Giving substance to the <i>Losanitsch</i> series

Grunder, Sergio,Stoddart, J. Fraser The Royal Society of Chemistry 2012 Chemical communications Vol.48 No.26

<P>A series of oligoparaxylene model compounds with two to six paraxylene units was synthesised and the resulting mixtures of atropisomers with one to five axes of chirality were analysed by dynamic <SUP>1</SUP>H NMR spectroscopy. The number of atropisomers was found to constitute part of the <I>Losanitsch</I> series.</P> <P>Graphic Abstract</P><P>A series of oligoparaxylenes with multiple axes of chirality was synthesized and their atropisomers were found by variable temperature <SUP>1</SUP>H NMR spectroscopy to obey the <I>Losanitsch</I> series. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cc17734j'> </P>

Dynamic imine chemistry

Belowich, Matthew E.,Stoddart, J. Fraser The Royal Society of Chemistry 2012 Chemical Society reviews Vol.41 No.6

<P>Formation of an imine—from an amine and an aldehyde—is a reversible reaction which operates under thermodynamic control such that the formation of kinetically competitive intermediates are, in the fullness of time, replaced by the thermodynamically most stable product(s). For this fundamental reason, the imine bond has emerged as an extraordinarily diverse and useful one in the hands of synthetic chemists. Imine bond formation is one of a handful of reactions which define a discipline known as dynamic covalent chemistry (DCC), which is now employed widely in the construction of exotic molecules and extended structures on account of the inherent ‘proof-reading’ and ‘error-checking’ associated with these reversible reactions. While both supramolecular chemistry and DCC operate under the regime of reversibility, DCC has the added advantage of constructing robust molecules on account of the formation of covalent bonds rather than fragile supermolecules resulting from noncovalent bonding interactions. On the other hand, these products tend to require more time to form—sometimes days or even months—but their formation can often be catalysed. In this manner, highly symmetrical molecules and extended structures can be prepared from relatively simple precursors. When DCC is utilised in conjunction with template-directed protocols—which rely on the use of noncovalent bonding interactions between molecular building blocks in order to preorganise them into certain relative geometries as a prelude to the formation of covalent bonds under equilibrium control—an additional level of control of structure and topology arises which offers a disarmingly simple way of constructing mechanically-interlocked molecules, such as rotaxanes, catenanes, Borromean rings, and Solomon knots. This <I>tutorial review</I> focuses on the use of dynamic imine bonds in the construction of compounds and products formed with and without the aid of additional templates. While synthesis under thermodynamic control is giving the field of chemical topology a new lease of life, it is also providing access to an endless array of new materials that are, in many circumstances, simply not accessible using more traditional synthetic methodologies where kinetic control rules the roost. One of the most endearing qualities of chemistry is its ability to reinvent itself in order to create its own object, as Berthelot first pointed out a century and a half ago.</P> <P>Graphic Abstract</P><P>This review highlights the utilisation of a 19th Century reaction—imine bond formation—and its exploitation in the construction of supramolecular and mechanostereochemical systems. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cs15305j'> </P>

Size-selective pH-operated megagates on mesoporous silica materials.

Xue, Min,Cao, Dennis,Stoddart, J Fraser,Zink, Jeffrey I RSC Pub 2012 Nanoscale Vol.4 No.23

<P>pH-responsive megagates have been fabricated around mesoporous silica material SBA-15 in order to mechanize the mesopores. These megagates remain closed in neutral conditions, but open at pH 5. The capping components of the megagates were designed to be capable of controlling pores up to 6.5 nm in diameter. Selectivity of payloads with different sizes can be achieved through the use of different capping components. The operation of the megagates was demonstrated by time-resolved fluorescence spectroscopy which is capable of monitoring the release of both the payload and the cap. This study opens up new possibilities in the field of controllable release.</P>

Cooperative self-assembly: producing synthetic polymers with precise and concise primary structures

Avestro, Alyssa-Jennifer,Belowich, Matthew E.,Stoddart, J. Fraser The Royal Society of Chemistry 2012 Chemical Society reviews Vol.41 No.18

