Nanoscale {Ln^III₂₄Zn^II₆} Triangular Metalloring with Magnetic Refrigerant, Slow Magnetic Relaxation, and Fluorescent Properties

The self-assembly of Ln­(ClO4)3·6H2O and Zn­(OAc)2·2H2O with pyrazine-2-carboxylic acid (HL) results in the formation of three novel nanosized {LnIII24ZnII6} triangular metallorings, [Gd24Zn6L24­(OAc)22­(μ3-OH)30­(H2O)14]­(ClO4)7­(OAc)·2CH3OH·26H2O (1), [Tb24Zn6L24­(OAc)22­(μ3-OH)30­(CH3O)2­(CH3OH)2­(H2O)10]­(ClO4)5(OH)·6CH3OH·12H2O (2), and (H3O)­[Dy24Zn6L24­(OAc)22­(μ3-OH)30­(H2O)14]­(ClO4)7­(OAc)2·4CH3OH·22H2O (3), having the largest nuclearity among any known Ln/Zn clusters. Magnetic and luminescent studies reveal the special prowess for each lanthanide complex. Magnetic studies reveal that 1 exhibits a significant cryogenic magnetocaloric effect with a maximum −ΔSm (isothermal magnetic entropy change) value of 30.0 J kg–1 K–1 at 2.5 K and 7 T and that a slow magnetization relaxation is observed for the dysprosium analogue. In addition, the solid-state photophysical properties of 2 display strong characteristic TbIII photoluminescent emission in the visible region, suggesting that TbIII-based luminescence is sensitized by the effective energy transfer from the ligand HL to the metal centers.