-functional materials via thermal linker removal.