Binuclear copper(II) complexes bridged by oxamidate and dithio-oxamidate groups. Synthesis, structure, and magnetism

Abstract

Binuclear copper(II) complexes bridged by oxamidate (oxd) or dithio-oxamidate (dto) groups [Cu₂L₂(oxd)]X₂ and [Cu₂L₂(dto)]X₂[L = 2,2′-bipyridyl (bipy) or 1,10-phenanthroline, X = NO₃ or ClO₄], have been synthesized. Single-crystal X-ray analysis for [Cu₂(bipy)₂(oxd)][ClO₄]₂·2dmf (dmf = dimethylformamide) reveals that the oxd^2– ion in trans conformation bridges two copper(II) ions with an intermetal distance of 5.192(2)Å. The configuration around each copper is square pyramidal with two nitrogens of bipy and one nitrogen and one oxygen of oxd in the basal plane and a perchlorate oxygen at the apex [2.433(9)Å]. The dto-bridged complexes are characterized by two intense i.r. bands at 1 540 and 1 440 cm^–1 and a charge-transfer band at (21.6–21.9)× 10³ cm^–1. Magnetic analyses in the temperature range 80–300 K on the basis of the Bleaney–Bowers equation gave –2J values (singlet–triplet energy separation) of 488–566 cm^–1 for the oxd-bridged complexes and 726–982 cm^–1 for the dto-bridged complexes, indicating that the dithio-oxamidate bridge mediates a stronger antiferromagnetic spin exchange between copper(II) ions than does the oxamidate bridge. Cyclic voltammograms for the oxd complexes show an irreversible couple at +0.1 V vs. the saturated calomel electrode which is assigned to the process Cu^II₂→ Cu^ICu^II.