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A Family of {CrIII2LnIII2} Butterfly Complexes: Effect of the Lanthanide Ion on the Single-Molecule Magnet Properties

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posted on 02.11.2015, 00:00 by Stuart K. Langley, Daniel P. Wielechowski, Nicholas F. Chilton, Boujemaa Moubaraki, Keith S. Murray
We report the synthesis of several heterometallic 3d–4f complexes which result from the replacement of the DyIII ions in the [CrIII2DyIII2­(OMe)2­(mdea)2­(O2CPh)4­(NO3)2] single-molecule magnet (SMM) by the trivalent Pr, Nd, Gd, Tb, Ho, and Er lanthanide ions. The parent {Cr2DyIII2} compound displayed an anisotropy barrier to magnetization reversal of 53 cm–1, with magnetic hysteresis observed up to 3.5 K and with large coercive fields at low temperatures (2.7 T at 1.8 K). Magnetic studies for the new complexes revealed significantly different static and dynamic magnetic behavior in comparison to the parent {CrIII2DyIII2} complex. When LnIII = Pr, a complete loss of SMM behavior is found, but when LnIII = Nd or Er, frequency-dependent tails in the out-of-phase susceptibility at low temperatures are observed, indicative of slow magnetic relaxation, but with very small anisotropy barriers and fast relaxation times. When LnIII = Tb and Ho, SMM behavior is clearly revealed with anisotropy barriers of 44 and 36 cm–1, respectively. Magnetic hysteresis is also observed up to 2.5 and 1.8 K (0.003 T/s) for the Tb and Ho complexes, respectively. A large loss of the magnetization is, however, observed at zero-field, and as a result, the large coercivity which is present in the {Cr2Dy2} example is lost. The {Cr2Tb2} and {Cr2Ho2} complexes are rare examples of Tb- and Ho-based SMMs which reveal both slow relaxation in the absence of a static dc field (ac susceptibility) and open hysteresis loops above 1.8 K.