Effect of coordination geometry on the magnetic properties of a series of Ln 2 and Ln 4 hydroxo clusters.

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 χM T(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 χM T(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 (Ueff ) 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 (-ΔSm ) 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.