The “Texas-Sized” Molecular Box: A Versatile Building Block for the Construction of Anion-Directed Mechanically Interlocked Structures

Abstract

Over the last two decades, researchers have focused on the synthesis and development of mechanically interlocked molecules (MIMs). The intramolecular motion of mechanical bonds and the ability to induce this effect with the choice of the proper external stimuli has prompted the development of macromolecular systems that possess the ability to "perform work" at the molecular level. Currently, researchers are working to incorporate interlocked species into complex structural systems, such as molecular frameworks and nanoparticles, and to create ever more elegant noncovalent architectures. This effort provides an incentive to generate new building blocks for the construction of MIMs. In this Account, we describe progress in the development of a new cationic building block inspired by the "blue box" of Stoddart and collaborators. The blue box (cylcobis(paraquat-p-phenylene) or CBPQT(4+)) is a tetracationic, electron-deficient macrocycle widely recognized for its role in the construction of MIMs. This venerable receptor displays a high affinity for a variety of π-donor guests, and researchers have used them to construct a wide range of molecular and supramolecular structures, including rotaxanes, catenanes, pseudorotaxanes, polypseudorotaxanes, pseudo[n]polyrotaxanes, and electrochemically switchable molecules. To date, several synthetic analogues of the basic CBPQT(4+) structure have been reported, including systems containing biphenylene linkers and chiral tetracationic cyclophanes. However, researchers have not yet fully generalized the promise of the blue box. In this Account, we chronicle the development of a larger, more flexible tetracationic macrocycle, referred to as the "Texas-sized" molecular box. To highlight its relatively increased size and to distinguish it from CBPQT(4+), we have chosen to color this new receptor burnt orange. The Texas-sized box (cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene), 1(4+)·4PF(6)(-)) acts as a dynamic molecular receptor that displays an ability to adjust its shape and conformation to accommodate anionic guests of different size and charge within its central core. The use of different guests can favor different binding modes and promote the formation of different macromolecular aggregates. Furthermore, the proper selection of the guest allows for the "turning on" or "turning off" of molecular threading and can be used to produce new kinds of threaded species. This dynamic behavior is a special feature of the Texas-sized molecular box, as is its ability to stabilize a range of polypseudorotaxanes, rotaxane-containing metal-organic frameworks (MORFs), and rotaxane-based supramolecular organic frameworks (RSOFs).

Biphen[n]arenes

“…1,2 To design and exploit novel macrocyclic hosts with unique structures and good host–guest properties is a permanent and challenging topic in this area. Some new molecular containers recently reported include Sessler's “Texas-sized” box, 3 Chun and Singh's calix[4]imidazolium, 4 Ogoshi's pillar[5]arene, 5 Stoddart's Ex-box, 6 Sindelar's bambus[6]uril, 7 Flood's cyanostar, 8 and others. 9,10 Among the supramolecular hosts, macrocyclic arenes based on methylene linked aromatic rings have been the focus of considerable recent research.…”

[2]Rotaxane with Multiple Functional Groups

“…The kinetically inert nature of the interlocked compound RTX was verified via slippage experiments. No evidence of dethreading was observed, as judged by variable-temperature (VT) 1 H NMR spectroscopy, even when RTX (4 mM) was heated for several hours at 353 K in DMSO-d 6 . The temperature of RTX solution was raised from 298 to 353 K, and the spectra are shown in Figure 8.…”

Donor-acceptor rotaxanes with tetracationic cyclophane ring

“…In investigations of rotaxanes [90][91][92][93][94][95] a growing attention is paid recently to radical-radical rotaxanes; 13,14,16,24,[96][97][98][99][100] one may hope that this area of radical-pairing interaction will become a topic of intense study. Owing to the constantly increasing amount of work concerning rotaxane chemistry, it would be impossible to cover all published reports; the present review is only a fragment of a wide research.…”