Rapid-scan FT-IR spectroscopy was employed to directly observe the one-electron and two-electron reduced intermediates during converting CO2 to CO by visible-light-sensitized catalysts, [CoIIN4H(MeCN)]2+ (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene) or Cu nanoparticles. In [CoIIN4H(MeCN)]2+ system, the CO2 reduction was conducted by using Ir(ppy)3 as photosensizer in wet acetonitrile (CD3CN), led to the observation of two sequential intermediates. The initially formed one-electron-reduced [CoIN4H]+−CO2 adduct was converted by the second electron to a transient [CoIN4H]+−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 [CoIN4H]+−CO2− complex and the preceding one-electron [CoIN4H]+−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 multielectron 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. In Cu nanoparticle system, the CO2 reduction was photosensitized by CdTe or CdSe quantum-dots in aqueous solution. Different with [CoIIN4H(MeCN)]2+, adduct of CO2-CO2- on Cu0, with partial electrons on Cu0 transferred to CO2-CO2-, was formed in the dark as one-electron-reduced intermediate. During photosensitization, the second electron injection split the adduct to CO and carbonate.

Mechanism of CO2 reduction monitored by rapid-scan FT-IR spectroscopy on two different catalysts: Cobalt(II) tetraaza macrocyclic complexes and copper nanoparticles

253rd National Meeting (2017)

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