Effect of coordination geometry on the magnetic properties of a series of Ln2 and Ln4 hydroxo clusters

Abstract

A series of three isostructural tetranuclear complexes with the general molecular formula [Ln₄(μ₃-OH)₄(L)₄(μ₂-piv)₄(MeOH)₄] (Ln = Gd 1, Dy 2 and Ho 3; LH = [1,3-bis(o-methoxyphenyl)-propane-1,3-dione]) were isolated and unambiguously characterized by single crystal XRD. Under similar reaction conditions, simply changing the co-ligand from pivalate to 2,6-bis(hydroxymethyl)-p-cresol (LH′₃) led to the isolation of dinuclear Ln(III) complexes with the general molecular formula [Ln₂(L)₄(μ₂-LH′₂)₂]·4DMF (Ln = Gd 4, Dy 5 and Ho 6). Direct current magnetic susceptibility data studies on the polycrystalline sample of 1–6 and the results reveal the existence of weak antiferromagnetic exchange interactions between the lanthanide ions in 1 which is evident from the spin Hamiltonian (SH) parameters (J₁ = −0.055 cm⁻¹ and g = 2.01) extracted by fitting χ_MT(T). On the other hand, though complex 4 exhibits weak antiferromagnetic coupling (J₁ = −0.048 cm⁻¹ and g = 1.99) between the Gd(III) ions, the χ_MT(T) data of complexes 5 and 6 unambiguously disclose the presence of ferromagnetic interactions between Dy(III) and Tb(III) ions at lower temperature. Magnetization relaxation dynamics studies performed on 2 show frequency dependent out-of-phase susceptibility signals in the presence of an optimum external magnetic field of 0.5 kOe. In contrast, complex 5 shows slow magnetization relaxation with an effective energy barrier (U_eff) of 38.17 cm⁻¹ with a pre-exponential factor (τ₀) of 1.85 × 10⁻⁶ s. The magnetocaloric effect (MCE) of complexes 1 and 4 was extracted from the detailed magnetization measurement and the change in the magnetic entropy (−ΔS_m) of 1 and 4 was found to be 25.57 J kg⁻¹ K⁻¹ and 12.93 J kg⁻¹ K⁻¹, respectively, at 3.0 K for ΔH = 70 kOe.