Formation mechanism of the secondary building unit in a chromium terephthalate metal-organic framework

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 molecules in the first chromium coordination shell. A simple kinetic model based on transition state theory gave a rate of SBU formation similar to a reported rate of MOF nucleation. The least energy path was identified with all linkers on the same face of the metal center added first. These first steps in developing a modeling framework for SBU formation will hopefully lay the groundwork for future comprehensive predictive models of the full MOF framework structure assembly and synthesis conditions required to support the self-assembly process.