Solution properties of tetrakis-(4-N-methyl)pyridylporphineiron(III)

Tetrakis-(4-N-methyl)pyridylporphineiron(III) (FeTMpyP) was synthesized and its aqueous solution properties studied. The visible spectrum of FeTMpyP was found to be dependent on pH and ionic strength. The following equilibria were sufficient to account for the titration data: H 2 O + FeP ( H 2 O ) 5+ ⥂ FeP ( OH )( H 2 O ) 4+ + H + ;p K a1 =4.7±0.2 FeP ( OH )( H 2 O ) 4+ ⥂ FeP (OH) 2 3+ + H + ; p K a2 =6.5±0.3 2 FeP ( OH ) 2 3+ ⥂O−( FePOH ) 2 6+ + H 2 O ; K D ≃9×10 5 M −1 . Magnetic susceptibility measurements and Mossbauer spectroscopy were used to characterize the FeTMpyP species at low and high pH. In the range pH 1–3, monomeric, high-spin, pentacoordinated iron(III) is the primary species. A μ-oxo dimer involving low-spin hexacoordinated iron(III) exists at pH 10–12. The rate of dissociation of the dimer at μ = 0.05 M and t = 25°C was found a two-term rate law: − d [ dimer ] d t =(k 1 +k′ 1 [ H + ])[ dimer ] where k1 = 0.42 sec−1 and K1 = 31 M−1 sec−1. These results are compared with those obtained for the dissociation of dimer of tetraphenylporphinesulfonatoferrate(III), FeTPPS.