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 Co‒N bond, which have distinct covalent features. Variable temperature Raman spectra reveal 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 nine solid solution structures of CoxZn1-x-ZIF-zni (x = 0.3 to 0.003) were synthesized, and a mere 4% Co-doping into Zn-ZIF-zni results in thermal decomposition instead of melting. These findings advocate significant chemical space for de-signing new MOF glasses either by engineering the chemical bonding with ligand functionalization or by avoiding defects at the metal site in the crystal lattice.