Two Discrete Ln₁₂ Shelf‐Shaped Clusters: Magnetic Studies Reveal a Significant Cryogenic Magnetocaloric Effect and Slow Magnetic Relaxation

Two unique dodecanuclear lanthanide clusters, [Ln III 12 Na 3 (µ 3 ‐OH) 2 (hmmp) 6 (piv) 12 (CO 3 ) 6 (MeOH) 6 ]OH · 5MeOH {Ln = Gd 3+ , Dy 3+ ; hmmpH 2 = 2‐[(2‐hydroxyethylimino)methyl]‐6‐methoxyphenol}, have been synthesized by treating the simple Schiff‐base ligand hmmpH 2 with the relevant lanthanide chloride and sodium pivalate (NaPiv). Single‐crystal X‐ray diffraction analysis revealed that the central cores of Gd 12 and Dy 12 are unprecedented and composed of a distorted four‐layer triangular shelf‐like molecular structure, which can be broken down into two types of four parallel triangular layers with an ABBA arrangement. Magnetic investigations revealed that the Gd 12 cluster is a good candidate for magnetic refrigeration with a significant entropy change (–Δ S m ) of 30.99 J kg –1 K –1 for H = 7 T at 4 K, whereas the Dy 12 cluster shows single‐molecule magnet behavior.