Photochemical and Electrochemical Reduction of Carbon Dioxide Catalyzed by a Polyoxometalate-Organic Photosensitizer Complex

A manganese substituted Anderson type polyoxometalate, [MnMo₆O₂₄]^9-, tethered with an anthracene photosensitizer was prepared and used as catalyst for CO₂ reduction. The polyoxometalate-photosensitizer hybrid complex, obtained by covalent attachment of the sensitizer to only one face of the planar polyoxometalate, was characterized by NMR, IR and mass spectroscopy. Cyclic voltammetry measurements show a catalytic response for the reduction of carbon dioxide, thereby suggesting catalysis at the manganese site on the open face of the polyoxometalate. Controlled potentiometric electrolysis showed the reduction of CO₂ to CO with a TOF of ~15 sec^-1. Further photochemical reactions showed that the polyoxometalate-anthracene hybrid complex was active for the reduction of CO₂ to yield formic acid and/or CO in varying amounts dependent on the reducing agent used. Control experiments showed that the attachment of the photosensitizer to [MnMo₆O₂₄]^9- is necessary for photocatalysis.