Electron and Hydrogen-Atom Self-Exchange Reactions of Iron and Cobalt Coordination Complexes

Yoder, Jeffrey C.; Roth, Justine P.; Gussenhoven, Emily M.; Larsen, and Anna S.; Mayer, James M.

doi:10.1021/ja0273905

Cited by 73 publications

(96 citation statements)

References 74 publications

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“…Of these systems perhaps the most well explored are the iron(II)tris(2-2′-biimidazoline) 2+ (Fe II H 2 bip) and iron(II)tris(2,2′-(tetrahydro)pyrimidine) 2+ (Fe II H 2 bip) 84,426–427428. The Fe II (H 2 bim) and Fe II (H 2 bip) systems have similar acid/base properties in MeCN, with p K a = 17.5.…”

Section: Thermochemistry Of Pcet Reagentsmentioning

confidence: 99%

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

2010

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Section: Thermochemistry Of Pcet Reagentsmentioning

confidence: 99%

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

2010

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“…Iron biimidazoline and bipyrimidine systems have k s.e. values a little slower (by 330 and 180 times) than k 4H , both in MeCN at 298 K 47,48…”

Section: Discussionmentioning

confidence: 98%

“…47,48 In the reactions of ruthenium polypyridyl oxo, hydroxo, and aquo complexes, the pseudo self-exchange reactions have substantial k H / k D values, 5.8–16.1,44,56 while the KIE for “true” self-exchange is apparently much smaller (Table 2). 21a As noted above, activation parameters are a more direct probe of tunneling than KIE values.…”

Section: Discussionmentioning

confidence: 99%

Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation

Mayer

2008

J. Am. Chem. Soc.

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The reaction of Ru II (acac) 2 (py-imH) (Ru II imH) with TEMPO • (2,2,6,6-tetramethyl-piperidine-1-oxyl radical) in MeCN quantitatively gives Ru III (acac) 2 (py-im) (Ru III im) and the hydroxylamine TEMPO-H by transfer of H • (H + + e − ) (acac = 2,4-pentanedionato, py-imH = 2-(2′-pyridyl) imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k 3H = 1400 ± 100 M −1 s −1 at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (ΔG°). Deuterium transfer from Ru II (acac) 2 (py-imD) (Ru II imD) to TEMPO • is surprisingly much slower at k 3D = 60 ± 7 M −1 s −1 , with k 3H /k 3D = 23 ± 3 at 298 K. Temperature dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E a3D − E a3H = 1.9 ± 0.8 kcal mol −1 . The large k 3H /k 3D and ΔE a values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru II imH and Ru III im, measured by 1 H NMR line broadening, occurs with k 4H = (3.2 ± 0.3) × 10 5 M −1 s −1 at 298 K and k 4H /k 4D = 1.5 ± 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A 4H /A 4D ) = −0.5 ± 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru II imH (D) + TEMPO • that are greater than those observed: k 3H,calc /k 3H = 31 ± 4 and k 3D,calc /k 3D = 140 ± 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.

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“…This is perhaps most evident in Shaik's two-state reactivity model for reactions of first-row metal-oxo species 27. Our laboratory found an example in the HAT self-exchange reaction Fe II (H 2 bip) + Fe III (Hbip), both of which are present in solution as mixtures of high-spin and low-spin forms (see eq 13 for compound drawings) 28 . k AH/A is slightly faster at lower temperatures, where the low-spin forms are more prevalent.…”

Section: Hat Self-exchange Reactionsmentioning

confidence: 90%

Understanding Hydrogen Atom Transfer: From Bond Strengths to Marcus Theory

2010

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CONSPECTUS Hydrogen atom transfer (HAT) is one of the most fundamental chemical reactions: A–H + B → A + H–B. It is a key step in a wide range of chemical, environmental, and biological processes. Traditional HAT involves p-block radicals such as tBuO• abstracting H• from organic molecules. More recently, it has been recognized that many transition metal species undergo HAT. This has led to a broader perspective, with HAT viewed as one type of proton-coupled electron transfer (PCET). When transition metal complexes oxidize substrates by removing H• (≡ e– and H+), typically the electron transfers to the metal and the proton transfers to a ligand. Two examples are shown in the Figure: iron-imidazolinate and vanadium-oxo complexes. Although such reagents do not “look like” main group radicals, they have the same pattern of reactivity. For instance, their HAT rate constants parallel the A–H bond strengths within a series of similar reactions. Just like main group radicals, they abstract H• much faster from O–H bonds than from C–H bonds of the same strength. This shows that driving force is not the only determinant of reactivity. We have found that HAT reactivity is well described using a Marcus-theory approach. In the simplest model, the cross relation, kAH/B = (kAH/AkBH/BKeqf)½, predicts the rate constant for AH + B in terms of the self-exchange rate constants (kAH/A for AH + A) and the equilibrium constant. For a variety of transition metal oxidants, kAH/B is predicted within one or two orders of magnitude with only a few exceptions. For 36 organic reactions of oxyl radicals, kAH/B is predicted with an average deviation of a factor of 3.8, and within a factor of 5 for all but six of the reactions. These reactions involve both O–H or C–H bonds, occur either in water or organic solvents, and over a range of 1028 in Keq and 1013 in kAH/B. The treatment of organic reactions of O–H bonds includes the well-established kinetic solvent effect on HAT reactions. This is one of a number of secondary effects that the simple cross relation does not include, such as hydrogen tunneling and the involvement of precursor and successor complexes. Various case studies are described, applying the cross relation and illustrating some of these additional issues. The success of the cross relation, despite it being a quite simplified treatment, shows that the Marcus approach based on free energies and intrinsic barriers captures much of the essential chemistry of HAT reactions. Among the insights derived from the analysis is that reactions correlate with ΔG°, not with bond enthalpies as has long been assumed. Also in contrast to common intuition, the radical character or spin state of an oxidant is not found to be a primary determinant of HAT abstracting ability. The intrinsic barriers for HAT reactions can be understood, at least in part, as Marcus-type inner-sphere reorganization energies. The intrinsic barriers for cross reactions are accurately derived from the HAT self-exchange rate constants, which is a remarkable and unprecedent...

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Electron and Hydrogen-Atom Self-Exchange Reactions of Iron and Cobalt Coordination Complexes

Cited by 73 publications

References 74 publications

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole Complex: Hydrogen Tunneling and the Applicability of the Marcus Cross Relation

Understanding Hydrogen Atom Transfer: From Bond Strengths to Marcus Theory

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