Multifunctional LnIII complexes with SMM, luminescence and MCE properties constructed using selected phosphine oxide and diketonate

Abstract

A series of mononuclear and binuclear lanthanide^III complexes with formulae [Dy(Cy₃PO)(tmhd)₃] (1), [Dy₂(dppeO₂)(tmhd)₆] (2) and [Ln₂(dppeO₂)(acac)₆(H₂O)₂] (3, Ln = Dy, 4, Ln = Gd, 5, Ln = Eu, 6, Ln = Tb) (Cy₃PO = tricyclohexyl phosphine oxide, tmhd = 2,2,6,6-tetramethyl-3,5-heptane dionate, acac = acetylacetone, dppeO₂ = 1,2-bis(diphenylphosphino)ethane dioxide) constructed using β-diketonate combined with mono- and bi-phosphine oxide ligands were structurally, optically and magnetically characterized. The easy to modify backbone of monodentate phosphine oxide and bidentate β-diketonate enables them to modulate local coordination geometries around lanthanide ions by means of an electronic effect and/or steric hindrance of substituents on the terminal of these ligands and consequently optimize the corresponding optical and magnetic properties. In this vein, a series of Dy containing single-molecule magnets (SMMs) were constructed, in which complex 3 possessed a relatively higher local geometry symmetry (D_4d) and displayed a significantly slow magnetic relaxation behavior at a hysteresis temperature of 3 K. In addition, the Gd congener complex 4 showed a pronounced magnetocaloric effect (MCE) with a –ΔS^max_m value of 20.4 J kg⁻¹ K⁻¹, and more strikingly, a field induced SMM behavior was observed in 4 despite the isotropic nature of Gd^III ions. Also of note is that the suitable triple state energy of ligands matches well with the excited states of Eu^III, Tb^III and Dy^III ions, which led to the corresponding characteristic metal-centered luminescence emission. In this work, the deliberately selected β-diketonate and phosphine oxide combined with different lanthanide ions led to a series of multifunctional materials with SMM, luminescence and MCE properties.