Electron self-exchange of low-spin iron octaethylporphyrin, chlorin, and isobacteriochlorin complexes

Dixon, Dabney W.; Woehler, S. E.; Hong, Xiaole; Stolzenberg, Alan M.

doi:10.1021/ic00293a054

Cited by 18 publications

(11 citation statements)

References 2 publications

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“…Actually, whereas numerous studies have been performed on heme proteins in order to determine the factors which control the rate of electron transfer, only a few experiments have been carried out directly measuring self-exchange electron transfer between hemes . The 1 H NMR investigations of Dixon and co-workers were particularly important in this area, using 1 H NMR line broadening analysis, − one of the limits being the complexity of the natural porphyrin proton spectra − All the measured rate constants by 1 H NMR range from 10 7 to 10 8 M -1 s -1 (Table ).…”

Section: 1 Iron Porphyrinsmentioning

confidence: 99%

Mechanism of Electron Transfer in Heme Proteins and Models: The NMR Approach

Simonneaux

Bondon

2005

Chem. Rev.

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Section: 1 Iron Porphyrinsmentioning

confidence: 99%

Mechanism of Electron Transfer in Heme Proteins and Models: The NMR Approach

Simonneaux

Bondon

2005

Chem. Rev.

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“…It has long been noted that the metal-centered oxidation of cobalt (Co 2+/3+ ) in porphyrins exhibits sluggish heterogeneous and homogeneous ET rates in comparison to porphyrin ring oxidation, to ring or metal reduction, or to the analogous Fe 2+/3+ oxidation in iron porphyrins . For example, self-exchange ET rate constants ( k ex ) for 5,10,15,20-tetraarylmetalloporphyrins fall in the range 10 -2 to 10 4 M -1 s -1 for the Co 2+/3+ process, and 10 7 to 10 8 M -1 s -1 for the Fe 2+/3+ process. − As studies of cobalt cytochrome c have demonstrated, the reduction in k ex for the oxidation of Co 2+ porphyrins is primarily due to large inner sphere reorganization energies (λ i ∼ 2.4 eV) . Because Co 2+ porphyrins are generally low spin ( S = 1 / 2 ), removal of an electron from the predominantly d z 2 -character HOMO results in large changes in the bonding about the cobalt ion, including significant alteration of the metal−nitrogen bond distances and an associated contraction of the macrocycle framework around the smaller ion.…”

Section: Introductionmentioning

confidence: 99%

Slow Electron Transfer Rates for Fluorinated Cobalt Porphyrins: Electronic and Conformational Factors Modulating Metalloporphyrin ET

2003

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The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF28TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenyl)porphyrinato]cobalt, CoF8TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF20TPP, and [5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric digital simulation, in situ UV−vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, 19F NMR spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte, the ET rate constants for the Co2+/3+ redox couples were found to be strongly substituent dependent; the heterogeneous ET rate constant (k el) varied by a factor of 104, and the ET self-exchange rate constants (k ex) varied over 7 orders of magnitude for the compounds studied. The remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical k el values for all compounds. UV−vis and IR spectroelectrochemistry, bulk electrolysis, and 19F NMR spectroscopic studies support attribution of different ET rates to widely varying inner sphere reorganization energies (λi) for these closely related compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP, CoF8TPP, CoF20TPP, and CoF28TPP first oxidations arises mainly from large nuclear reorganization energies primarily associated with core contraction and dilation. Taken together, these studies provide rational design principles for modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar manipulation of ET rates in other porphyrin-based systems: substituents that lower C−C, C−N, and N−M vibrational frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron population will increase k ET. The heme ruffling apparent in electron transfer proteins such as cytochrome c is interpreted as nature's exploitation of this design strategy.

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“…So far, experimental investigations have mostly focused on the electronic structure of low-spin iron(III) porphyrins − This is related to the synthetic difficulty of preparing pure compounds due to the oxygen sensitivity of reduced macrocycle.…”

Section: Introductionmentioning

confidence: 99%

“…3 Studies of the electronic structures with simple iron chlorins which might serve as models for naturally occurring iron chlorin proteins have been more limited. [4][5][6][7][8][9][10][11][12][13] This is related to the synthetic difficulty of preparing pure compounds due to the oxygen sensitivity of reduced macrocycle.…”

Section: Introductionmentioning

confidence: 99%