Slow Magnetic Relaxation of a Ferromagnetic Ni II 5 Cluster with an S ) 5 Ground State Athanassios K. Boudalis,* ,† Michael Pissas, † Catherine P. Raptopoulou, † Vassilis Psycharis, † Bele ´n Abarca, ‡ and Rafael Ballesteros ‡ Institute of Materials Science, NCSR “Demokritos”, 153 10 Aghia ParaskeVi Attikis, Greece, and Departamento de Quı ´mica Orga ´nica, Faculdad de Farmacia, UniVersidad de Valencia, AVda. Vicente Andre ´s Estelle ´s s/n, 46100 Burjassot (Valencia), Spain Received July 31, 2008 Complex [Ni 5 {pyCOpyC(O)(OMe)py} 2 (O 2 CMe) 4 (N 3 ) 4 (MeOH) 2 ] · 2MeOH · 2.6H 2 O(1 · 2MeOH · 2.6H 2 O) was synthesized by the reaction of Ni(O 2 CMe) 2 · 4H 2 O with pyCOpyCOpy and NaN 3 in refluxing MeOH. It crystallizes in the monoclinic C2/c space group and consists of five Ni II atoms in a helical arrangement. Direct current magnetic susceptibility studies reveal ferromagnetic interactions between the Ni II (S ) 1) ions, stabilizing an S ) 5 ground state. Alternating current susceptibility experiments revealed the existence of out-of-phase signals indicative of slow magnetic relaxation. Analysis of the signals showed that they are composite, suggesting more than one relaxation process, while analysis of their magnitudes suggests not all molecules undergo slow magnetic relaxation. Magnetization field-sweep experiments revealed hysteresis at 1.8 K, and magnetization decay experiments clearly verified the appearance of slow magnetic relaxation at that temperature. Introduction Recent years have witnessed an impressive growth in the study of molecular magnetic materials. These materials exhibit interesting new magnetic phenomena, such as single- molecule magnetism (SMM) 1 and quantum tunneling of the magnetization (QTM), 2 which test the limits of current models for magnetism of condensed matter. Besides their theoretical interest, they have been proposed as potential candidates for various technological applications such as qubits for quantum computing, 3 coolants for magnetic refrigeration, 4 and MRI contrast agents. 5 Because the structure-property relations for molecular magnetic materials are still very elusive, there is a constant need for new structures, belonging to the same structural type, and for new structural types for us to study. Given the nature of this chemistry, which is based on the interaction of metal ions with organic or inorganic ligands, the proper choice of such ligands may provide us with the new structural types desired. Our previous studies on the chemistry of the ligand (py) 2 CO (di-2-pyridyl ketone, dpk) showed that its carbonyl function can undergo metal-assisted solvolysis to yield its hemiacetal form in the case of alcoholic solvents or its gem-diol form in the case of nonalcoholic solvents. Subsequent deprotonations of these forms give rise to a rich coordination chemistry, 6 which has yielded polynuclear complexes with interesting properties. 7 Inspired by the chemistry of this ligand, we have previously started exploring the coordination chemistry of the ligand pyCOpyCOpy 8 [2,6- di-(2-pyridylcarbonyl)-pyridine, dpcp] shown in Scheme 1. Our synthetic endeavors, which have already afforded a Co II 20 * To whom correspondence should be addressed. E-mail: tbou@ ims.demokritos.gr. Tel: (+30) 210-6503346. Fax: (+30) 210-6503365. † NCSR “Demokritos”. ‡ Universidad de Valencia. (1) (a) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 2000, 25, 66. (b) Gatteschi, D.; Sessoli, R. Angew. Chem., Int. Ed. 2003, 42, 268. (2) (a) Friedman, J. R.; Sarachik, M. P.; Tejada, J.; Ziolo, R. Phys. ReV. Lett. 1996, 76, 3830. (b) Thomas, L.; Lionti, F.; Ballou, R.; Gatteschi, D.; Sessoli, R.; Barbara, B. Nature 1996, 383, 145–147. (3) (a) Leuenberger, M. N.; Loss, D. Nature 2001, 410, 789. (b) Hill, S.; Edwards, R. S.; Aliaga-Alcalde, N.; Christou, G. Nature 2003, 302, 1015. (c) Lehmann, J.; Gaita-Arin ˜o, A.; Coronado, E.; Loss, D. Nat. Nanotechnol. 2007, 2, 312. (4) Evangelisti, M.; Luis, F.; de Jongh, L. J.; Affronte, M. J. Mater. Chem. 2006, 16, 2534. (5) Cage, B.; Russek, S. E.; Shoemaker, R.; Barker, A. J.; Stoldt, C.; Ramachandaran, V.; Dalal, N. S. Polyhedron 2007, 26, 2413. (6) Papaefstathiou, G. S.; Perlepes, S. P. Comments Inorg. Chem. 2002, 23, 249. (7) Boudalis, A. K.; Sanakis, Y.; Clemente-Juan, J. M.; Donnadieu, B.; Nastopoulos, V.; Mari, A.; Coppel, Y.; Tuchagues, J.-P.; Perlepes, S. P. Chem.sEur. J. 2008, 14, 2514. (8) Abarca, B.; Ballesteros, R.; Elmasnaouy, M. Tetrahedron 1998, 54, 15287. Inorg. Chem. 2008, 47, 10674-10681 10674 Inorganic Chemistry, Vol. 47, No. 22, 2008 10.1021/ic801441d CCC: $40.75  2008 American Chemical Society Published on Web 10/17/2008