Integrating Polyoxometalate into Dy(III)-based Single-molecule Magnets with Pentagonal Bipyramidal Symmetry

Polyoxometalates (POMs) with various coordination fashions are versatile ligands for constructing single-ion magnets (SIMs), but enforcing POM-SIMs with a specific geometry remains a synthetic challenge. Herein, we synthesized a POM-cocrystallized Dy^III-SIM [Dy(OPPh₃)₄(H₂O)₃][PW₁₂O₄₀]·4EtOH (1Dy) and a POM-ligated Dy^III-SIM [{Dy(OPPh₃)₃(H₂O)₃}{PW₁₂O₄₀}]·Ph₃PO·H₂O (2Dy) with pentagonal bipyramidal local coordination geometry. Magnetic measurements indicate that 1Dy displays field-induced single-molecule magnet (SMM) behavior and the relaxation is dominated by under-barrier processes. 2Dy exhibits spin-lattice relaxation at a broader temperature region with a reversal barrier over 300 K. Magneto-structural analysis reveals that the enhancement of SMM behavior originated from the equatorial replacement of Ph₃PO by POM, which strengthens the axial anisotropy in 2Dy. Luminescent experiments indicate that the characteristic Dy^III emissions of 1Dy are covered up by the strong π-π* emission of Ph₃PO at low-temperature regions. As for 2Dy, partial Dy^III emission persists thanks to the antenna effect between Dy^III and POM.