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 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5,10,15,20-tetraphenylporphyrin (H(2)tBuTPP) and the reaction with superoxide (KO(2)) 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 cm(3) 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) × 10(3) M(-1), and does not change significantly in the presence of the externally added crown ether in DMSO (11 ± 4) × 10(3) M(-1). The rate constants for the superoxide binding (k(on) = (1.30 ± 0.01) × 10(5) 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.

Transition Metal Complexes and the Activation of Dioxygen

“…Kinetic and equilibrium studies showed the reaction between the Fe(II) precursor and O À 2 to be fast, with the equilibrium strongly in favor of the bound species. New experimental and theoretical evidence suggests that 16 actually exists in equilibrium with its (η 1 -superoxo)Fe(II)(DMSO) isomer rather than a resonance form of 16 [45].…”

“…Axial ligation at the heme center has also been demonstrated to have an effect on the binding mode of the peroxo bridge. In a recent example, addition of 1,5-dicyclohexylimidazole (DCHim) to compound 42 ( Figure 18) was shown to result in the fully end-on η 1 :η 1 coordination of the O 2 À 2 moiety to both metal centers (45), which was supported on the basis of rR, EXAFS, and UV-vis spectroscopy [131]. Values of ν(O-O) and ν(Fe-O) for 45 are similar to those observed for the η 2 :η 1 bound species.…”

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

“…183 The addition of excess potassium superoxide to Fe II (tBuTPP) (tBuTPP = meso -tetrakis( p-tert -butylphenyl)porphyrinato 2− ) generated the peroxo species [Fe III (O 2 2− )(tBuTPP)] − . This process could be reversed by addition of a proton source, triflic acid (HOTf).…”

Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids

“…As it is much slower than the first reaction step, the second reaction step was observed only at higher concentrations of superoxide (≥2 mM), and the final spectrum is that of the (su)peroxo adduct with an absorption maximum at 440 nm. 4,23,45,46,48,49 From the concentration-dependent measurements ( Figure 7), the second-order rate constants for the first and the second reaction steps were found to be 18.6 ± 0.5 M −1 s −1 and 2.8 ± 0.1 M −1 s −1 , respectively. Strikingly, these reaction rates are four to five orders of magnitude lower than the corresponding reaction rates for other porphyrin complexes in conventional organic solvents.…”