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

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 Fe^II forms have been quantified (K_DMSO = 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 (k_on = (1.30 ± 0.01) × 10⁵ M⁻¹ s⁻¹) and release (k_off = 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.