Electron self-exchange in dicyanoiron porphyrins

Abstract

a strong case cannot be made for S,N,0 chelation by cysteine and penicillamine.A substantial contribution of S,N chelation to kinetic Cu(II)-S bond stability is readily apparent from the fc2((Me6tren)Cu-SR, (tmpa)Cu-S-pen) and A:3((tmpa)Cu-S-cys and -S-cme) rate constants. The presence of an optimal, five-membered S,N chelate unit certainly would stabilize the Cu(II) oxidation state and retard Cu(II)-S bond breaking in a reductive-elimination pathway leading to Cu(I) and a monodentate, N-bonded thiyl radical. Oxidation rate constants of S,N-bonded cysteine and its methyl ester are not greatly different in complexes with both (tmpa)Cu11 and (Me6tren)Cun, but the reactivity of penicillamine is lower by more than 1 order of magnitude in both systems. This observation lends further support to a reductive-elimination mechanism requiring Cu(II)-S bond cleavage in the rate-determining step. Considering the surprisingly small rate of ring closure within (20) Voltammograms generated from 1 mM Cu(Me6tren)2+ in pH 6.0, / = 0.1 M (MES) buffer showed a cathodic current maximum at -217 mV vs. NHE (50 mV/s sweep rate). E6(Cu(tmpa)2+/+) is -147 mV under these conditions. MES = morpholinoethanesulfonic acid. (21) Coates,

Applications of the Marcus cross relation to inner sphere O2 reduction: Implications in Small-Molecule Activation

“…The experimentally determined Fe II/III potential, self-exchange rate constant, and equilibrium and rate constants for the reversible reaction with O2 and are in Table 1. 28,32 The reversible adduct formation between PP-Fe(Im) and O2 have kon = 5.5 x 10 7 M -1 s -1 and koff = 310 s -1 . By determining the associated equilibrium constants for OSET and ISET reactions with O2 (Scheme 5 and Table 1), the MCR can then be used to determine rate constants and compared against experiment.…”

“…It is reasonable to estimate a self-exchange rate constant based on literature precedents (e.g., porphyrin ≈ 10 5 -10 7 M -1 s -1 and non-heme ≈ 10 3 -10 4 M -1 s -1 ). 32,36,45 When experimental equilibrium constants for O2 binding are not known, one can use density functional theory to compute an estimated BDFEFe-O2. To demonstrate, a DFT calculation (B3LYP 6-31G*) affords a BDFEFe-O2 of 32 kcal/mol in MeCN (13 kcal/mol in water) for a protoporphyrin IX derivative "PP" (Chart 2; See SI) that is close enough to the experimental value 27 kcal/mol to place a complex on the threshold lines.…”

High sensitivity of the fluorine NMR signals of difluorovinyl analogs of natural hemin: reconstituted heme proteins and self‐exchange electron transfer in model compounds

“…Such an analysis of the solvent influence on the rate constants has previously been successfully applied to a related compound, dicyanoiron porphyrin, in DMSO and methanol. 34 However, in the case of acetone as solvent, there is no direct relation between the solvent parameters and the observed rate constant. At the present stage of investigation, there is no satisfactory explanation for this result.…”

“…29 -32 Some measurements have also been performed by Dixon and co-workers using 1 H NMR line broadening analysis. 33 -36 Only one study reported the direct determination of dicyanoiron(II, III)protoporphyrin and deuteroporphyrin self-exchange electron rate constants, 34 (for a recent review, see Ref. 37), one of the limits being the complexity of the natural porphyrin proton spectra.…”

Methodology to determine the NMR chemical shifts of carbon atoms with radical character: A case of low-spin bis(tert-butylisocyanide) complex of (meso-tetrapropylporphyrinato)iron(III)

“…1. This method is based on selfexchange electron transfer between Fe(II) and Fe(III), which has been used for the determination of the kinetic rate constants [16][17][18][19][20]. Under the assumption that the rate for electron exchange between paramagnetic 1 and diamagnetic 2 is fast on the NMR time scale, the observed 1 H and 13 C NMR chemical shifts(δ obs ) of 2 containing x equiv of 1 can be expressed by Eq.…”