Insights into the Molecular Assembly of Zeolitic Imidazolate Frameworks by ESI-MS

ZIFs are an interesting class of MOFs with zeolite-like topologies, owing to their structural similarities. Rational design and synthesis of ZIFs are highly challenging due to limited understanding of their formation process from solution, as previous studies were largely on crystal growth and properties. In this contribution, we describe a systematic approach to synthesize well-defined ZIF-8 crystals at different rates of nucleation by adjusting zinc to 2-methylimidazole ratios. For the first time, we report discrete chemical species detected in ZIF-8 synthesis and trace their transition with time using ESI-MS. Cravillon et al. have previously described three essential steps in ZIF-8 nucleation: (i) complex formation, (ii) complex deprotonation, and (iii) ligand exchange and oligomerization. In our work, we were able to identify species undergoing the various steps and correlate their evolution to the different nucleation rates. Applying ESI-MS in both positive and negative modes, we identified two species, “[Zn₄(C₄N₂H₅)₄(C₄N₂H₆)₂(NO₃)₃]⁺” and “[Zn₄(C₄N₂H₅)₅(C₄N₂H₆)₅(NO₃)₄(CH₃OH)]⁻”, with changes in intensities corresponding to onset of nucleation processes. These species could potentially form the four-membered ring which is a basic structural unit in the sodalite framework. The usefulness of ESI-MS in studying ZIF formation is further demonstrated by the successful application of this technique to probe ZIF-67 syntheses