Michael W. Wemple

Manganese carboxylate clusters: From structural aesthetics to single-molecule magnets

Abstract An overview is provided of some recent developments in manganese carboxylatecluster chemistry. A variety of synthetic methodologies are described together with the structuresof the resultant products, which span metal nuclearities of 4 to 18 and oxidation states of II to IV,including mixed-valency. The spins of the ground states of these products are often large andsometimes abnormally large, and in certain cases when this is coupled to a sufficiently largemagnetoanisotropy, the clusters function as single-molecule magnets i.e., they can be magnetizedby an external magnetic field below a critical temperature. These complexes display hysteresis inmagnetization vs . magnetic field studies, and clear evidence for quantum tunnelling ofmagnetization. Such results establish single-molecule magnetism as a new magnetic phenomenon,and one that holds great promise for next-century technological applications.

Site-specific ligand variation in manganese–oxide cubane complexes, and unusual magnetic relaxation effects in [Mn4O4X(OAc)3(dbm)3](X = N3–, OCN– Hdbm = dibenzoylmethane)

Treatment of [Mr4O2(OAc)6(py)2(dbm)2]1(py = pyridine, Hdbm = dibenzoylmethane) in hot MeCN with Me3SiX (X = N3–, OCN –) yields [Mn4O3X(OAc)3(dbm)3] containing extremely rare η1, µ3-N3– or -OCN– groups; alternating-current magnetic susceptibility studies reveal unusual magnetic relaxation effects and indicate these species to be the newest and smallest examples yet of truly molecular magnets.

Single-Molecule Magnets: Characterization of Complexes Exhibiting Out-of-Phase AC Susceptibility Signals

Abstract The structures and characterization are described of complexes that exhibit an out-of-phase AC magnetic susceptibility x m” The dodecanuclear complexes [Mn12O12(O2CR)16(H2O)4]z contain 8MnIII, 4MnIV or MnII, 7MnIII, 4MnIV and possess ground state spins of S = 9 or 10 (z = 0) or S = 19/2 (z = −1). The distorted-cubane complexes [Mn4O3X(O2CR)3(dbm)3] contain 3MnIII, MnIV and have been prepared with a variety of X groups; in every case, the ground state is S = 9/2. The anion [Mn11O10X6(O2CPh)11(H2O)4]2− (9MnIII, 2MnIV; X = NCO−, N3 −) contains two [Mn4O3X] distorted-cubanes linked by a [Mn3O4]+ unit; the ground state is S = 5. The cation [V4O2(O2CEt)7(bpy)2]+ (4VIII) has a [V4O2]8+ butterfly core and a S = 3 ground state. All these complexes exhibit out-of-phase AC magnetic susceptibility signals at low temperatures (<8 K). These signals are indicative of slow relaxation of the magnetization, unable to keep in phase with the oscillating AC magnetic field, and are taken as evidence for single-molecul...

Resonant magnetization tunneling in single-molecule magnets

Abstract Data are presented for three different types of single-molecule magnets (SMMs): [Mn12O12(O2CR)16(H2O)4],[Cation][Mn12O12(O2CR)16(H2O)4], and the distorted cubane [MnIV MnIII 3O3X(O2CR)3L3] complexes. All three types of complexes exhibit slow magnetization relaxation at temperatures below 5 K. Each molecule can change the direction of its magnetization only slowly at these low temperatures. Manisfetations of this are seen in magnetization hysteresis loops and in the presence of frequency-dependent out-of-phase ac magnetic susceptibility signals (X′M). It is shown that the neutral Mn12 SMMs exist in isomeric forms that differ in the positoning of the 4H2O and 16 carboxylate ligands. The orgin of the two X′M signals for the neutral Mn12 complexes is discussed. All Mn12 complexes exhibit magnetization hysteresis loops with steps seen at regular increments of magnetic field. The step heights and coercive fields for the loops vary from one Mn12 SMM to another. Step-structured hysteresis loops are als...

High spin molecules: (NBun4)2[Mn8O4(O2CPh)12(Et2mal)2(H2O)2], a mixed-valence managenese(II/III) aggregate with dicarboxylate ligation, an unusual linked-butterfly structure, and an S= 3 ground state

Treatment of (NBun4)[Mn4O2(O2CPh)9(H2O)] with Na2(Et2mal) in MeCN leads to a nuclearity and oxidation level change to give (NBun4)2[Mn8O4(O2CPh)12(Et2mal)2(H2O)2]1; the crystal structure and magnetochemical studies show the core of the anion to have a unique topology consisting of two linked [Mn4O2] butterfly units and to possess an S = 3 ground state, respectively.