Molecular Biophysics and Integrated Bioimaging Division, Lawrence Berkeley National Laboratory, University of California , Berkeley, California 94720, United States.
J Am Chem Soc. 2016 Aug 10;138(31):9959-67. doi: 10.1021/jacs.6b05248. Epub 2016 Jul 28.
In the search for the two-electron-reduced intermediate of the tetraaza catalyst Co(II)N4H(MeCN) (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene) for CO2 reduction and elementary steps that result in the formation of CO product, rapid-scan FT-IR spectroscopy of the visible-light-sensitized catalysis, using Ir(ppy)3 in wet acetonitrile (CD3CN) solution, led to the observation of two sequential intermediates. The initially formed one-electron-reduced Co(I)N4H-CO2 adduct was converted by the second electron to a transient Co(I)N4H-CO2(-) complex that spontaneously converted CO2 to CO in a rate-limiting step on the second time scale in the dark under regeneration of the catalyst (room temperature). The macrocycle IR spectra of the Co(I)N4H-CO2(-) complex and the preceding one-electron Co(I)N4H-CO2 intermediate show close similarity but distinct differences in the carboxylate modes, indicating that the second electron resides mainly on the CO2 ligand. Vibrational assignments are corroborated by (13)C isotopic labeling. The structure and stability of the two-electron-reduced intermediate derived from the time-resolved IR study are in good agreement with recent predictions by DFT electronic structure calculations. This is the first observation of an intermediate of a molecular catalyst for CO2 reduction during the bond-breaking step producing CO. The reaction pathway for the Co tetraaza catalyst uncovered here suggests that the competition between CO2 reduction and proton reduction of a macrocyclic multi-electron catalyst is steered toward CO2 activation if the second electron is directly captured by an adduct of CO2 and the one-electron-reduced catalyst intermediate.
在寻找四氮杂轮烷催化剂Co(II)N4H(MeCN)（N4H = 2,12-二甲基-3,7,11,17-四氮杂环十八烷-1(17),2,11,13,15-五烯）的两电子还原中间体和导致 CO 产物形成的基元步骤时，使用 Ir(ppy)3 在湿乙腈（CD3CN）溶液中对可见光敏化催化的快速扫描傅里叶变换红外光谱研究导致观察到两个连续的中间体。最初形成的单电子还原Co(I)N4H-CO2加合物通过第二个电子转化为瞬态Co(I)N4H-CO2(-)配合物，该配合物在黑暗中在第二个时间尺度上以限速步骤自发地将 CO2转化为 CO，同时在催化剂再生下（室温）。Co(I)N4H-CO2(-)配合物和前一个单电子Co(I)N4H-CO2中间物的大环 IR 光谱在羧酸盐模式上显示出密切的相似性，但在特征上存在明显差异，表明第二个电子主要位于 CO2配体上。振动分配得到（13）C 同位素标记的证实。时间分辨红外研究得出的两电子还原中间体的结构和稳定性与最近的 DFT 电子结构计算预测结果非常吻合。这是在产生 CO 的键断裂步骤中观察到分子催化剂 CO2还原中间物的首次报道。这里揭示的 Co 四氮杂轮烷催化剂的反应途径表明，如果第二个电子直接被 CO2 和单电子还原催化剂中间物的加合物捕获，则大环多电子催化剂的 CO2 还原和质子还原之间的竞争将转向 CO2 活化。
