Halogen-Bonded

: Seventeen new halogen-bonded co-crystals characterized by single crystal X-ray analysis are presented from 8 × 4 combinations using methyl-substituted pyridine N -oxides and 1, ω -diiodoperﬂuoroalkanes. The N − O group in six of 17 co-crystals is monodentate and 11 have µ - O,O bidentate halogen bond acceptor modes. Remarkably, the N − O group in co-crystals of 3-methyl-, 4-methyl-and 3,4-dimethylpyridine N -oxides with octaﬂuoro-1,4-diiodobutane acted as a µ - O,O,O,O halogen and hydrogen bond acceptor, while acting as a µ - O,O,O acceptor in the co-crystal of 2,5-dimethylpyridine N -oxide and tetraﬂuoro-1,2-diiodoethane. The C − H ··· O − N hydrogen bonds demonstrated the polydentate cooperativity of the N − O group as a mixed halogen-hydrogen bond acceptor. The co-crystal of 2,4,6-trimethylpyridine N -oxide and dodecaﬂuoro-1,6-diiodohexane exhibited C − I ··· O − − N + halogen bonds with R XB value 0.76, the shortest of its kind compared to previously reported structures. The R XB values between 0.76 and 0.83 suggested that the C − I ··· O − − N + halogen bonds are moderately strong compared to our previously studied N − − I + ··· O − − N + system, with R XB in the order 0.66.


Introduction
The halogen bond (XB), analogous to the hydrogen bond (HB), has been defined [1] as a net attractive interaction between an electrophilic region of a halogen atom bound to a molecular entity and a nucleophilic site in another moiety, e.g., a nitrogen, oxygen or sulphur atom [2].This region of positive electrostatic potential, called the "sigma hole" [3,4], is characteristic for halogen atoms attached to an organic backbone, with its magnitude generally decreasing in the order I > Br > Cl > F. Despite similar geometrical features between HB and XB, the halogen bond is still not as well-explored compared to the ubiquitous HB [5][6][7][8][9].Halogen bonds are frequently studied using nitrogen compounds, which typically display monovalent N•••X (X = I, Br) interactions and are well understood as crystal engineering tools for self-assembly processes.Their precedence from discrete structures to increased dimensionality through the controlled reactivity of substrates is well-reported [10][11][12][13].In a solid-state XB complex, R−X•••B−Z, where X is the donor and B is the acceptor atom, the ratio of the short distance between X and B atoms (d X-B ) to sum of the Van der Waals radii of X and B atoms (d vdW ) is defined as the normalized strength parameter, R XB = d X-B /d vdW [1].Knowledge of R XB values provides an opportunity to roughly estimate the strengths of XB complexes.For example, the X•••N distances in bis(pyridine)iodonium(I) tetrafluoroborate constitute an R XB value of 0.65, and such compounds are classified as halogen bonds of covalent nature [14].Carefully designed nitrogen compounds are successfully utilized as molecular building blocks engaging in (N−X−N) + X − XBs to construct supramolecular structures resembling metal coordination frameworks [15].However, weaker C−I•••N XBs with typical R XB values ranging from 0.75 to 0.90 are still of considerable importance for applications in materials chemistry, e.g., for triggering liquid crystallinity and gelation behaviour [16][17][18].
Strategic exploitation of XB acceptor properties for heteroatoms, such as oxygen, remains very much unknown in the literature.A Cambridge Structural Database (CSD) search for pyridine N-oxides functioning as XB acceptors revealed only a handful of structures (see supporting information for more details), while their systematic investigations remain especially scarce [32][33][34].Previously, monodentate strong N − −I + •••O − −N + XBs (R XB as low as 0.66) of coordinative nature between pyridine N-oxides and N-haloimides were studied both in solution and in the solid-state [35].Here, we aimed to investigate C−I•••O − −N + XBs using 1,ω-diiodoperfluoroalkanes (DI2-DI8) and methyl-substituted aromatic N-oxides (1-8), as shown in Figure 1.Haloperfluoroalkanes are robust XB donors, and their ability to steer the supramolecular assembly by XBs and F•••F interactions is well described [36].However, the volatile nature of these compounds often results in oily or waxy substances, which are difficult to characterize using single crystal X-ray diffraction [18,37].Despite their reluctance to crystallize, our attempts from 8 × 4 (acceptor × donor) combinations resulted in 17 crystal structures, providing an impressive crystallization success rate to analyze and understand the interactions at play in their solid-state structures.
Crystals 2017, 7, 214 2 of 11 compounds are classified as halogen bonds of covalent nature [14].Carefully designed nitrogen compounds are successfully utilized as molecular building blocks engaging in (N−X−N)⁺X − XBs to construct supramolecular structures resembling metal coordination frameworks [15].However, weaker C−I•••N XBs with typical RXB values ranging from 0.75 to 0.90 are still of considerable importance for applications in materials chemistry, e.g., for triggering liquid crystallinity and gelation behaviour [16][17][18].
Strategic exploitation of XB acceptor properties for heteroatoms, such as oxygen, remains very much unknown in the literature.A Cambridge Structural Database (CSD) search for pyridine Noxides functioning as XB acceptors revealed only a handful of structures (see supporting information for more details), while their systematic investigations remain especially scarce [32][33][34].Previously, monodentate strong N − −I + •••O − −N + XBs (RXB as low as 0.66) of coordinative nature between pyridine N-oxides and N-haloimides were studied both in solution and in the solid-state [35].Here, we aimed to investigate C−I•••O − −N + XBs using 1,ω-diiodoperfluoroalkanes (DI2-DI8) and methyl-substituted aromatic N-oxides (1-8), as shown in Figure 1.Haloperfluoroalkanes are robust XB donors, and their ability to steer the supramolecular assembly by XBs and F•••F interactions is well described [36].However, the volatile nature of these compounds often results in oily or waxy substances, which are difficult to characterize using single crystal X-ray diffraction [18,37].Despite their reluctance to crystallize, our attempts from 8 × 4 (acceptor × donor) combinations resulted in 17 crystal structures, providing an impressive crystallization success rate to analyze and understand the interactions at play in their solid-state structures.

