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

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 Fe3+:1,10-phenanthroline solution in 0.1 M HNO3 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 Fe3+ 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-(1,10-phenanthroline)iron(II/III) complex is attributed to the formation of a dimeric ore-bridging iron(III) species, [(H2O)(phen)(2)FeOFe(phen)(2)(H2O)](4+), which is irreversibly reduced at potentials more negative than similar to+0.20 V presumably to the unstable ferrous form, which converts to tris-(1,10-phenanthroline)iron(II). (C) 1998 Elsevier Science S.A.