A Study of Magnetic Relaxation in Dysprosium(III) Single-Molecule Magnets

Although the development of single-molecule magnets (SMMs) is rapid, there are only two families of high energy barrier (U_eff ) dysprosium(III) SMMs known so far: the cyclopentadienyl (Cp) family with a sandwich structure and the pentagonal-bipyramidal (PB) family with D_5h symmetry. These high-barrier SMMs, which usually possess U_eff >500 cm^-1 allow the separate study of the four magnetic relaxation paths, namely, direct, quantum tunnelling, Raman and Orbach processes, in detail. Whereas the first family is chemically more challenging to modify the Cp rings, it is shown herein that the latter family, with the common formulae [DyX¹ X² (L_eq )₅ ]⁺ , such as X¹ /X² =^- OCMe₃ , ^- OSiMe₃ , ^- OPh, Cl^- or Br^- ; L_eq =THF/pyridine/4-methylpyridine, can be readily fine-tuned with a range of axial and equatorial ligands by simple substitution reactions. This allows unambiguous confirmation that the U_eff mainly depends on the identity of X¹ and X² , rather than on L_eq . More importantly, the fitted parameters are barrier dependent. If X¹ is an O donor and X² is a halide, 500eff <600 cm^-1 , log τ_0avg (s)=-10.66, log C_avg (s^-1  K^-n )= -5.05, n_avg =4.1 and T_H =9 K (in which τ₀ is the pre-exponential factor for the Orbach relaxation process, C and n are parameters used to describe Raman relaxation, and T_H is the highest temperature at which magnetic hysteresis is observed). For cases in which both X¹ and X² are O donors, 900eff <1300 cm^-1 , log τ_0avg (s)=-11.63, log C_avg (s^-1  K^-n )= -6.03, n_avg =4.1 and 18H <25 K. Based on these results, it can be further concluded that U_eff not only has a linear correlation to the axial Dy-X bond lengths, but also to T_H for these PB SMMs. This represents the first systematic study of a family of lanthanide SMMs and derives the first magneto-structural correlation in Dy SMMs.