X-ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal-Organic Frameworks.

Glass-forming metal-organic frameworks (MOFs) have novel applications, but the origin of their peculiar melting behavior is unclear. Here, we report synchrotron X-ray diffraction electron densities of two zeolitic imidazolate frameworks (ZIFs), the glass-forming Zn-ZIF-zni and the isostructural thermally decomposing Co-ZIF-zni. Electron density analysis shows that the Zn-N bonds are more ionic than the Co-N bonds, which have distinct covalent features. Variable-temperature Raman spectra reveal the onset of significant imidazolate bond weakening in Co-ZIF-zni above 673 K. Melting can be controlled by tuning the metal-ligand and imidazole bonding strength as shown from thermal analysis of nine solid-solution Co Zn -ZIF-zni (x=0.3 to 0.003) MOFs, and a mere 4 % Co-doping into Zn-ZIF-zni results in thermal decomposition instead of melting. The present findings demonstrate the key role of the metal-ligand bonds and imidazolate bonds in controlling the delicate balance between melting and decomposition processes in this class of ZIF compounds.