A Wheel-Shaped Single-Molecule Magnet of [MnII3MnIII4]: Quantum Tunneling of Magnetization under Static and Pulse Magnetic Fields
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The reaction of N-(2-hydroxy-5-nitrobenzyl)iminodiethanol (=H3(5-NO2-hbide)) with Mn(OAc)2·4H2O in methanol, followed byrecrystallization from 1,2-dichloroethane, yielded a wheel singlemolecule magenet (SMM) of [MnII3MnIII4(5-NO2-hbide)6]·5C2H4Cl2 (1).In 1, seven manganese ions are linked by six tri-anionic ligandsand compose the wheel in which the two manganese ions on the rimand the one in the center are MnII and the other four manganese ionsare MnIII ions. Powder magnetic susceptibility measurements showeda gradual increase with χmT values as the temperature was lowered,reaching a maximum value of 53.9 emu mol-1 K. Analyses of magneticsusceptibility data suggested a spin ground state of S = 19/2. Thezero-field splitting parameters of D and B40 were estimated to-0.283(1) K and -1.64(1) × 10-5 K, respectively, by high-field eprmeasurements (HF-EPR). The anisotropic parameters agreed with theones estimated from magnetization and inelastic neutron scatteringexperiments. AC magnetic susceptibility measurements showedfrequency dependent in- and out-of-phase signals, characteristicof an SMM, and an Arrhenius plot of the relaxation time gave are-orientation energy barrier (ΔE) of 18.1 K and a pre-exponentialfactor of 1.63 × 10-7 s. Magnetization experiments on aligned singlecrystals below 0.7 K showed a stepped hysteresis loop, confirming4the occurrence of quantum tunneling of the magnetization (QTM).QTM was, on the other hand, suppressed by rapid sweeps of themagnetic field even at 0.5 K. The sweep-rate dependence of the spinflips can be understood by considering theLandau-Zener-Stückelberg (LZS) model.
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