Solution chemistry of NN′-ethylenebis(salicylideneiminato)iron(III). Part 1. Deprotonation equilibria and reversible decomposition in acid medium of NN′- ethylenebis(salicylideneimine). Stability constant of NN′- ethylenebis(salicylideneiminato)iron(III)

Abstract

NN′-Ethylenebis(salicylideneimine), H₂ salen, behaves as a weak diprotic acid in dimethyl sulphoxide (dmso)–H₂O (80 : 20 w/w) solution. The values of the overall association constants are β_j1=(1.63 ± 0.05)× 10¹² dm³ mol^–1 and β_j2=(2.04 ± 0.02)× 10²³ dm⁶ mol^–2. H₂ salen undergoes slow hydrolytic decomposition in acid medium [equations (i) and (ii)](Hsal = salicylaldehyde, en = ethylenediamine); β_h1=(2.60 ± 0.05)× 10⁴ and β_h2=(1.60 ± 0.02)× 10⁷. H₂ salen + H⁺ [graphic omitted] Hsal + HOC₆H₄CHN(CH₂)₂NH₃⁺(i), H₂ salen + 2 H⁺ [graphic omitted] 2 Hsal + H₂en²⁺(ii) The favourable thermodynamics for the hydrolysis are provided by the protonation of en. In neutral or basic media the hydrolysis is not spontaneous in spite of the large water content of the solvent. Solutions of [{Fe(salen)Cl}₂] in dmso–H₂O contain the cation [Fe(salen)]⁺. The chloride is completely displaced from the co-ordination sphere of Fe^III by a solvent molecule, and [Fe(salen)]⁺ decomposes, in acid medium, very slowly [equation (iii)]. [Fe(salen)]⁺+ 4 H⁺⇄ Fe³⁺+ 2 Hsal + H₂en²⁺(iii) The study of this equilibrium allows the stability constant of the complex, Fe³⁺+ salen^2–⇄[Fe(salen)]⁺, β=(7.1 ± 0.1)× 10²⁵ dm³ mol^–1 to be obtained. This is the first reported stability constant of a metal complex of H₂ salen. All equilibrium constants were determined at 25 °C and 0.1 mol dm^–3 KClO₄ in dmso–H₂O (80 : 20 w/w).