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Unformatted text preview: Single-Molecule Magnetism Properties of the First Strontium-Manganese Cluster [SrMn 14 O 11 (OMe) 3 (O 2 CPh) 18 (MeCN) 2 ] Abhudaya Mishra , Yulia Pushkar , Junko Yano , Vittal K. Yachandra , Wolfgang Wernsdorfer , Khalil A. Abboud , and George Christou * , Department of Chemistry, Uni V ersity of Florida, Gaines V ille, Florida 32611, Mel V in Cal V in Laboratory, Physical Biosciences Di V ision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Institut Nel, CNRS/UJF, BP 166, 25 A V enue des Martyrs, 38042 Grenoble Cedex 9, France Received July 6, 2007 The preparation and properties of the first strontium-manganese molecular complex are described. The reaction of (NBu n 4 )[Mn 4 O 2 (O 2 CPh) 9 (H 2 O)] (4Mn III ) with Sr(ClO 4 ) 2 in MeCN/MeOH led to the isolation of [SrMn 14 O 11- (OMe) 3 (O 2 CPh) 18 (MeCN) 2 ] ( 1 ; 13Mn III , Mn II ). The structure of 1 consists of two [Mn 4 O 3 (OMe)] cubane units attached to a central, near-planar, trinuclear [Mn 3 O 4 ] unit, to which are also attached a Mn and a Sr above the plane and a [Mn 2 O(OMe)] rhomb below the plane. Peripheral ligation is provided by 18 bridging benzoate and two terminal MeCN groups. Variable-temperature and -field dc magnetization ( M ) data were collected in the 1.810 K and 0.14.0 T ranges and fit by matrix diagonalization methods to give S ) 9/2, D ) - 0.50(5) cm- 1 , and g ) 1.88(10), where S is the ground-state spin and D is the axial zero-field splitting parameter. Magnetization versus dc field sweeps at various temperatures and scan rates exhibited hysteresis loops, confirming 1 to be a new single-molecule magnet. Because complex 1 is the initial molecular example of intimately associated Mn and Sr atoms, Sr EXAFS studies have been performed for the first time on a synthetic Sr-containing molecule. This has also allowed comparisons with the EXAFS data on the Sr-substituted water oxidizing complex (WOC) of Photosystem II (PS II), which contains a SrMn 4 complex. Introduction Molecular manganese carboxylate cluster chemistry con- tinues to attract a lot of attention from many groups around the world. The reasons for this are many, but the two most important ones are the relevance of the chemistry of this metal to Mn bioinorganic chemistry and to the ongoing efforts to develop new routes to nanoscale magnetic materi- als. The latter area primarily involves the synthesis of single- molecule magnets (SMMs), which are molecules that retain their magnetization below a blocking temperature ( T B ) in the absence of an applied field. 1,2 This thus represents a molecular, bottom-up approach to nanoscale magnets, complementary to the standard top-down approaches to nanoparticles of traditional magnetic materials such as certain metals and metal oxides. For a molecule to function as an SMM, it must possess a large ground-state spin ( S ) and a negative (easy-axis or Ising) magnetoanisotropy ( D ). To- gether, these provide a barrier to relaxation of the magnetiza-...
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