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

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posted on 2010-01-18, 00:00 authored by Oindrila Das, N. N. Adarsh, Ankan Paul, Tapan Kanti Paine
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.