Magneto-structural correlations in Ni(ii) [2 × 2] metallogrids featuring a variable number of μ-aquo or μ-hydroxo extra bridges

Abstract

Four new [2 × 2] grid-type metallosupramolecular species have been obtained by using the ditopic 3,6-bis(2′-pyridyl)pyridazine ligand (dppn) and nickel(II) salts containing poorly coordinating anions. Three of them have the formula [Ni₄(μ-dppn)₄(μ-OH)₂(μ-H₂O)₂]X₆·nH₂O [with X = ClO₄⁻ (1), NO₃⁻ (2) and CF₃SO₃⁻ (3), and n = 6.5 (1), 14 (2) and 4 (3)]. Their crystal structure shows the same tetranuclear core, constituted by four six-coordinate metal ions and four dppn molecules. Two hydroxo groups and two water molecules efficiently interact forming two hydrated hydroxide (H₃O₂⁻) supramolecular bridging anions, further stabilizing the grid. The other compound, [Ni₄(μ-dppn)₄(μ-OH)₃(μ-H₂O)](ClO₄)₅·5H₂O·2EtOH (4), also exhibits the same tetranuclear core but with three hydroxo groups and one water molecule as supporting bridges, thus featuring only one hydrated hydroxide ion. Cryomagnetic measurements on polycrystalline samples of 1–4 in the temperature range 1.9–300 K reveal an overall antiferromagnetic behaviour. The values of the intramolecular magnetic coupling (J) cover the range −50.4(3) to −63.6(4) cm⁻¹, the Hamiltonian being defined as H = −J(S₁·S₂ + S₂·S₃ + S₃·S₄ + S₁·S₄) + gβH(S₁ + S₂ + S₃ + S₄). The magneto-structural data of 1–4 show that the larger the average value at the hydroxo/aquo bridges is, the greater the antiferromagnetic coupling.