In Situ Mossbauer Spectroscopy of a Species Irreversibly Adsorbed on an Electrode Surface

The redox properties of μ-oxobis[iron meso-tetrakis(methoxyphenyl)porphyrin] (FeTMPP)20, adsorbed on high-area carbon in alkaline solutions have been examined in situ by using Mossbauer spectroscopy at cryogenic temperatures. The spectrum obtained for the electrode polarized at 0.0 V vs Hg/HgO,OH-yielded at 100 K a doublet with parameters resembling those of bulk crystalline (FeTMPP)20. Similar measurements with the electrode polarized at -0.9 V at the same temperature displayed in addition to the original feature two new doublets with Mossbauer parameters consistent with the presence of a high-spin ferrous (S = 2) and either a high-spin ferric (5 = 5/2) or an intermediate-spin ferrous porphyrin species (S = 1). As the temperature was increased, these new features collapsed into a single doublet with an isomer shift intermediate between those observed at 100 K. Two possible models have been suggested to account for these results: (i) the formation of a mixed-valence μ-oxo-type porphyrin, which below the spin trapping temperature would give rise to ferrous and ferric sites, and (ii) the full reduction and dissociation of the dimer following the first electron transfer, to yield an intermediate-spin and a high-spin ferrous porphyrin. The possibility of impurities being responsible for some of the spectroscopic features has also been considered.