Abstract
Polyoxometalates (POMs) provide rigid and highly symmetric coordination sites and can be used as a strategy for the stabilization of magnetic ions. Herein, we report a new member of the Keggin archetype, the Cr-centered Keggin anion [α-CrW12O40]5– (CrW12), with the unusual tetrahedral coordination of CrIII reported for the first time in POMs conferring unattended magnetic properties. POM chemistry has recently presented excellent examples of single-molecule and single-ion magnets (SMMs and SIMs) as well as molecular spin qubits; however, the majority of POM-based SIMs reported to date contain lanthanoid ions. CrW12, as the first example of a chromium(III) SIM, exhibits slow relaxation of magnetization and quantum tunneling with a single-ion magnetic behavior even above 10 K with an energy barrier for the reversal of the magnetization of 3.0 K. The first 3d-metal SIM based on a nonlacunary Keggin anion is the foundation for a new research area in POM chemistry.
Highlights
Polyoxometalates (POMs) provide rigid and highly symmetric coordination sites and can be used as a strategy for the stabilization of magnetic ions
A handful of chromiumcontaining POMs are known to date, with all exhibiting an octahedral coordination of CrIII.[3−5] In 1962, Brown reported the preparation of 12-tungstochromic(III) acid H5[αCrW12O40]·nH2O with CrIII ions in a tetrahedral ligand field.[6]
single-molecule magnet (SMM) based on POMs reported so far can be divided into three groups, representing: (1) a number of 3d-transition metal ions, which can be connected through oxo-bridges forming magnetic clusters of variable nuclearities and high symmetries;[9] (2) a number of lanthanoid ions giving rise to lanthanoid complexes in which 4f-magnetic ions are submitted to the crystal field created by POM ligands;[8] (3) a mixed-valence framework hosting a number of electrons that are usually delocalized over all the framework structure.[10]
Summary
The spectral quality was moderate at best, a low-temperature, multifrequency experiment in the V-band range (Figure 3) delivered a direct measure of zfs by observing a near-zero field absorption between the ±1/2 and ±3/2 Kramers doublets at 65 ± 1 GHz = 2.16(3) cm−1 This value represents the parameter Δ = 2[(D2 + 3E2)1/2] for an S = 3/2 spin state. The existence of CrW12 demonstrates that CrIII can adopt a tetrahedral coordination in the Keggin cavity and opens a new perspective for Cr-centered lacunary POMs. The Keggin cavity is capable of stabilizing an a priori unstable tetrahedral geometry for a high-spin d3 ion, which results in a single-ion magnet behavior.
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