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]<middle dot>4EtOH (1Dy) and a POM-ligated Dy-III-SIM [{Dy(OPPh3)(3)(H2O)(3)}{PW12O40}]<middle dot>Ph3PO<middle dot>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 pi-pi* 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.