Zero Field SMM Behavior and Magnetic Refrigeration in Rare Heterometallic Double Stranded Helicates of Cu2Ln2 (Ln = Dy, Tb, Gd)

Abstract

The strategic design and synthesis of new double stranded helical structures, namely [Cu2Ln2(H2L1)2(L2)2(OMe)(NO3)]·8MeOH (1: Ln = DyIII), [Cu2Ln2(H2L1)2(L2)2(H2O)(OMe)(NO3)]·xH2O·7MeOH (2: Ln = TbIII, x = 0.5; 3: Ln = GdIII, x = 1) and [Cu2Ln2(HL1)2(L2)2(MeOH)2]·xMeOH (4: Ln = DyIII, x = 8.5; 5: Ln = TbIII, x = 8; 6: Ln = GdIII, x = 8.66), has been achieved by using a flexible helical ligand and a rigid tridentate ligand. Butanedihydrazide‐bridged bis(3‐ethoxysalicylaldehyde) (H4L1) was used as a primary ligand due to its flexible nature for constructing the helical strand and another rigid tridentate ligand, 2‐salicylideneaminophenol (H2L2), was used to accommodate the transition metal. The two series of complexes are analogous and differ only in terms of coordination geometry around the LnIII centers. Complexes 1–3 are composed of two non‐equivalent Cu–Ln pairs, where one LnIII center is eight‐coordinate and the other LnIII center is nine‐coordinate; whereas complexes 4–6 are made up of two equivalent Cu–Ln pairs, where both the LnIII centers are eight‐coordinate. Crystal structural analysis revealed that in all the complexes the deprotonated form of H4L1 binds in a helical fashion forming a mixture of left‐handed (Λ) and right‐handed (Δ) configurations (“meso”‐relation). AC magnetic susceptibility measurements demonstrate slow magnetic relaxation behaviors of complexes 1 and 4 in the absence of any external magnetic field. Complexes 3 and 6 exhibit ferromagnetic interaction and show magnetic refrigeration, notable in heterometallic 3d‐4f helicates.