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

Sandhya Kapurwan; Arpan Mondal; Pradip Kumar Sahu; Sanjit Konar

doi:10.1021/acs.inorgchem.2c02298

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

Inorg Chem. 2022 Nov 7;61(44):17459-17468. doi: 10.1021/acs.inorgchem.2c02298. Epub 2022 Oct 24.

Authors

Sandhya Kapurwan¹, Arpan Mondal¹, Pradip Kumar Sahu¹, Sanjit Konar¹

Affiliation

¹ Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal462066Madhya Pradesh, India.

PMID: 36278960
DOI: 10.1021/acs.inorgchem.2c02298

Abstract

Herein, two polyoxometalate (POM)-ligated tetranuclear rare-earth metal complexes having the molecular formula [Cs_xK_24-x{Ln₄(H₂O)₈(α-AsW₉O₃₃)₄}]·yH₂O {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 LnCl₃·6H₂O using di-lacunary [As₂W₁₉O₆₇(H₂O)]^14- precursor and characterized. The structural analysis shows that the building units [α-AsW₉O₃₃]^9- are bridged by four rare-earth ions, where one [α-AsW₉O₃₃]^9- bridged two Ln(III) centers asymmetrically by μ₂-O and terminal oxygen atoms. The [α-AsW₉O₃₃]^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.