Dinuclear Co(iii)/Co(iii) and Co(ii)/Co(iii) mixed-valent complexes: synthetic control of the cobalt oxidation level

Abstract

The reactivity of cobalt(II) salts towards H₃L (2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) was studied in different reaction conditions. Accordingly, the interaction of cobalt(II) acetate with H₃L in methanol gives rise to the discrete complex [Co^III₂L(OAc)₂(OMe)]·1.5H₂O·MeOH, 1. Reaction of cobalt(II) acetylacetonate with H₃L in the presence of dicarboxylic acids was also investigated. Thus, when cobalt(II) acetylacetonate and H₃L are mixed with terephthalic or malonic acid in 4 : 2 : 1 molar ratios, the mixed valent [Co^II/III₂L(acac)(p-O₂CC₆H₄CO₂H)][Co^II/III₂L(acac)(OH)]·2H₂O·2MeOH, 2 and [Co^II/III₂L(acac)(O₂CCH₂CO₂H)][Co^II/III₂L(acac)(OH)]·7H₂O, 3 complexes are isolated. Decreasing the pH of the medium, by addition of a second mol of dicarboxylic acid, leads to [Co^II/III₂L(O₂CCH₂CO₂)(MeOH)]·2MeOH, 4, while the reaction with terephthalic acid does not proceed. 1, 2 and 4 were crystallographically characterised and all the complexes are dinuclear, with hydrogen bonds that expand the initial nodes. The magnetic characterisation, as well as the NMR spectroscopy, indicates a diamagnetic nature for 1, in agreement with the presence of Co(III), showing the aerial oxidation suffered by the cobalt(II) ions. Nevertheless, 2–4 are paramagnetic. Temperature variable magnetic measurements were recorded for the crystallographically characterised complexes 2 and 4 and these studies confirm the mixed valence Co(II)/Co(III) nature of the compounds. The best fits of the magnetic data give an axial distortion parameter Δ = 628.7 cm⁻¹ for 2 and 698.8 cm⁻¹ for 4, and spin–orbit coupling constant λ = −117.8 cm⁻¹ for 2 and −107.0 cm⁻¹ for 4. Therefore, this study shows that the oxidation degree of the initial cobalt(II) salt by atmospheric oxygen can be controlled according to the pH of the medium.