Photocatalytic conversion of CO2 to CO by Ru(II) and Os(II) octahedral complexes: a DFT/TDDFT study.

The reaction mechanisms of the photocatalytic reduction of CO2 to CO catalyzed by [(en)M(CO)3Cl] complexes (M = Ru, Os, en = ethylenediamine) in the presence of triethanolamine (TEOA), R3N (R = -CH2CH2OH), in DCM and DMF solvents, were studied by means of DFT/TDDFT electronic structure calculations. The geometric and free energy reaction profiles for two possible reaction pathways were calculated. Both reaction pathways studied, start with the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ generated from the first triplet excited state, T1 upon reductive quenching by TEOA which acts as a sacrificial electron donor. In the first possible pathway, TEOA- anion binds to the metal center of the catalytically active intermediate, [(en)M(CO)3]˙+ followed by CO2 insertion into the M-OCH2CH2NR2 bond. The latter upon successive protonations releases a metal 'free' [R2NCH2CH2OC(O)(OH)] intermediate which starts a new and final catalytic cycle, leading to the formation of CO and H2O while regenarating TEOA. In the second possible pathway, the 17e-, catalytically active intermediate, [(en)M(CO)3]˙+ captures CO2 molecule, forming an η1-CO2 complex. Upon 2H+/2e- successive protonations and reductions, CO product is obtained along with regenarating the catalytically active intermediate [(en)M(CO)3]˙+. The nature of the proton donor affects the reaction profiles of both mechanisms. The nature of the solvent does not affect significantly the reaction mechanisms under study. Finally, since photoexcitation and T1 reductive quenching are common to both pathways, we have srutinized the photophysical properties of the [(en)M(CO)3Cl] complexes along with their T1 excited states reduction potentials, . The [(en)M(CO)3Cl] complexes absorb mainly in the UV region while the absolute are in the range 6.4-0.9 eV.

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.