The "Texas-sized" molecular box: a versatile building block for the construction of anion-directed mechanically interlocked structures

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).

π-Metal complexes of i-propyldinaphthoporphycene

The synthesis and characterization of four metalloporphycenes containing a [MCp*]+ (Cp* = pentamethylcyclopentadiene; M = Rh , Ir , and Ru ) fragment directly bonded to the π-framework of the macrocycle were obtained. The electrochemical and optical properties of these complexes, studied in their respective free-base forms, revealed that the electron acceptor properties of the macrocycle can be enhanced by the coordination of an organometallic fragment to the π-electron face. In the case of the "fused" ruthenocene adduct, the corresponding nickel complex was also prepared. Here, less electronic communication between the ruthenocene moiety and the macrocycle was seen; however, the basic electron acceptor properties of the macrocycle were retained.

Cyclo[m]pyridine[n]pyrroles: hybrid macrocycles that display expanded π-conjugation upon protonation

Novel hybrid cyclo[m]pyridine[n]pyrroles have been synthesized using Suzuki coupling. Their NMR and optical spectroscopic features and solid state structural parameters provide support for the proposal that these species are best described as locally aromatic compounds devoid of long-range intersubunit conjugation. However, an extension of the π-conjugation in the macrocycles can be realized through protonation, as inferred from optical spectroscopic and X-ray diffraction-based structural studies.

Synthesis and study of a calixpyrrole-texaphyrin chimera: A new oligopyrrolic chloride anion receptor

Reported here is the synthesis and characterization of a new class of macrocycle that represents a hybrid between texaphyrin and calixpyrrole. These polypyrrolic macrocycles were prepared in one step by the acid-catalyzed condensation between diformyl dipyrromethanes and o-phenylenediamines in high yields. The X-ray crystal structure of one of these new “chimeras” was solved. It reveals that the diprotonated Schiff-base macrocycle has a V-shape in which one chloride anion is bound. Isothermal titration calorimetry (ITC) studies served to confirm the ability of the hybrid macrocycles to bind chloride anion in acetonitrile solution. These compounds may be useful as easy-to-make anion receptors.

Synthesis and characterization of a hexaphyrin(1.0.1.0.0.0) bearing both meso- and β-substituents

The synthesis of an isoamethyrin-type expanded porphyrin bearing both meso- and β-substituents is presented. The diprotonated form of this macrocycle was characterized by conventional spectroscopic means and via a single crystal X-ray diffraction analysis. This species is observed to adopt a planar conformation in the solid state. Nonetheless, the inner ring current, as inferred from 1 H NMR spectroscopic studies, is found to be severely diminished as compared to isoamethyrin, by the presence of the two meso-phenyl moieties. A second crystal structure, in which a molecule of water is hydrogen-bound to a pyrrole NH , was also solved.

Schiff-base porphyrin and expanded porphyrin analogs

In recent years, the field of porphyrin chemistry has expanded to include several new analogs of the original four pyrrole, carbon-bridged systems. This review traces the development of one such class of new macrocycles, namely Schiff-base porphyrin analogs. The review's focus is on the synthesis and properties of these new Schiff-base macrocycles that for ease of division have been divided into four classes: systems with two pyrrole rings, systems with three pyrrole or heterocyclic rings, systems with four pyrrole or heterocyclic rings, and systems with five or more pyrrole or heterocyclic rings. In addition to the role of metal ions as templating agents in the synthesis of these complexes, the rich coordination chemistry of several of the macrocycles is also discussed. X-ray crystallography has played an important role in determining the structures of many of these Schiff-base porphyrin analogs and many of the available structures have been incorporated into this review. Aromaticity of the macrocycles is discussed and has been evaluated from available electronic and NMR spectra. Finally, several potential applications of these molecules are discussed, briefly.

