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 βj1=(1.63 ± 0.05)× 1012 dm3 mol–1 and βj2=(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); βh1=(2.60 ± 0.05)× 104 and βh2=(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 FeIII 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++ salen2–⇄[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).