Electrochemistry of various substituted aminophenyl iron porphyrins

Abstract

Abstract The redox properties of dissolved, adsorbed and electropolymerized films of iron(III) tetrakis( o -aminophenyl)porphyrin (Fe(III) ( o -NH 2 )TPP), iron(III) tetrakis( p -aminophenyl)porphyrin (Fe(III) ( p -NH 2 )TPP) and iron(III) tetrakis( p - N, N ' -dimethylaminophenyl)porphyrin (Fe(III) ( p -NMe 2 TPP) have been studied in aqueous solutions. Cyclic voltammetry at glassy carbon electrodes for the complexes dissolved in 1 M HCl showed two Fe(III)/Fe(II) waves with cathodic peak potentials of −0.14 and −0.38 V for Fe( o -NH 2 )TPP, −0.15 and −0.36 V for Fe( p -NH 2 )TPP, and −0.17 and −0.45 V for Fe( p -NMe 2 )TPP. Differential pulse voltammetry also revealed two reduction waves for Fe( o -NH 2 )TPP adsorbed on a glassy carbon electrode (−0.16 and +0.08 V) and only one reduction process for Fe( p -NH 2 )TPP (−0.24 V) and Fe(NMe 2 )TPP (−0.25 V). The reduction peak potentials for the polymeric films at pH 3.2 ±0.2 are −0.22 and +0.12 V for poly[Fe( o -NH 2 )TPP], −0.22 and -0.07 V for poly[Fe( p -NH 2 )TPP and −0.18 and −0.02 V for poly[Fe( p -NMe 2 )TPP]. Thin-layer spectroelectrochemical techniques for the dissolved iron porphyrins and cyclic voltammetry for the polymeric films in strong acidic solutions indicated irreversible changes resulting from a demetallation process occurring upon reduction. The demetallation rate for the Fe(II) ( p -NMe 2 TPP compound (8×10 −4 s −1 ) was about 20-fold smaller than that of the Fe(II) ( p -NH 2 )TPP complex. In contrast, the polymeric films showed high stability at pH > 3: the cyclic voltammetric peak currents decreased by only 2% after 100 subsequent cycles in the potential range +0.4 to −0.8 V.