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 .

Low-Coordinate Dinuclear Dysprosium(III) Single Molecule Magnets Utilizing LiCl as Bridging Moieties and Tris(amido)amine as Blocking Ligands

“…Dy 2 , on the other hand, shows a gradual decrease of the χT(T) related to the thermal depopulation of the m J states. The room temperature χT value of 27.7 cm 3 •K•mol −1 corresponds well with the expected 28.34 cm 3 •K•mol −1 for two isolated Dy III ions ( 6 H 15/2 , g J = 4/3) [28]. Both experimental dependences χT(T) and M(H) for Dy 2 (black points and red circles, respectively, in Figure 3) are well reproduced by the CASSCF calculations (red solid lines in Figure 3; for details see section Ab initio calculations) which confirm the significant magnetic anisotropy of Dy 2 .…”

“…Polynuclear lanthanide (Ln) complexes often exhibit slow magnetic relaxation and related single molecule magnet (SMM) behavior similar to their more famous mononuclear relatives [1][2][3] but show also the influence of the superexchange interactions. Similarly to mononuclear Ln-SMMs, the field of the polynuclear congeners is also dominated by Dy III [4][5][6][7][8].…”

“…Obviously, the employment of longer bridging ligands leads to a good separation of the magnetic centers and therefore the possible slow magnetic relaxation becomes a single-ion effect [10]. Due to the weakness of the magnetic interactions in lanthanide-based compounds, only short molecular bridges could provide sufficient communication between the magnetic centers that could influence the slow magnetic relaxation of the compound [3]. Still, the exchange coupling constants between the lanthanide ions in Ln-SMMs, even with very short bridges, are one or two orders of magnitude smaller than the typical values of the energy barrier for the magnetization reversal (J-values are typically in the range 0.1-3 cm −1 for diamagnetic bridging ligands).…”

“…The magnetic interactions between the lanthanide ions can lead to a shift of otherwisedegenerate m J sublevels to different energies and reduce the probability of the QTM (quantum tunneling of magnetization) [3]. QTM is frequently the reason for a very fast relaxation acceleration under zero magnetic field, so its exclusion is crucial to improve single molecule magnet performance.…”

Switching the Local Symmetry from D5h to D4h for Single-Molecule Magnets by Non-Coordinating Solvents

“…There are considerable high performance single-molecule magnets (SMMs) containing dysprosium(III) reported in recent years [1][2][3][4][5][6], ranging from the cyclopentadienyl (Cp) based system [7][8][9][10] to the pentagonal-bipyramidal (PB) family [11][12][13][14]. Among these dysprosium(III)-based complexes, highly axial crystal field [14,15] is essential to achieve good performance of SMMs [16,17].…”