SINGLE-MOLECULE MAGNETS Iron Lines up for More Than a Decade, Single-Molecule Magnets Have Relied on Multinuclear Transition Metal Clusters and Lanthanide Compounds

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SINGLE-MOLECULE MAGNETS Iron lines up For more than a decade, single-molecule magnets have relied on multinuclear transition metal clusters and lanthanide compounds. Now, a mononuclear, two-coordinate iron(i) complex has shown that single transition metals can compete with the lanthanides when certain design principles from magnetochemistry are borne in mind. Eckhard Bill

he bottom-up synthesis of magnetic z then, f elements have been considered ‘silver materials is a key aim of research in bullets’ for SMM developments. Tmolecular magnetism. The goal is xz/yz In their present work, Long and not to compete with bulk magnets, such as co-workers have tuned the ligand field those decorating the fridge door at home splitting of a ‘simple’ iron complex to or operating valves in your car, whose 2 2 achieve similar properties — a magnetic Fe x −y /xy magnetism depends on the collective moment that has a suitable magnitude and contributions of neighbouring magnetic strong anisotropy. The complex shows a atoms arranged with long-range order. spin relaxation barrier of 328 K, and is the Instead, molecules that are intrinsically z 2 first complex with a cheap and abundant magnetic — single-molecule magnets transition element that may be able to (SMMs) — are attractive because of their compete as an SMM with lanthanide small size, versatile chemical composition Figure 1 | A single-molecule magnet has been compounds. This is particularly noteworthy and adaptable fabrication methods. They devised based on an iron(i) centre with linear because the spin–orbit coupling (and thus hold promise for spin-dependent electronics symmetry. Schematic representation of the anisotropy) of first-row transition metal or magnetic information storage, with fields complex, emphasizing its axial symmetry (left). ions is intrinsically lower than that of the that put forth devices such as reading heads The corresponding ligand field at the metal lanthanides, and their larger ligand field for hard disks or non-volatile computer affords a particular orbital scheme for the seven effects in most coordination environments memory components. 3d electrons (right) that is the origin of a strong usually quench the orbital moment In particular, SMMs that contain magnetic moment along the z direction. necessary for persistent magnetism to arise. just one spin centre should be ideal Also, molecular vibrations in transition for future miniaturized spin-based metal coordination complexes often computational devices that can store and of the magnetization. To be a good SMM, perturb the degeneracy of d orbitals (for process information by making use of the a compound must combine a high-spin example through the Jahn–Teller effect); fascinating fundamental phenomena of ground state and a ZFS that is large and this weakens the ZFS and further limits the quantum mechanics1. Jeffrey R. Long and negative and provides a well-defined axis for magnetic anisotropy. his collaborators from spectroscopy and easy magnetization. The selection of the successful iron(i) theoretical chemistry have now described2 An established strategy for building SMM was guided by the consideration in Nature Chemistry a ‘simple’ mononuclear, oligonuclear SMMs relies on clusters of that a low coordination number of the two-coordinate iron(i) compound with transition metal ions with suitable local d-metal ion, and a low oxidation state, impressive SMM properties. Their findings spins and large ZFS. To optimize the should exhibit a high-spin d-electron outline a few remarkable design principles global cluster spin, ferro- or ferrimagnetic configuration with unpaired electrons in for future advances. spin coupling is required between the close-lying d orbitals. This could provide The SMM nature of a compound metal centres, and these centres need to unquenched orbital angular momentum, so manifests itself, among other features, be organized in a geometry that favours that strong spin–orbit coupling is expected, through magnetic hysteresis: magnetization build-up of large magnetic anisotropy. which maximizes the magnitude and induced by an external field relaxes only The latter in particular is not trivial, the anisotropy of the magnetic moment. slowly after the field is removed. Below but successful schemes have been In the new compound, this mechanism a certain ‘blocking temperature’, the derived from the principles of classical holds in particular for the dx2–y2 and dxy magnetization may even be stable for a magnetochemistry3. orbitals (Fig. 1). The fact that the complex period of time, from minutes to years, Recently, however, mononuclear is linear inevitably provides a unique axis before the compound relaxes. This slow complexes of terbium and dysprosium for easy magnetization and the preferential magnetic relaxation arises from coupling have surpassed all transition metal alignment of the magnetic moment. of the electronic spins with the orbital complexes by showing SMM behaviour with A proof of this principle has already moments of the atoms. Among other extremely large spin relaxation barriers4 been obtained5–8, when SMM behaviour effects, this spin–orbit coupling causes corresponding to thermal energies of more was observed for linear iron(ii) compounds zero-field splitting (ZFS) of the atomic than 900 K — 10 times as high as those of with spin S = 2. These compounds exhibit electronic ground states, which generates transition metal clusters. This is because a impressive relaxation barriers of up to directionality for the molecular magnetic single lanthanide spin centre can display 260 K or so, but their magnetization was moments (magnetic anisotropy) and large enough axial magnetic anisotropy to short-lived — even at low temperatures creates an energy barrier towards reversal lead to high spin relaxation barriers. Since — unless a polarizing magnetic field was

