Windmill-like Ln 4 Clusters [Ln = Tb(III) and Dy(III)] Bridged by [α-AsW 9 O 33 ] 9- Unit Showing Zero-Field SMM Behavior: Experimental and Theoretical Investigation.

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 }]· y H 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.