Studies on phenoxo-bridged macrocyclic dicopper complexes. EPR spectral behaviour of mixed-valence systems [Cu(II)Cu(I)L]⁺ and magnetic exchange interaction in complexes [Cu₂L](ClO₄)₂

Abstract

EPR spectra of two series of electrochemically generated macrocyclic mixed-valence copper complex species [Cu(II)Cu(I)L]⁺ have been investigated. Complexes belonging to series 1 [Cu₂L](ClO₄)₂ are derived from 6,12,18,24-substituted (H,H,H,H, (a); Me,Me,Me,Me, (b); Pr,Pr,Pr,Pr (c); Ph,Ph,Ph,Ph (d); H,H,Me,Me (e)) macrocycle 7,11,19,23-dimetheno-9,21-dimethyl-1,5,13,17-tetra-azacyclotetracosa-5,7, 9,12,17,19,21,14-octaene-25,26-diol (H₂L). Complexes belonging to series 3, [Cu₂L](ClO₄)₂, are related to those of series 1 by replacement of two of the CH=N linkages by---CH₂---NH---groups of the segment containing two azomethine linkages having the 6,12-substituents H,H (a); Me,Me (b); Me,Ph (c). In the mixed-valence species derived from 1b-1e and 3a-3c in MeCN or CH₂Cl₂ solutions the single unpaired electron is localized on one of the metal centres at room temperature on the EPR time scale. Only the species derived from 1a gives a seven-line EPR spectrum at room temperature indicating that the unpaired electron is delocalized over both metal centers. Variable temperature magnetic measurements on complexes 1b and 1c indicate strong antiferromagnetic exchange (-2J = 835 ± 5 cm^-1 and 806 ± 9 cm^-1, respectively) in both cases.