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

Though the development of SMMs is rapid, there are only two families of high Ueff Dy(III) SMMs known so far: the cyclopentadienyl (Cp) family with sandwich structure and the pentagonal-bipyramidal (PB) family with D5h symmetry. We show here the latter family with the common formulae DyX1X2(Le)5]+ can be readily fine tuned with a range of axial and equatorial ligands by simple substitution reactions, such as X1/X2 = -OCMe3 , -OSiMe3, -OPh, Cl- or Br-; Le = THF/py/4-methylpy. This allows to unambiguously reveal that the Ueff mainly depends on the identity of X1 and X2 rather than Leq. More importanty, the fitted parameters are barrier dependent. Where X1 is an O-donor and X2 is a halide we find 500 < Ueff < 600 cm-1; log[tau0 (s)]avg = -10.66 +/- 0.93; log[C (s-1 K-n)]avg = 5.05 +/- 0.70; navg = 4.1 +/- 1.0; TH = 9 K (where tau0 is the pre-exponential factor for the Orbach relaxation process, C and n are parameters used to describe the Raman relaxation and TH is the highest temperature at which magnetic hysteresis is observed). For cases where both X1 and X2 are O-donors we find 900 < Ueff < 1300 cm-1; log[tau0 (s)]avg = -11.63 +/- 0.57; log[C (s-1 K-n)]avg = 6.03 +/- 0.52; navg = 4.1 +/- 1.0; 18 < TH < 25 K. Based on this, we further conclude that Ueff not only has a linear correlation to the axial Dy-X bond lengths, but also to the TH for these PB SMMs. This represents the first systematic study of a family of lanthanide SMMs and derives the first magnetostructural correlation in Dy SMMs.