Preface

Chemists are often trained to think about bonding as a feature governed by attractive forces, such as those associated with electron sharing between atoms or electrostatic attractions between ions. If we look up the word "bond" in a dictionary, however, we find atop the list of many definitions, "something that binds or restrains". It is this kind of non-chemical bond, which is governed in the final resort by repulsive forces that prevent chemical bonds from intersecting, that is known in chemistry as the mechanical bond. As a topic of research, the mechanical bond has received much less attention than chemical bonds, to the point of being all but anonymous until the 1960s, when chemistry was already a relatively mature discipline. Even more surprising is the fact that a comprehensive book on the mechanical bond has not, to the best of our knowledge, been written. Of course, we do not mean to diminish the momentous importance of influential monographs, such as Gottfried Schill's 1971 Catenanes, Rotaxanes, and Knots, or the 1999 Molecular Catenanes, Rotaxanes, and Knots volume, edited by Jean-Pierre Sauvage and Christiane Dietrich-Buchecker, which pay considerable attention to mechanical bonding in mechanically interlocked molecules (MIMs). In our chronicle of the birth and life of molecules with mechanical bonds, such as catenanes and rotaxanes, we summarize the strategies for synthesizing them, the supramolecular/host-guest motifs utilized in their templation, their various molecular topologies and architectures, their chiralities, isomerism, and dynamics in solution and in condensed phases, not forgetting their switching properties under the influence of external stimuli and their many current and potential applications in science and technology. Taken as a whole, we hope that this book will illuminate the nature of the mechanical bond in chemistry and beyond.

When charged with the opportunity to write a book on the mechanical bond, we asked ourselves what we would like to see in a book. What would be useful to us and hopefully the readers? With a rapidly growing body of reviews in the chemical literature that cover almost every nook and cranny of contemporary research, a book ought to offer something more than a collection of reviews. Naturally, a book should be educational so as to allow students and newcomers to familiarize themselves with the field. It should consolidate and organize the seminal contributions and key fundamental topics that define the field. A book should look backwards as well as forwards, offering a perspective on the history and evolution of the field, as well as where it appears to be heading in the future. We have tried to go beyond these bare minimum requirements by paying a considerable amount of attention to presentation, with the goal of minimizing the time and effort required by a reader to learn almost anything about the mechanical bond. Because chemistry is blessed with a visual repertoire of symbolic language that enables us to communicate molecular structures and even their dynamics in a highly concise and effective manner, we have made every effort to compile an image-driven manuscript. The Nature of the Mechanical Bond commands 82 Tables, 382 Schemes, 426 Figures, and >4000 citations contained in an excess of a quarter of a million words (not counting captions or references) in six chapters. Not to feel discouraged, we suspect that a reader can assimilate 80% or more of the book's information from viewing the images and reading the captions alone.

How to Read this Book

We have written this book with the contemporary formats of the digital age in mind. Readers of the electronic version of the book are privy to several advantages. Explained below are a handful of features to help readers make the most of the experience.

Compound Numbers. The systematic notation adopted in this book for compound numbers involves a letter to denote the type of structure (M for macrocycle, C for catenane, R for rotaxane, and L for 'linear' or 'ligand', which encompass everything else), followed by a number corresponding to the Chapter (1-6), followed by a number which is assigned sequentially in order of appearance. Compound numbers appear in bold text, whereas complexes, such as pseudorotaxanes, are italicized. We employ the mathematical notation of subsets (?) to denote the inclusion of a guest within a host-e.g., guest???host.

Color. We have made extensive use of color to aid in the representation of MIMs. In some cases, the colors have significance-p-donors are typically shaded in warmer colors such as red, orange, and green, for example, whereas p-acceptors embody cool shades such as blue and purple. In other cases, the colors are meant to reflect the actual color of the corresponding compound in solution, as in the case of p-associated radicals and several of the transition metal templates. Homocatenanes, which are composed of identical interlocking rings, have been shaded with differentiated ring colors (e.g., black and green) to help readers identify the component parts.

Hyperlinks. We have introduced a highly interconnected web of hyperlinks that enhance the navigability of the book in the digital version, with the goal of establishing a seamless connection between themes and concepts, as well as the primary literature. In the main text, a click on a reference number will redirect the reader to the reference section. By clicking on the chapter numbers in the top corners of any page, readers will be redirected back to the chapter's Table of Contents (ToC). The ToC and cross-references embedded within the main text navigate to their respective Section headings.

References. At a total count of 3400 unique references, this book is a repository for many of the most important references related to the mechanical bond, dating to early 2016. Each Chapter of the book is intended to stand on its own as a useful compendium on the featured topic. References are therefore compiled at the end of each Chapter to sustain the modularity of the book. A click on a reference number within the reference section redirects the reader back to the first appearance of the citation in the main text. A click on a journal name in the reference section opens a weblink, which leads to the landing page of the primary article on the internet.

In writing this book, we decided to grasp the opportunity to tighten up, unify, and consolidate the language associated with the chemistry of mechanical bonds. We have attempted to create a resource that defines clearly the vocabulary and terminology by including a Glossary (Appendix A). In some cases, we felt it was necessary to introduce new terms for which an appropriate word or phrase was unavailable, or to emphasize distinctions that are often overlooked. For example, catenanes and rotaxanes possess mechanical bonds whereas molecular knots do not, yet all are grouped under the umbrella of MIMs. We have introduced the word 'mechanomolecule' as a way to describe molecules with mechanical bonds explicitly. In other cases, there was a need to describe unnamed phenomena that are related conceptually to phenomena with literature precedence. For example, we have introduced the term 'co-configuration' to describe the assignment of chirality that arises from mechanical bonds-a term inspired by the descriptor, 'co-conformation', which has been adopted widely in discussions of isomerism that arises from dynamic motions constrained by the mechanical bond. A similar scenario arose in the cases of 'mechanically axial chirality' and 'mechanical helicity', which we have coined in solidarity with the precedent for use of the term 'mechanically planar chirality'. Still other terms that are employed frequently in the literature are either misleading or have not been clearly defined until now. For example, the molecular entities engaged in mechanical bonding in catenanes and rotaxanes have been given many names, which include hoops, loops, rings, wheels, rods, axles, dumbbells, and components, among many others. We have chosen to define some of these terms in the context of mechanostereochemistry, our word of choice for the stereochemistry of mechanomolecules. We have also selected the term 'component part' as a generic phrase for describing these entities because 'component' has its own unrelated definition in chemistry.

Despite our own involvement in research on the mechanical bond, neither one of us appreciated fully the breadth and diversity of the field at the outset of writing this book. We have discovered that it has flourished in little more than 20 years into one of the most vibrant and interdisciplinary themes in chemistry, after a long induction period in the esoteric backwaters of unnatural product synthetic organic chemistry. For one of us (CJB), the intimate engagement with the mechanical bond rapidly became addictive, leading to many 16-hour sessions of reading, illustrating, and writing during a period of 30 months, whereas for the other (JFS), the commitment and dedication to unveiling the secrets behind the mechanical bond-an odyssey that began 30 years ago-continues unabated with no end in sight. Although we have been at considerable pains to present a comprehensive and unbiased account of the mechanical bond, we apologize in advance to any of our colleagues in the field whose work we may have...