adduct (a, BENZBR01), bromine/acetone adduct (b, ACETBR), hexamethylenetetramine/iodoform adduct (c, HEXAIF10), and tetraiodoethylene/pyrizine adduct (d, IETPYA10). CSD ref codes are provided after the location description. Dotted lines represent halogen bond contacts, and space‐filling diagrams are drawn using default van der Waals radii in OLEX2.
A definitive solid‐state review written by Bent in 1968 compiles many early solid‐state halogen bond studies [17] and highlights several characteristics that have been conclusively shown in modern studies. For example, Bent noted a hierarchy of interaction strengths highlighting contact distances of halogen bond complexes with diselenane (I2 > diiodoacetylene > tetraiodoethylene). To expand the analysis, he compiled a hierarchy of donor and acceptor strengths from the compiled data (Figure 1.4). With the Cambridge Structural Database (CSD) recently surpassing one million structures, it is impressive to see that Bent was identifying and proposing trends from just 27 structures that were later verified to be correct (from much larger data sets).
In 1983 Dumas, Gomel, and Guerin presented a review primarily composed of various solution‐based studies (e.g. UV–vis, nuclear magnetic resonance (NMR), Raman, IR) of intramolecular interactions of haloorganics with Lewis bases [49]. The message from this review was that the distinctive features of the halogen bond identified in the solid state persist in solution phase. One notable idea in the review was their consideration of halogen and hydrogen bond interplay in their solution studies: The simultaneous presence of hydrogen atom(s) with acidic character and halogen atom(s) able to interact with a base leads to a new kind of “isomeric complex” in which the C–X⋯Y interaction competes with the C–H⋯Y interaction. This observation is topical, and maintains to this day, a design consideration for modern solution‐based halogen bonding chemists. To elaborate, many halogen bonding designs often incorporate strong electron‐withdrawing groups to elicit stronger halogen bond interactions. However, in many instances, strong CH hydrogen bond donors are also formed, highlighting a need to ensure molecule performance is largely dictated by halogen bonding and not CH hydrogen bonding or other competing interactions. A modern study addressing this concern comes from the Huber lab [50].
Figure 1.4 Table from the 1968 solid‐state review by Bent.
Source: From Bent [17]. © 1968 American Chemical Society.
Experimentally, the gas phase behavior of dihalogen bonding adducts with various Lewis bases was extensively studied by Legon using rotational microwave spectroscopy in the late 1980s and through the 1990s [51,52]. They acknowledged the similarities between the hydrogen and halogen bond geometries in the gas phase but noted the distinct linearity of the latter. The study of these complexes in the gas phase revealed structural parallels to the solid state, reinforcing that halogen bond contacts were not a byproduct of lattice effects.
Contributing to the collection of halogen bonding data during this time were notable theoretical studies. The concept of the “σ‐hole” discussed above was largely driven by the computational works of Politzer and Murray [5,6]. Specifically, they demonstrated the anisotropic charge distribution of halogen atoms forming one covalent bond, the details of which are elaborated on in the computational section.
While not comprehensive, this section illustrates that the accumulation of data showing the attractive noncovalent behavior of halogens is consistent across the three primary phases of matter and in silico. These seminal studies and others provided the groundwork for the “rediscovery” of the halogen bond in the early 2000s.
1.2.1 Rediscovery
Dihalogens contributed extensively to the early identification of the halogen bond interaction. However, diatomic halogens (and interhalogens) limit the possible functional applications because they tend to be reactive and offer little in the way of tunability. This has led to the extensive study of haloorganics since the mid‐1980s, as these species are readily adaptable to systematic study. For example, Weiss described interesting early instances of organic and cationic halogen donor complexes [53–55]. Additionally, a team from Politecnico di Milano in Italy, led by Pierangelo Metrangolo and Giuseppe Resnati, have studied many different haloorganics, including haloperfluorocarbons. Their contributions have aided in stimulating interest in halogen bonding. In particular, a review paper [56] and concept article [57] compile much of their early works and largely unify the field. The paper that many cite as the “rediscovery” of halogen bonding is titled “Halogen bonding: A Paradigm in Supramolecular Chemistry.” Here, Metrangolo and Resnati describe several halogen bonding concepts that remain topical today such as hydrogen and halogen bonding selectivity, donor selectivity, acceptor selectivity, hard–soft acid–base favorability, the ability of the halogen bond to outcompete the hydrogen bond, and the ability of the halogen bond to perform admirably in aqueous media. This unification of halogen bond topics also details various methods for quantifying and identifying halogen bond interactions, including the use of heteronuclear NMR resonances (e.g. 14N and 19F). Lastly, they highlight several crystallographic studies providing the groundwork for the surge in subsequent solid‐state halogen bonding studies. The design principles outlined by Metrangolo and Resnati highlight advantageous features of the halogen bond for the construction of predictable supramolecular architectures. For example, the effectiveness of dihalogen perfluorocarbons paired with various di‐Lewis basic molecules to engender the formation of linear one‐dimensional (1D) chains (Figure 1.5). Taken together, the studies from the groups of Metrangolo and Resnati have been instrumental in making the halogen bond topical to so many diverse research areas.
Figure 1.5 ChemDraw figure highlighting the use of alkyl‐ and aryl‐dihaloperfluorocarbon halogen bond donors to form predictable 1D networks in the solid state.
Source: From Metrangolo and Resnati [57]. © 2001 John Wiley & Sons.
1.3 Crystallographic Studies
X‐ray crystallography has been critical to establishing the field of halogen bonding and remains an integral part of modern analyses. Solid‐state investigations often complement solution studies and provide perspective for the following chapters. For example, crystallography can reveal the atomic positioning of the atoms involved in a halogen bond. In fact, crystal structures will be presented in the ensuing chapters as they often reveal likely solution binding modes or binding stoichiometry. From a crystal engineering standpoint, the linearity of the halogen bond favors predictable structure directing contacts – a lauded feature that has been exploited in the construction of numerous 1D, 2D, and 3D halogen bond architectures [7]. In general, solid‐state studies have largely focused on fundamental investigations evaluating how this interaction “fits” into established crystal engineering concepts, the generation of new halogen bonding concepts, and application of the halogen bond in designed and functional crystalline materials. As such, this section is loosely organized from fundamental to functional studies and begins by highlighting a few CSD evaluations. The section provides a quick survey on halogen bonding in the solid state, and those desiring more extensive treatment of the topic are referred to the following reviews [7,58–65].
1.3.1 CSD Evaluations
Possibly, the first CSD evaluation
