Single-Molecule Magnet Behavior and Magnetocaloric Effect in Ferromagnetically Coupled Ln^III-Ni^II-Ni^II-Ln^III (Ln^III = Dy^III and Gd^III) Linear Complexes

New types of linear tetranuclear Ln^III-Ni^II-Ni^II-Ln^III (Ln^III = Dy (<b>1</b>), Gd (<b>2</b>)) complexes have been prepared using the multidentate ligand <i>N</i>,<i>N</i>′-bis­(3-methoxysalicylidene)-1,3-diaminobenzene, which has two sets of NO and OO′ coordination pockets that are able to selectively accommodate Ni^II and Ln^III ions, respectively. The X-ray structure analysis reveals that the Ni^II ions are bridged by phenylenediimine groups forming a 12-membered metallacycle in the central body of the complex, whereas the Ln^III ions are located at both sides of the metallacycle and linked to the Ni^II ions by diphenoxo bridging groups. Phenylenediimine and diphenoxo bridging groups transmit ferromagnetic exchange interactions between the two Ni^II ions and between the Ni^II and the Ln^III ions, respectively. Complex <b>1</b> shows slow relaxation of the magnetization at zero field and a thermal energy barrier <i>U</i>_eff = 7.4 K with <i>H</i>_DC = 1000 Oe, whereas complex <b>2</b> exhibits an S = 9 ground state and significant magnetocaloric effect (−Δ<i>S</i>_m = 18.5 J kg^–1 K^–1 at <i>T</i> = 3 K and Δ<i>B</i> = 5 T)