Electron transfer in a trinuclear oxo-centred mixed-valence iron complex, in solid and solution statesDedicated to Professor Dieter Sellmann on the occasion of his 60th birthday.

Abstract

Complexes [Fe III 2 M II OL 3 ] (M = Fe, Co, Ni, Cu; 2, 3, 4, 5) have been synthesised in which L 2Ϫ is a pentadentate ligand designed to coordinate all three metal atoms in the central cluster and to inhibit dissociation and solvent exchange processes. Crystal structures for 2, 4 and 5 show threefold symmetry, attributed to rotational disorder. Magnetisation data for 2 indicate strong superexchange between basis oxidation states Fe(3ϩ, 3ϩ, 2ϩ). Comparisons of IR spectra across the series of complexes confirm the non-threefold symmetry of the mixed-valence cluster on the vibrational time scale, both in the solid state and in solution. Proton NMR spectra in solution at room temperature do not distinguish the three iron sites, suggesting that pseudo-rotation by thermal electron transfer also operates. Cyclic voltammetry and spectroelectrochemical measurements show that the mixed-valence iron complex 2 can be oxidised reversibly to give the tri-iron() complex [Fe 3 OL 3 ] ϩ and reduced reversibly and quasireversibly to give respectively [Fe 3 OL 3 ] Ϫ and tri-iron() [Fe 3 OL 3 ] 2Ϫ , E 0 = 85, Ϫ635, Ϫ1230 mV (versus Fc ϩ/0 ) in dichloromethane (T = 298 K, 0.1 M [n-Bu 4 N][PF 6 ]). Mössbauer spectra of 2 indicate significant valence delocalisation even at low temperature (4.2 K) with estimated valences Fe(2.9ϩ, 2.9ϩ, 2.2ϩ) in the solid state. At higher temperatures no lifetime broadening is observed but additional Mössbauer absorptions are consistent with increasing proportions of trimer molecules with greater delocalisation, i.e. Fe(2.75ϩ, 2.75ϩ, 2.5ϩ). In frozen solution (THF) the spectra indicate increasing proportions of molecules fully valence-delocalised on the Mössbauer time scale. The data are accounted for with a model which places the complex at the Robin-Day class III/II borderline. It combines strong superexchange with significant double exchange even at the lowest temperatures, while at higher temperatures in solution complete valence delocalisation occurs through intramolecular electron transfer at rates intermediate between the IR and NMR time scales.

Mechanistic studies on the intramolecular electron transfer in an adduct species of the oxo-centred trinuclear iron(III) cation and l-ascorbic acid in aqueous solution

“…The reduction wave was found to be dependent on scan rate (which is typical of irreversible electrode reactions) and pH. The simultaneous reduction in the three Fe(III) centres points to the equivalency of the three metal centres, and the irreversibility of the reduction is consistent with reports of similar complexes [36,37], where reversibility (mostly quasi) is only observed in strong donor-type solvents such as pyridine and its derivatives [1][2][3]. At a higher pH, the reduction potential is more negative.…”

“…The water ligand that is coordinated to the iron(III) centre is sensitive to pH, and it contributes significantly to the charge transfer nature of the complex, as this portion of the spectrum is sensitive to the terminal ligand [1][2][3]. As the pH is increased, the concentration of the hydroxo species of iron(III) (Fe-OH) increases.…”

“…Following the electron transfer, the trinuclear core becomes highly unstable, due the enhanced lability of Fe(II) when compared to Fe(III) [1][2][3], and this spontaneously disintegrates to yield iron in aqueous acetate.…”

“…Electron transfer reactions involving oxo-centred trinuclear transition metal complexes have been investigated by several techniques and by various groups [1][2][3][4]. However, most of these studies involve complexes in which the terminal water ligands have been substituted for by organic molecules to stabilize the complexes and to make them more soluble in organic media.…”

“…John Cousins & Laurence Herbert, TgK Scientific Limited, UK 2. Carboxylate based buffers are best suited for this system, as other buffers such as citrate, PO4 3-or dimethylglutaric acid, reacted with the complex as evidenced in the UV/Vis spectra 3.…”