Syntheses, Structures, and Magnetic Properties of a Family of Tetranuclear Hydroxido-Bridged NiII2LnIII2 (Ln = La, Gd, Tb, and Dy) Complexes: Display of Slow Magnetic Relaxation by the Zinc(II)–Dysprosium(III) Analogue

A new family of [2 × 2] tetranuclear 3d–4f heterometallic complexes have been synthesized. These are [Zn2Dy2L2(μ3-OH)2(μ4-OH)­(dbm)2(MeOH)2]­(NO3)·2H2O·MeOH (3), [Ni2Dy2L2(μ3-OH)2(μ4-OH)­(dbm)2(MeOH)2]­(NO3)·MeOH (4), [Ni2La2L2(μ3-OH)2­(μ4-OH)­(dbm)2(MeOH)2]­(ClO4)·H2O·2MeOH (5), [Ni2Tb2L2(μ3-OH)2(μ4-OH)­(dbm)2 (MeOH)2]­(NO3)·MeOH (6), and [Ni2Gd2L2(μ3-OH)2(μ4-OH)­(dbm)2(MeOH)2]­(NO3)·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)]·2CH3OH (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 μ4-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···Ni (JNi = −50 cm–1), Ni···Gd (JNiGd = −4.65 cm–1), and Gd···Gd (JGd = −0.02 cm–1) exchange interactions in 7, involving the gadolinium­(III) ions. The ZnII2DyIII2 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 NiII2DyIII2 compound 4 in which both the participating metal ions possess large single ion anisotropy, has failed to show up any slow magnetic relaxation.