Halogen Bonding in Solution

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Edited by Stefan Huber

Editor

Stefan Huber

Ruhr‐Universität Bochum

Organische Chemie I

Universitätsstr. 150

NC 4/171

44801 Bochum

Germany

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Halogen bonding has been “known” since the onset of modern chemistry, and thus for about as long as hydrogen bonding, even though a clear structural and theoretical understanding was first developed much later. But while hydrogen bonding has been studied intensively in the past and has obviously been an integral part of countless applications in all fields of chemistry, halogen bonding has remained some sort of curiosity for a long time. Even after it had received increased interest in crystal engineering and theory from the 1990s on, solution‐phase applications have been essentially absent until 10–15 years ago. Since then, however, inspired by many successful solid‐state studies, the use of halogen bonding in solution has thrived quite remarkably, and interesting studies appear at ever‐increasing frequency. Thus, it seems timely to provide an overview on this matter, which is the aim of this book.

I am very happy that we could recruit the perfect lineup of authors for this task, as all groups involved are renowned for their expertise in the respective field and are considered pioneers on the respective topic.

The introduction by Berryman and coworkers is intended to get readers – particularly those not very familiar with the topic – up to speed on the general features of halogen bonding and the latest developments in crystal engineering, which is arguably still its core field of application. For a rational design of strong halogen bond donors, a clear understanding of their binding to various substrates is essential, and so the next four chapters focus on various halogen bonding motifs, the recognition of substrates, and the associated thermodynamics. Taylor first discusses the interaction of structurally diverse “small molecule” halogen bond donors with Lewis bases and analyzes the underlying thermodynamic trends to provide a basis for the further applications described in later chapters. Subsequently, Beer discusses structurally complex halogenated Lewis acids, especially macrocycles and interlocked systems. The latter, in particular, are a vivid demonstration of the level of sophistication that has already been achieved with halogen bonding in solution. While all these halogen bond donors are based on terminal halogen substituents, strong halogen bonds can also be formed in NXN systems, in which X +may be considered as the key electrophile, as discussed by Erdelyi in the following chapter. The same author has then teamed up with Herrebout to describe in more detail the currently available techniques to spectroscopically study halogen bonding in solution. Nuclear magnetic resonance (NMR) spectroscopy, most of all, allows a wealth of information to be extracted on the nature of halogen‐bonded complexes.

The subsequent four chapters deal with more specific applications of halogen bonding extending pure binding and recognition in several different directions. Vargas‐Jentzsch and Matile illustrate how the binding of anions can be used for transport processes and how just the right strength of halogen bonding is crucial for optimal performance. The two ensuing chapters focus on the use of halogen bonding in catalysis, with Breugst et al. discussing applications of elemental iodine in organic reactions. While iodine has been known as a potent catalyst for some time, in most cases, its exact mode of action has remained unknown until very recently. A disadvantage of elemental iodine is the fact that it is not very susceptible to modification, and so Sutar subsequently discusses the use of more complex organic (and sometimes inorganic) halogenated compounds in organocatalysis. Finally, Schöllhorn and Fave describe how electrochemistry can be used to severely alter the halogen bonding strength of halogenated Lewis acids and to detect the formation of adducts.