Synthesis, Structure, and Magnetic Properties of Dy2Co2L10(bipy)2 and Ln2Ni2L10(bipy)2, Ln = La, Gd, Tb, Dy, and Ho: Slow Magnetic Relaxation in Dy2Co2L10(bipy)2 and Dy2Ni2L10(bipy)2

The 3,5-dichlorobenzoate anion, L–, serves as a bridging ligand and 2,2′-bipyridine, bipy, as a terminal bidentate ligand to yield, through hydrothermal syntheses, the tetranuclear clusters Dy2Co2L10(bipy)2, 1, and Ln2Ni2L10(bipy)2, where Ln is the trivalent La, 2, Gd, 3, Tb, 4, Dy, 5, or Ho, 6, ion. Single-crystal X-ray diffraction reveals that the six complexes are all isomorphous with the monoclinic P21/c space group and with lattice parameters that decrease with the lanthanide contraction. The two cobalt(II) or nickel(II) and two Ln(III) cations are linked by the 10 L– anions to generate Dy2Co2 or Ln2Ni2 3d–4f cationic heteronuclear clusters with a slightly bent Co···Dy···Dy···Co or Ni···Ln···Ln···Ni arrangement. Direct current magnetic susceptibility studies reveal that the complexes are essentially paramagnetic, with room-temperature χMT values close to the expected values for two cobalt(II) or nickel(II) and two Ln(III) cations. The temperature dependence of χMT for 1 and 5 is well reproduced by ab...