Photochemical CO2-reduction catalyzed by mono- and dinuclear phenanthroline-extended tetramesityl porphyrin complexes

Abstract

We here present a comprehensive study on the light-induced catalytic CO₂ reduction employing a number of mono- and dinuclear complexes with a phenanthroline-extended tetramesityl porphyrin ligand (H₂-1). A stepwise synthesis of heterodinuclear complexes is possible because the phenanthroline moiety of the ligand H₂-1 can selectively coordinate a second metal center, e.g. Ru(tbbpy)₂²⁺ fragment, while any other metal can reside in the porphyrin cavity. We expanded our former studies to cobalt and iron compounds and synthesized the complexes Co-1, FeCl-1 and Co-1-Ru, FeCl-1-Ru. Thorough catalytic investigation on the light-driven CO₂ reduction of all M-1(-Ru) compounds (M = 2H, Cu, Pd, Co, FeCl) was performed in a DMF solution in the presence of triethylamine (TEA) as a sacrificial electron donor. A very surprising wavelength dependence of the catalytic performance was observed. Turnover numbers (TONs) of CO were quantified and showed that redox active metals (i.e.M = Co and FeCl) in the porphyrin cavity caused the highest catalytic activity. After 24 hours of illumination with light λ > 305 nm FeCl-1-Ru reached a TON_CO of 11.4 with our experimental setup without showing much decomposition. This value is twice as high as the TON_CO determined for CoTPP (5.8) under the same conditions, which represented the most active porphyrinic system so far for photocatalytic CO₂ reduction.