The slow magnetic relaxation regulated by ligand conformation of a lanthanide single-ion magnet [Hex4N][Dy(DBM)4]

Abstract

A mononuclear Dysprosium(III) complex [Hex₄N][Dy(DBM)₄] (1) was synthesized using dibenzoylmethane (DBM) anion ligand with a tetrahexylammonium (Hex₄N⁺) cation balancing the charge. Complex 1 was structurally and magnetically characterized. The local geometry of Dy(III) ions is close to the ideal D_4d symmetry. The temperature and frequency-dependent out-of-phase ac susceptibility peaks were observed in the absence of a static dc field. The relaxation energy barrier U_eff = 27.7 K and τ₀ = 1.3 × 10⁻⁷ s were obtained by Arrhenius fitting. It is interesting that the quantum tunneling of the magnetization was suppressed when two optimum dc fields (300 and 1500 Oe) were applied. Two distinct thermal relaxation processes were observed with U_eff = 56.6 K, τ₀ = 6.6 × 10⁻¹⁰ s for 300 Oe and U_eff = 68.1 K, τ₀ = 3.4 × 10⁻¹¹ s and U_eff = 88.0 K, τ₀ = 5.0 × 10⁻¹⁰ s for 1500 Oe. The two thermal relaxation processes were also recognized clearly under zero dc field for the analogue with 20 times magnetic site dilution by Y(III). Nevertheless there is only one crystallographically independent Dy(III) ion in this system. Further inspection of the crystallographic structure reveals that the benzene disorder within the conjugated system of the β-diketonate ligand could change the delocalized electron distribution on the carbonyl coordination oxygen atoms and result in small different ligand fields, which account for the multiple relaxation processes. Ab initio calculations confirm the two energy barriers derived from two disordered structures.