Incorporation of Cr(III) into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {Cr(III)O(4)} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties

[Image: see text] 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-CrW(12)O(40)](5–) (CrW(12)), with the unusual tetrahedral coordination of Cr(III) 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. CrW(12), 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.