Thermodynamics and electrochemistry of manganese ortho-tetrakis(N-n-butylpyridinium-2-yl)porphyrin in aqueous solutions

Abstract

Spectrophotometric pH-titrations (1<pH<12) of manganese ortho-tetrakis(N-n-butylpyridinium-2-yl)porphyrin (MnTnBu-2-PyP) have been performed in aqueous solution at various temperatures and ionic strengths, studying complexes of various manganese oxidation states (Mn(II)–(IV)). The observed results indicate three absorbing species of MnIIITnBu-2-PyP, but only two absorbing species of MnIITnBu-2-PyP and MnIVTnBu-2-PyP within the studied acidity range. The first and second deprotonation constants are related to fully protonated and monodeprotonated species, i.e., the aqua and hydroxo complexes, respectively. The same complexes have been studied by cyclic-voltammetry and the results can be summarized as follows: E 0′(MnIIITnBu-2-PyP/MnIITnBu-2-PyP(H2O)4+) = +0.203 V, E 0′(MnIIITnBu-2-PyP(H2O)(OH)4+/MnIITnBu-2-PyP(OH)3+) = +0.191 V, and E 0′(MnIVTnBu-2-PyP(H2O)(OH)5+/MnIIITnBu-2-PyP) = +0.62 V, all versus NHE. In neutral and acidic media the oxidation of the aqua Mn(III) complex may also occur as a one-electron two-proton process yielding the oxoaqua Mn(IV) complex.