Synthesis, structure, and spectral and magnetic properties of trinuclear copper(II) complexes bridged by glyoximate groups

Abstract

Copper(II) complexes of composition Cu₃L₂L′₂(ClO₄)₂ or Cu₃L₂L′₂(CH₃OH)₂(NO₃)₂ were obtained where L = dimethylglyoximate (dmg), diphenylglyoximate (dpg), or o-benzoquinone dioximate (bqd) dianion, L′= 2,2′-bipyridyl (bipy) or 1,10-phenanthroline (phen). The crystal structures of Cu₃(dmg)₂(bipy)₂(CH₃OH)₂(NO₃)₂ and Cu₃(dpg)₂(bipy)₂(CH₃OH)₂(NO₃)₂ were solved by the single-crystal X-ray method. Both have an essentially similar trinuclear structure where the [CuL₂]^2– dianion functions as a bridge between two copper(II) ions through its deprotonated oximate oxygens. The configuration around the central copper (with two L^2– ions) is an elongated octahedron with two NO₃^– ions above and below the [CuL₂]^2–. The configuration around the terminal copper is a square pyramid with two nitrogens of bipy and two oxygens of oximate groups in the basal plane and the methanol oxygen at the apical site. Cryomagnetic investigations (80–300 K) revealed the operation of a very strong antiferromagnetic spin exchange through the oximate bridges, causing complete or nearly complete spin coupling even at room temperature. Exchange integrals (–J) larger than 300 cm^–1 were evaluated for all the complexes. Based on e.s.r. spectra in methanol, it is suggested that the unpaired electron is localized on the terminal copper atom. The complexes dimerized in dimethylformamide, especially at a low temperature, and their frozen solutions each showed an e.s.r. spectrum typical of the spin-triplet state.