Proton-Coupled Electron Transfer between 4-Cyanophenol and Photoexcited Rhenium(I) Complexes with Different Protonatable Sites
Proton-Coupled Electron Transfer between 4-Cyanophenol and Photoexcited Rhenium(I) Complexes with Different Protonatable Sites
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DOI：
10.1021/ic300834c
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发表时间：
2012-08-06
影响因子：
4.6
通讯作者：
Wenger, Oliver S.
中科院分区：
文献类型：
--
作者：
Bronner, Catherine;Wenger, Oliver S.
合成了两个含2，2‘-联吡嗪(Bpz)和吡啶(Py)配体的三甲酰二亚胺配合物([Re(Bpz)-(CO)(3)(Py)](+))和一个含2，2’-联吡啶(Bpy)和1，4-吡嗪(Pz)配体的三羰基配合物[(Re(Bpy)(CO)(3)(Pz)(+)])，并研究了它们与4-氰基苯酚在乙腈溶液中的光化学行为。金属-配体电荷转移(MLCT)激发发生在[Re(Bpz)-(CO)(3)(Py)](+)络合物中朝向可质子化的bpz配体，而在[Re(Bpy)(CO)(3)(Pz)](+)络合物中，相同类型的激发促进电子远离质子化的pz配体。本研究旨在探讨这种电子激发态结构的差异如何影响4-氰基苯酚和两种Re(I)配合物之间的光致质子耦合电子转移(PCET)的速率和反应机理。瞬时吸收光谱为PCET反应产物提供了明确的证据，在一些发光猝灭实验中观察到了显著的H/D动力学同位素效应。在这两种情况下，协同的质子-电子转移可能起到重要作用，但对于4-氰基苯酚/[Re(Bpz)(CO)(3)(Py)](+)反应对，不能完全排除质子转移和电子转移步骤的反应序列。有趣的是，两种Re(I)配合物的双分子激发态猝灭的速率常数在相同的数量级。
Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2'-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls ([Re(bpz)-(CO)(3)(py)](+)), and one tricarbonyl complex with a 2,2'-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand [(Re(bpy)(CO)(3)(pz)(+)) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the [Re(bpz)-(CO)(3)(py)](+) complex while in the [Re(bpy)(CO)(3)(pz)](+) complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/[Re(bpz)(CO)(3)(py)](+) reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium(I) complexes.