1 | INTRODUCTION
Halogen bonds have recently been extensively studied as they play an essential role in medicinal chemistry1, molecular recognition2, material science3, and crystal engineering4, 5. Halogen bonds are weak, noncovalent interactions between an electrophilic region of a halogen and a nucleophile6. Halogen bonds can schematically be depicted R‒X⋯Y, where X is a halogen (typically I, Br, Cl, and rarely F), and Y is defined as the halogen bond donor7, 8. X acts as an electron acceptor for the interaction, whereas Y is typically an electron rich O, N, S, or Y‒donor functional group (e.g. π‒electron systems or aromatic surfaces). A halogen atom may be covalently bound to one or several atoms and can additionally form one or several halogen bonds simultaneously6, 9.
Halogen bonds have similarities with hydrogen bonds and involve the same mechanisms10. In 2008, Metrangolo et al. summarized the similarities and differences between halogen bonding and hydrogen bonding complexes4. Kirk et al. investigated the competition between hydrogen bonding and halogen bonding for the (Y = Cl, Br, I, At)/halogenabenzene/NH3 systems and concluded that hydrogen bonding has an advantage when the halogen is Cl, while halogen bonds tend to be formed when the halogen atom Y = I11. Halogen bonds have been observed in crystal structures containing halogen atoms12. Due to their geometric properties, halogen bonds are considered efficient tools in designing the structures of crystals4, 13. The concept of halogen bonds in crystal engineering attracts increasing attention as they can be pivotal in the stability of crystals14, 15.
Although halogen bonding was first observed two centuries ago7, the fundamentals of its nature and its potential applications in crystal engineering have remained unexplored until recently. In most cases reported in the literature, halogen bonds were studied in co-crystals between two different compounds, one of which being the halogen bond donor and the other is the acceptor12, 16. On the basis of crystallographic and computational studies, this type of interaction was shown to predominantly originate from charge transfer and electrostatics. Later, the interaction was also found to possess polarization and dispersion contributions. In 2014, Deepa et al. carried out a theoretical study of a series of organic crystal structures containing various halogen bonds and found the strongest halogen bonds involed iodine as both halogen-bond donor and acceptor17. In the following year, Koskinen et al. carried out a detailed study of unexpectedly strong I+…S halogen bonds in [I(2-imidazolidinethione)2]­+with the results supporting the coordinative nature of the halogen bond18.
Furthermore, topological properties, vibrational frequencies, interaction energies, and charge transfer in halogen-bond-containing systems have been studied using both Bader’s quantum theory of atoms in molecules (QTAIM)19 and Weinhold’s “natural bond orbital NBO” methods. A variety of important concepts, such as critical points, bond path, gradient path of electron density, and the Laplacian of electron density are generated through QTAIM theory. Clark et al. calculated the electrostatic potential of the series of molecules CF3X (X = F, Cl, Br and I) and found that the three unshared pairs of electrons produced a belt of negative electrostatic potential around the central region of the X atom (except for F), leaving the outermost region positive, which designated the “σ-hole”20. They also discovered that the strength of the σ-hole depends on the nature of the halogen atom. The more polarizable and the lower the electronegativity of the halogen atom, the more positive the σ-hole. Thus, the interaction strength of halogen bonds increases in the order of F < Cl < Br < I. It is the σ-hole that allows the halogen atom to form a halogen bond with a Lewis base21 and that makes the A X-R angle tend towards a linear configuration10. In 2010, Zeng et al. comparatively analyzed the properties of halogen bonds and hydrogen bonds by QTAIM calculations and concluded that the two interactions were coincident in topological properties22. In the following year they analyzed halogen bonds between sulfides and dihalogen molecules and found that electrostatic interactions played an important role in these halogen bonds23. Grabowski et al. calculated the QTAIM characteristics of halogen bonds, dihalogen bonds and halogen-hydride bonds24. In 2018, Bauzá et al. analyzed the interplay between π‐hole and lone pair⋯π/X-H⋯π interacitons through QTAIM calculations25. In 2019, Benito et al. first reported the cocrystallization of an adenine derivative which acts as a halogen bond acceptor; the calculations were carried out via DFT calculations and the QTAIM method26. In 2020, Wzgarda-Raj et al. investigated several observed types of halogen bonding interactions in a series of cocrystals in detail based on QTAIM27. In addition to the traditional halogen bonds, Domagala et al. built a model of a double (+/-) charge-assisted halogen bridge for a set of quinuclidine-like cation derivatives and anions, these charged fragments were observed to form strong halogen bonding complexes, with interaction energies high as 100 kcal/mol28.
In this work, calculations carried out on new crystal structures of 1,2-diiodoolefines are reported, including nine monomers and nine dimers. These crystal structures were previously synthesised by Hettstedt et al . in 201529 (see Figure 1). The purpose of this study is to investigate the characteristics and properties of halogen bonds (i.e. I…I, I…O, and I…C(π), where C(π) can be aromatic, aliphatic or acetylenic π-systems) and other noncovalent interactions such as I…H and O…O observed in these crystal structures.