Towards New Molecular Photocatalysts for CO₂ Reduction: Photo‐Induced Electron Transfer versus CO Dissociation within [Os(NN)(CO)₂Cl₂] Complexes

Abstract

Optical excitation in the visible region of trans-(Cl)-[Os(bpy)(CO)(2)Cl(2)] (bpy=2,2'-bipyridine; C1) and trans-(Cl)-[Os(dmbpy)(CO)(2)Cl(2)] (dmbpy=4,4'-dimethyl 2,2'-bipyridine; C2) is known to induce the common CO dissociation reaction. However, the quantum yield of the reactions is less than 0.15, although C1 and C2 display pronounced photoluminescence in the visible region at room temperature with a lifetime of few tens of nanoseconds. Taking into account the characteristics of their emitting state, we have investigated the capability of C1 and C2 to act as a photosensitiser in redox reactions in different solvents (MeCN, PrCN and DMF). The efficient oxidation and reduction of both complexes under continuous irradiation in the presence of a sacrificial electron acceptor or donor is reported here. The photo-induced transformations and the nature of the resulting compounds were analysed by UV/Vis and IR spectroscopies and cyclic voltammetry. Photo-induced oxidation of C1 and C2 leads to the corresponding monocarbonyl oxidised species, whereas photo-induced reduction under argon leads mainly to the formation of the corresponding Os-bonded molecular wires P1 and P2 after exchange of two electrons associated with the loss of two chloro ligands. The chemical yield of the latter reaction (around 65%) becomes quantitative by adding [Ru(bpy)(3)](2+) as an external redox photosensitiser. This behaviour has been used to photocatalyse the two electron, two proton conversion of CO(2) to CO. Turnover numbers (TON) of 11.5 and 19.5 have been obtained respectively for C1 and C2 after 4.5 h of irradiation under CO(2) in DMF with triethanolamine as the electron donor. TON can be slightly increased by adding [Ru(bpy)(3)](2+) to the solution.

Electro‐ and Photo‐driven Reduction of CO2 by a trans‐(Cl)‐[Os(diimine)(CO)2Cl2] Precursor Catalyst: Influence of the Diimine Substituent and Activation Mode on CO/HCOO− Selectivity

“…The presence of the monocarbonyl species at the end of irradiation induced a distortion of the maximum absorption bands in the visible region by comparison of the UV/Visible spectrum of P1 – P4 obtained after exhaustive electrolysis. Previous experiments have shown that the monocarbonyl related complexes of C1 and C2 are not reduced by TEOA under irradiation . The amount of the monocarbonyl species after irradiation remains difficult to determine owing to the large absorption of the polymers in the same range.…”

“…An alternative could be given by the parent osmium [Os(NN)(CO) 2 Cl 2 ] complexes, for which the heavier metal center stabilizes the complexes and offers more attractive reactivity upon optical excitation. These complexes exhibit a mixed metal/halide to diimine charge‐transfer excited state with lifetimes of a few tens of nanoseconds enabling an efficient competitive electron transfer of the CO ligand photosubstitution process. We have already demonstrated that trans ‐(Cl)‐[Os(bpy)(CO) 2 Cl 2 ] ( C1 ) and [Os(dmbpy)(CO) 2 Cl 2 ] ( C2 ) (dmbpy=4,4′‐dimethyl 2,2′‐bipyridine) can undergo net photoinduced oxidation or reduction in the presence of an irreversible electron acceptor or donor, respectively, under visible‐light irradiation .…”

“…These complexes exhibit a mixed metal/halide to diimine charge‐transfer excited state with lifetimes of a few tens of nanoseconds enabling an efficient competitive electron transfer of the CO ligand photosubstitution process. We have already demonstrated that trans ‐(Cl)‐[Os(bpy)(CO) 2 Cl 2 ] ( C1 ) and [Os(dmbpy)(CO) 2 Cl 2 ] ( C2 ) (dmbpy=4,4′‐dimethyl 2,2′‐bipyridine) can undergo net photoinduced oxidation or reduction in the presence of an irreversible electron acceptor or donor, respectively, under visible‐light irradiation . For instance, photoinduced reduction of C1 in argon‐saturated PrCN+1 m TEA (triethylamine) or DMF+1 m TEOA (triethanolamine) solution leads mainly to the formation of the corresponding Os 0 ‐bonded molecular wire after transfer of two electrons and the release of two Cl − into the solution.…”

Rhenium(i) trinuclear rings as highly efficient redox photosensitizers for photocatalytic CO2 reduction

“…, Ru( ii ), 23–26 Co( ii ), 27 Ni( ii ), 28,29 Re( i ), 30–33 Fe( ii ), 34,35 Os( ii ), 36 Ir( iii ) 37 and Mn( i ), 38 have been reported mostly with Ru( ii )–trisdiimine complexes, typically [Ru(bpy) 3 ] 2+ or [Ru(4dmb) 3 ] 2+ (4dmb = 4,4′-dimethyl-2,2′-bipyridine) as PSs. We selected three efficient and widely studied catalysts, i.e.…”

Comparison of CO2 Photoreduction Systems: A Review

“…9. Chauvin et al (2011) reported the use of type 2 osmium complexes for the photoreduction of CO 2 in visible light with the use of an amine sacrificial agent. It was reported that conversion rates were comparable to rhenium based systems thus making osmium complexes a possible alternative.…”