Opening Magnetic Hysteresis by Axial Ferromagnetic Coupling: From Mono‐Decker to Double‐Decker Metallacrown

Abstract

Combining Ising-type magnetic anisotropyw ith collinear magnetic interactions in single-molecule magnets (SMMs) is as ignificant synthetic challenge.H erein we report aD y[15-MC Cu-5] (1-Dy)S MM, where aD y III ion is held in ac entral pseudo-D 5h pocket of ar igid and planar Cu 5 metallacrown (MC). Linking two Dy[15-MC Cu-5] units with as ingle hydroxideb ridge yields the doubledecker {Dy[15-MC Cu-5]} 2 (2-Dy)S MM where the anisotropya xes of the two Dy III ions are nearly collinear,resulting in magnetic relaxation times for 2-Dy that are approximately 200 000 times slower at 2Kthan for 1-Dy in zero external field. Whereas 1-Dy and the Y III-diluted Dy@2-Y analogue do not show remanence in magnetic hysteresis experiments,t he hysteresis data for 2-Dy remain open up to 6Kwithout asudden drop at zero field. In conjunction with theoretical calculations,these results demonstrate that the axial ferromagnetic Dy-Dy coupling suppresses fast quantum tunneling of magnetization (QTM). The relaxation profiles of both complexes curiously exhibit three distinct exponential regimes,a nd hold the largest effective energy barriers for any reported d-f SMMs up to 625 cm À1 .

A Linear "Head-to-Tail" Chlorido-Bridged Didysprosium Single-Molecule Magnet Exhibiting 100-s Blocking Temperature near 8 K

“…But for the non-radical bridged dimeric complexes the bridge is usually neither a single atom nor a single (200 Oe/s). 48 All these recent progresses reinforce the importance of linear "head-to-tail" bridging mode in designing high-performance SMMs. Just as Tong and co-worker mentioned that combining Ising-type magnetic anisotropy with collinear magnetic interactions in SMMs is a significant synthetic challenge.…”

“…Considering that the steps on hysteresis curves are the consequence of level-crossing between the ground state and the first-excited state of the dimer, eqn (2) could be applied to calculate the J value. 46,48 The first derivative of the magnetization of 1 reveals that the crossing field could be estimated as being 1.8 kOe. The other method involves fitting the dc magnetic susceptibility of compound 1 during 2 K to 300 K using the program POLY_ANISO.…”

“…Just as Tong and co-worker mentioned that combining Ising-type magnetic anisotropy with collinear magnetic interactions in SMMs is a significant synthetic challenge. 48 Moreover, only those with unpaired electron strongly delocalized on the ligand can enforce the magnetic anisotropy axes of the metal center collinear. For weaker radicals or non-radical bridges the axiality is always weakened.…”

“…38 Unlike other cases, the recently reported 3d-4f SMM {Dy }2 (MC = Cu5 metallacrown) by linking two Dy units with a single hydroxide bridge yields the double-decker complex with a slightly distorted Dy-O-Dy linking angle of 158.2°. Such a "head-to-tail" bridging style successfully enhances the Ueff up to 894 K and magnetic hysteresis remains opening up to 6 K (200 Oe/s) without a sudden drop at zero field, 48 indicating the axial ferromagnetic Dy-Dy coupling somewhat restricts the fast QTM effect and holds the largest effective energy barriers in reported d-f SMMs. We reason if the exchange-coupling direction can be perfectly aligned in the axial position of the individual metal center the QTM effect should be further reduced.…”

“…However, this simple idea has not been succeeded because the coordination geometry of the lanthanide is very flexible considering the large radii and ionic bonding nature of the lanthanide ions. [48][49][50] Herein, by using the equatorial 4- 1 and S1). Further theoretical insight shows that this is mainly attributed to the regular PB coordination geometry with perfect collinear alignments of both principle magnetic-axes and the exchange coupling direction of two adjacent Dy ions transferred by this µ-Cl bridge.…”