Halogen bonding in the crystal structures of 1,2-diiodo alkenes

Halogen bonding is currently a widely discussed topic in the literature. In the last 15 years an increasing number of reviews (approximately 50) have been published on this subject. This type of intermolecular interaction was discovered about two centuries ago but was investigated in detail only in the last few years (for detailed mechanism and the theory of halogen bonding see ref. 2–6). Halogen bonds are relevant in the field of crystal engineering as well as in synthetic chemistry and material science. Many theoretical investigations have been published and halogen bonding has proved to be important in medicinal chemistry as well as in chemical biology. In 2013, the official IUPAC definition was released which states amongst others that typical halogen bonds R–X Y–Z are formed between a halogen bond donor (R–X, X = electrophilic halogen atom, e.g. I2, CH3Br, halonium ion) and a halogen bond acceptor Y (Y = lone pair possessing atom, a p-system or an anion). Additionally, the contact distance of X Y has to be shorter than the sum of the van der Waals radii, the R–X bond length should be elongated and the contact angle R–X Y should be near to 1801. Generally, iodine containing molecules form stronger halogen bonds than corresponding bromine, chlorine or fluorine containing derivatives. In most cases reported in the literature structures showing halogen bonding were obtained by forming co-crystals between two different compounds, one of which acts as the halogen bond donor and the other as the acceptor. A typical example is provided by the co-crystals of phenazine and 1,2,3,4-tetrafluoro-5,6-diiodo benzene or 4,40-bipyridine and tetraiodoethene. The other possible case – halogen bonds in crystals where the halogen bond donor and acceptor are included in the same molecule – is much less discussed. This includes structures of compounds containing e.g. iodine as halogen bond donor and nitrogen, oxygen, a halogen or a p-system as acceptor (I I, I O, I N, I C(p)). Diiodoolefines having additional halogen bond acceptor functionalities in the organic framework provide good examples for such structures. In these compounds halogen bonding should play a key role in determining the features of the crystal structure. In order to put light on the importance of halogen bonding as directing interaction in the formation of the structure in the solid state we have determined and investigated the crystal structures of a series of 1,2-diiodoolefines (Fig. 1). In this paper our aim is to investigate weak halogen bonds of iodine with the less strong halogen bond acceptors oxygen, iodine itself and aromatic, aliphatic or acetylenic p-systems. The systems we include in this study consist of the new crystal structures of nine 1,2-diiodoolefines determined in our laboratory and the crystal structures of several 1,2-diiodolefines from the literature that complement our data.