A family of {CrIII2LnIII2} butterfly complexes: effect of the lanthanide ion on the single-molecule magnet properties

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