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challenge for future research in this direction is extending the scope of the reaction to sulfur-containing rings, for example to synthesize the drug Clopidogrel (Plavix) — used in the prevention of blood clotting.

The Berkeley team have invested much time and effort in demonstrating that their catalyst is indeed heterogeneous, and show the stability and effectiveness of the Pt40 cluster. When capped with a polyamidoamide (so-called PAMAM) dendrimer, and supported on mesoporous silica (in this case SBA-15) the Pt40 nanoparticles could be recycled four times using simple filtration, reduction and repeated treatment with PhICl2. The catalysts were still active a month later, although the turnover number (TON) — the number of reactant molecules converted to product per metal atom — was a rather low 400.

Quite remarkably, in one case, the heterogeneous catalysts gave qualitatively different results than their homogeneous analogues, steering the reaction of a

phenylurea derivative away from the ‘traditional’ cyclization and towards a less common indole-ring dimerization with a yield of approximately 85%. This unexpected ‘bonus’ is thought to arise from a sort of cascade of reactions catalysed by the entrapped Pt nanoparticles. In this respect, the trapped nanoparticles mimic an enzyme’s active site, where intermediates that form have no time to ‘escape’ from the reactive zone and so remain and react further.

Notwithstanding these achievements, the real test for such ‘caged’ catalysts would be their use in a continuous fixed-bed system, and for situations where they spend a long time on-stream — continuously flowing reactants over a stable catalyst. With Pt prices currently topping $46 per gram, high TONs and atom-efficient recycling will be crucial for an economically viable catalytic process. Note that the TON of 400 quoted above is considered ‘low’ at least partly because of the high cost of the

catalyst and for cheaper catalysts this could be acceptable. Nevertheless, making your catalyst stay put is an essential step towards bridging the gap between homogeneous and heterogeneous catalysis. This approach may find applications in the synthesis of active pharmaceutical intermediates, where catalyst separation down to ppb levels counts for more than the price of the metal. ❐

Gadi Rothenberg is in the Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018WV Amsterdam, The Netherlands. e‑mail: g.rothenberg@uva.nl

References1. Blaser, H.-U. Adv. Synth. Catal. 344, 17–31 (2002).2. Knowles, W. S. Angew. Chem. Int. Ed. 41, 1998–2007 (2002).3. de Smit, E. et al. Nature 456, 222–225 (2008).4. Maldonado, A. G. & Rothenberg, G. CEP magazine

105, 26–32 (2009).5. Rothenberg, G. Catalysis: Concepts and Green Applications

(Wiley-VCH, 2008).6. Witham, C. A. et al. Nature Chem. 2, 36–41 (2010).

Altering the properties of materials by external perturbation is highly attractive for the construction of

switches and memory devices. In particular, an electrical bias has been widely explored as an external stimulus, and although this has led to the development of materials with electrically tunable properties such as electro–optical effects, research is now focusing on investigating other novel mechanisms that can be induced by an electric field. Such exotic electric-field-induced phenomena — nonlinear resistance effects or metal–insulator transitions, for example — have recently been reported1,2, and now, writing in the Journal of the American Chemical Society, Gábor Molnár, Azzedine Bousseksou and co-workers describe3 an interesting example of an electric-field effect in two cyanometallate complexes.

Cyano-bridged metallocomplexes were among the first materials to be recognized as coordination polymers — materials built by linking metal ions with organic ligands. They have been extensively studied and applied; ‘Prussian blue’

(Fe4[Fe(CN)6]3∙xH2O), for example, is used as a pigment in paints. Recently, however, they have attracted a great deal of attention as molecular magnets4.

In particular, cyano-bridged metallocomplexes can exhibit intervalence charge transfers, in which an electron transfer occurs between metal centres of different valences. The charge-transfer phase transition between these two states can be thermally induced, and in some cases it is accompanied by a large thermal hysteresis loop. This means that, within the hysteresis region, the compounds can exist in either of two different states — the high-temperature (HT) or the low-temperature (LT) phases. These are separated by a potential energy barrier, but they have similar energy levels around the phase-transition temperature. An external stimulus when the system is close to this temperature could therefore be expected to induce the inter-conversion between the two states, triggering a nonlinear change in the material’s properties5.

The two materials investigated by Molnár, Bousseksou and co-workers — the Prussian

blue analogue Rb0.8Mn[Fe(CN)6]0.93∙1.62H2O (Fig. 1a), and the complex Co3[W(CN)8]2 (pyrimidine)4∙6H2O, denoted as the MnFe and CoW systems, respectively — are examples of such compounds3,6; their HT and LT phases are known to display drastically different magnetic, optical and electrical properties.

