Spectroelectrochemical study of the generation of tris-(1,10-phenanthroline) iron(II/III) from μ-oxo-bis[aquabis(1,10-phenanthroline) iron(III)

Abstract

Through the use of an optically transparent thin-layer electrochemical cell with an Au minigrid working electrode, the redox behavior of 1:2 mole ratio Fe 3+:1,10-phenanthroline solution in 0.1 M HNO 3 was investigated via visible spectroelectrochemistry. Holding the Au working electrode at reducing potentials is required to achieve both a nominally reversible cyclic voltammogram and spectroscopic evidence of oxidation and reduction of tris-(1,10-phenanthroline)iron(II/III) from a solution prepared with an Fe 3+ source. Once formed, the tris-(1,10-phenanthroline)iron(II/III) complex exhibits the expected formal redox potential, 0.88 ± 0.01 V vs. Ag|AgCl|sat'd KCl. The spectroscopic and electrochemical signatures of the tris-(1,10-phenanthroline)iron complex both indicate depletion of this complex with continuous cycling. The need to reduce the solution electrochemically and the gradual depletion of the tris-(1,10phenanthroline)iron(II/III) complex is attributed to the formation of a dimeric oxo-bridging iron(III) species, [(H 2O)(phen) 2FeOFe(phen) 2(H 2O)] 4+, which is irreversibly reduced at potentials more negative than ∼ +0.20 V presumably to the unstable ferrous form, which converts to tris-(1,10-phenanthroline)iron(II).