Formation of out of plane oxime metallacycles in [Cu 2] and [Cu 4] complexes

Abstract

The reaction of Cu(ClO 4) 2·6H 2O with dimethylglyoxime (H 2dmg) in a 1:1 mole ratio in aqueous methanol at room temperature affords the dinuclear complex [Cu 2( μ-Hdmg) 4] ( 1). Reaction of 1 with [Cu(bpy)(H 2O) 2](ClO 4) 2 (bpy = 2,2′-bipyridine) in a 1:1 mole ratio in aqueous methanol at room temperature yields the tetranuclear complex [Cu 4( μ-Hdmg) 2( μ-dmg) 2(bpy) 2(H 2O) 2](ClO 4) 2 ( 2). The direct reaction of Cu(ClO 4) 2·6H 2O with H 2dmg and bpy in a 2:2:1 mole ratio in aqueous methanol at room temperature also yields 2 quantitatively. The complexes 1 and 2 were structurally characterized by X-ray crystallography. Unlike the binding in Ni/Co-dmg, two different types of N−O bridging modes during the oxime based metallacycle formation and stacking of square planar units have been identified in these complexes. The neutral dinuclear complex 1 has CuN 4O coordination spheres and complex 2 consists of a dicationic [Cu 4( μ-Hdmg) 2( μ-dmg) 2(bpy) 2(H 2O) 2] 2+ unit and two uncoordinated ClO 4 − anions having CuN 4O and CuN 2O 3 coordination spheres. The two copper(II) ions are at a distance of 3.846(8) Å in 1 for the trans out of plane link and at 3.419(10) and 3.684(10) Å in 2 for the trans out of plane and cis in plane arrangements, respectively. The average Cu–N oxime distances are 1.953 and 1.935 Å, respectively. The average basal and apical Cu−O oxime distances are 1.945, 2.295 and 2.429 Å. The UV–Vis spectra of 2 is similar to the spectrum of the reaction mixture of 1 and [Cu(bpy)(H 2O) 2] 2+. Variable temperature magnetic properties measurement shows that the interaction between the paramagnetic copper centers in complex 1 is antiferromagnetic in nature. The EPR spectra of frozen solution of the complexes at 77 K consist of axially symmetric fine-structure transitions (Δ M S = 1) and half-field signals (Δ M S = 2) at ca. 1600 G, suggesting the presence of appreciable Cu–Cu interactions.