Bifurcated and Monocentric Halogen Bonds in Cocrystals of Metal(II) Acetylacetonates with p-Dihalotetrafluorobenzenes

In order to test metal diketonates as potential acceptors of bifurcated halogen bonds, the series of acetylacetonates (acac) of divalent cations Cu(acac)(2) (1), Pd(acac)(2) (2), VO(acac)(2) (3), Ni(acac)(2) (H2O)(2) (4), Co(acac)(2) (H2O)(2) (5), and Zn (acac)(2) (H2O) (6) were cocrystallized with 1,4-diiodotetrafluorobenzene (tfib) and 1,4-dibromotetrafluorobenzene (tfbb) as halogen donors. This has yielded a series of 10 cocrystals, tfib having formed cocrystals with all six acceptors and tfbb with all except for 4 and 5. In eight cocrystals a pair of acac oxygen atoms acts as a bifurcated halogen bond acceptor, the bond being symmetric in cocrystals of 1 and 2 and asymmetric in cocrystals of 3 and 6. The only cocrystals in which a halogen bond was formed with alternative acceptor sites were cocrystals of tfib with 4 and 5, where coordinated water molecules form hydrogen bonds with all available acac oxygen atoms, leaving only the water molecules themselves as halogen bond acceptors. The favorability of the bifurcated halogen bond was also confirmed by QM computations, which have shown the bifurcated bonds to be the most favorable interactions in vacuo, with bond energies in the range of 29-37 kJ mol(-1) for tfib and 20-25 kJ mol(-1) for tfbb. This also reflects on the thermal stability of the cocrystals of 1-3 (which do not contain coordinated water) with tfib, which melt/ decompose between ca. 180 and 220 degrees C.