Characterisation of nanoscopic [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] single-molecule magnets: physicochemical properties and LDI- and MALDI-TOF mass spectrometry{ Daniel Ruiz-Molina, a Philippe Gerbier, a Evan Rumberger, b David B. Amabilino, a Ilia A. Guzei, c Kirsten Folting, d John C. Huffman, d Arnold Rheingold, c George Christou,* d Jaume Veciana* a and David N. Hendrickson* b a Institut de Cie `ncia de Materials de Barcelona (CSIC), Campus Universitari, 08193 Bellaterra, Catalonia, Spain. Tel: 34 93 580 1853; Fax: 34 93 580 5729; E-mail: vecianaj@icmab.es b Department of Chemistry and Biochemistry-0358, University of California at San Diego, La Jolla, California, 92093-0358, USA c Department of Chemistry, University of Delaware, Newark, Delaware, 19716, USA d Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana, 47405-4001, USA Received 25th May 2001, Accepted 11th January 2002 First published as an Advance Article on the web 8th February 2002 The syntheses of the two new single molecule magnets, [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ]S (R ~ CHCHCH 3 ,S ~ H 2 O (2) and R ~ C 6 H 4 C 6 H 5 ,S ~ 2C 6 H 5 C 6 H 4 COOH (3)) and X-ray crystal structure of the first are described. Complex 2 crystallizes in the orthorhombic space group Ibca, which at 198 K has a ~ 21.2208(4), b ~ 21.2265(4), c ~ 42.2475(6) A ˚ , and Z ~ 8. Frequency-dependent out-of-phase ac signals are seen for both complexes, which indicates that these two complexes function as single-molecule magnets. Dc magnetization measurements for both complexes also exhibit hysteresis in the magnetization vs. external magnetic field plots with regular steps characteristic of quantum mechanical tunnelling of the direction of magnetization. The techniques of LDI- and MALDI-TOF mass spectrometry have been investigated to prospect their utility for the chemical characterization of Mn 12 clusters. The technique is applied to known clusters as well as to two new compounds, and characteristic signals are found, especially predominant being that of the [Mn 12 O 12 (O 2 CR) 14 ] cation or anion, showing the potential interest of this technique for those preparing this type of compounds. Introduction The rapid growth of high-speed computers and the miniatur- ization of magnetic technology have led to much interest in the field of nanoscale magnetic materials. 1 In a nanomagnet there can be several domains where the spins within one domain are aligned and behave collectively because the spins are strongly coupled by magnetic exchange interactions. In response to an external magnetic field, domains sluggishly change the orientation of their magnetic moments to become aligned with the external magnetic field. As a consequence interesting magnetic properties such as magnetization hysteresis loops appear making such particles suitable candidates to be used as bits of information storage at the nanometer scale. Several synthetic methods have been developed to prepare these materials. One such technique is the fragmentation of bulk ferro- and ferri-magnetic materials. The main disadvantage of this ‘‘top-down’’ approach is that the nanoscale magnetic materials exhibit a distribution of particle size, anisotropy and shapes. This situation leads to a distribution of the barrier heights for the inter-conversion of the spins ‘‘up’’ to the spins ‘‘down’’ within the domains, which is not desirable for application in devices. From a fundamental point of view a distribution in barrier heights masks properties such as those associated with resonant magnetization tunnelling. Another technique involves growing crystallites of a known magnetic material in a micelle. 2 Since the size of the micelle cavity is tractable, predictable crystallite growth can be afforded. This method allows the synthesis of particles of controllable size, but thus far, has been limited to the miniaturization of known magnetic materials rather than the discovery of new structural classes of molecules. The discovery of large metal cluster complexes with interesting magnetic properties characteristic of nanoscale magnetic particles, such as magnetization hysteresis loops and out-of-phase ac magnetic susceptibility signals, is an exciting breakthrough. Indeed, the advantages of using a synthetic approach (the so-called ‘‘bottom-up’’ approach) to obtain molecular nanomagnets are numerous: 1) metal clusters are normally prepared by a solution method and, once purified, are composed of single, sharply-defined size; 2) they are readily amenable to variations in peripheral ligands (small vs. bulky, hydrophilic vs. hydrophobic, etc.); 3) they are normally soluble in common solvents providing advantages in potential applications; 4) since each molecule has sub-nanoscale dimen- sions, such materials could potentially be used for high density information storage; and 5) from a theoretical point of view, understanding the magnetic properties of these molecules is important to help bridge the gap between the quantum and classical understanding of magnetism. 3 In 1993 it was discovered for the first time that [Mn 12 O 12 (O 2 CCH 3 ) 16 (H 2 O) 4 ]?4H 2 O?2CH 3 CO 2 H(1) (complex 1 or Mn 12 Ac for short), functions as a nanoscale molecular magnet. 4 Such a molecule has been termed a single-molecule {LDI- and MALDI-TOF are acronyms for Laser Desorption/ Ionisation and Matrix Assisted Laser Desorption/Ionisation Time-of- Flight. 1152 J. Mater. Chem., 2002, 12, 1152–1161 DOI: 10.1039/b104607c This journal is # The Royal Society of Chemistry 2002