Direct Observation by Rapid-Scan FT-IR Spectroscopy of Two-Electron-Reduced Intermediate of Tetraaza Catalyst [CoIIN4H(MeCN)]2+ Converting CO2 to CO

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• 作者：Hua Sheng ; Heinz Frei
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：August 10, 2016
• 年：2016
• 卷：138
• 期：31
• 页码：9959-9967
• 全文大小：623K
• 年卷期：0
• ISSN：1520-5126

文摘

In the search for the two-electron-reduced intermediate of the tetraaza catalyst [Co^IIN₄H(MeCN)]²⁺ (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^IN₄H]⁺–CO₂ adduct was converted by the second electron to a transient [Co^IN₄H]⁺–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^IN₄H]⁺–CO₂^– complex and the preceding one-electron [Co^IN₄H]⁺–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 ¹³C 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.