Improved slow magnetic relaxation in optically pure helicene-based Dy^III single molecule magnets

Abstract

Racemic and optically pure [Dy(hfac)(L)] complexes with L = 3-(2-pyridyl)-4-aza[6]-helicene have been synthesized and characterized. Both the racemic and enantiopure forms behave as single molecule magnets in their crystalline phase, while electronic circular dichroism activity is evidenced. Ab initio calculations on isolated complexes followed by the determination of intermolecular dipolar couplings allowed the rationalization of the different low-temperature magnetic behaviours. The enantiopure SMM differs from the racemic one by the presence of a hysteresis loop in the former system.

Slow Magnetic Relaxation in Chiral Helicene-Based Coordination Complex of Dysprosium

“…Upon cooling, χMT decreases monotonically down to 11.20 cm 3 •K•mol −1 due to the thermal depopulation of the MJ states. Below 5 K, the more rapid decrease could be attributed to the presence of weak dipolar antiferromagnetic interactions as determined by quantum calculations on the analogue complex involving hfac − ancillary ligands [27]. The field dependence of the magnetization measured at 2.0 K reaches the value of 5.12 Nβ under a magnetic field of 50 kOe, in agreement with the expected value (5 Nβ) for an Ising ground state (Inset of Figure 3).…”

“…In the case of the [Dy(L)(hfac) 3 ] SMM [27], the chirality of the ligand results in two possible crystal structures (racemic and enantiopure) with similar molecular arrangement but different packings. Interestingly, racemic and enantiopure crystals show notable different magnetic behavior, with the opening of a magnetic hysteresis only in the case of the enantiopure.…”

“…Relaxation times on the order of few seconds is slow enough to observe the opening of the hysteresis loop at 0.50 K (Figure 5) which remains opened at higher temperatures ( Figure S6). One must mention that the hysteresis loop of the racemic form of [Dy(L)(hfac)3] [27] was closed at the same temperature. …”

“…The N2O6 coordination polyhedron can be described as a distorted square antiprism environment (D4d symmetry on the basis of SHAPE analysis, Table S2) [41]. Thus, the replacement of the 1,1,1,5,5,5-hexafluoroacetylacetonate anions with tta − ones confers an higher symmetry for the coordination environment [27]. As already noted, such observation was already done for another complex stemming from this group and a significant positive impact on the magnetic properties was observed [37,38].…”

“…A first example of the coupling between a Dy III -based SMM and a chiral antenna has been reported recently with the complex [Dy(L)(hfac) 3 ] with L = 3-(2-pyridyl)-4-aza [6]-helicene and hfac = 1,1,1,5,5,5-hexafluoroacetylacetone [27]. The Dy III ion, with its 6 H 15/2 ground state, easily leads to Ising type of magnetic anisotropy in coordination spheres like N 2 O 6 and this is achieved with the common bidentate 2,2 -bipyridine (bpy) ligand and three hfac − ligands [13,[28][29][30].…”