Title: Ion association and charge-transfer excitation between <i>N</i>-heterocyclic cations and cyanoiron complexes
Abstract:N-Heterocyclic cations form with substituted pentacyanoferrates a series of outer-sphere complexes of general formula Fe(CN) 5 L//N-Het, suitable for systematic studies in aqueous solution. The equili...N-Heterocyclic cations form with substituted pentacyanoferrates a series of outer-sphere complexes of general formula Fe(CN) 5 L//N-Het, suitable for systematic studies in aqueous solution. The equilibrium constants for the association of dipositive cations (e.g. N,N′-dimethyl-4,4-bipyridyl, or paraquat ion) and monopositive cations (e.g. N-methylpyrazinium) with the hexacyanoferrate(II) anion are typically in the range of 30–40 M −1 and 10–13 M −1 . The optical charge-transfer energies depend on the nature of the N-heterocyclic acceptor, and on the binding properties of the ligand L as they modify the ionization potentials of the Fe(CN) 5 L n− complexes. A linear correlation between the optical charge-transfer energies and ΔE 0 was found, with a slope (ΔE op /ΔG 0 ) of 1.03 ± 0.03. The results were interpreted on the light of Hush's theory for intervalence transitions, with the aid of the equation E op = 2(ΔG 11 * + ΔG 22 *) + ΔG 12 0 , which correlates the optical energy (E op ) for electron-transfer with the intrinsic barriers (ΔG 11 * + ΔG 22 *) of the donor and acceptor ions, and the free energy change (ΔG 12 0 ) for the process.Read More