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Halogen bonded cocrystals of divalent metal acetylacetonates with 1, 4-diiodotetrafluorobenzene

Coordination compounds of transition metals with β-diketones (acetylacetone (Hacac) in particular) have been extensively studied for many decades. Recently metal β-diketonates started attracting considerable attention as building blocks in crystal engineering, using either functionalised diketones which can further connect into supramolecular architectures or metal ions which remain coordinationaly unsaturated after the formation of a neutral diketonate complex which can bind additional ligands. In this presentation we shall report on our study of the potential of unmodified metal bis(acetylacetonates) to act as halogen bond acceptors. Seven complexes have been prepared (Cu(acac)2, Pd(acac)2, VO(acac)2, Zn(acac)2(H2O), Co(acac)2(H2O)2 and Ni(acac)2(H2O)2) and cocrystallised with 1, 4-diiodotetrafluorobenzene (tfib). In all seven cases cocrystals have formed in which the complex molecules are bonded with tfib via O∙∙∙I halogen bonds into supramolecular chains, however the stoichiometry of the formed cocrystal, as well as the binding site of the iodine atoms, was found to be dependent on the presence of the coordinated water molecules. Thus in the case of Cu(acac)2, Pd(acac)2 and VO(acac)2 1:1 cocrystals were formed with each iodine atom forming a bifurcated halogen bond with oxygen atoms of two acetylacetonate ligands. Interestingly VO2+ oxygen atoms did not participate in halogen bonding, but rather coordinated to the sixth coordination site of neighbouring vanadium atoms forming linear coordination polymers. Co(acac)2(H2O)2 and Ni(acac)2(H2O)2 also form 1:1 cocrystals with tfib, however, here the coordinated water molecules form hydrogen bonds with the oxygen atoms of acetylacetonate ligands of adjacent molecules rendering them inaccessible to the tfib iodine atoms. This hydrogen bonding however leaves the oxygen atoms of the water molecules free to bind with the iodine. The resulting structure thus comprises of sheets of molecules hydrogen bonded in one direction and halogen bonded in another. Similar sheets are also present in the structure of Zn(acac)2(H2O)– tfib cocrystal, Here, however, the presence of only one coordinated water molecule leads to hydrogen bonding of only one pair of acetylacetonate oxygen atoms per Zn(acac)2(H2O) molecule, leaving the other free to form bifurcated halogen bonds with the tfib iodine atoms, equivalent to those in Cu(acac)2, Pd(acac)2 and VO(acac)2. The result is a cocrystal of 2:1 stoichiometry with double chains of hydrogen bonded Zn(acac)2(H2O) molecules interconnected by tfib molecules.

1, 4-diiodotetrafluorobenzene ; cocrystals ; coordination compounds ; halogen bond

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