Copper(II) and Nickel(II) Complexes of β-Aminoketoxime Ligand: Syntheses, Crystal Structures, Magnetism, and Nickel(II) Templated Coupling of Oxime with Nitrile

The syntheses, molecular structures, and magnetic properties of a dicopper(II) complex, [Cu₂(HL¹)₂](ClO₄)₂ (1), and its nickel(II) analog, [Ni₂(HL¹)₂](ClO₄)₂ (2), of a β-amino ketoxime ligand (H₂L¹ = 4,4,9,9-tetramethyl-5,8-diazadodecane-2,11-dione dioxime) are discussed. The metal centers in out-of-plane oximate bridged dinuclear complexes (1 and 2) display distorted trigonal bipyramidal geometry and form a six-membered M₂(NO)₂ ring oriented in a boat conformation. The two copper(II) centers in 1 interact ferromagnetically giving rise to a triplet-spin ground state whereas the two nickel(II) centers in 2 interact antiferromagnetically to stabilize a singlet-spin state. Variable temperature magnetic susceptibility measurements establish the presence of a weak ferromagnetic coupling (J = 13 cm⁻¹) in 1 and a weak anitiferromagnetic coupling (J = −12 cm⁻¹) in 2. The exchange coupling constant derived from B3LYP computations in conjunction with broken symmetry spin-projection techniques for the oximate bridged dinuclear copper(II) complex shows excellent agreement with the corresponding experimental value. A square-planar mononuclear nickel(II) complex of the dioxime ligand, [Ni(H₂L¹)](ClO₄)₂ (3), is reported along with its crystal structure, which reacts with acetonitrile to produce a six-coordinate mononuclear complex, [Ni(L²)](ClO₄)₂ (4). The ligand (L²) in complex 4 is the iminoacyl derivative of oxime, where the coupling of oxime and acetonitrile takes place via a proton-assisted pathway. The iminoacylation of H₂L¹ works with other nitriles like butyronitrile and benzonitrile. Computational studies support a proton-assisted coupling of oxime with nitrile. The critical transition states have been located for the iminoacylation reaction. Complex 4 can be converted back to complex 3 by reacting with sodium acetate in methanol.