Wu Adam, Mayer James M
Department of Chemistry, University of Washington, Campus Box 351700, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2008 Nov 5;130(44):14745-54. doi: 10.1021/ja805067h. Epub 2008 Oct 9.
The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO() (2,2,6,6-tetramethylpiperidine-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 (Delta G(o)). 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 DeltaE(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) x 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.
Ru(II)(acac)2(py-imH)（Ru(II)imH）与TEMPO()（2,2,6,6-四甲基哌啶-1-氧基自由基）在乙腈中反应，通过H()（H(+) + e(-)）的转移定量生成Ru(III)(acac)2(py-im)（Ru(III)im）和羟胺TEMPO-H（acac = 2,4-戊二酮，py-imH = 2-(2'-吡啶基)咪唑）。通过紫外可见停流技术对该反应进行动力学测量表明，在298 K时双分子速率常数k(3H) = 1400 ± 100 M(-1) s(-1)。该反应通过协同氢原子转移（HAT）机制进行，排除了由于初始质子或电子转移的热化学性质非常不利（ΔG(o)）而导致的逐步途径。从Ru(II)(acac)2(py-imD)（Ru(II)imD）到TEMPO()的氘转移出奇地慢，k(3D) = 60 ± 7 M(-1) s(-1)，在298 K时k(3H)/k(3D) = 23 ± 3。对该氘动力学同位素效应（KIE）进行的温度相关测量表明，表观活化能之间存在很大差异，E(a3D) - E(a3H) = 1.9 ± 0.8 kcal mol(-1)。较大的k(3H)/k(3D)和ΔE(a)值似乎大于半经典极限，因此表明存在隧道效应机制。通过(1)H NMR谱线展宽测量的Ru(II)imH和Ru(III)im之间的自交换HAT反应，在298 K时k(4H) = (3.2 ± 0.3) × 10(5) M(-1) s(-1)，k(4H)/k(4D) = 1.5 ± 0.2。尽管KIE较小，但根据阿仑尼乌斯预指数因子的比值log(A(4H)/A(4D)) = -0.5 ± 0.3，表明存在隧道效应。这些数据对马库斯交叉关系在涉及大量隧道效应的反应的一系列温度范围内对H和D转移的适用性进行了检验。交叉关系计算出的Ru(II)imH(D) + TEMPO()的速率常数大于观察到的速率常数：在298 K时k(3H,calc)/k(3H) = 31 ± 4和k(3D,calc)/k(3D) = 140 ± 20。在这些速率常数和活化参数中，与H转移相比，马库斯交叉关系与D转移的一致性更好，尽管H的隧道效应更为普遍。交叉关系没有明确包含隧道效应，因此不应期望有密切的一致性。鉴于这些结果，讨论了将交叉关系应用于HAT反应的优点和缺点。
