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AgCN it is the electrostatic interactions interaction potential corresponds to an corresponding variations of the magnetic that dominate, leading to the displacement unconventional spin nematic state, in exchange interactions within the analogous of neighbouring chains to exactly avoid which the spins align along a preferred magnetic phases. Compellingly, this may those metal–metal contacts. An important axis without demonstrating any long-range reveal the types of emergent phenomena point noted by the researchers is that the order of the absolute spin direction along we can expect to observe in frustrated distinct supramolecular self-interaction that axis. This type of magnetic order has magnetic systems that are currently out of potentials of the two different polymer been theoretically predicted for XY spins our experimental reach. chain assemblies are well approximated on a triangular lattice, but experimental Finally, it is important to emphasize the by a simple Fourier series expansion. This realizations of such a state are yet to be truly symbiotic nature of this study. Not allowed them to demonstrate that the found. To fully account for the experimental only will it be of great potential benefit to

mapping to the different phases of the data on Au0.5Ag0.5CN, the researchers found supramolecular systems chemists who may triangular XY magnet is no coincidence, that it was necessary to include thermal seek to employ this approach to allow for the and that the supramolecular interaction effects within their model, which in the case informed design of assemblies with specific potentials of the AgCN and AuCN of the mapped spin triangular XY system emergent properties, but undoubtedly, it assemblies directly correspond to antiferro- corresponds to the presence of spin vortices will also be enormously beneficial to hard- and ferromagnetic nearest-neighbour that emerge from the nematic ground state condensed-matter physicists, who are always exchange energies, respectively. at finite temperatures. Systems that can driven to uncover new ways to reveal the As pointed out by the researchers, this play host to such spin vortices — a type of collective emergent behaviours of the most is not the first example of a conceptual collective magnetic structure that exhibits fascinating theoretically predicted phases mapping between purely structural and an in-plane curling of spins — are attracting of frustrated magnets. One could even complex magnetic systems in frustrated considerable attention as potential data argue, given the seemingly unrelated nature magnetism8. However, what is remarkable storage devices9. of these two fields, that the unexpected in this instance is the use of the simple The significance of this study is, mutually beneficial relationship discovered mapping developed for the AuCN and therefore, twofold. First, it clearly between them here is a prime example of AgCN polymer systems to begin to demonstrates that the emergence of emergence in and of itself! ❐ understand the altogether more complicated a variety of supramolecular polymer collective behaviour of supramolecular structures is driven by their chemically Lucy Clark and Philip Lightfoot are at the School assemblies of bimetallic gold-and-silver tunable interaction potentials, and that the of Chemistry and EaStChem, University of cyanide, the true structure of which has nature of these structures can be reliably St Andrews, St Andrews, Fife, KY16 9ST, UK. eluded us up until now. The bimetallicity predicted through a mapping to different e-mail: [email protected] introduces a greater complexity within phases of frustrated magnets. Second, it the overall system by allowing for both shows that through this mapping these References heterophilic and homophilic metal–metal supramolecular assemblies act as structural 1. Balazs, A. C. & Epstein, I. R. Science 325, 1632–1634 (2009). 2. Rasmussen, S. et al. Science 303, 963–965 (2004). interactions. The determination of the analogues of highly complex magnetic 3. Mattia, E. & Otto, S. Nature Nanotech. 10, 111–119 (2015). form of the supramolecular interaction states of matter that are otherwise yet 4. Nitschke, J. R. Nature 462, 736–738 (2009). 5. Szostak, J. W., Bartel, D. P. & Luisi, P. L. Nature 409, 387–309 (2001). potentials in the Au0.5Ag0.5CN assembly, and to be discovered. There is great scope to 6. Cairns, A. B. et al. Nature Chem. 8, 442–447 (2016). hence a suitable mapping to a triangular explore how the collective structures of the 7. Lee, S.-H. et al. Nature 418, 856–858 (2002). XY magnet, allowed the structure of the bimetallic polymer assemblies respond to 8. Harris, M. J., Bramwell, S. T., McMorrow, D. F., Zeiske, T. bimetallic system to be elucidated from its external perturbations, including changes & Godfrey, K. W. Phys. Rev. Lett. 79, 2554–2557 (1997). experimental powder X-ray diffraction data. in temperature, pressure and chemical 9. Mühlbauer, S. et al. Science 323, 915–919 (2009). Particularly exciting is the observation substitution. By determining the variations Acknowledgements that the magnetic phase obtained from the in the ensuing supramolecular interaction We thank the Leverhulme Trust for an award mapping of the bimetallic supramolecular potentials, one could map out the (RPG-2013-343) to support L.C.