The charge-transfer phase transition of the MnFe system is expressed by a reversible transition between the HT state, FeIII(S = 1/2)–CN–MnII(S = 5/2), and the LT state, FeII(S = 0)–CN–MnIII(S = 2), where S is the spin state of the metal centre. The colours of the HT and LT phases of the MnFe system are not blue, despite the fact that it is a Prussian blue analogue, but light and dark brown, respectively. The phase transition of the CoW system is expressed by a transition between the red-colour HT state, WV(S = 1/2)–CN–CoII(S = 3/2), and the blue-colour LT state, WIV(S=0)–CN–CoIII(S=0)

In their approach, Molnár, Bousseksou and co-workers have focused on the thermal hysteresis region. For both compounds, the researchers observed that applying an

SWITCHABLE MATERIALS

An electric effectElectrically tunable materials are used to construct switches and memory devices. Applying an electric field within a specific temperature range to cyanometallate complexes triggers their charge-transfer phase transition, altering their optical and magnetic properties.

osamu Sato

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electric field to the HT phase within this temperature range induced a phase transition to the LT phase (Fig. 1b). They only observed this effect when the electric field exceeded a given threshold value, which shows that the electric-field-induced conversion is a nonlinear process. The electric-field-induced transition involved an abrupt increase of the current flow in these compounds. When the current flow was impeded, no phase change was observed, indicating the importance of the charge-injection process.

Furthermore, the reverse phenomenon — the transition from the LT to the HT phase — could not be induced by the electric field. It could be achieved, however, by heating the system above the phase-transition temperature. Once the HT state was recovered, the phase transition could be induced again with an electric field. The HT-LT transformation could thus be cycled reversibly by alternate application of the electric field and heating. Because the phase transitions involve a change both in magnetization and colour, the compounds can be regarded as displaying electric-field-induced magnetism and chromism switching.

For both compounds, Molnár, Bousseksou and co-workers observed a paraelectric-to-ferroelectric transition during the HT-LT phase transition. On the basis of the existence of the ferroelectric phase with a non-zero dipole moment ρ, they proposed a theoretical model taking into account elastic interactions and electrostatic interactions that stabilize the ferroelectric LT phase by an energy equal to ρE (E is electric field). This model can explain well the observed one-way transition from the HT to the LT phase. As described by the researchers, however, the suggested model cannot explain all of the

experimental findings, such as the fact that an electric field of opposite sign could not induce the phase transition.

These findings, as well as other recent examples of electric-field-induced exotic effects, show that focusing on materials with multistability can lead to unusual phenomena by applying subtle external perturbations1–3. The two electrically switchable systems described here are promising components for future molecular devices. As the authors suggested, the reverse transition can be induced by a thermal–electrical effect3, but it might also be possible to induce it by light irradiation. Indeed, this photo-induced phase transition has been reported for several molecular compounds5,7.

Before the present work, Bousseksou and co-workers had also reported a spin-crossover complex for which both the phase transition and its reverse could

be photo-induced at room temperature7. Hence, the present findings might lead to the development of new molecular devices with optical and magnetic properties that can be switched by using electric field and thermal–electrical effects, or, potentially, an electric field and light irradiation. ❐

Osamu Sato is at the Institute for Materials Chemistry and Engineering, Kyushu University, 6‑1 Kasuga‑koen, Kasuga, Fukuoka 816‑8580, Japan. e‑mail: sato@cm.kyushu‑u.ac.jp

References1. Potember, R. S., Poehler, T. O. & Cowan, D. O. Apply. Phys. Lett.

34, 405–407 (1979).2. Sawano, F. et al. Nature 437, 522–524 (2005).3. Mahfoud, T. et al. J. Am. Chem. Soc. 131, 15049–15054 (2009).4. Verdaguer, M. et al. Coord. Chem. Rev. 190–192, 1023–1047 (1999).5. Koshihara, S., Tokura, Y., Takeda, K. & Koda, T. Phys. Rev. Lett.

68, 1148–1151 (1992).6. Ohkoshi, S., Tokoro, H. & Hashimoto, K. Coord. Chem. Rev.

249, 1830–1840 (2005).7. Bonhommeau, S. et al. Angew. Chem. Int. Ed. 46, 4069–4073 (2005).

Molecular photoswitches use light to swap between states that can have different colours or absorption/emission properties. Diarylethylene switches have been used to control chemical and biochemical reactivity, but have not previously been used in vivo.

Now, Neil Branda and colleagues have shown (J. Am. Chem. Soc. 131, 15966–15967; 2009) that a molecular photoswitch can reversibly control paralysis in Caenorhabditis elegans nematode worms. The transparent worms were incubated in a mixture of the bipyridinium dithienylethene switch and a 10% dimethylsulfoxide buffer. One group

of worms was exposed to the ring-open form and another to the ring-closed.

The ‘ring-closed’ group showed the characteristic blue/green photoswitch colour demonstrating its absorption into the worms (pictured). Although the ‘ring-open’ group did not, a 2-minute exposure to UV light flipped the photoswitch and the worms changed colour. Worms fed the ring-closed form for 60 minutes appeared immobile, and this paralysis could be turned on and off by alternating their exposure to UV and visible light.

NEIL WITHERS

The worm that turned offMoLECuLAR PHoToSWITCHES

© 2

009

AC

S

Figure 1 | Crystal structure and schematic representation of the reversible phase transition of electrically switchable cyanometallate complexes. a, Crystal structure of Rb0.8Mn[Fe(CN)6]0.93∙1.62H2O. Reprinted with permission from ref. 3, © 2009 ACS. b, Electric-field-induced phase transition from the HT (FeIII-CN-MnII) to the LT (FeII-CN-MnIII) phase. After application of electrical bias, the reverse process (from LT to HT) can be induced by heating the compound above its phase-transition temperature. This process can be cycled reversibly.

Heating

Electric field

FeMnCNRb

a b

MnIIIFeII FeIII MnII

LT HT

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