Formation Mechanism of the Secondary Building Unit in a Chromium Terephthalate Metal–Organic Framework

Abstract

A detailed mechanism, based on density functional theory calculations and simulation, is presented outlining the formation of the secondary building unit (SBU) of MIL-101, a chromium terephthalate metal organic framework (MOF). Formation of the metal core and of the SBU is key to MOF nucleation, the rate-limiting step in the synthesis process of many MOFs. A series of reactions that lead to the formation of the SBU of MIL-101 is proposed in this work. The highest barrier (∼35 kcal/mol) involves the formation of a dimetal-linker intermediate and high to low spin transition as a third Cr-linker moiety joins to form a three metal-linker group joined by a central oxygen. The terephthalate linkers play an important, key mechanistic role with the carboxylates first joining chromium atoms prior to the formation of bridging oxygens. Subsequent to metal core formation, stepwise linker addition reactions generate different assembly pathways due to structural isomers that are limited by the removal of water molecule...