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.