Wheel-like Ln₁₈ Cluster Organic Frameworks for Magnetic Refrigeration and Conversion of CO₂

Abstract

Two isostructural 2D MOFs ([Ln₇(CDA)₆(HCOO)₃(μ₃-OH)₆(H₂O)₈] _n, abbreviated as 1-Gd and 2-Dy) were successfully synthesized under solvothermal conditions. The self-assembly of lanthanide(III) nitrate and 1,1'-cyclopropane-dicarboxylic acid (H₂CDA) resulted in wheel-like Ln₁₈ cluster second building units (SBU), which are further linked to six neighboring wheels to generate a 2D ordered honeycomb array. Both 1-Gd and 2-Dy exhibit high thermal stability and decompose above 330 °C. Moreover, they have good solvent stability in ten common solvents and pH stability with pH values from 1 to 13. Magnetic studies reveal that 1-Gd exhibits weak antiferromagnetic coupling between adjacent Gd³⁺ ions and has a large magnetocaloric effect of 47.30 J kg^-1 K^-1 (Δ H = 7.0 T at 2 K), while 2-Dy shows ferromagnetic interaction between adjacent Dy³⁺ ions. Interestingly, 1-Gd and 2-Dy can catalyze the cycloaddition of CO₂ to epoxides under mild conditions and can be reused at least five rounds with negligible loss of catalytic performance.