Two unprecedented 3d–4f hexagonal metallo-rings {Ln₂₄Cu₃₆} (Ln = Dy and Gd) with a diagonal dimension of about 4.6 nm were facilely synthesized by self-assembly of Cu^II, Ln^III nitrates and benzoate in the presence of triethylamine. Magnetic studies show slow relaxation behaviour for the dysprosium analogue and a large magnetocaloric effect (MCE) for the gadolinium analogue.

The large cryogenic magnetocaloric effect of a 3D oxydiacetate-bridged gadolinium–manganese MOF material, [Mn(H₂O)₆][MnGd(oda)₃]₂·6H₂O (1), was evaluated by magnetization and heat capacity measurements. A maximum −ΔS_m of 50.1 J kg⁻¹ K⁻¹ for ΔH = 70 kG along with significant entropy change at lower field was found on account of the weak Mn⋯Gd ferromagnetic interactions and the small molecular mass. This suggests that 1 could be considered as a potential coolant for liquid helium temperature applications.