Results and Discussion
The methods used to obtain the single crystals suitable for X-ray analysis are shown in Table S1.

Results and Discussion
The methods used to obtain the single crystals suitable for X-ray analysis are shown in Table S1.with alternate acceptors and donors.The XB interaction bond parameters are shown in Table 1.In 1•DI6, 1•DI8 and 3•DI8, the aromatic rings and donors were essentially coplanar, in contrast to the orthogonal alignment typically observed in coordination compounds [30,31].The N−O groups were µ-O,O bidentate, bridging the donors to form remarkably similar 1-D polymeric chains, as shown in Figure 2. Further analysis of the crystal packing revealed the donor-acceptor parallel arrangement to be a result of F•••F aggregation [38][39][40][41][42] between perfluorinated donor chains, which, though weaker than C−I•••O − −N + XBs, play a crucial role to yield a robust 3-D crystal structure.with alternate acceptors and donors.The XB interaction bond parameters are shown in Table 1.In 1•DI6, 1•DI8 and 3•DI8, the aromatic rings and donors were essentially coplanar, in contrast to the orthogonal alignment typically observed in coordination compounds [30,31].The N−O groups were μ-O,O bidentate, bridging the donors to form remarkably similar 1-D polymeric chains, as shown in Figure 2. Further analysis of the crystal packing revealed the donor-acceptor parallel arrangement to be a result of F•••F aggregation [38][39][40][41][42] between perfluorinated donor chains, which, though weaker than C−I•••O − −N + XBs, play a crucial role to yield a robust 3-D crystal structure.

S.No Code
with alternate acceptors and donors.The XB interaction bond parameters are sh 1•DI6, 1•DI8 and 3•DI8, the aromatic rings and donors were essentially coplana orthogonal alignment typically observed in coordination compounds [30,31].Th μ-O,O bidentate, bridging the donors to form remarkably similar 1-D polymeric Figure 2. Further analysis of the crystal packing revealed the donor-acceptor para be a result of F•••F aggregation [38][39][40][41][42] between perfluorinated donor chains, wh than XBs, play a crucial role to yield a robust 3-D crystal structure.with alternate acceptors and donors.The XB interaction bond parameters are shown in Table 1.In 1•DI6, 1•DI8 and 3•DI8, the aromatic rings and donors were essentially coplanar, in contrast to the orthogonal alignment typically observed in coordination compounds [30,31].The N−O groups were μ-O,O bidentate, bridging the donors to form remarkably similar 1-D polymeric chains, as shown in Figure 2. Further analysis of the crystal packing revealed the donor-acceptor parallel arrangement to be a result of F•••F aggregation [38][39][40][41][42] between perfluorinated donor chains, which, though weaker than C−I•••O − −N + XBs, play a crucial role to yield a robust 3-D crystal structure.

