Highly Efficient and Selective Photocatalytic CO₂ Reduction by Iron and Cobalt Quaterpyridine Complexes

The design of highly efficient and selective photocatalytic systems for CO₂ reduction that are based on nonexpensive materials is a great challenge for chemists. The photocatalytic reduction of CO₂ by [Co(qpy)(OH₂)₂]²⁺ (1) (qpy = 2,2′:6′,2″:6″,2'-quaterpyridine) and [Fe(qpy)(OH₂)₂]²⁺ (2) have been investigated. With Ru(bpy)₃ ²⁺ as the photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant in CH₃CN/triethanolamine solution under visible-light excitation (blue light-emitting diode), a turnover number (TON) for CO as high as 2660 with 98% selectivity can be achieved for the cobalt catalyst. In the case of the iron catalyst, the TON was >3000 with up to 95% selectivity. More significantly, when Ru(bpy)₃ ²⁺ was replaced by the organic dye sensitizer purpurin, TONs of 790 and 1365 were achieved in N,N-dimethylformamide for the cobalt and iron catalysts, respectively.