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To understand how the IUPAC definition of the halogen bond developed, one can look to the past. In fact, some have traced the observation of the halogen bond back to around the discovery of iodine. Consider what chemistry was probably like during the late Napoleonic era: mixing compounds, observing color changes, evolving gases, minimal safety concerns, etc. In fact, observing changes in color was how the first halogen bond complexes were detected (although not referred to as halogen bonds). The following is a brief commentary on a select number of historical studies considered to involve the halogen bond.

Early halogen bond observations occurred near the start of the nineteenth century in France, around the discovery and isolation of a new substance by Bernard Courtois in 1812. Samples of this material were given to a few chemists, including Sir Humphry Davy and Joseph Louis Gay‐Lussac. Shortly thereafter (December 1813), both Davy and Gay‐Lussac identified (independently) and quarreled who was first to establish the new substance, iodine [35]. Less than a year later (July 1814), J. J. Colin, working for Gay‐Lussac, reported the formation of a liquid with a metallic luster when mixing the newly identified material (I₂) with dry gaseous ammonia [36]. At the time the composition of the substance was unknown, but was eventually established by Frederick Guthrie in 1863 as Iodide of Iodammonium [37]. While the nature and atomic positioning of the two components remained unknown, Guthrie correctly predicted the formula of NH₃I₂. We now understand this material as a complex formed by a halogen bond between an iodine atom and the nitrogen atoms of the ammonia (I–I⋯NH₃). Similar 1 : 1 dimers between Br₂, Cl₂, and various amines were later reported by Remsen and Norris [38], while Rhoussopoulos provided initial evidence of iodoform participating in unique noncovalent interactions with quinoline [39].

The interest in I₂ continued into the early 1900s, resulting in numerous observations that are now understood to be rooted in the halogen bond phenomena. For example, Lachman in the early twentieth century noted various colors from solutions of diatomic iodine [40]. These colors range from brown or red brown solutions when combined with acetone, alcohols, ethers, amines, and benzene to more violet solutions with aliphatic hydrocarbons, carbon tetrachloride and chloroform. The diverse color palette of iodine solutions is now attributed to I₂⋯solvent complexes driven by the halogen bond. More importantly, studies of dihalogen complexes with various Lewis bases would have influences on two chemistry Nobel Prizes. The following few paragraphs will identify some of these impactful solution, solid, gas, and computational investigations leading to the rediscovery of the halogen bond in the late twentieth to early twenty‐first century.

The works of Benesi and Hildenbrand in 1948 detailed that “…new evidence has been found for the presence of addition compounds of iodine and the solvent molecule” [41]. These studies evaluating aromatic hydrocarbons and their π‐systems as acceptors (e.g. I₂⋯benzene) were influential in the development of conceptual models to explain halogen and Lewis base adducts. In fact, a couple years later in 1950, Mulliken evaluated carbonyl derivatives and ethers with diatomic iodine that helped developed electron donor–acceptor concepts used to understand these complexes [42]. A key component of the above studies was the use of UV–vis spectroscopy to closely monitor the changes, quantify behavior, and understand the nature of these early halogen bonded complexes. Ultimately, the widespread contributions of Mulliken led to him winning the 1966 Nobel Prize in Chemistry for “fundamental work concerning chemical bonds and the electronic structure of molecules by the molecular‐orbital method” [43].

While many of these early studies observed spectroscopic changes, little was known about the atomic arrangements of these halogen bonding complexes. X‐ray crystallographic studies began to reveal structural features of the halogen bond. Numerous cocrystal structures reported by Hassel in the 1950s were critical to elucidating structural features of the halogen bond. Early structures included Br₂⋯dioxane [44], Br₂⋯benzene [45], Cl₂⋯benzene [46], and Br₂⋯acetone [47] adducts (Figure 1.3). Hassel noted the distinctive features of the halogen bond common to all solid‐state studies: R–X⋯Y angles of near 180° and contacts shorter than the sum of their respective vdW radii. Hassel's 1970 Nobel lecture provides perspective on early solid‐state studies of halogen interactions and highlights themes still topical today such as hydrogen and halogen bond interplay [48]. In his lecture he also discusses a number of early halocarbon⋯Lewis basic cocrystals such as 1 : 1 hexamethylenetetramine/iodoform adduct and 1 : 1 tetraiodoethylene/pyrizine adduct (Figure 1.3).

Figure 1.3 Early halogen bonding cocrystals from Hassel. Bromine/benzene 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 CH hydrogen bond donors are also formed, highlighting a need to ensure molecule performance is largely dictated by halogen bonding and not CH 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.