Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University , 1-1 Minami-osawa, Hachioji, Tokyo 192-0397, Japan.
J Am Chem Soc. 2014 Apr 23;136(16):6021-30. doi: 10.1021/ja500403e. Epub 2014 Apr 10.
Photochemical CO2 reduction sensitized by rhenium-bipyridyl complexes has been studied through multiple approaches during the past several decades. However, a key reaction intermediate, the CO2-coordinated Re-bipyridyl complex, which should govern the activity of CO2 reduction in the photocatalytic cycle, has never been detected in a direct way. In this study on photoreduction of CO2 catalyzed by the 4,4'-dimethyl-2,2'-bipyridine (dmbpy) complex, [Re(dmbpy)(CO)3Cl] (1), we successfully detect the solvent-coordinated Re complex [Re(dmbpy)(CO)3DMF] (2) as the light-absorbing species to drive photoreduction of CO2. The key intermediate, the CO2-coordinated Re-bipyridyl complex, [Re(dmbpy)(CO)3(COOH)], is also successfully detected for the first time by means of cold-spray ionization spectrometry (CSI-MS). Mass spectra for a reaction mixture with isotopically labeled (13)CO2 provide clear evidence for the incorporation of CO2 into the Re-bipyridyl complex. It is revealed that the starting chloride complex 1 was rapidly transformed into the DMF-coordinated Re complex 2 through the initial cycle of photoreduction of CO2. The observed induction period in the time profile of the CSI-MS signals can well explain the subsequent formation of the CO2-coordinated intermediate from the solvent-coordinated Re-bipyridyl complex. An FTIR study of the reaction mixture in dimethyl sulfoxide clearly shows the appearance of a signal at 1682 cm(-1), which shifts to 1647 cm(-1) for the (13)CO2-labeled counterpart; this is assigned as the CO2-coordinated intermediate, Re(II)-COOH. Thus, a detailed understanding has now been obtained for the mechanism of the archetypical photochemical CO2 reduction sensitized by a Re-bipyridyl complex.
过去几十年，人们通过多种方法研究了偕二吡啶铼配合物敏化的光化学 CO2 还原。然而，在光催化循环中控制 CO2 还原活性的关键反应中间体——CO2 配位的 Re-联吡啶配合物，从未被直接检测到。在本研究中，我们使用 4,4'-二甲氧基-2,2'-联吡啶（dmbpy）配合物[Re(dmbpy)(CO)3Cl]（1）作为催化剂，对 CO2 的光还原进行了研究。我们成功地检测到溶剂配位的 Re 配合物[Re(dmbpy)(CO)3DMF]（2）作为吸光物种，从而驱动 CO2 的光还原。我们还首次通过冷喷雾电离质谱（CSI-MS）成功检测到关键中间体 CO2 配位的 Re-联吡啶配合物[Re(dmbpy)(CO)3(COOH)]。对于含有同位素标记（13）CO2 的反应混合物的质谱，为 CO2 掺入 Re-联吡啶配合物提供了明确的证据。结果表明，起始的氯配合物 1 通过 CO2 的初始光还原循环迅速转化为 DMF 配位的 Re 配合物 2。CSI-MS 信号时间曲线中观察到的诱导期很好地解释了溶剂配位的 Re-联吡啶配合物中 CO2 配位中间产物的后续形成。二甲基亚砜中反应混合物的 FTIR 研究清楚地表明，在 1682cm-1 处出现了一个信号，对于（13）CO2 标记的对应物，该信号移至 1647cm-1；这被分配为 CO2 配位的中间产物，Re(II)-COOH。因此，我们现在对偕二吡啶铼敏化的典型光化学 CO2 还原的反应机制有了更详细的了解。
