[α-AsWO] bridged hexagonal clusters of Ln(III) showing field induced SMM behavior: experimental and theoretical insight.

Polyoxometalates (POM), as inorganic polydentate oxygen donors, provide binding opportunities for oxophilic lanthanide metal centers to construct novel Ln-substituted POM materials with exciting structures and attractive properties. Herein, we have reported four arsenotungstate [α-AsWO] based lanthanide-containing polyoxometalates [CsK{Ln(HO)(α-AsWO)}]·HO (Ln = Er (1), Gd (2), Ho (3), and Tb (4)), which are synthesized in an alkaline medium. Complexes 1-3 are the dimeric structures of [Ln(HO)(α-AsWO)] polyanions, whereas complex 4 is a hexamer of the polyanion [Tb (HO)(α-AsWO)] as a building unit. In all the complexes, [α-AsWO] units are staggered up and down and give rise to the chair conformation, where one [α-AsWO] unit bridges two Ln(III) centers through four μ-oxygen and two terminal oxygen atoms, resulting in the hexagonal arrangement of lanthanides. The dynamic magnetic measurement indicates that only complex 1 exhibits slow relaxation of magnetization with an applied dc field (1500 Oe). To gain insight into the slow relaxation of magnetization in complex 1, the ligand-field parameters and the splitting of the ground-state multiplet of the Er(III) ions have been estimated. The calculation results confirm that the ground state wave function of these molecules (1, 3, and 4) is mainly composed of a mixture of states, and the non-axial crystal field (CF) terms are more predominant than the axial CF term. The solid-state fluorescence spectra of 1-4 reveal that the photoexcitation O → M ligand-to-metal charge-transfer (LMCT) of arsenotungstate fragments is effectively quenched due to the spatial coordination environment around the Ln(III) ion.