Solution behavior of iron(III) and iron(II) porphyrins in DMSO and reaction with superoxide. Effect of neighboring positive charge on thermodynamics, kinetics and nature of iron-(su)peroxo product

K. Duerr,  O. Troeppner,  J. Olah,  J. Li,  A. Zahl,  T. Drewello,  N. Jux,  J. N. Harvey  and  I. Ivanović-Burmazović

doi: 10.1039/C1DT11521A

Dalton Trans., 2012,41, 546-557

Abstract

The solution behavior of iron(III) and iron(II) complexes of 54,104,154,204-tetra-tert-butyl-5,10,15,20-tetraphenylporphyrin (H2tBuTPP) and the reaction with superoxide (KO2) in DMSO have been studied in detail. Applying temperature and pressure dependent NMR studies, the thermodynamics of the low-spin/high-spin equilibrium between bis- and mono-DMSO FeII forms have been quantified (KDMSO = 0.082 ± 0.002 at 298.2 K, ΔH° = +36 ± 1 kJ mol−1, ΔS° = +101 ± 4 J K−1 mol−1, ΔV° = +16 ± 2 cm3 mol−1). This is a key activation step for substitution and inner-sphere electron transfer. The superoxide binding constant to the iron(II) form of the studied porphyrin complex was found to be (9 ± 0.5) × 103 M−1, and does not change significantly in the presence of the externally added crown ether in DMSO (11 ± 4) × 103 M−1. The rate constants for the superoxide binding (kon = (1.30 ± 0.01) × 105 M−1 s−1) and release (koff = 11.6 ± 0.7 s−1) are not affected by the presence of the external crown ether in solution. The resulting iron(II)-superoxide adduct has been characterized (mass spectrometry, EPR, high-pressure UV/Vis spectroscopy) and upon controlled addition of a proton source it regenerates the starting iron(II) complex. Based on DFT calculations, the reaction product without neighboring positive charge has iron(II)-superoxo character in both high-spin side-on and low-spin end-on forms. The results are compared to those obtained for the analogous complex with covalently attached crown ether, and more general conclusions regarding the spin-state equilibrium of iron(II) porphyrins, their reaction with superoxide and the electronic structure of the product species are drawn.

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