Lanthanide-Directed Fabrication of Four Tetranuclear Quadruple Stranded Helicates Showing Magnetic Refrigeration and Slow Magnetic Relaxation

A rare class of four tetranuclear
lanthanide based quadruple stranded helicates namely, [Ln4L4(OH)2]­(OAc)2·xH2O (Ln = GdIII(1), DyIII(2) and x = 4, 5 respectively), [Er4L4(OH)2]­(NO3)2·9H2O (3), and [Dy4L4(NO3)]­(NO3)2·2CH3OH·H2O (4) were synthesized by
employing succinohydrazone derived bis-tridentate ligand (H2L) and characterized. Structures of 1–3 are similar to each other except the nature of counterions and number
of lattice water molecules. In 4, a distorted nitrate
ion was arranged in a hexagonal manner holding four dysprosium centers
in a slightly twisted manner. Because of the symmetrical nature of
each complex, the C4 axis crosses the
center of helicate resulting a pseudo-D4 coordination environment. Each ligand coordinates to lanthanide
centers in helical manner forming mixture of left (Λ) and right
(Δ) handed discrete units. Complex 1 exhibits antiferromagnetic
exchange interaction between nearby GdIII centers and shows
magnetic refrigeration (−ΔSm = 24.4 J kg–1 K–1 for ΔH = 7 T at 3 K). AC magnetic susceptibility measurements
of 2 and 4 demonstrate slow relaxation behavior,
with Ueff (effective energy barrier) of
20.5 and 4.6 K, respectively. As per our knowledge, complexes 1, 2, and 4 represent the first
examples of aesthetically pleasing quadruple stranded helicates showing
potential magnetocaloric effect and single-molecule-magnet-like behavior.