Abstract
Chemical reactions in solution almost always take place via a series of minute intermediates that are often in rapid equilibrium with each other, and hence hardly characterizable at the level of atomistic molecular structures. We found that single-molecule atomic-resolution real-time electron microscopic (SMART-EM) video imaging provides a unique methodology for capturing and analyzing the minute reaction intermediates, as illustrated here for single prenucleation clusters (PNCs) in the reaction mixture of metal–organic frameworks (MOFs). Specifically, we found two different types of PNCs are involved in the formation of MOF-2 and MOF-5 from a mixture of zinc nitrate and benzene dicarboxylates at 95 °C and 120 °C, respectively. SMART-EM identified a small amount of 1-nm-sized cube and cube-like PNCs in the MOF-5 synthesis, but not in the MOF-2 synthesis. In the latter, we instead found only linear and square PNCs, suggesting that the MOF-2/-5 bifurcation takes place at the PNC stage.
Highlights
Chemical reactions in solution almost always take place via a series of minute intermediates that are often in rapid equilibrium with each other, and hardly characterizable at the level of atomistic molecular structures
To stop the metal–organic frameworks (MOFs) forming reactions taking place at high temperature, we cooled down the reaction mixture quickly to 25 °C, immediately followed by removal by filtration of the solvent and the soluble materials to prevent further reactions
Through the combined use of dynamic light scattering (DLS) analysis in solutions and microscopic analysis of the solvent-free products, we found that only two kinds of molecular assemblies formed in the zinc MOF syntheses: MOF crystals and 1-nm-sized prenucleation clusters (PNCs)
Summary
Chemical reactions in solution almost always take place via a series of minute intermediates that are often in rapid equilibrium with each other, and hardly characterizable at the level of atomistic molecular structures. The formation of D (MOF-5 node) from C requires the addition of one zinc cation and three molecules of BDC, with a considerable entropy loss[25,26] This chemical diagram suggests that the linear and square (lower order, LO) PNCs made only of Zn and BDC should form readily under mild conditions, while the cube and cube-like (higher order, HO) clusters requiring nodes C and D should increase in number as DMF decompose upon prolonged heating at 120 °C. We verified this hypothesis experimentally by SMART-EM studies of the clusters isolated from the reaction mixture as described below. The SMART-EM technique recently revealed the feasibility of single-molecule level kinetics[27,28,29,30], and allows us to investigate atomistic structures of minute intermediates of chemical reactions
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