Decoding the Gate Opening Mechanism of the Flexible Framework RPM3-Zn upon Hydrocarbon Inclusion

Although the existence of a gate-opening phenomenon in the flexible RPM3-Zn is well known, the actual mechanism remains a mystery. Here, we provide a full picture that unambiguously identifies and explains the gate-opening mechanism in RPM3-Zn upon exposure to various hydrocarbons-acetylene, ethylene, ethane, propane, and butane-by combining insights gained from calorimetry, adsorption isotherms, PXRD, in situ infrared spectroscopy, and ab initio simulations. We find that the key to gate opening in this framework is the stretching of a bond between an O and the Zn metal center (COO-Zn), acting as a "stabilizer", which weakens the necessary support required for the structure to remain intact. Consequently, an increasing concentration of guest molecules exerts sufficient internal pressure to induce strong structural transformations in the unit cell shape and volume, thus triggering the gate opening. Our results are critical to understanding the gate opening in several other flexible frameworks and provide an opportunity to fine-tune hydrocarbon separation.