Functionalized Zeolitic Imidazolate Frameworks

Abstract

Metal-organic frameworks (MOFs) are showing various structure flexibility in response to external stimuli (e.g., pressures and temperatures). Their responsiveness can be manipulated by the substitution of the functional groups at the organic linker of the frameworks. Some MOFs are recently reported to be able to melt and form glasses, and their melting behavior is also related to functionalizing the linkers. Thus, the mechanisms of controlling responsiveness and melting behaviors of MOFs by linker functionalization (using benzo and cyano groups) have been studied in this work. The high-pressure behavior of a series of Zeolitic Imidazolate Frameworks (ZIFs) of ZIF 62 (M(im)2-x(bim)x, M = Zn2+/Co2+, im– = imidazolate, bim– = benzimidazolate, 0.02  x  0.37) has been studied through in situ high pressure powder X-ray diffraction. The ZIF-62 derivatives are observed to contract reversibly from an open pore (op) to a closed pore (cp) phase under hydrostatic mechanical pressure. Importantly, the observed op-to-cp phase transition switches from the typical first order (discontinuous) to second order (continuous) with increasing the bim– fraction (to 17.5%). Rietveld refinements revealed that the second order transition is achieved by a continuous linker rotation. The mechanism that the void volume and the pore size of the material can be tuned continuously by adjusting the pressure was demonstrated. To investigate the melting mechanism, a series of derivates of ZIF-4 (Zn(im)2) containing various amounts of cyano-functionalized imidazolate linkers was synthesized. The incorporation of electron-withdrawing cyano groups results in a drastic decrease in the melting temperatures. Density function theory (DFT) calculations revealed that the cyano groups weaken the dissociation of Zn–N bonds in the melting process. Remarkably, a particular liquid-liquid transition (LLT) of prototypical ZIF-4 remains in all cyano derivatives. The kinetic fragility of ZIF liquids has been demonstrated to be correlated with their micropore volume. Overall, this work provides a guideline for controlling the mechanical responsiveness and melting behavior of MOFs by linker functionalization. The mechanically tunable MOFs open new possibilities for their application in pressure-switchable devices, membranes, and actuators. The porous glasses produced through controllable melting can further facilitate the manufacture of functionalized MOF glasses.