Syntheses, Structures, and Magnetic Properties of a Family of Tetranuclear Hydroxido-Bridged Ni2IILn2III (Ln = La, Gd, Tb, and Dy) Complexes: Display of Slow Magnetic Relaxation by the Zinc(II)-Dysprosium(III) Analogue

A new family of [2 X 2] tetranuclear 3d-4f heterometallic complexes have been synthesized. These are [Zn2Dy2L2(mu 3-OH)(2)(mu(4)-OH)(dbm)(2)(MeOH)(2)](NO3)center dot 2H(2)O center dot MeOH (3), [Ni2Dy2L2(mu(3)-OH)(2)(mu(4)-OH)(dbm)(2)(MeOH)(2)](NO3)center dot MeOH (4), [Ni2La2L2(mu(3)-OH)(2)(mu(4)-OH)(dbm)(2)(MeOH)(2)](ClO4)center dot H2O center dot 2MeOH (5), [Ni2Tb2L2(mu(3)-OH)(2)(mu(4)-OH)(dbm)(2) (MeOH)(2)](NO3)center dot MeOH (6), and [Ni2Gd2L2(mu(3)-OH)(2)(mu(4)-OH)(dbm)(2)(MeOH)(2)](NO3)center dot MeOH (7), [H2L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine and Hdbm = dibenzoylmethane] obtained through a single-pot synthesis using [Zn(HL)(dbm)] (for 3)/[Ni(HL)(dbm)]center dot 2CH(3)OH (for 4, 5, 6, and 7) as 3d-metal ion precursors. Single-crystal X-ray diffraction analysis and electrospray ionization (ESI) mass spectroscopy have been used to establish their identities. Compounds are isostructural, in which the metal ions are all connected together by a bridging hydroxido ligand in a rare mu(4)-mode. In complexes 3-7, the metal ions are antiferromagnetically coupled. Taking a cue from the results of 3 and 5, precise estimations have been made for the antiferromagnetic Ni center dot center dot center dot Ni (J(Ni) = -50 cm(-1)), Ni center dot center dot center dot Gd (J(NiGd) = -4.65 cm(-1)), and Gd center dot center dot center dot Gd (J(Gd) = -0.02 cm(-1)) exchange interactions in 7, involving the gadolinium(III) ions. The (Zn2Dy2III)-Dy-II compound 3 has shown the tail of an out-of-phase signal in alternating current (AC) susceptibility measurement, indicative of slow relaxation of magnetization. Interestingly, the (Ni2Dy2III)-Dy-II compound 4 in which both the participating metal ions possess large single ion anisotropy, has failed to show up any slow magnetic relaxation.