DFT insights into the photocatalytic reduction of CO2 to CO by Re(i) complexes: the crucial role of the triethanolamine “magic” sacrificial electron donor

The reaction mechanism for the photocatalytic reduction of CO2 to CO catalyzed by the [Re(en)(CO)3Cl] complex in the presence of triethanolamine, R3N (R = CH2CH2OH) abbreviated as TEOA, in DMF solution was studied in-depth with the aid of DFT computational protocols by calculating the geometric and free energy reaction profiles for several possible reaction pathways. The reaction pathways studied start with the "real" catalytic species [Re(en)(CO)3], [Re(en)(CO)3]- and/or [Re(en)(CO)2Cl]- generated from the excited triplet T1 state upon single and double reductive quenching by a TEOA sacrificial electron donor or photodissociation of a CO ligand. The first step in all the catalytic cycles investigated involves the capture of either CO2 or the oxidized R2NCH2CH2O˙ radical. In the latter case, the CO2 molecule is captured (inserted) by the Re-OCH2CH2NR2 bond forming stable intermediates. Next, successive protonations (TEOA also acts as a proton donor) lead to the release of CO either from the energy consuming 2e- reduction of [Re(en)(CO)4]+ or [Re(en)(CO)2Cl]+ complexes in the CO2 capture pathways or from the released unstable diprotonated [R2NCH2CH2OC(OH)(OH)]+ species regenerating TEOA and the catalyst. The CO2 insertion reaction pathway is the favorable pathway for the photocatalytic reduction of CO2 → CO catalyzed by the [Re(en)(CO)3Cl] complex in the presence of TEOA manifesting its crucial role as an electron and proton donor, capturing CO2 and releasing CO.