Interrogating Kinetic versus Thermodynamic Topologies of Metal-Organic Frameworks via Combined Transmission Electron Microscopy and X-ray Diffraction Analysis

Synthetic protocols that preferentially result in metal-organic framework (MOF) crystallization of one topology over another remain an elusive, empirical process. This is primarily because of a lack of fundamental insights into MOF crystal growth and the effect of various experimental parameters on the resulting topologies. In this Communication, we demonstrate the temperature-topology relationship of MOFs constructed from hexanuclear oxozirconium cluster nodes and tetrakis(4-carboxylphenyl)porphyrin linkers via a combined transmission electron microscopy and powder X-ray diffraction study. Synthesis at room temperature led to a mixed phase consisting of 12-connected (assuming no defects) MOF-525 and 6-connected PCN-224, possessing ftw and she topologies, respectively. When the temperature was raised to 145 °C, 8-connected PCN-222 (csq topology) was found, with a possible concurrence of another 8-connected NU-902 (scu topology) and 12-connected PCN-223 (shp topology), in addition to MOF-525 and PCN-224. With an increase in reaction time at 145 °C, a change in product distribution was observed where PCN-222 remained the major crystal phase after 7 days, while the contribution from MOF-525 and PCN-224 decreased. These data suggest that MOF-525 and PCN-224 are the kinetic products while PCN-222 is the thermodynamic product.