Effects of electron donating/withdrawing groups in the 5-substituted-2-hydroxybenzaldehyde on the synthesis of neutral cubanes with a NiII4O4 core: synthesis, crystal structures and magnetic properties

Abstract

Two tetranuclear cubane-like nickel(II) complexes of formula [Ni₄(CH₃O)₄(L¹)₄(CH₃OH)₄] (1) and [Ni₄(CH₃O)₄(L²)₄(CH₃OH)₄] (2) (HL¹ = 2-hydroxybenzaldehyde and HL² = 2-hydroxy-5-methylbenzaldehyde) have been prepared by the reaction of NiCl₂·6H₂O with methanolic solutions of HL¹ and HL² in the presence of triethylamine at room temperature. Complexes 1 and 2 have been characterized by elemental analyses, IR spectra and single-crystal X-ray diffraction. The X-ray crystal structure analysis revealed that 1 and 2 have a cubane-type structural topology with four Ni(II) ions and four methoxo-oxygen atoms regularly alternating at the corners. Each metal ion in 1 and 2 is six-coordinate in a distorted octahedral geometry defined by three bridging methoxo-oxygens, one oxygen atom from a methanol molecule and two oxygen atoms from the bidentate L¹ (1)/L² (2) ligands. The magnetic properties of 1 and 2 were investigated in the temperature range 1.9–300 K. They are indicative of an overall ferromagnetic behaviour, the decrease in the very low temperature region being due to zero-field splitting and intermolecular interactions. A detailed analysis of the magnetic data of 1 and 2 assuming an idealized D_2d symmetry for the Ni₄O₄ unit shows the occurrence of a low-lying spin nonet and two opposite magnetic interactions, one being ferromagnetic [J_a = +12.6 (1) and +10.5 cm⁻¹ (2)] and the other antiferromagnetic [J_b = −4.10 (1) and −2.10 cm⁻¹ (2)]. These values fit well the linear correlation between the exchange coupling parameter (J) and the Ni–O–Ni angle (α) which was established for Ni₄O₄ cubane motifs in previous works.