MECHANICAL CHIRALITY A chiral catalyst with a ring to it A chiral [2]rotaxane in which the asymmetry is derived from the way in which the two components are mechanically interlocked — rather than being encoded in the covalent connectivity of the components themselves — has been shown to act as an enantioselective organocatalyst. Stephen M. Goldup

otaxanes are a class of mechanically macrocycle and axle, if the end groups of somewhat of a chemical curiosity, have interlocked molecules, the simplest the dumbbell are large enough to prevent been developed as prototypical molecular Rexamples of which consist of a the macrocycle from escaping, the two machines and are now receiving increased macrocycle encircling a linear, dumbbell- components cannot separate and the link attention as catalysts. shaped axle component. Although there between them is termed a mechanical Although early studies focused on using is no direct covalent bond between the bond. These compounds, initially the sterically crowded environment created

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a when either reaction was performed in O O the presence of the achiral axle (2), no enantioselectivity was observed. In contrast, N N R H H R rotaxane (S)-1 catalysed both the Michael O NH O NH addition and α-amination reactions with H H Ph N N N Ph reasonable levels of enantioselectivity — H N 36% and 42% e.e., respectively, in the best Ph O H H = O Ph cases reported. H H (S)-1 N N This is the first time that an enantioselective catalyst that is chiral solely O O as a result of a mechanical bond has been demonstrated. In this context, the e.e. b R R values achieved in this proof-of-concept NH study are certainly respectable. Because O NH O H H Ph N the catalyst itself is not enantiopure, it N N Ph H N = should be noted that the values reported Ph H H O O Ph underestimate the underlying selectivity; if 2 the catalyst were enantiopure the e.e. values would be expected to rise to 43% and c 50% e.e. for the iminium and enamine O O Cbz Iminium O O Enamine Cbz pathways, respectively, assuming that NN catalysis catalysis Cbz NH the catalyst is acting monomerically. It is Ph Ph Ph Ph Cbz N + somewhat surprising, however, that the + O Catalyst O Catalyst enantioselectivities observed are not higher, O O (S)-1 36% e.e. (S)-1 42% e.e. given that the difference in steric bulk 2 0% e.e. 2 0% e.e. between the encircled and free succinamide units is significant. Figure 1 | A mechanically point chiral organocatalyst. a, The S enantiomer of the mechanically point One possible explanation is that although chiral organocatalytic rotaxane 1. b, The achiral amine organocatalyst (2), which is the same as the axle the macrocycle is a very large ‘substituent’, in rotaxane 1. c, Organocatalytic reactions that occur via iminium and enamine intermediates used to it may be positioned too far from the amine compare the stereoselectivity of rotaxane (S)-1 with axle 2. organocatalyst to exert the maximum influence on the reaction. If this were the case, rigidifying and/or shortening the link by the mechanical bond to influence the substituent is too large for the macrocycle between the stereogenic centre bearing enantioselectivity of a catalytic process1, to pass over, these two enantiomers are the amine and region of the axle where the the majority of recent reports have used the stable and can be separated. This form machine-like movement of the macrocycle of chirality is dubbed ‘mechanical point a along the axle to produce catalysts with chirality’ by analogy with the much more unusual reactivity2 or stimuli-responsive common covalent point chirality. The behaviour3. Now, writing in the Journal of absolute configuration of1 was assigned the American Chemical Society, David Leigh as S by treating the macrocycle as a and co-workers have taken advantage4 of one substituent of the right-hand portion of of the least explored structural properties the axle where it is localized and applying b of the mechanical bond — the ability of the familiar Cahn–Ingold–Prelog rules for rotaxanes to display molecular asymmetry stereogenic centres. even in the absence of any covalent chiral To demonstrate the potential of such information — in order to create a new class ‘mechanochirogenesis’ in catalyst design, of enantioselective catalyst. Leigh and co-workers prepared (S)-1 in c Leigh and colleagues synthesized highly enantioenriched form (84% e.e.) with rotaxane (S)-1 (Fig. 1a) and compared the stereochemical information derived its catalytic behaviour with that of from a chiral-pool starting material, namely the corresponding non-interlocked d-asparagine. The1 H NMR spectrum of axle, 2 (Fig. 1b). Importantly, the covalent (S)-1 clearly showed the desymmetrization structure of 2 (which is identical to the of the axle component caused by the Figure 2 | Examples of mechanical chirality axle component of 1) is achiral — the position of the macrocycle and the circular exhibited by rotaxanes and catenanes. amine organocatalytic moiety (yellow) dichroism spectrum of the compound a, Enantiomeric rotaxanes comprised of a is bonded to a carbon bearing to two also exhibited a large Cotton effect. macrocycle that lacks rotational symmetry and identical succinamide (green) substituents. Rotaxane (S)-1 and the non-interlocked an axle with two different ends.b , Enantiomeric The presence of the macrocycle in1 axle 2 were then investigated separately catenanes comprised of macrocycles that lack desymmetrizes the structure, meaning in two different, yet well-understood, rotational symmetry. c, Enantiomeric catenanes that the rotaxane exists in two mirror- organocatalytic reactions (Fig. 1c) — one containing two macrocycles that are facially image forms depending on which of the that proceeds through the formation of an non-symmetrical. In each case the dotted succinamide binding sites the macrocycle iminium species and one that involves an line represents a mirror plane. Figure adapted encircles. Moreover, because the amine enamine mode of activation. As expected, from ref. 11, RSC.