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applied. This was not entirely surprising, and weakest interactions for magnetic through the ligand field model (derived because this type of integer-spin system decay and deserves further study. ab initio), the calculations provide a solid (S = 2, 4…) provides magnetic decay Detailed spectroscopic and quantum- understanding of the compound and a pathways ‘around’ the spin relaxation chemical characterization of the linear wealth of chemical information, which will barrier. In contrast, half-integer spins iron(i) compound revealed an electronic stir things up in the field. ❐ (with S = 3/2, 5/2…) are not at risk from structure that many inorganic chemists that, owing to the different polarization familiar with ligand-field models would Eckhard Bill is at the Max Planck Institute for of their magnetic levels. The Long group probably not have guessed correctly. Chemical Energy Conversion, Stiftstrasse 34–36, therefore chemically reduced one of the The theoreticians in the research team D-45470 Mülheim/Ruhr, Germany. known iron(ii) precursor complexes5–7 showed — with spectroscopy-oriented e-mail: [email protected] to obtain an iron(i) compound with spin multireference methods9 and ligand- state S = 3/2. The resulting complex not field analyses based on computation References 7 1. Stamp, P. C. E. & Gaita-Arino, A. J. Mater. Chem. only shows, as anticipated, a record high of all electronic multiplets for the 3d 19, 1718–1730 (2009). relaxation barrier (for iron) of 328 K, configuration — that strong s4 –3dz2 2. Zadrozny, J. M. et al. Nature Chem. 5, 577–581 (2013). significant magnetic hysteresis below 29 K mixing readily stabilizes the dz2 orbital 3. Glaser, T. Chem. Commun. 47, 116–130 (2011). and magnetic ‘blocking’ below 4.5 K, but in the linear symmetry, as sketched in 4. Rinehart, J. D. & Long, J. R. Chem. Sci. 2, 2078–2085 (2011). 5. Reiff, W. M., LaPointe, A. M. & Witten, E. H. V. J. Am. Chem. Soc. also has improved permanence in zero field. Fig. 1 (right). The result is well supported 126, 10206–10207 (2004). Unfortunately, the stability at weak fields by comprehensive spectroscopic 6. Power, P. P. Chem. Rev. 112, 3482–3507 (2012). close to zero was not as good as expected, characterization, and the calculated 7. Atanasov, M., Zadrozny, J. M., Long, J. R. & Neese, F. Chem. Sci. 4, 139–156 (2013). but this deviation touches on fundamental orbital degeneracies nicely explain the 8. Zadrozny, J. M. et al. Chem. Sci. 4, 125–138 (2013). questions about the role of spin tunnelling unquenched orbital moment. Interpreted 9. Atanasov, M. et al. Inorg. Chem. 50, 7460–7477 (2011).

POLYMER CHEMISTRY Proteins in a pill Protein drugs are important therapies for many different diseases, but very few can be administered orally. Now, a cationic dendronized polymer has been shown to stabilize a therapeutic protein for delivery to the gut. Heather D. Maynard

roteins are widely used as drugs — over administration because their action is of mitigating the pathology of gluten 130 are approved by the US Food and required within the gastrointestinal tract2, intolerance. Bacteria-derived PEPs target PDrug Administration (FDA). These for example enzymes for treating maladies proline-rich regions of the gluten and have therapeutics make up for deficiencies, such as pancreatic insufficiency and lactose been explored as a way of reducing its supplement or boost existing pathways, intolerance. Some are relatively stable in immunotoxicity5. However, some of the and block deleterious agents1. Proteins are the gut, but most are not. Various strategies candidate enzymes are easily inactivated found in vaccines and other treatments have been investigated to retain activity in the stomach, and yet any therapeutic for diseases that range from diabetes, of these proteins; however, it has proved would require prolonged enzymatic activity osteoporosis and cancer. Most of these difficult to preserve biological activity, there to ensure that the gluten is effectively drugs must be given by injection, and this especially within specific places in the broken down before it moves into the small has associated difficulties including pain, gastrointestinal tract. Now writing in intestine. Leroux and co-workers show that discomfort and inconvenience for patients. Nature Chemistry, Jean-Christophe Leroux by attaching a positively charged polymer to Oral administration in the form of a pill and co-workers report3 that by covalently a PEP, the enzyme was not only active in the would be a much better alternative, yet there attaching a polymer to a proline-specific stomach, it was retained there. are many challenges associated with this endopeptidase (PEP), the enzyme was Conjugation of synthetic polymers to method of delivery. stabilized and retained in the upper proteins is often used to protect injected The gastrointestinal tract has evolved gastrointestinal tract (Fig. 1). proteins from enzymatic degradation to cleave proteins into small fragments Proline-specific endopeptidases are a and to increase the time it takes for the and to denature them, a beneficial feature promising treatment for coeliac disease — biomolecule to be excreted out of the that enables us to digest our food and commonly known as gluten intolerance. body6. This results in increased circulation obtain essential nutrients. Unfortunately Coeliac disease comes about when a person times and reduced dosage frequency. for delivery of therapeutic proteins, launches an immune response against large The polymer that is most widely utilized this degradation also results in loss of gluten fragments in the small intestine4. for this purpose is the linear polyether biological activity. Another problem These fragments typically contain high poly(ethylene glycol) (PEG), and many facing oral delivery is the rapidity with proportions of the amino acids proline so-called PEGylated proteins and peptides which materials are moved through the and glutamine, which our natural enzymes have been approved as treatments by gastrointestinal system by gastric emptying do not recognize. Cleaving gluten into the FDA. Leroux and co-workers tried and peristalsis. Despite these problems, tiny peptides before passing them to the PEGylation of the PEP but found, however, there are a few proteins that need oral intestine has been proposed as one method that although this standard polymer

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