Yoder Jeffrey C, Roth Justine P, Gussenhoven Emily M, Larsen Anna S, Mayer James M
Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195-1700, USA.
J Am Chem Soc. 2003 Mar 5;125(9):2629-40. doi: 10.1021/ja0273905.
Reported here are self-exchange reactions between iron 2,2'-bi(tetrahydro)pyrimidine (H(2)bip) complexes and between cobalt 2,2'-biimidazoline (H(2)bim) complexes. The (1)H NMR resonances of Fe(II)(H(2)bip)(3) are broadened upon addition of Fe(III)(H(2)bip)(3), indicating that electron self-exchange occurs with k(Fe,e)(-) = (1.1 +/- 0.2) x 10(5) M(-1) s(-1) at 298 K in CD(3)CN. Similar studies of Fe(II)(H(2)bip)(3) plus Fe(III)(Hbip)(H(2)bip)(2) indicate that hydrogen-atom self-exchange (proton-coupled electron transfer) occurs with k(Fe,H.) = (1.1 +/- 0.2) x 10(4) M(-1) s(-1) under the same conditions. Both self-exchange reactions are faster at lower temperatures, showing small negative enthalpies of activation: DeltaH++(e(-)) = -2.1 +/- 0.5 kcal mol(-1) (288-320 K) and DeltaH++(H.) = -1.5 +/- 0.5 kcal mol(-1) (260-300 K). This behavior is concluded to be due to the faster reaction of the low-spin states of the iron complexes, which are depopulated as the temperature is raised. Below about 290 K, rate constants for electron self-exchange show the more normal decrease with temperature. There is a modest kinetic isotope effect on H-atom self-exchange of 1.6 +/- 0.5 at 298 K that is close to that seen previously for the fully high-spin iron biimidazoline complexes.(12) The difference in the measured activation parameters, E(a)(D) - E(a)(H), is -1.2 +/- 0.8 kcal mol(-1), appears to be inconsistent with a semiclassical view of the isotope effect, and suggests extensive tunneling. Reactions of Co(H(2)bim)(3)-d(24) with Co(H(2)bim)(3) or Co(Hbim)(H(2)bim)(2) occur with scrambling of ligands indicating inner-sphere processes. The self-exchange rate constant for outer-sphere electron transfer between Co(H(2)bim)(3) and Co(H(2)bim)(3) is estimated to be 10(-)(6) M(-1) s(-1) by application of the Marcus cross relation. Similar application of the cross relation to H-atom transfer reactions indicates that self-exchange between Co(H(2)bim)(3) and Co(Hbim)(H(2)bim)(2) is also slow, < or =10(-3) M(-1) s(-1). The slow self-exchange rates for the cobalt complexes are apparently due to their interconverting high-spin Co(II)(H(2)bim)(3) with low-spin Co(III) derivatives.
本文报道了铁的2,2'-联（四氢）嘧啶（H₂bip）配合物之间以及钴的2,2'-联咪唑啉（H₂bim）配合物之间的自交换反应。在加入[Fe(III)(H₂bip)₃]³⁺后，[Fe(II)(H₂bip)₃]²⁺的¹H NMR共振峰变宽，这表明在298 K的CD₃CN中发生了电子自交换，其电子自交换速率常数k(Fe,e⁻) = (1.1 ± 0.2) × 10⁵ M⁻¹ s⁻¹。对[Fe(II)(H₂bip)₃]²⁺与[Fe(III)(Hbip)(H₂bip)₂]²⁺的类似研究表明，在相同条件下发生了氢原子自交换（质子耦合电子转移），其氢原子自交换速率常数k(Fe,H·) = (1.1 ± 0.2) × 10⁴ M⁻¹ s⁻¹。这两种自交换反应在较低温度下都更快，表现出较小的负活化焓：ΔH⁺⁺(e⁻) = -2.1 ± 0.5 kcal mol⁻¹（288 - 320 K）和ΔH⁺⁺(H·) = -1.5 ± 0.5 kcal mol⁻¹（260 - 300 K）。得出这种行为是由于铁配合物的低自旋态反应更快，而随着温度升高，低自旋态的数量减少。在约290 K以下，电子自交换的速率常数随温度呈现更正常的下降。在298 K时，氢原子自交换存在适度的动力学同位素效应，为1.6 ± 0.5，这与之前在完全高自旋的铁联咪唑啉配合物中观察到的相近。(12) 测得的活化参数之差E(a)(D) - E(a)(H)为 -1.2 ± 0.8 kcal mol⁻¹，这似乎与同位素效应的半经典观点不一致，并表明存在广泛的隧穿效应。[Co(H₂bim)₃]²⁻ - d₂₄与[Co(H₂bim)₃]³⁺或[Co(Hbim)(H₂bim)₂]²⁺的反应发生配体的交换，表明是内球过程。通过应用Marcus交叉关系，估计[Co(H₂bim)₃]²⁺与[Co(H₂bim)₃]³⁺之间外层电子转移的自交换速率常数为10⁻⁶ M⁻¹ s⁻¹。将交叉关系类似地应用于氢原子转移反应表明，[Co(H₂bim)₃]²⁺与[Co(Hbim)(H₂bim)₂]²⁺之间的自交换也很慢，≤10⁻³ M⁻¹ s⁻¹。钴配合物自交换速率慢显然是由于它们的高自旋[Co(II)(H₂bim)₃]²⁺与低自旋Co(III)衍生物之间的相互转化。
