Defective Dicubane-like Tetranuclear Nickel(II) Cyanate and Azide Nanoscale Magnets

Four tetrameric nickel(II) pseudohalide complexes have been synthesized and structurally, spectroscopically, and magnetically characterized. Compounds 1−3 are isostructural and exhibit the general formula [Ni₂(dpk·OH)(dpk·CH₃O)(L)(H₂O)]₂A₂·2H₂O, where dpk = di-2-pyridylketone; L = N₃⁻, and A = ClO₄⁻ for 1, L = NCO⁻ and A = ClO₄⁻ for 2, and L = NCO⁻ and A = NO₃⁻ for 3. The formula for 4 is [Ni₄(dpk·OH)₃ (dpk·CH₃O)₂(NCO)](BF₄)₂·3H₂O. The ligands dpk·OH⁻ and dpk·CH₃O⁻ result from solvolysis and ulterior deprotonation of dpk in water and methanol, respectively. The four tetramers exhibit a dicubane-like core with two missing vertexes where the Ni^II ions are connected through end-on pseudohalide and oxo bridges. Magnetic measurements showed that compounds 1−4 are ferromagnetic. The values of the exchange constants were determined by means of a theoretical model based on three different types of coupling. Thus, the calculated J values (J₁ = J₂, J₃, and D) were 5.6, 11.8, and 5.6 cm⁻¹ for 1, 5.5, 12.0, and 5.6 cm⁻¹ for 2, 6.3, 4.9, and 6.2 cm⁻¹ for 3, and (J₁, J₂, J₃, and D) 6.9, 7.0, 15.2, and 4.8 cm⁻¹ for 4.