Assembly of heterobimetallic NiII-LnIII (LnIII=DyIII, TbIII, GdIII, HoIII, ErIII, YIII) complexes using a ferrocene ligand: slow relaxation of the magnetization in DyIII, TbIII and HoIII analogues

A family of dinuclear 3d 4f heterobimetallic complexes [LNi(H2O)(mu-OAc)Ln(NO3)(2)]center dot CH3CN; {Ln=Dy-III (1), Tb-III (2), Ho-III (3), Gd-III (4), Er-III (5), Y-III (6)} have been synthesized by utilizing a ferrocene-based, dual compartmental ligand H2L. 1-6 are isostructural and crystallize in the triclinic (P (1) over bar) space group. In these complexes Ni-II is present in the inner coordination sphere of the dianionic [L](2-) ligand; Ln(III) is encapsulated in the outer coordination pocket. Ni-II shows a 2N, 4O coordination environment in a distorted octahedral geometry, while the LnIII ion possesses a 9O coordination environment in a distorted tricapped trigonal prismatic geometry. ESI-MS studies suggest that the structural integrity of 1-6 is retained in solution. Electrochemical studies reveal that these complexes show a reversible one-electron response typical of the ferrocene motif along with an irreversible one-electron oxidation involving the NiII/NiIII couple. Magnetic studies revealed the presence of ferromagnetic exchange coupling between NiII and LnIII centers as shown by the increase of chi M-T value upon cooling below 50 K for compounds 1, 2, 4 and 5. Further, dynamic magnetic susceptibility measurements (1-3) confirm the absence of an out-of-phase (chi'') signal at zero dc fields. However, when these measurements were carried out at 1000 Oe dc field the chi'' signal was observed, although maxima could not be detected up to 2 K.