Zero field SMM behavior and magnetic refrigeration in rare heterometallic double stranded helicates of Cu₂Ln₂ (Ln = Dy, Tb, Gd)

The strategic design and synthesis of new double stranded helical structures, namely [Cu₂Ln₂(H₂L¹)₂(L²)₂(OMe)(NO₃)].8MeOH (1: Ln = Dy^III), [Cu₂Ln₂(H₂L¹)₂(L²)₂(H₂O)(OMe)(NO₃)].xH₂O.7MeOH (2: Ln = Tb^III, x = 0.5; 3: Ln = Gd^III, x = 1) and [Cu₂Ln₂(HL¹)₂(L²)₂(MeOH)₂].xMeOH (4: Ln = Dy^III, x = 8.5; 5: Ln = Tb^III, x = 8; 6: Ln = Gd^III, 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 Ln^III centers. Complexes 1-3 are composed of two non-equivalent Cu-Ln pairs, where one Ln^III center is eight-coordinate and the other Ln^III center is nine-coordinate; whereas complexes 4-6 are made up of two equivalent Cu-Ln pairs, where both the Ln^III 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. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)