Slow Magnetic Relaxation in a Dysprosium Ammonia Metallocene Complex

We report the serendipitous discovery and magnetic characterization of a dysprosium bis(ammonia) metallocene complex, [(C₅Me₅)₂Dy(NH₃)₂](BPh₄) (1), isolated in the course of performing a well-established synthesis of the unsolvated cationic complex [(C₅Me₅)₂Dy][(μ-Ph)₂BPh₂]. While side reactivity studies suggest that this bis(ammonia) species owes its initial incidence to impurities in the DyCl₃(H₂O)_x starting material, we were able to independently prepare 1 and its tetrahydrofuran (THF) derivative, [(C₅Me₅)₂Dy(NH₃)(THF)](BPh₄) (2), from the reaction of [(C₅Me₅)₂Dy][(μ-Ph)₂BPh₂] with ammonia in THF. The low-symmetry complex 1 exhibits slow magnetic relaxation under zero applied direct-current (dc) field to temperatures as high as 46 K and notably exhibits an effective barrier to magnetic relaxation that is more than 150% greater than that previously reported for the [(C₅Me₅)₂Ln][(μ-Ph)₂BPh₂] precursor. On the basis of fitting of the temperature-dependent relaxation data, magnetic relaxation is found to occur via Orbach, Raman, and quantum-tunneling relaxation processes, and the latter process can be suppressed by the application of a 1400 Oe dc field. Field-cooled and zero-field-cooled dc magnetic susceptibility measurements reveal a divergence at 4 K indicative of magnetic blocking, and magnetic hysteresis was observed up to 5.2 K. These results illustrate the surprises and advantages that the lanthanides continue to offer for synthetic chemists and magnetochemists alike.