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 {Ln^III₂₄Zn^II₆} 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 −ΔS_m (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 Tb^III photoluminescent emission in the visible region, suggesting that Tb^III-based luminescence is sensitized by the effective energy transfer from the ligand HL to the metal centers.