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Mechanical Chirality news & views 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 404 NATURE CHEMISTRY | VOL 8 | MAY 2016 | www.nature.com/naturechemistry ©2016 Mac millan Publishers Li mited. All ri ghts reserved. ©2016 Mac millan Publishers Li mited. All ri ghts reserved. news & views 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 of 1 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.
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