Heteronuclear Ni(II)-Ln(III) (Ln = La, Pr, Tb, Dy) complexes: synthesis and single-molecule magnet behaviour

The reaction of hydrated nickel(II) salts (chloride or nitrate) and hydrated lanthanide nitrate salts with the Schiff base ligand 2-methoxy-6-[(E)-phenyliminomethyl] phenol (HL) in methanol resulted in the isolation of three isostructural linear heterometallic trinuclear complexes and a heterometallic tetranuclear complex. The molecular structures of these complexes were determined via single crystal X-ray diffraction revealing molecular structures of formulae [Ni2La(L-)(6)](NO3)(0.55)(OH)(0.45) (1), [Ni2Pr(L-)(6)]-(NO3)(0.48)(OH)(0.52) (2), [Ni2Tb(L-)(6)](NO3)(0.5)(Cl)(0.5) (3) and [Ni2Dy2(L-)(2)(o-vanillin)(2)(CO3)(2)(NO3)(2)(MeOH)(2)] (4). Structural analysis for 1-3 reveals that the lanthanide ion is sandwiched between two Ni(II) ions and the Ni center dot center dot center dot Ln center dot center dot center dot Ni metallic core displays a linear arrangement, with an average angle Ni center dot center dot center dot Ln center dot center dot center dot Ni bond angle of 179.7 degrees. Analysis of 4 reveals the metal ions are arranged such that two Ni-Dy subunits are bridged by two carbonate ligands via the Dy sites. Direct current magnetic susceptibility measurements for complexes 1-4 reveal that the Ni(II) ions are coupled ferromagnetically with the Tb(III) (3) and Dy(III) (4) ions, and antiferromagnetically with the Pr(III) ion (2). For complex 1 a long range intramolecular ferromagnetic interaction is witnessed between the Ni(II) ions (Ni center dot center dot center dot Ni = 6.873(9) angstrom) via a closed shell La(III) ion. The magnetic data of 1 were fitted using the HDVV Hamiltonian revealing the following parameters; J = +0.46 cm(-1), g = 2.245, D = +4.91 cm(-1). Alternating current magnetic susceptibility measurements performed on complexes 2-4 revealed that 3 and 4 displayed frequency dependent chi(M)'' signals (H-ac = 3.5 Oe and H-dc = 0 Oe) which is a characteristic signature of a single-molecule magnet behaviour.