The effect of the electronic structure and flexibility of the counteranions on magnetization relaxation in [Dy(L)2(H2O)5]3+ (L = phosphine oxide derivative) pentagonal bipyramidal SIMs

Abstract

We report here a new Dy^III-SIM [Dy(OPCy₃)₂(H₂O)₅](CF₃SO₃)₃·2OPCy₃ (OPCy₃ = tricyclohexylphosphine oxide) with a pentagonal bipyramidal geometry, which exhibits a blocking temperature T_B = 8.5 K and an anisotropy barrier U_eff = 562 K. Ab initio calculations show that this complex exhibits the largest U_calc = 732 K among the Dy^III-SIM complexes containing the [Dy(L)₂(H₂O)₅]³⁺ (L = phosphine oxide derivative) cationic unit, which is essentially due to the electronic effects of the triflate anion that increase the charge difference between the oxygen atoms of the ligands L coordinated in axial positions and those belonging to the equatorial water molecules. This charge difference enhancement, which is also reflected in a larger difference between the corresponding Dy–O distances (Δ), appears to be the driving force to increase the U_calc value. The comparatively smaller experimental U_eff value observed for this compound has been justified by the flexibility of the structural network due to the size of the triflate counteranions. The absence of a clear correlation between T_B and U_eff (or U_calc) suggests the involvement of Raman and QTM mechanisms in the magnetization relaxation process.