Ligand Exchange in the Synthesis of Metal–Organic\nFrameworks Occurs Through\nAcid-Catalyzed Associative Substitution

Abstract

The syntheses of\nmetal–organic frameworks (MOFs) can be\nimproved through modulated synthesis, synthesis employing precursors,\nand postsynthetic exchange (PSE) modifications, all of which share\nligand exchange as a common and crucial reaction. To date, however,\nthe mechanism of ligand exchange and the underlying principles governing\nit have remained elusive. Herein, we report energy landscapes for\nthe ligand exchange processes of 1,4-benzenedicarboxylic acid and\n2,3,5,6-tetrafluoro-1,4-benzenedicarboxylic acid with Zr₆O₄(OH)₄(OMc)₁₂ (OMc = methacrylate),\nas calculated using density functional theory (DFT). The rate-limiting\nstep of ligand exchange follows an associative-substitution mechanism\ncatalyzed by protons, consistent with previous kinetic data. Our calculations\nsuggest that the acid catalysis is dependent on the relative basicities\nof the incoming and outgoing ligands coordinated in the complex, allowing\nmolecular-level rationalization of many seminal MOF syntheses that\nhad previously been interpreted macroscopically. Our results provide\nnew insights for MOF synthesis and new clues for the rational de novo\nsynthesis of MOFs.