A Family of {Cr^III₂Ln^III₂} Butterfly Complexes: Effect of the Lanthanide Ion on the Single-Molecule Magnet Properties

Abstract

We report the synthesis of several heterometallic 3d–4f complexes which result from the replacement of the Dy^III ions in the [Cr^III₂Dy^III₂­(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 {Cr₂Dy^III₂} 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 {Cr^III₂Dy^III₂} complex. When Ln^III = Pr, a complete loss of SMM behavior is found, but when Ln^III = 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 Ln^III = 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