Sequential Hydrolysis of Metal Oxo Clusters Drives Polymorphism in Electrodeposited Zirconium Metal–Organic Frameworks

Over the past few decades, significant developments have been made in the electrodeposition of nanomaterials, mainly due to its streamlined process for producing thin films which are deployed toward various catalysis applications. Concurrently, advances in the electrodeposition of porous materials such as metal–organic frameworks (MOFs) in the past decade have aimed at optimizing their performance for gas storage and catalysis applications. Despite being a relatively recent fabrication method, electrodeposition of MOFs has seen success in only a few instances, which is limited by the formation of unwanted oxide/hydroxide in the metallic component during the linker attachment step. Some studies have shown how to prevent these unwanted metal oxides/hydroxides by controlling solution acidity (or pH) and temperature, resulting in the successful demonstration of the electrochemical synthesis of a subset of MOFs (paddlewheel-based Cu and Zn MOFs) on conductive substrates. However, a comprehensive understanding of the electrochemical synthesis pathway for these porous frameworks is still lacking. We address this gap by presenting, for the first time, a detailed deprotonation mechanism outlining the evolution of Zirconium (Zr) oxo cluster directing the synthesis of porphyrin-based zirconium MOFs. These MOFs are known to exhibit diverse polymorphic topologies that are influenced by modulator type and concentrations. In this work, we show that applied current can influence the polymorphic topologies of Zr MOF by varying local cathodic pH. While synthesizing these MOFs electrochemically, the modulator concentrations are maintained constant to demonstrate the effect of applied current density and solution pK_a leading to phase-pure polymorphs of MOF-525 at a higher current density and PCN-222 at a lower current density. The density functional theory calculations reveal that zirconium-oxo clusters undergo sequential hydrolysis, with the pK_a of the cluster dictating the extent of deprotonation. The degree of deprotonation, in turn, determines the 12- and 8-connections in MOF-525 and PCN-222, respectively. Finally, the study demonstrates the robustness of the electrochemical protocol by applying it to pyrene- and tricarboxylic-linker-based Zirconium MOFs.