Solution chemistry of NN′-ethylenebis(salicylidenimeinato)iron(III). Part 2. Formation equilibria of µ-oxo-bis[NN′-thylenebis(salicylideneiminato)iron(III)] and catecholato[NN′-ethylenebis(salicylideneiminato)]ferrate(III)

Abstract

The complex [Fe(salen)]⁺[H₂salen =NN′-ethylenebis(salicylideneimine)] can dimerize in dimethyl sulphoxide–H₂O (80 : 20 w/w) solution according to the following equation 2 [Fe(salen)]⁺+ H₂O [graphic omitted] [{Fe(salen)}₂O]+ 2 H⁺. Although the µ-oxo-complex has been known for some time, this equilibrium has not been studied up to now; the equilibrium constant,K_D=(5.00 ± 0.05)× 10^–13 mol dm^–3(0.1 mol dm^–3 KClO₄, 25 °C). [Fe(salen)]⁺ interacts in solution with catechol (H₂cat) to give [Fe(salen)(cat)]^–. We have determined the complex formation constant, [Fe(salen)]⁺+ cat^2– [graphic omitted] [Fe(salen)(cat)]^–, K=(4.79 ± 0.02)× 10¹⁴ dm³ mol^–1(0.1 mol dm^–3 KClO₄, 25 °C). In this complex, cat^2– is co-ordinated as a bidentate ligand. This fact implies a marked structural change in the salen^2– ligand, from the normal planar configuration to a very distorted one. There is no kinetic hindrance to this change, as shown by the fast formation equilibrium, but the distorted anion causes reduced stability compared to the planar anion.