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macrocycle is localized could be expected rotaxanes and catenanes can exhibit other Steve Goldup is in the Department of to enhance the enantioselectivity of the forms of mechanical chirality in the absence Chemistry, University of Southampton, catalyst. Alternatively, projecting the steric of covalent chirality (Fig. 2) when there is a Southampton, SO17 1BJ, UK. bulk of the macrocycle back towards the directionality associated with the covalent e-mail: [email protected]; catalyst by adding large groups to the frameworks of the sub-components5–7. Twitter: @sgoldup flanking p-xylyl aromatic rings of the Looking to the future, by combining macrocycle could achieve the same effect. elements of mechanical chirality with the References It may also be that rotaxane (S)-1 is less well-developed chemistry of rotaxane 1. Tachibana, Y., Kihara, N. & Takata, T. J. Am. Chem. Soc. 3 126, 3438–3439 (2004). active than other highly enantioselective molecular shuttles , it may be possible to 2. De Bo, G. et al. J. Am. Chem. Soc. 136, 5811–5814 (2014). 3 8 secondary-amine catalysts . If the reactions create switchable catalysts that can generate 3. Blanco, V., Leigh, D. A., Marcos, V., Morales-Serna, J. A. investigated in this study can also take either mirror-image form of a chiral target & Nussbaumer, A. L. A. J. Am. Chem. Soc. 136, 4905–4908 (2014). place via non-selective uncatalysed in response to external stimuli. Perhaps 4. Cakmak, Y., Erbas-Cakmak, S. & Leigh, D. A. J. Am. Chem. Soc. 138, 1749–1751 (2016). pathways, then low catalyst activity may more prosaically — but no less exciting from 5. Wasserman, E. & Frisch, H. L. J. Am. Chem. Soc. enable these to compete with the desired the point of view of catalyst development, 83, 3789–3795 (1961). rotaxane-catalysed process thus lowering given that the crowded environment of the 6. Moulin, E. & Giuseppone, N. Nature Nanotech. the observed enantioselectivity. mechanical bond has been demonstrated to 9, 331–332 (2014). 7. Bordoli, R. J. & Goldup, S. M. J. Am. Chem. Soc. These points notwithstanding, based on influence challenging catalytic processes such 136, 4817–4820 (2014). 9,10 Leigh and co-workers’ preliminary results, as gold(i)-mediated reactions — it seems 8. Zhao, D., Neubauer, T. M. & Feringa, B. L. Nature Commun. it can be argued that such mechanically likely that chiral reaction fields generated 6, 6652 (2015). chiral molecules have a bright future in by the mechanical bond in rotaxanes and 9. Lee, A.‑L. Nature Chem. 8, 8–9 (2015). 10. Galli, M., Lewis, J. E. M. & Goldup, S. M. Angew. Chem. Int. Ed. catalysis. In addition to the mechanical catenanes could be used to address existing 54, 13545–13549 (2015). point chirality exploited in rotaxane (S)-1, challenges in asymmetric catalysis. ❐ 11. Neal, E. A. & Goldup, S. M. Chem. Commun. 50, 5128–5142 (2014).

