Octanuclear Ni(ii) cubes based on halogen-substituted pyrazolates: synthesis, structure, electrochemistry and magnetism

Abstract

The employment of halogen-substituted pyrazole ligands in nickel(II) clusters afforded three anionic Ni(II) cubes, (HNEt₃)₂[Ni₈(Xpz)₁₂(OH)₆] (X = Cl, 1·2CH₃CN; X = Br, 2·2CH₃CN; X = I, 3·12CH₃CN; pz = pyrazolate). Clusters 1–3 are isolated as dianionic compounds which have similar cube geometry with each vertex occupied by a Ni(II) atom and eight μ₄-OH⁻ capped on eight square faces. The Xpz ligands adopt a bidentate mode to bind paired Ni(II) atoms on the edge of the cube. Interestingly, the three Ni₈ cubes have similar symmetry, size and shape but pack together to form different lattice symmetry, which should be dictated by the halogen-related supramolecular interactions (C–X⋯H, C–X⋯π and X⋯X halogen bonding) in different crystals. The electrochemistry of 1–3 showed a Ni(II)/Ni(I) redox couple with E_1/2 at ca. 900 mV and the oxidization potential order is 1 > 3 > 2 depending on the halogen substituents. 1–3 exhibit excellent electrocatalytic performances toward the oxidation of nitrite. The different packing of Ni₈ cubes in 1–3 also has an important influence on their magnetic behaviors. Complex 2 showed antiferromagnetic couplings between the Ni(II) ions within the cubes, whereas 1 and 3 exclusively exhibited mixed ferromagnetic and antiferromagnetic properties, leading to frustration and typical spin glass behavior.