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 [Cu(2)Ln(2)(H2L1)(2)(L-2)(2)(OMe)(NO3)]8MeOH (1: Ln = Dy-III), [Cu(2)Ln(2)(H2L1)(2)(L-2)(2)(H2O)(OMe)(NO3)]xH(2)O7MeOH (2: Ln = Tb-III, x = 0.5; 3: Ln = Gd-III, x = 1) and [Cu(2)Ln(2)(HL1)(2)(L-2)(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 () and right-handed () 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.