Search

A detailed mechanism, based on densityfunctional theory calculationsand simulation, is presented outlining the formation of the secondarybuilding unit (SBU) of MIL-101, a chromium terephthalate metal organicframework (MOF). Formation of the metal core and of the SBU is keyto MOF nucleation, the rate-limiting step in the synthesis processof many MOFs. A series of reactions that lead to the formation ofthe SBU of MIL-101 is proposed in this work. The highest barrier (∼35kcal/mol) involves the formation of a dimetal-linker intermediateand high to low spin transition as a third Cr-linker moiety joinsto form a three metal-linker group joined by a central oxygen. Theterephthalate linkers play an important, key mechanistic role withthe carboxylates first joining chromium atoms prior to the formationof bridging oxygens. Subsequent to metal core formation, stepwiselinker addition reactions generate different assembly pathways dueto structural isomers that are limited by the removal of water moleculesin the first chromium coordination shell. A simple kinetic model basedon transition state theory gave a rate of SBU formation similar toa reported rate of MOF nucleation. The least energy path was identifiedwith all linkers on the same face of the metal center added first.These first steps in developing a modeling framework for SBU formationwill hopefully lay the groundwork for future comprehensive predictivemodels of the full MOF framework structure assembly and synthesisconditions required to support the self-assembly process. [ABSTRACT FROM AUTHOR]