Asymmetric Assembly of Chiral Lanthanide(III) Tetranuclear Cluster Complexes Using Achiral Mixed Ligands: Single-molecule Magnet Behavior and Magnetic Entropy Change

Abstract

It is challenging to use achiral ligands to spontaneously construct chiral molecular magnets. In this work, two new Ln 4 cluster complexes based on N , N ′-(1,3-propanediyl)bis[ N -[1,1-bis(hydroxymethyl)-2-hydroxyethyl]amine] (H 6 L) have been assembled, which are crystallized in a chiral space group due to the asymmetric distribution of acetate (OAc – ) groups and hexafluoroacetylacetonate (F 6 acac – ) groups on both sides of the parallelogram-like Ln 4 core. Complex 1 , [Dy 4 (H 3 L) 2 (OAc) 3 (F 6 acac) 3 ]·5MeOH·2H 2 O, exhibits single-molecule magnet properties at the zero field with the U eff / k value of 48.4 K; notably, besides the Orbach process, the Raman process is also prominent for the magnetic relaxation of 1 . Complex 2 , [Gd 4 (H 3 L) 2 (OAc) 3 (F 6 acac) 3 ]·4MeOH·2.5H 2 O, displays a large magnetocaloric effect, whose largest −Δ S m value is 21.88 J kg –1 K –1 (when T = 2 K and Δ H = 5 T); it thus can be utilized as a good magnetic refrigeration molecular material.

Two Pairs of Homochiral Parallelogram-like Dy4 Cluster Complexes with Strong Magneto-Optical Properties

“…26 The large C value (28.27 s −1 K −2.14 ) confirms the significant Raman process in D-1. 28,48,51,52 The U eff /k of D-1 (57.5 K) is comparable with those of other Dy 4 complexes derived from the L1 2− ligand. 48,49 The Cole−Cole curves at 5−10 K could be fitted with a generalized Debye model (Figure 4d), 53,54 giving the α values of 0.077−0.185, which are relatively small, suggesting the narrow relaxation time distribution for D-1.…”

“…Notably, the χ″−ν plot not only shows a wide peak shape at low temperatures but also a platform shape appears in the high-frequency region as the temperature increases (Figure 4b), indicating the presence of a significant Raman process. 28,51,52 The ln(τ) versus 1/T plot based on the χ″−ν curve peaks at different temperatures is a curve rather than a straight line (Figure 4c), therefore, τ −1 = τ QTM −1 + CT n + τ 0 −1 exp(−U eff /kT), which includes the quantum tunneling effect, the Orbach process, and the Raman process, was used to fit this curve, giving τ QTM = 0.00738 s, n = 2.14, C = 28.27 s −1 K −2.14 , U eff /k = 57.5 K and τ 0 = 1.0 × 10 −6 s. The n value of 2.14 (1 < n < 6) and the τ 0 value of 1.0 × 10 −6 s (10 −6 −10 −11 s) are reasonable. 26 The large C value (28.27 s −1 K −2.14 ) confirms the significant Raman process in D-1.…”

Chiral Zn3Ln3 Hexanuclear Clusters of an Achiral Flexible Ligand

“…Similarly, the construction of chiral SMMs using chiral additives is facile; however, it is a challenge to obtain two enantiomeric complexes via spontaneous resolution utilizing achiral ligands. 35,36 R e c e n t l y , a c h i r a l d i n u c l e a r [ C o I I I ( H 0 . 5 L 1 ) -Dy III (DBM) 2 (H 2 O)](ClO 4 ) 0.5 •3H 2 O SMM, 37 4)), and the different enantiomers (Λ and Δ) are isolated and characterized, respectively.…”

“…Currently, new multifunctional crystalline materials with optical, electrical, magnetic, and other properties have been widely used in high-efficiency catalysis, nonlinear optics, high-density information storage, molecular converters, chiral recognition, biomedicine, and so on. ,,− Three main methods have been used for the construction of common chiral SMMs: chiral ligands via chirality transmitting, chiral additives via chirality induction, and achiral ligands via spontaneous resolution. − Among the available chiral SMMs, most are chiral ligands-based ones, which are easy to synthesize. Similarly, the construction of chiral SMMs using chiral additives is facile; however, it is a challenge to obtain two enantiomeric complexes via spontaneous resolution utilizing achiral ligands. , …”

Effects of Counterions, Coordination Anions, and Coordination Solvent Molecules on Single-Molecule Magnetic Behaviors and Nonlinear Optical Properties of Chiral Zn2Dy Schiff Base Complexes

“…Single-molecule magnets (SMMs) are nanomagnets composed of single molecules that exhibit magnetic bistability at the magnetic blocking temperature. , Although much difficult to synthesize than achiral SMMs, chiral SMMs have attracted more and more attention in recent years as a special class of multifunctional molecular materials. − They not only have advantages in high-density information storage but also perform well in chirality-induced properties such as ferroelectrics, − second-order nonlinear optics, , magnetochiral dichroism (MChD), , magneto-optical Faraday effect, , and circularly polarized luminescence. , Except for very few examples obtained by spontaneous chiral resolution after assembly with achiral ligands, ,,− most chiral SMMs derive their chirality directly from bonded chiral ligands − or even from the chiral organic molecules participating in the cocrystal . However, it is still difficult to obtain chiral SMMs through the coordination of chiral ligands due to the tendency of chiral molecules to spontaneously form racemates in solution .…”