Direct Observation by Rapid-Scan FT-IR Spectroscopy of Two-Electron-Reduced Intermediate of Tetraaza Catalyst [Co~ⅡN_4H(MeCN)]~(2+) Converting CO_2 to CO

摘要

In the search for the two-electron-reduced intermediate of the tetraaza catalyst [Co~ⅡN_4H(MeCN)]~(2+)(N_4H = 2,12-dimethyl-3,7,11,17-tetraazabicydo[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene) for CO_2 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 (CD_3CN) solution, led to the observation of two sequential intermediates. The initially formed one-electron-reduced [Co~ⅠN_4H]~+-CO_2 adduct was converted by the second electron to a transient [Co~ⅠN_4H]~+-CO_2~- complex that spontaneously converted CO_2 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~ⅠN_4H]~+-CO_2~- complex and the preceding one-electron [Co~ⅠN_4H]~+-CO_2 intermediate show close similarity but distinct differences in the carboxylate modes, indicating that the second electron resides mainly on the CO_2 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 CO_2 reduction during the bond-breaking step producing CO. The reaction pathway for the Co tetraaza catalyst uncovered here suggests that the competition between CO_2 reduction and proton reduction of a macrocyclic multi-electron catalyst is steered toward CO_2 activation if the second electron is directly captured by an adduct of CO_2 and the one-electron-reduced catalyst intermediate.