Slow relaxation of the magnetization in two cobalt(iii)/cobalt(ii) dimers

Abstract

Here we describe the synthesis and characterization of two new mixed-valence Co^II/Co^III dimers:[(N₃)Co^III(L^1r)(μ-O₂CR¹)Co^II(N₃)][(N₃)Co^III(L^1r)(μ-O₂CR¹)Co^II(Cl)]·2MeOH (1) and [(N₃)Co^III(L^2r)(μ-O₂CR²)Co^II(N₃)][(N₃)Co^III(L^2r)(μ-O₂CR¹)Co^II(OH₂)]ClO₄·2H₂O (2) with R¹CO₂H = 4-nitrobenzoic acid and R²CO₂H = 3-methyl-4-nitrobenzoic acid. These compounds have the same structural Co^IIIO₂Co^IIcore topology and have been obtained with two related reduced compartmental Schiff base ligands: H₂L^1r = (1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol) and H₂L^2r = (1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol), with the same N₂O₂O′₂ donor set. The single crystal structures of both compounds display octahedral cobalt(II) centres and cobalt(III) centres with an intermediate geometry between octahedral and trigonal prism with rhombic distortion. The cobalt(III) and cobalt(II) centres are located at the inner N₂O₂ and outer O₂O′₂ compartments of the reduced Schiff base ligands, respectively. Magnetic properties show the presence of a field-induced slow relaxation of the magnetization following Direct, Raman and Orbach mechanisms.