Assembly of Hydrazine-Bridged Cyclic FeIII4LnIII4 Octanuclear Complexes

The elaborate design and synthesis of three isomorphous cyclic octanuclear clusters [Fe4Ln4(L)8(H2O)4(NO3)4](NO3)4 (Ln = Dy (1·5CH3CN·14H2O), Gd (2·5CH3CN·19H2O), Y (3·6CH3CN·16H2O)) with the Schiff base ligand H2L (2-(hydroxyimino)propionic acid (dipyridin-2-ylmethylene)hydrazide) have been achieved. The compartmental ligand H2L has N3-inner and N2O-outer coordination sites, which chelate low-spin Fe(III) and Ln(III), respectively. The remaining coordination sites around the rare-earth ions are occupied by nitrate ions and water molecules. This is the first report on the use of a hydrazine group within a ligand to connect FeIII and LnIII ions, which can not only stabilize the skeleton of the molecular ring but also transmit a weak antiferromagnetic interaction between FeIII and LnIII (Ln = Dy, Gd) ions. ac magnetic susceptibility measurements under an external applied dc field of 1600 Oe reveal the SMM behavior of complex 1 with Ueff = 25.8 K. The magnitude of the magnetic exchange between Fe(III) and Gd(III) in complex 2 is evaluated to be 2JFeGd = −0.324 cm–1; meanwhile, complex 2 shows a moderate magnetocaloric effect (MCE) with −ΔSm = 17.06 J kg–1 K–1 under 5 T at 2 K.