The role of 3d–4f exchange interaction in SMM behaviour and magnetic refrigeration of carbonato bridged Cu II2Ln III2 (Ln = Dy, Tb and Gd) complexes of an unsymmetrical N2O4 donor ligand

Abstract

The strategic design and synthesis of a new Cu(II) complex, [CuL] (1) of an asymmetrically dicondensed N₂O₄ donor ligand (where H₂L = N-3-ethoxysalicylidene-N′-3-methoxysalicylidene-1,3-propanediamine) and its utilization as a metalloligand towards the synthesis of three new tetra-nuclear carbonato bridged Cu–Ln complexes [(μ₄-CO₃)₂{(CuL)(MeOH)Ln(NO₃)}₂] (where Ln = Dy, Tb and Gd for complexes 2, 3 and 4 respectively) have been achieved. Each tetranuclear Cu^II₂Ln^III₂ structure can be described as two μ₂-phenoxo bridged Cu^IILn^III dinuclear units connected via two μ₄–κ²:η¹:η¹CO₃²⁻. Single crystal X-ray diffraction studies show that complexes 2, 3 and 4 are isostructural and isomorphic. The Ln^III centres are nona-coordinated with spherical capped-square antiprism geometry and the geometry around the Cu^II centre is distorted octahedral. The DC magnetic susceptibility and field dependent magnetization measurements reveal the ferromagnetic interactions between the Cu^II and Ln^III centres in all three complexes. The ac susceptibility measurements under an applied bias field reveal the SMM behaviour of complexes 2 and 3 with U_eff 6.5 and 4.5 K, respectively. Complex 4 shows a notable magnetocaloric effect (MCE) with −ΔS_m 23.5 J kg⁻¹ K⁻¹ for ΔH = 7 T at 3 K. The ab initio CASSCF/RASSI-SO/SINGLE_ANISO calculations predict that the observed SMM behaviour under an external dc field in 2 and 3 does not arise solely from the single ion anisotropy of the lanthanide ions. The exchange interactions (Cu^II–Ln^III and Ln^III–Ln^III) lower the energy of the exchange coupled states as well as enhance QTM or tunnel splitting and play an important role in the overall magnetic properties of the complexes. Furthermore, to obtain more insight into the exchange interaction, BS-DFT calculations have been performed for complexes 2–4.