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 [Cu 2 Ln 2 (H 2 L 1 ) 2 (L 2 ) 2 (OMe)(NO 3 )] · 8MeOH ( 1 : Ln = Dy III ), [Cu 2 Ln 2 (H 2 L 1 ) 2 (L 2 ) 2 (H 2 O)(OMe)(NO 3 )] · x H 2 O · 7MeOH ( 2 : Ln = Tb III , x = 0.5; 3 : Ln = Gd III , x = 1) and [Cu 2 Ln 2 (HL 1 ) 2 (L 2 ) 2 (MeOH) 2 ] · x MeOH ( 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) (H 4 L 1 ) was used as a primary ligand due to its flexible nature for constructing the helical strand and another rigid tridentate ligand, 2‐salicylideneaminophenol (H 2 L 2 ), 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 H 4 L 1 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.