Seven-coordinate LnIII complexes assembled from a bulky MesacacH ligand: their synthesis, structure, photoluminescence and SMM behaviour

Abstract

The reaction of a bulky acetyl acetone ligand 1,3-dimesitylpropane-1,3-dione (^MesacacH) with hydrated lanthanide chlorides in the presence of tetramethylammonium hydroxide afforded a new family of neutral mononuclear Ln^III complexes [Ln(^Mesacac)₃(DMF)] (Ln = Dy (1); Tb (2); Y_0.91Dy_0.09 (3); and Er (4)). The molecular structures of these complexes were confirmed by single crystal X-ray diffraction studies. The coordination geometries of the Ln^III centre were analysed by SHAPE analysis which revealed a capped octahedral geometry in 1–4. Photoluminescence studies showed ligand-sensitized green emissions for 2 with an appreciable quantum yield of 0.83%. Static (dc) and dynamic (ac) magnetic studies of complexes 1 and 3 were performed. The dynamic magnetic study revealed that complex 1 exhibits zero-field slow relaxation of the magnetization without showing a clear maximum in the out-of-phase ac susceptibility plots. However, magnetic dilution of 1 with the Y^III metal ion (complex 3) and/or the application of a dc magnetic field induces a strong frequency dependence of the ac susceptibility signals with χ′′_M peaks in the 3–10 K temperature range, thus supporting field-induced SMM behaviour of 1. The relaxation process takes place through a combination of the Orbach and Raman mechanisms. The fitting of the temperature dependence of the relaxation time to the equation τ⁻¹ = τ₀⁻¹ exp(–U_eff/k_BT) + BTⁿ, allows the extraction of the effective energy barrier U_eff/k_B = 70 K (48.7 cm⁻¹) and pre-exponential parameter of τ₀ = 2.7 × 10⁻⁷ s for the Orbach mechanism (first term) and the parameters B = 0.04 s⁻¹ K⁻ⁿ and n = 6.11, for the Raman mechanism (second term).