MOLECULAR MATERIALS Captivating COFs

Heavy metals are well-known environmental contaminants and are Hg2+ receptor Fluorophore released through a variety of industrial UV processes with the main risk of human exposure stemming from the Hg2+( ) contamination of natural water sources. S2– Small-molecule fluorescent sensors make 2– it possible to detect such metal ions in [HgS2] aqueous solutions, and they can then be removed using a plethora of adsorption COF Strong fluorescence Quenched fluorescence materials, including hydrogels and SOCIETY CHEMICAL AMERICAN porous silicas. A significant challenge is combining these processes to allow for co-condensation of a thioether monomer mercury ions through their absorption the simultaneous detection and removal under mild conditions, the researchers into its porous channels, where the ions of heavy metal ions. Metal–organic synthesized a COF with functional interact with the thiol branches. This frameworks (MOFs) exhibit the same thioether side-chain branches embedded system showed better performance ion-capturing capability as hydrogels within the framework that act as cation for mercury ion removal than its and silicas, and recently their metal-free receptors (pictured). The material MOF counterpart. cousins, covalent organic frameworks fluoresces under normal conditions but Wang and colleagues also showed (COFs), have come to light as potential exhibits strong fluorescence quenching that their COF was robust to recycling. candidates for ion capture because in response to the addition of mercury Experiments were carried out to exchange they share the well-exploited porosity ions — attributed to their interactions with the mercury ions out of the framework

of MOFs. the thioether groups — demonstrating its using Na2­S and the crystalline structure Now, Wei Wang and a team detecting ability. and fluorescence sensitivity were of researchers from Lanzhou and When Wang and colleagues tested the preserved during a number of cycles. Sun Yat-Sen universities in China have fluorescence response of the material to a With its combined sensing and capture designed a system that combines the number of different metal ions, they found properties, this material shows promise sensing capabilities of thiol groups that significant quenching was exhibited in expanding the role of COFs towards with the robust stabilizing network only for mercury, showing the COF to be environmental applications. provided by COFs, creating a material selective for this ion. Furthermore, they capable of detecting and removing found that when suspended in a dilute LORNA G. CHRISTIE

mercury ions from solution (J. Am. Chem. aqueous solution of Hg(ClO4)2, the COF Lorna G. Christie is a PhD student at the Soc. 138, 3031–3037; 2016). By the could take up over 98 percent of the University of Glasgow.

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