Preparation and Spectral, Magnetic, and Electrochemical Characterization of a Flexible Phenoxo-Bridged Binuclear Copper(II) Complex

The preparation of a pentadentate binucleating ligand, 2,6-bis((salicylideneamino)methyl)-4-methylphenol, is described together with the corresponding pyrazolate-bridged copper(II) complex, Cu2L. Spectroscopic evidence suggests square-planar coordination of the copper ions, and the magnetic data when fitted to the Bleaney-Bowers equation give an exchange coupling (2J) of -457 ± 5 cm-1. The complex has unusual stereochemical flexibility, which enables reactions coupled with reduction to be considered in some detail. The electrochemical reduction of the complex Cu2L has been examined at mercury and platinum electrodes. Two overall one-electron-reduction steps are observed, which appear to be Cu2L + e- ⇋ [Cu2L]- and [Cu2L]- + e- ⇋ [Cu2L]2-. However, there are considerable differences in the electrochemical behavior of the compound at the two electrodes and the temperature dependence is not consistent with the commonly employed one-electron reversible or quasi-reversible theoretical models. Rather, a more complex reaction scheme in which a structural change accompanies the charge transfer in addition to the reaction [Cu2L]2- + Cu2L ⇋ 2[Cu2L]- is needed to provide a rational explanation of the data. The stereochemical and/or magnetic changes accompanying electron transfer may have relevance to electron transfer in molecules of biological importance.