Single-molecule magnet behaviour in metal–organic nanospheres generated by simple precipitation of Mn 12 O 12 clustersw Inhar Imaz, a Fernando Luis, b Chiara Carbonera, b Daniel Ruiz-Molina c and Daniel Maspoch* a Received (in Cambridge) 17th October 2007, Accepted 11th December 2007 First published as an Advance Article on the web 21st January 2008 DOI: 10.1039/b716071b Metal–organic sub-50 nm spherical particles that exhibit single- molecule magnet behaviour have been fabricated by direct precipitation of Mn 12 O 12 clusters in a mixture of acetonitrile and toluene. Micro- and nanoparticles have evolved into a powerful family of materials for many applications ranging from data storage, catalysis, photonics, electronics, lithography and microlenses to biosensors. 1 Therefore, widespread attention has been paid to new strategies for fabricating particles with novel composi- tions and properties. Metal–organic solids are hybrid materi- als created by the association of metal ions and organic ligands, which have recently shown a wide range of promising properties in gas sorption, sensing, catalysis, ion exchange, magnetism, optics, etc. 2 Because of the vast range of proper- ties, one of the current objectives is the synthesis of nanoscale metal–organic materials (NMOMs). 3 To date, there are at least two possible routes to fabricating NMOMs. One involves the use of microemulsion techniques, which have already enabled the synthesis of Prussian blue- and triazol-based magnetic nanoparticles, and Gd(III) nanorods that can be used as multimodal contrast enhancing agents. 3–8 The second is based on precipitation processes. 9–13 For example, nanoparti- cles of neutral metal–salen complexes have been fabricated by using compressed antisolvent technology with supercritical carbon dioxide as the precipitant. 9 More recently, a method consisting of both coordination polymerization and precipita- tion in a poor solvent has allowed the synthesis of colloidal amorphous particles from infinite coordination polymers that show interesting optical properties and ion-exchange capabil- ities. 10,11 We have been inspired by the latter route to show that one can use a simple precipitation process to obtain functional nanoparticles from pre-synthesized magnetic metal–organic clusters. Manganese oxide clusters of general formula [Mn 12 O 12 (RCOO) 16 (H 2 O) 4 ] (Mn 12 ; R = alkyl, aryl) present a remark- able single-molecule magnet behaviour. These complexes pre- sent a large spin ground state together with a strong uniaxial anisotropy, resulting in an energy barrier for spin reversal. Therefore, Mn 12 clusters can be considered as magnetic units of a sharply defined size that may offer new possibilities for high-density information storage molecular devices, spintronic systems and quantum-computing applications. 14 However, before these applications can be realized, micro- and nano- sized Mn 12 -based systems, either in the form of structured surfaces or particles, must be fabricated. To date, while Mn 12 clusters have been structured on surfaces 15–18 or synthesized in the form of sub-micron crystals, 19–21 less work has been reported on the obtaining of nanosized particles. The synthesis of Mn 12 -based nanoparticles would be important, for exam- ple, to determine how the magnetic properties are modified in the transition from the macroscopic to the nanoscopic world, i.e., size, crystallinity and dimensionality effects. In this com- munication, we report the fabrication of sub-50 nm Mn 12 - based spheres that show SMM behaviour by a direct precipi- tation process. A series of solvents, such as hexane, chloro- form, dimethylformamide, acetonitrile, toluene and dichoromethane, and respective solvent mixtures were system- atically studied to determine the optimum conditions for nanoparticle formation. Under the studied conditions, Mn 12 - based nanospheres were only obtained when an acetonitrile solution of crystalline Mn 12 clusters were mixed with an anti- solvent such as toluene (Scheme 1). In a typical experiment, a macroscopic crystalline sample of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]2CH 3 COOH4H 2 O (Mn 12 -Ac, Fig. 1a) was dissolved into acetonitrile, and the resulting solution was added to an aliquot of toluene under vigorous stirring at room temperature (see ESIw). The mixture was then stirred for 1 h. A gradual opacity indicative of precipitation was observed. The resulting precipitate was collected by centrifugation, washed several times with acetonitrile and toluene, and finally dispersed in toluene. The final brown Scheme 1 a Institut Catala ` de Nanotecnologia, Campus Universitari, 08193 Bellaterra, Catalonia, Spain. E-mail: daniel.maspoch.icn@uab.es; Fax: +34 93 581 4411; Tel: +34 93 581 4299 b Instituto de Ciencia de Materiales de Arago ´n, CSIC-Universidad de Zaragoza, 50009 Zaragoza, , Spain c Centre d’Investigacio ´ en Nanocie `ncia i Nanotecnologia, Campus Universitari, 08193 Bellaterra, Catalonia, Spain w Electronic supplementary information (ESI) available: Detailed experimental data, EELS, IR, DLS and additional TEM and FE-SEM images of Mn 12 -based nanoparticles. See DOI: 10.1039/ b716071b 1202 | Chem. Commun., 2008, 1202–1204 This journal is  c The Royal Society of Chemistry 2008 COMMUNICATION www.rsc.org/chemcomm | ChemComm