<P>The quest to construct mechanically interlocked polymers, which present precise monodisperse primary structures that are produced both consistently and with high efficiencies, has been a daunting goal for synthetic chemists for many years. Our ability to realise this goal has been limited, until recently, by the need to develop synthetic strategies that can direct the formation of the desired covalent bonds in a precise and concise fashion while avoiding the formation of unwanted kinetic by-products. The challenge, however, is a timely and welcome one, as a consequence of, primarily, the potential for mechanically interlocked polymers to act as dynamic (noncovalent) yet robust (covalent) new materials for a wide array of applications. One such strategy which has been employed widely in recent years to address this issue, known as Dynamic Covalent Chemistry (DCC), is a strategy in which reactions operate under equilibrium and so offer elements of “proof-reading” and “error-checking” to the bond forming and breaking processes such that the final product distribution always reflects the thermodynamically most favourable compound. By coupling DCC with template-directed protocols, which utilise multiple weak noncovalent interactions to pre-organise and self-assemble simpler small molecular precursors into their desired geometries prior to covalent bond formation, we are able to produce compounds with highly symmetric, robust and complex topologies that are otherwise simply unobtainable by more traditional methods. Harnessing these strategies in an iterative, step-wise fashion brings us ever so much closer towards perfecting the controlled synthesis of high order main-chain mechanically interlocked polymers. This <I>tutorial review</I> focuses (i) on the development of DCC—namely, the formation of dynamic imine bonds—used in conjunction with template-directed protocols to afford a variety of mechanically interlocked molecules (MIMs) and ultimately (ii) on the synthesis of highly ordered poly[<I>n</I>]rotaxanes with high conversion efficiencies.</P> <P>Graphic Abstract</P><P>Emergent positive cooperativity accelerates the efficient template-directed assembly of mechanically interlocked molecules with contiguous [π···π] stacks in high yields. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cs35167f'> </P>

Mechanostereochemistry and the mechanical bond

Barin, Gokhan,Forgan, Ross S.,Stoddart, J. Fraser The Royal Society 2012 Proceedings, Mathematical, physical, and engineeri Vol.468 No.2146

<P> The chemistry of mechanically interlocked molecules (MIMs), in which two or more covalently linked components are held together by <I>mechanical bonds</I> , has led to the coining of the term <I>mechanostereochemistry</I> to describe a new field of chemistry that embraces many aspects of MIMs, including their syntheses, properties, topologies where relevant and functions where operative. During the rapid development and emergence of the field, the synthesis of MIMs has witnessed the forsaking of the early and grossly inefficient statistical approaches for template-directed protocols, aided and abetted by molecular recognition processes and the tenets of self-assembly. The resounding success of these synthetic protocols, based on templation, has facilitated the design and construction of artificial molecular switches and machines, resulting more and more in the creation of integrated functional systems. This review highlights (i) the range of template-directed synthetic methods being used currently in the preparation of MIMs; (ii) the syntheses of topologically complex knots and links in the form of stable molecular compounds; and (iii) the incorporation of bistable MIMs into many different device settings associated with surfaces, nanoparticles and solid-state materials in response to the needs of particular applications that are perceived to be fair game for mechanostereochemistry. </P>

Dynamic clicked surfaces based on functionalised pillar[5]arene

Zhang, Huacheng,Strutt, Nathan L.,Stoll, Ragnar S.,Li, Hao,Zhu, Zhixue,Stoddart, J. Fraser Royal Society of Chemistry 2011 Chemical communications Vol.47 No.41

<P>A dynamic smart surface which was constructed by the self-assembly of an azobenzene-functionalised pillar[5]arene exhibits reversibly responsive morphologies towards UV and visible light as observed by TEM, SEM and AFM.</P> <P>Graphic Abstract</P><P>Pillar[5]arene monofunctionalized with azobenzene was used to prepare a photoresponsive surface which can switch between vesicles and solid particles. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1cc14934b'> </P>

A self-complexing and self-assembling pillar[5]arene

Strutt, Nathan L.,Zhang, Huacheng,Giesener, Marc A.,Lei, Juying,Stoddart, J. Fraser The Royal Society of Chemistry 2012 Chemical communications Vol.48 No.11

<P>A monofunctionalised pillar[5]arene derivative carrying a viologen side chain which exhibits self-complexation in dilute dichloromethane solutions forms supramolecular daisy chain polymers and eventually organogels as its concentration is increased three-fold over the range from 0.1 to 100 mM.</P> <P>Graphic Abstract</P><P>A viologen-functionalized pillar[5]arene exhibits self-complexation in dilute solutions and forms supramolecular daisy chain polymers and organogels at high concentrations. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cc16030g'> </P>

Efficient Long-Range Stereochemical Communication and Cooperative Effects in Self-Assembled Fe₄L₆ Cages

Ousaka, Naoki,Grunder, Sergio,Castilla, Ana M.,Whalley, Adam C.,Stoddart, J. Fraser,Nitschke, Jonathan R. American Chemical Society 2012 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.134 No.37

