Phosphonate-assisted tetranuclear lanthanide assemblies: observation of the toroidic ground state in the TbIII analogue

Abstract

The reaction of LnCl₃·6H₂O with a multidentate flexible Schiff base ligand (LH₄), H₂O₃P^tBu and trifluoroacetic acid (tfaH) afforded a series of homometallic tetranuclear complexes, [Ln₄(LH₂)₂(O₃P^tBu)₂(μ₂-η¹η¹tfa)₂][2Cl] (Ln = Dy^III (1), Tb^III (2) and Gd^III (3)). The tetranuclear lanthanide core contains two structurally different lanthanide centres, one being in a distorted trigonal dodecahedron geometry and the other in a distorted trigonal prism. Complexes 1–3 were investigated via direct and alternating current (DC and AC) magnetic susceptibility measurements. Only 1 revealed a weak single-molecule magnet (SMM) behaviour. Alternating current (ac) magnetic susceptibility measurements on 1 reveal a frequency-dependent out-of-phase signal. However, the absence of distinct maxima in the χ′′ peak (within the temperature/frequency range of our experiments) prevented deduction of the experimental energy barrier for magnetization reversal (U_eff) and the relaxation time. We have carried out extensive ab initio (CASSCF + RASSI-SO + SINGLE_ANISO + POLY_ANISO) calculations on complexes 1–2 to gain deeper insights into the nature of magnetic anisotropy. Our calculations yielded only one exchange coupling parameter between the two Ln^III centres bridged by the ligand (neglecting the exchange between the Ln^III centres that are not proximal wrt each other). All the extracted J values indicate a weakly antiferromagnetic coupling between the metal centres (J = −0.025 cm⁻¹ for 1 and J = −0.015 cm⁻¹ for 2). Calculated exchange coupled U_cal values of ∼5 and ∼1 cm⁻¹ in 1 and 2 respectively nicely corroborated the experimental observations regarding weak and no SMM characteristics. Our calculations indicated the presence of a net single-molecule toroidal (SMT) behaviour in complex 2. On the other hand, fitting the magnetic data (susceptibility and magnetization) in the isotropic cluster 3 revealed weak AFM exchange couplings of J₁ = 0.025 cm⁻¹ and J₂ = −0.020 cm⁻¹ which are consistent with those for Gd^III ions.