Direct Detection of Key Reaction Intermediates in Photochemical CO₂Reduction Sensitized by a Rhenium Bipyridine Complex

Abstract

Photochemical CO2 reduction sensitized by rhenium–bipyridyl complexes has been studied through multiple approaches during the past several decades. However, a key reaction intermediate, the CO2-coordinated Re–bipyridyl complex, which should govern the activity of CO2 reduction in the photocatalytic cycle, has never been detected in a direct way. In this study on photoreduction of CO2 catalyzed by the 4,4′-dimethyl-2,2′-bipyridine (dmbpy) complex, [Re(dmbpy)(CO)3Cl] (1), we successfully detect the solvent-coordinated Re complex [Re(dmbpy)(CO)3DMF] (2) as the light-absorbing species to drive photoreduction of CO2. The key intermediate, the CO2-coordinated Re–bipyridyl complex, [Re(dmbpy)(CO)3(COOH)], is also successfully detected for the first time by means of cold-spray ionization spectrometry (CSI-MS). Mass spectra for a reaction mixture with isotopically labeled 13CO2 provide clear evidence for the incorporation of CO2 into the Re–bipyridyl complex. It is revealed that the starting chloride complex 1 was rapidly transformed into the DMF-coordinated Re complex 2 through the initial cycle of photoreduction of CO2. The observed induction period in the time profile of the CSI-MS signals can well explain the subsequent formation of the CO2-coordinated intermediate from the solvent-coordinated Re–bipyridyl complex. An FTIR study of the reaction mixture in dimethyl sulfoxide clearly shows the appearance of a signal at 1682 cm–1, which shifts to 1647 cm–1 for the 13CO2-labeled counterpart; this is assigned as the CO2-coordinated intermediate, ReII–COOH. Thus, a detailed understanding has now been obtained for the mechanism of the archetypical photochemical CO2 reduction sensitized by a Re–bipyridyl complex.

The concept of photochemical enzyme models – State of the art

“…Similar to the enzymatic substrate activation process described above, in many of the investigated systems the formation of an inner-sphere CO 2 adduct is part of the proposed reaction mechanisms [161][162][163][164][165][166][167]. Only a few photocatalysts are able to reach a certain degree of selectivity for the type of CO 2 reduction products.…”

One-dimension-based spatially ordered architectures for solar energy conversion

“…5,7,[187][188][189][190] One-dimension-based spatially ordered architectures as efficient photocatalysts in artificial photosynthesis could help to address the issue of rising levels of greenhouse gas emissions and provide an alternative source of renewable energy.…”

“…2,3 Combustion of fossil fuels causes environmental pollution and enormous CO 2 emission, with drastic implications for human life. 4,5 Thus, human society must look towards alternative renewable, carbon-neutral energy sources for storage and conversion of solar energy. 3 Semiconductor-based heterogeneous photocatalysis, which allows direct conversion of solar energy into chemical energy via a renewable route, could be the most viable long-term solution with potential to address these environmental and energy issues.…”

Photocatalytic reduction of CO2 with H2O to CH4 over ultrathin SnNb2O6 2D nanosheets under visible light irradiation

“…Meanwhile, some organic semiconductors (such as g-C 3 N 4 [16][17][18][19] and metal organic frameworks (MOFs) [20][21][22] ) and some non-semiconductors (such as zeolite molecular sieve 23,24 and layered double hydroxides 25 ) have also been developed. Furthermore, the dye-sensitization, [26][27][28] precious metal modification 29 and the co-catalyst loading 30,31 are used for the enhancement of the photocatalytic activity. However, sustainable utilization of solar energy by the reduction of CO 2 calls for the development of more abundant and stable catalysts working with visible light, and this has remained challenging so far.…”