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Halogen-bonded metal-organic cocrystals: structure and synthesis

Currently, the most important studies of halogen bonding that could find great use in the construction of supramolecular metal-organic architectures have generally been performed on single-component systems. Multicomponent halogen bonded metal-organic systems, where an organic ligand functions as a halogen bond acceptor are still rare in literature.[1, 2] Since our previous research has indicated the acetyl fragment as an acceptor species of interest, [3, 4] in this work, we were interested in cocrystallizing robust metal complexes containing that fragment on its periphery. We first synthesized two coordination compounds of our selected ligand, n4aa (derived from 2-hydroxy-1-naphthaldehyde and 4-aminoacetophenone), with copper(II) and nickel(II) ions, Cu(n4aa)2 and Ni(n4aa)2, respectively, after which they were cocrystallized with selected perflourinated halogen bond donors: iodopentafluorobenzene (ipfb), 1, 4-diiodotetrafluorobenzene (1, 4-tfib), 1, 3-diiodotetrafluorobenzene (1, 3-tfib), and 1, 4-dibromotetrafluorobenzene (1, 4-tfbb). Cocrystal screening experiments were performed by liquid-assisted grinding mixtures of the metal complexes with the selected halogen bond donors, and the products were then characterized by powder X-ray diffraction. Powder patterns have revealed that cocrystals are formed in all these cases, and also that the corresponding Cu(n4aa)2 and Ni(n4aa)2 cocrystals are isostructural. Single crystals were obtained by traditional solution based crystallization methods. The supramolecular bonding topology of our obtained cocrystals was found out to be dependent on the topicity of the used halogen bond donor. Cocrystals with the monotopic halogen bond donor, ipfb, contain a discrete I∙∙∙O halogen bonded complex (0D). These discrete units are then connected into chains that alternates two types of the metal complex subunit, one which participates in two I∙∙∙O halogen bonds and the other which participates exclusively in C–H∙∙∙F interactions. In metal complex cocrystals with linear ditopic halogen bond donors, 1, 4-tfib and 1, 4-tfbb, the dominant supramolecular interaction is the I∙∙∙O halogen bond which leads to the formation of chains (1D). The chains are then stacked via C–H∙∙∙O hydrogen bonds, perpendicularly to the halogen bond, and further into a 3D network via C–H∙∙∙F interactions. In metal complex cocrystals with 1, 3-tfib, a longer and a shorter I∙∙∙O halogen bond are formed and lead to the formation of a layered architecture (2D). The layers are then stacked via long C–H∙∙∙O interactions.

cocrystals, coordination compounds, crystal engineering, halogen bond, mechanochemistry

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