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

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 photoinduced 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 photo-sensitiser. This behaviour has been used to photocatalyse the two electron, two proton conversion of CO2 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 CO2 in DMF with triethanolamine as the electron donor. TON can be slightly increased by adding [Ru-(bpy)(3)](2+) to the solution.