Researchers from the Department of Chemistry Luka Fotović, Nikola Bedeković, and Vladimir Stilinović have published the study Keggin-Type Anions as Halogen Bond Acceptors in the ACS journal Crystal Growth & Design (IF = 4.076).
Polyoxometalate anions have drawn considerable interest over the past five decades due to not only their structural versatility but also numerous (both potential and actual) applications in catalysis, molecular electronics, and medicine. Given the wide spread of applications of halogen bonding over the past decades, it is somewhat surprising that there have been very few and unsystematic studies of the halogen bonding proclivities of oxometalate and polyoxometalate anions. In order to supply these data, a combined experimental and theoretical study was devised in which a series of halogen-bonded solids in which polyoxometallate anions of the classical Keggin type bind to structurally and electronically different halogen bond donating cations was prepared, while the electrostatic potential of the anions was studied by quantum-mechanical computations.
The crystallisation of the selected anions with protonated and N-alkylated iodopyridinium cations, as well as bis(halopyridine) cations (which can bridge between two POMs forming two halogen bonds) yielded 10 solids with supramolecular architectures varying from discrete complexes to 3D networks. The distribution of the electrostatic potential on the anion was found to favour the formation of multicentric bonds with faces of the anion, whereas sterically it is most favourable for a halogen to bind on a terminal M=O oxygen, and this apparently determines the observed distribution of halogen bond acceptor sites. These observations indicate that Keggin ions (and probably polyoxometalates in general) can be used as building blocks in halogen-bonded solids in a predictable (and at least somewhat controllable) fashion. Furthermore, the possibility of preparing 2D and 3D networks with large separation between constituent ions indicates that this might be a feasible pathway for the systematic preparation of porous structures. This, in conjunction with the well-documented catalytic activity of polyoxometalates as well as low solubility of such materials, leads to a tantalizing idea of potential applications of such materials in heterogeneous catalysis.
The research was performed as a part of the research project New building blocks for the supramolecular design of complex multi-component molecular crystals based on halogen bonds (HaloBond IP-2019-04-1868) funded by the Croatian Science Foundation.
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