Windmill-like Ln₄ Clusters [Ln = Tb(III) and Dy(III)] Bridged by [α-AsW₉O₃₃]^9– Unit Showing Zero-Field SMM Behavior: Experimental and Theoretical Investigation

Abstract

Herein, two polyoxometalate (POM)-ligated tetranuclear rare-earth metal complexes having the molecular formula [Cs x K 24−x {Ln 4 (H 2 O) 8 (α-AsW 9 O 33 ) 4 }]•yH 2 O {x = 5, y = 20, and Ln = Tb(III) (1); and x = 6, y = 28, and Ln = Dy(III) (2)} were synthesized by a one-pot reaction with LnCl 3 •6H 2 O using di-lacunary [As 2 W 19 O 67 (H 2 O)] 14− precursor and characterized. The structural analysis shows that the building units [α-AsW 9 O 33 ] 9− are bridged by four rare-earth ions, where one [α-AsW 9 O 33 ] 9− bridged two Ln(III) centers asymmetrically by μ 2 -O and terminal oxygen atoms. The [α-AsW 9 O 33 ] 9− units are orthogonal to each other, resembling as vanes of a windmill. The magnetic studies disclosed the presence of large magnetic anisotropy and slow relaxation of magnetization behavior [U eff = 15.2 K (1) and 26 K (2)] in the absence of an external magnetic field. Detailed analysis of relaxation dynamics confirmed that the QTM process in 2 (τ QTM = 2.50 × 10 −4 s) is slower as compared to complex 1 (τ QTM = 2.38 × 10 −4 s), and the relaxation process mainly follows the shortcut pathways (such as QTM, optical, and acoustic phonon process) rather than the thermally activated Orbach process. Further, the ab initio calculations show high axial ground states with minimum transverse anisotropy and provide a good agreement between calculated and experimental magnetic data for both complexes. It has also been observed that the local symmetry (D 4d subgroup) around the metal centers in 1 provides higher axiality and stabilizes m J = ±6 of Tb(III) more as compared to m J = ±15/2 of Dy (III) in 2, resulting in higher energy splitting of the ground state in the former complex. The combined experimental and theoretical observations suggest that the high axial nature of the ground state with minimum transverse anisotropy resulting from local ligand field symmetry is responsible for the observed zero-field single-molecule magnet (SMM) behavior in the studied complexes. Notably, complex 1 is the first example of a POM-based terbium complex that shows SMM behavior in the absence of an external field.

Slow magnetic relaxation and luminescence properties in β-diketonate lanthanide(iii) complexes. Preparation of Eu(iii) and Yb(iii) OLED devices

“…Therefore, the crystal field of compound 3-Tb stabilizes a ground state different than m j = 0, though there is transverse contribution in the ground state wavefunction because of QTM presence at H dc = 0 T and even applying an optimal external magnetic field, slow relaxation of the magnetization is recorded but in a very narrow temperature range. 47 Compounds 5-Er and 6-Yb show clear maxima of the w M 00 component in a rather low temperature range (1.8-2.7 K for 5-Er and of 2.1-4.3 K for 6-Yb). For 5-Er and 6-Yb, in the plots of the out-of-phase magnetic susceptibility components versus temperature and frequency, the w M 00 peaks move progressively to higher frequencies upon increasing the temperature.…”

“…Therefore, the crystal field of compound 3-Tb stabilizes a ground state different than m j = 0, though there is transverse contribution in the ground state wavefunction because of QTM presence at H dc = 0 T and even applying an optimal external magnetic field, slow relaxation of the magnetization is recorded but in a very narrow temperature range. 47…”

[α-AsW9O33]9− bridged hexagonal clusters of Ln(iii) showing field induced SMM behavior: experimental and theoretical insight

“…13 The encapsulation of lanthanide ions in the pockets of ATs can result in a rigid coordination sphere which may help fulfill the required equatorial ligand field and minimize the vibrational relaxation, thus reinforcing the relaxation barrier. 9,14–16 However, only a few Ln-containing POMs with anisotropic metal centers have been reported to date.…”

Supramolecular Chemistry of Windmill-like Al10 Clusters: Adjustable Guests and Hydrogen-Bonding Networks for Enhanced Optical Limiting

“…Unlike fully dense polyoxometalates or hollow structures represented by wheel clusters, this type of structure is characterized by a compact core and discrete curved arms toward the periphery. Typical examples include {Ag@Ag 12 } reported by Sun et al, Ti 13 -oxo-clusters reported by Dai et al, {V 8 } clusters synthesized by Xu et al and Youichi Ishii et al, heptanuclear copper­(I) iodide clusters reported by Park et al, 17-nuclear Zr/Hf oxide clusters synthesized by Wang et al, two types of Ln 4 III clusters reported by Konar et al and Wu et al, chiral lanthanide clusters synthesized by Zou et al, Ln-containing organophosphonate-based polyoxomolybdates reported by Niu et al, and cadmium-containing windmill-like heteropolyoxoniobate synthesized by Zheng et al Commonly, the dense inner core confers some stability to the structure, while the open metal–ligand (or metal–oxygen) sites on the periphery are beneficial for functionalization or performance properties, such as catalysis. , On the other hand, the windmill-like configuration can be regarded as a special multi-paddle wheel-type structure, which makes the cluster a potential secondary building unit for coordination or supramolecular assembly . Up to now, windmill-like structures have not been reported in the main group metal oxo clusters.…”