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N-pyrid-3-yl fragment as a halogen bond acceptor: one-pot combined covalent and supramolecular synthesis of an imine cocrystal

Molecules containing pyridyl fragments have been shown as reliable building blocks in the field of crystal engineering halogen bonded structures (Cavallo et al., 2016). In this work, we have synthesized a cocrystal of 3-aminopyridine (3ap) with 1, 4-tetrafluorodiiodobenzene (tfib). In the (3ap)(tfib) cocrystal the pyridine ring nitrogen atom proved its potential as an acceptor species, since it participates in a strong halogen bond with an iodine atom (d(I∙∙∙N) = 2.81 Å, angle(C–I∙∙∙N) = 175°). Considering that building blocks derived from 3ap and containing an intact pyridyl fragment could as well be considered good halogen bond acceptors, we first obtained an imine by reacting 3ap and 2-hydroxy-1-naphthaldehyde (n). The next goal of our crystal engineering approach was to establish whether this bigger building block retains its functionality as a halogen bond acceptor. We have synthesized the n3ap and tfib cocrystal via both liquid-assisted grinding of a mixture of n3ap and tfib in a 2:1 molar ratio, as well as by recrystallizing the ground cocrystal sample. Then, we utilized a simpler and faster way of obtaining the cocrystal: by one-pot liquid-assisted grinding of a mixture of napht, 3ap and tfib in a 2:2:1 molar ratio, combining both imine formation and cocrystallization in the solid state. In the (n3ap)2(tfib) cocrystal structure, each tfib molecule forms two strong halogen bonds with pyridyl nitrogen atoms (d(I∙∙∙N) = 2.92 Å, angle(C–I∙∙∙N) = 175 °). Additionally, each n3ap molecule participates in two C–H∙∙∙O hydrogen bonds with another n3ap molecule, leading to the formation of an R22(16) bonding motif, and, as a result, of chains that are stacked in parallel along the a crystallographic axis. The crystal contains two symmetrically independent sets of chains held together only by weak intermolecular interactions.

1, 4-diiodotetrafluorobenzene ; cocrystals ; halogen bond ; mechanochemistry

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