X‐ray Electron Density Study of the Chemical Bonding Origin of Glass Formation in Metal–Organic Frameworks**

Abstract

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 ZnÀ N bonds are more ionic than the CoÀ 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 metalligand and imidazole bonding strength as shown from thermal analysis of nine solid-solution Co x Zn 1À x -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.

Effects of Ligand Substituents on the Character of Zn-Coordination in Zeolitic Imidazolate Frameworks

“…Despite the general consensus regarding the ionic nature of the Zn-N bond in ZIFs, [27][28][29][30] the actual strength of this bond and the influence exerted on it by the chemical environment are still a matter of debate. 15,31,32 In particular, recent studies have unraveled non-trivial characteristics in the electronic structure of ZIF-8, [33][34][35] which point to the existence of some degree of covalence in the bond between Zn and N. This hypothesis is supported by ab initio calculations on the transport properties 33 and by the vibrational activity of ZIFs. 34,35 Experimental evidence of the (partial) covalent character of Zn-N coordination is, to date, only indirect.…”

Impact Of Local Structural Characteristics On Glass-formation In Metal-organic Frameworks

“…The final major peaks at 1363 and 1422 are attributed to C=C 𝜈 (202). During heating, the peaks generally shift to a lower wavenumber, which is associated with increased bond length in the organic units (171). This is an expected finding, though the minimal change to the lattice volume suggests that the Zn-N bonds remain largely constant throughout heating.…”

“…It should be noted that not all Cu/Zn-ZIF-4 samples were able to be melted and quenched into glasses without decomposition occurring, which complicates the analysis. Based on the work of Sarkar et al (171), it is likely that this is tightly associated with the nature of the coordination bond itself. Based on their work, it can be inferred that the Cu-N bonds are more covalent in nature when compared to the Zn-N bonds, which is feasible due to the difference in electronegativity between the two metals (Zn=1.65, Cu=1.9 (172)).…”

“…During the VT-Raman analysis (Figure 8.2), several findings were made. First, the signal quality deteriorated significantly above 400 °C, likely due to a combination of bond weakening (171) as well as thermal decomposition due to the insufficient protective atmosphere. As, dmbIm substitutes for bIm at increasing levels (0.13 and 0.24-ZIF-62) the stability is noticeably lower, as the signal-to-noise ratio is significantly lower at 400 °C than that of conventional ZIF-62.…”

“…In accordance with the findings of chapter 5 (17), stronger Zn-N bonds would result in lower meltability. The bond strength of these could either be investigated by electron density analysis, like the work of Sarkar et al (171) or by far-IR spectroscopy similar to Hou et al (102). The proposed effect of the electron-donating nature of dmbIm is an additional parallel to the mixed metal node effect discussed in chapters 4 and 6.1. specifically, the nature of the chemical metal-to-ligand bond has a profound effect on the specific melting characteristics.…”