S-shaped decanuclear heterometallic [Ni8Ln2] complexes [Ln(iii) = Gd, Tb, Dy and Ho]: theoretical modeling of the magnetic properties of the gadolinium analogue

Abstract

The reaction of 8-quinolinol-2-carboaldoxime (LH₂) with Ni^II and Ln^III salts afforded the heterometallic decanuclear compounds [Ni₈Dy₂(μ₃-OH)₂(L)₈(LH)₂(H₂O)₆](ClO₄)₂·16H₂O (1), [Ni₈Gd₂(μ₃-OH)₂(L)₈(LH)₂(H₂O)₄(MeOH)₂](NO₃)₂·12H₂O (2), [Ni₈Ho₂(μ₃-OH)₂(L)₈(LH)₂(H₂O)₄(MeOH)₂](ClO₄)₂·2MeOH·12H₂O (3) and [Ni₈Tb₂ (μ₃-OH)₂(L)₈(LH)₂(MeOH)₄(OMe)₂]·2CH₂Cl₂·8H₂O (4). While compounds 1–3 are dicationic, compound 4 is neutral. These compounds possess an S-shaped architecture and comprise a long chain of metal ions bound to each other. In all the complexes, the eight Ni^II and two Ln^III ions of the multimetallic ensemble are hold together by two μ₃-OH, eight dianionic (L²⁻) and two monoanionic oxime ligands (LH⁻) whereas compound 4 has two μ₃-OH, eight dianionic (L²⁻), two monoanionic oxime ligands (LH⁻) and two terminal methoxy (MeO⁻) ligands. The central portion of the S-shaped molecular wire is made up of an octanuclear Ni^II ensemble which has at its two ends the Ln^III caps. Magnetic studies on 1–4 reveal that the magnetic interactions between neighboring metal ions are negligible at room temperature. On the other hand, at lower temperatures in all the compounds anti-ferromagnetic interactions seem to be dominated. Analysis of the magnetic data for the Gd^III derivative indicates Ni^II–Ni^II anti-ferromagnetic interactions and Gd^III–Ni^II ferromagnetic interactions at low temperatures. A theoretical density functional study on the magnetic behavior of the Gd^III derivative suggests that while the weak ferromagnetic interaction between Gd^III and Ni^II is in line with the expectation of the magnetic interactions between orthogonal d and f orbitals, antiferromagnetic Ni^II–Ni^II interactions are related to the wide Ni–O–Ni angles (∼102°) and quasi-planar conformation of the Ni₂O₂ core.