The kinetics and mechanisms of reactions of iron(III) with caffeic acid, chlorogenic acid, sinapic acid, ferulic acid and naringin

The kinetics and mechanisms of the reactions of iron(III) with the hydroxy cinnamic acid based ligands caffeic, chlorogenic, sinapic and ferulic acids and the flavonoid naringin have been investigated in aqueous solution. The mechanisms for caffeic and chlorogenic acid are generally consistent with the formation of a 1:1 complex that subsequently decays through an electron transfer reaction. On reaction with iron(III), ferulic and sinapic acids undergo an electron transfer without the prior formation of any complex. There was no evidence of electron transfer occurring in the complex formed when iron(III) is reacted with naringin. Rate constants for k₁ (formation) and k _-1 (dissociation) have been evaluated for the complex formation reactions of [Fe(H₂O)₆(OH)]²⁺ with caffeic acid, chlorogenic acid and naringin. Analysis of the kinetic data yielded stability constants, equilibrium constants for protonation of the iron(III) chlorogenic acid complex initially formed, together with the rate constants for complex decomposition through intramolecular electron transfers and in the case of caffeic acid and chlorogenic acid, rate constants for the iron(III) assisted decomposition of the initial complex formed. Some of the suggested mechanisms and calculated rate constants are validated by calculations carried out using global analysis of time dependent spectra.