<P>A series of large, optically active Fe<SUB>4</SUB>L<SUB>6</SUB> cages was prepared from linear 5,5′-bis(2-formylpyridines) incorporating varying numbers (<I>n</I> = 0–3) of oligo-<I>p</I>-xylene spacers, chiral amines, and Fe<SUP>II</SUP>. When a cage was constructed from the ligand bridged by one <I>p</I>-xylene spacer (<I>n</I> = 1) and a bulky chiral amine, both a homochiral Fe<SUB>2</SUB>L<SUB>3</SUB> helicate and Fe<SUB>4</SUB>L<SUB>6</SUB> cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe<SUB>4</SUB>L<SUB>6</SUB> cage was observed. In the case of larger cages (<I>n</I> = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands’ geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2012/jacsat.2012.134.issue-37/ja306615d/production/images/medium/ja-2012-06615d_0014.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja306615d'>ACS Electronic Supporting Info</A></P>

A Metal–Organic Framework-Based Material for Electrochemical Sensing of Carbon Dioxide

Gassensmith, Jeremiah J.,Kim, Jeung Yoon,Holcroft, James M.,Farha, Omar K.,Stoddart, J. Fraser,Hupp, Joseph T.,Jeong, Nak Cheon American Chemical Society 2014 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.136 No.23

<P>The free primary hydroxyl groups in the metal–organic framework of CDMOF-2, an extended cubic structure containing units of six γ-cyclodextrin tori linked together in cube-like fashion by rubidium ions, has been shown to react with gaseous CO<SUB>2</SUB> to form alkyl carbonate functions. The dynamic covalent carbon–oxygen bond, associated with this chemisorption process, releases CO<SUB>2</SUB> at low activation energies. As a result of this dynamic covalent chemistry going on inside a metal–organic framework, CO<SUB>2</SUB> can be detected selectively in the atmosphere by electrochemical impedance spectroscopy. The “as-synthesized” CDMOF-2 which exhibits high proton conductivity in pore-filling methanolic media, displays a ∼550-fold decrease in its ionic conductivity on binding CO<SUB>2</SUB>. This fundamental property has been exploited to create a sensor capable of measuring CO<SUB>2</SUB> concentrations quantitatively even in the presence of ambient oxygen.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2014/jacsat.2014.136.issue-23/ja5006465/production/images/medium/ja-2014-006465_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja5006465'>ACS Electronic Supporting Info</A></P>

Ground-State Kinetics of Bistable Redox-Active Donor–Acceptor Mechanically Interlocked Molecules

Fahrenbach, Albert C.,Bruns, Carson J.,Li, Hao,Trabolsi, Ali,Coskun, Ali,Stoddart, J. Fraser American Chemical Society 2014 Accounts of chemical research Vol.47 No.2

<P>The ability to design and confer control over the kinetics of theprocesses involved in the mechanisms of artificial molecular machines is at the heart of the challenge to create ones that can carry out useful work on their environment, just as Nature is wont to do. As one of the more promising forerunners of prototypical artificial molecular machines, chemists have developed bistable redox-active donor–acceptor mechanically interlocked molecules (MIMs) over the past couple of decades. These bistable MIMs generally come in the form of [2]rotaxanes, molecular compounds that constitute a ring mechanically interlocked around a dumbbell-shaped component, or [2]catenanes, which are composed of two mechanically interlocked rings. As a result of their interlocked nature, bistable MIMs possess the inherent propensity to express controllable intramolecular, large-amplitude, and reversible motions in response to redox stimuli. In this Account, we rationalize the kinetic behavior in the ground state for a large assortment of these types of bistable MIMs, including both rotaxanes and catenanes. These structures have proven useful in a variety of applications ranging from drug delivery to molecular electronic devices.</P><P>These bistable donor–acceptor MIMs can switch between two different isomeric states. The favored isomer, known as the ground-state co-conformation (GSCC) is in equilibrium with the less favored metastable state co-conformation (MSCC). The forward (<I>k</I><SUB>f</SUB>) and backward (<I>k</I><SUB>b</SUB>) rate constants associated with this ground-state equilibrium are intimately connected to each other through the ground-state distribution constant, <I>K</I><SUB>GS</SUB>. Knowing the rate constants that govern the kinetics and bring about the equilibration between the MSCC and GSCC, allows researchers to understand the operation of these bistable MIMs in a device setting and apply them toward the construction of artificial molecular machines.</P><P>The three biggest influences on the ground-state rate constants arise from (i) ground-state effects, the energy required to breakup the noncovalent bonding interactions that stabilize either the GSCC or MSCC, (ii) spacer effects, where the structures overcome additional barriers, either steric or electrostatic or both, en route from one co-conformation to the other, and (iii) the physical environment of the bistable MIMs. By managing all three of these effects, chemists can vary these rate constants over many orders of magnitude. We also discuss progress toward achieving mechanostereoselective motion, a key principle in the design and realization of artificial molecular machines capable of doing work at the molecular level, by the strategic implementation of free energy barriers to intramolecular motion.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/achre4/2014/achre4.2014.47.issue-2/ar400161z/production/images/medium/ar-2013-00161z_0012.gif'></P>