The energy frameworks of aufbau synthon modules in 4-cyanopyridine co-crystals

The supramolecular arrangement of 4-cyanopyridine (4CNpy) in its native crystal form and its co-crystals with halogen bond (XB) donors is discussed in terms of long-range synthon aufbau modules (LSAMs) and energy frameworks. Dissociations of 2D zigzag chains of parent 4CNpy into 1D dimers observed in its co-crystals are in good agreement with the aufbau model. Its co-formers, XB donors 1,4-I2(CF2)4, C2I2, 1,3- and 1,4-I2C6F4 (m- and p-DITFB), provide equal energy of I⋯N XBs, but perfluorinated iodo-alkane 1,4-I2(CF2)4 and diiodoacetylene C2I2 cannot achieve the same strength of homomolecular aggregation as π–π stacking in columnar DITFB modules. As a result, DITFBs form I⋯N XBs with both Npy and NCN nitrogen atoms of 4CNpy, while 1,4-I2(CF2)4 and C2I2 only with the Npy atom. This is not a particular case of 4CNpy co-crystals, but in general, DITFB appears to be a more effective XB donor co-former than C2I2, 1,4-I2(CF2)n and other iodo-XB donors, which has similar potential to an iodine atom but lacks homomolecular aggregation. In supramolecular reactions of p-DITFB with (η6-Ar)RuX2(4CNpy) (Ar = p-cymene, X = Cl, I) bearing Npy-coordinated 4CNpy, the former gives definite preference to the XBs with the halogen atoms, but not to the CN group of the 4CNpy ligand.