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 (1), Gd (2)) complexes have been prepared using the multidentate ligand N,N’-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 1 shows slow relaxation of the magnetization at zero field and a thermal energy barrier Ueff = 7.4 K with HDC = 1000 Oe, whereas complex 2 exhibits an S = 9 ground state and significant magnetocaloric effect (-triangle S-m = 18.5 J kg(-1) K-1 at T = 3 K and triangle B = 5 T).