Zeolitic polyoxometalates metal organic frameworks (Z-POMOF) with imidazole ligands and ε-Keggin ions as building blocks; computational evaluation of hypothetical polymorphs and a synthesis approach

Abstract

We investigate here a new family of zeolitic Metal Organic Frameworks (MOFs) based on imidazole (im) as the ligand and ε-type Keggin PolyOxoMetalates (POMs) as building units. The POM used in this study is the ε-{PMo₁₂O₄₀} Keggin isomer capped by four Zn(II) ions (noted ε-Zn) in tetrahedral coordination. We describe here our methods to first construct and then evaluate the stability of hypothetical 3-D POMOFs possessing a tetrahedral network, typified by dense silica polymorphs and zeotypes and referred here to as Z-POMOFs. We use the analogy between the connectivity of silicon ion in dense minerals or zeolites and the ε-Zn, using imidazolate ligands to mimic the role of oxygen atoms in zeolites. Handling the ε-Keggin and imidazole as the constitutive building-blocks, a selection of 40 polymorphs were constructed and their relative stabilities computed. Among these Z-POMOFs, the cristobalite-like and zni-structure were identified as the most stable candidates. In parallel, we have attempted to synthesize Z-POMOF structures with ε-Zn POMs, synthesized in situ under hydrothermal conditions, and imidazole ligands. We present our first experimental result, the extended material [NBu₄][PMo^V₈Mo^VI₄O₃₇(OH)₃Zn₄(im)(Him)], named ε(im)₂. The structure of the hybrid framework is built by the connection of dimerized ε-Zn POMs to imidazole ligands in two directions. The obtaining of the first POMOF based on imidazole ligand is an encouraging step towards the synthesis of a new family of POMOFs.