Chemical and electrochemical oxidation of N-alkyl cyclo[n]pyrroles

Cyclo[n]pyrroles ( n = 6, 7, and 8), are expanded porphyrins that lack bridging meso carbon atoms. They adopt conformations that are near-planar and show features that are consistent with global aromaticity. In previous work, involving studies of cyclo[8]pyrroles, it was found that, in contrast to what is true for porphyrins, reductive per- N -alkylation led to the formation of non-planar products that displayed few signs of extended conjugation. These octa- N -alkylated species could be reoxidized electrochemically to produce dicationic, octa- N -substituted analogues of the starting aromatic cyclo[8]pyrroles. We have now extended this study to cyclo[6]pyrrole, and have found that reductive per- N -methylation can be readily effected using methyl iodide. However, in contrast to what is true in the case of cyclo[8]pyrroles, efforts to obtain the fully N -ethylated or N -benzylated products proved unsuccessful. Moreover, in the case of the hexa- N -methylated cyclo[6]-pyrrole product no viable chemical means of oxidizing it to the putative aromatic dication could be found. However, evidence for the formation of such species was obtained by electrochemical means. The differences between cyclo[6]- and cyclo[8]pyrrole highlight the effect of ring size in mediating the redox and reactivity chemistry within what is otherwise a closely matched pair of porphyrin analogues.

Novel β-substituted calix[4]pyrroles

Calix[4]pyrroles have attracted considerable attention within the macrocyclic, supramolecular, and porphyrin analog communities due to their ability to act as simple-to-synthesize receptors for common anions, such as chloride and fluoride. Reported here is what we believe is a new, potentially generalizable route to fully β-alkyl substituted calixpyrroles. These products are accessible from the starting pyrrolic esters, and are obtained via simple reaction with CH 3 Li , followed by subsequent quenching with HCl in ether. A representative system has been characterized by X-ray diffraction analysis. The present route allows access to calix[4]pyrroles bearing functionality, such as double bonds, off the so-called C-rim; such systems are of interest because they could allow for further synthetic elaborations and permit incorporation of calix[4]pyrroles into more complex structures.

Hepatocyte-targeting single galactose-appended naphthalimide: a tool for intracellular thiol imaging in vivo

We present the design, synthesis, spectroscopic properties, and biological evaluation of a single galactose-appended naphthalimide (1). Probe 1 is a multifunctional molecule that incorporates a thiol-specific cleavable disulfide bond, a masked phthalamide fluorophore, and a single galactose moiety as a hepatocyte-targeting unit. It constitutes a new type of targetable ligand for hepatic thiol imaging in living cells and animals. Confocal microscopic imaging experiments reveal that 1, but not the galactose-free control system 2, is preferentially taken up by HepG2 cells through galactose-targeted, ASGP-R-mediated endocytosis. Probe 1 displays a fluorescence emission feature at 540 nm that is induced by exposure to free endogenous thiols, most notably GSH. The liver-specificity of 1 was confirmed in vivo via use of a rat model. The potential utility of this probe in indicating pathogenic states and as a possible screening tool for agents that can manipulate oxidative stress was demonstrated in experiments wherein palmitate was used to induce lipotoxicity in HepG2 cells.

A texaphyrin-oxaliplatin conjugate that overcomes both pharmacologic and molecular mechanisms of cisplatin resistance in cancer cells

A texaphyrin-oxaliplatin conjugate, oxaliTEX, was designed to test the concept that a platinum analog can overcome defects in drug accumulation and p53-dependent DNA damage response in a tumor model expressing multifactorial mechanisms of cisplatin resistance. Cytotoxic studies resulted in a resistance factor of only 1.2, which essentially indicated complete reversal of resistance in 2780CP cells expressing a factor of 22 with cisplatin. Unlike cisplatin, oxaliTEX accumulated and formed DNA adducts, stabilized and activated p53 at similar levels in both sensitive and resistant cells, and induced apoptosis in both models. The ability and importance of a designer drug, such as oxaliTEX, to overcome cisplatin resistance by targeting two dominant resistance mechanisms is discussed.