Selective and Efficient Light-Driven CO2 Reduction to CO with a Heptacoordinated Polypyridine Iron(II) Catalyst

The selective generation of carbon-based products in the presence of proton donors currently represents one of the major goals in the catalysis of the CO2 reduction reaction (CO2RR). Within this framework, the iron complex of the 1-([2,2′-bipyridin]-6-yl)-N-([2,2′-bipyridin]-6-ylmethyl)-N-(pyridin-2-ylmethyl) methanamine ligand (DBPy-PyA) turns out to be a selective and efficient catalyst to promote the conversion of CO2 into CO. In the present work, we report a detailed experimental and computational investigation of the CO2RR by this metal complex. Efficient formation of CO (selectivity >90%) was attained under electrochemical conditions (applied potential of −2.0 V vs Fc+/Fc) using trifluoroethanol as the proton donor, which provides the best balance, among those tested, in terms of Lewis and Brønsted acidity. This is indeed instrumental in accelerating CO2 activation while minimizing the parallel generation of hydrogen byproduct. The high activity and selectivity toward CO formation were shown to arise from (i) the ability of the ligand to assist via intramolecular routes the formation of the metallacarboxylic acid catalytic intermediate, (ii) the favorable and almost barrierless detachment of the CO product from the putative iron(II) carbonyl intermediate, and (iii) the weak tendency of the two-electron-reduced complex to form the metal-hydride species. The CO2RR by the titled complex was further investigated under light-driven catalytic conditions with [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) as the sensitizer and N,N-diisopropylethylamine (DIPEA) as the electron donor, leading to unprecedented performances under 1 sun irradiation (0.85 mL CO per mL of solution, quantum yield of 9.4%, selectivity >97%, solely limited by degradation of the sensitizer). Transient absorption spectroscopy suggested that, for the three-component photochemical system examined, catalyst activation by the photogenerated reductant represents the rate-determining step of the photosynthetic process. With this information in hand, by carefully modulating the photon flux, we succeeded in achieving a more than 3-fold enhancement in the quantum yield of CO formation (up to 28%). All in all, our study showcases the great, but often underestimated, potential of molecular catalysis to target efficient and selective transformations.