Conclusions
Halogen bonding between pyridine N-oxides and 1,ω-diiodoperfluoroalkanes was found to be a reliable tool for crystal engineering, as witnessed by the successful structural characterization of 17 co-crystals reported here.The N-oxide functionality was able to act as either a monodentate

Conclusions
Halogen bonding between pyridine N-oxides and 1,ω-diiodoperfluoroalkanes was found to be a reliable tool for crystal engineering, as witnessed by the successful structural characterization of 17 co-crystals reported here.The N-oxide functionality was able to act as either a monodentate

Figure 6 .
Figure 6.The 1-D Hydrogen bond tapes bridged by DI2 donors (a), and a view along the b-axis showing connecting modes of DI2 (b).Color representation: Gold capped sticks are DI2 donors, black and red broken lines are respectively XB and HB interactions.

Figure 6 .Figure 6 .
Figure 6.The 1-D Hydrogen bond tapes bridged by DI2 donors (a), and a view along the b-axis showing connecting modes of DI2 (b).Color representation: Gold capped sticks are DI2 donors, black and red broken lines are respectively XB and HB interactions.
5•DI4, 8•DI2 and 8•DI6 also formed 2:1 acceptor-donor discrete structures (Figure 7a-c), with monodentate XB acceptor modes for N−O groups.In 5•DI4, the 1:2 discrete units propagated along the b-axis by C−H•••O interactions between N−O and methyl groups.Further, the 1-D motifs (Figure 7d) extended three dimensionally through C−H•••F and π•••π interactions.Co-crystal 8•DI2 had an interesting 1-D ladder structure (Figure 7f), with N-oxides forming 1-D tapes through C−H•••O interactions as the side rails (Figure 7g) connected by halogen bonds via DI2.These 1-D ladders further packed through stacking of the aromatic rings as depicted schematically in Figure 7h.On the other hand, 8•DI6 with its 3:1 acceptor-donor generated a more complex structure, extended by π•••π interactions between 2:1 discrete units and the additional, "passive" molecule of N-oxide 8, not involved in XB, as depicted in Figure 7e.While the "passive" molecule of 8, situated near an inversion centre, was disordered over two components with 50:50 occupancies, the anti-gauche conformation of DI6 was not compatible with the presence of an inversion centre in the middle of the C3-C4 bond of the donor.Crystals 2017, 7, 214 7 of 11 crystal 8•DI2 had an interesting 1-D ladder structure (Figure 7f), with N-oxides forming 1-D tapes through C−H•••O interactions as the side rails (Figure 7g) connected by halogen bonds via DI2.These 1-D ladders further packed through stacking of the aromatic rings as depicted schematically in Figure 7h.On the other hand, 8•DI6 with its 3:1 acceptor-donor generated a more complex structure, extended by π•••π interactions between 2:1 discrete units and the additional, "passive" molecule of N-oxide 8, not involved in XB, as depicted in Figure 7e.While the "passive" molecule of 8, situated near an inversion centre, was disordered over two components with 50:50 occupancies, the antigauche conformation of DI6 was not compatible with the presence of an inversion centre in the middle of the C3-C4 bond of the donor.
(6/17 cocrystals) or μ-O,O bidentate (11/17 co-crystals) halogen bond acceptor.Monodentate C−I•••O − −N + halogen bonds were stronger than bidentate C−I•••O − −N + halogen bonds.Based on the observed RXB values, ranging from 0.76 to 0.83, C−I•••O − −N + halogen bonds can be classified as moderately strong compared to e.g.very strong monodentate N − −I + •••O − −N + type halogen bonds, previously studied by us, which display RXB values as low as 0.66.In addition to that, an important role of weak interactions, such as weak C−H•••O hydrogen bonds and aromatic ring stacking, has been established.In particular, the N-oxide oxygen atom was shown to simultaneously engage in both the hydrogen and halogen bonding as a mixed acceptor.However, the C−H•••O hydrogen bonds were fairly weak, as witnessed by the observation of two polymorphs of 4•DI4, where only one of the two exhibited C−H•••O hydrogen bonds.The ability of N-oxide oxygen to act as a μ2-(one XB and one HB), μ3-(one XB and two HB) and μ4-acceptor (two XB and two HB) is a complex process.For example, the
(6/17 co-crystals) or µ-O,O bidentate (11/17 co-crystals) halogen bond acceptor.Monodentate C−I•••O − −N + halogen bonds were stronger than bidentate C−I•••O − −N + halogen bonds.Based on the observed R XB values, ranging from 0.76 to 0.83, C−I•••O − −N + halogen bonds can be classified as moderately strong compared to e.g.very strong monodentate N − −I + •••O − −N + type halogen bonds, previously studied by us, which display R XB values as low as 0.66.In addition to that, an important role of weak interactions, such as weak C−H•••O hydrogen bonds and aromatic ring stacking, has