Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium-Substituted Polyoxometalate

A polyoxometalate of the Keggin structure substituted with Ru-III, (6)Q(5)[Ru-III(H2O)SiW11O39] in which (6)Q=(C6H13)(4)N+, catalyzed the photoreduction of CO2 to CO with tertiary amines, preferentially Et3N, as reducing agents. A study of the coordination of CO2 to (6)Q(5)[Ru-III(H2O)SiW11O39] showed that 1) upon addition of CO2 the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g(x)=2.146, g(y)=2.100, and g(z)=1.935), and 3) the C-13 NMR spectrum had a broadened peak of bound CO2 at 105.78 ppm (Delta(1/2)=122 Hz). It was concluded that CO2 coordinates to the Ru-III active site in both the presence and absence of Et3N to yield (6)Q(5)[Ru-III(CO2)SiW11O39]. Electrochemical measurements showed the reduction of Ru-III to Ru-II in (6)Q(5)[Ru-III-(CO2)SiW11O39] at -0.31 V versus SCE, but no such reduction was observed for (6)Q(5)[Ru-III(H2O)SiW11O39]. DFT-calculated geometries optimized at the M06/PC1//PBE/AUG-PC1//PBE/PC1-DF level of theory showed that CO2 is preferably coordinated in a side-on manner to Ru-III in the polyoxometalate through formation of a Ru-O bond, further stabilized by the interaction of the electrophilic carbon atom of CO2 to an oxygen atom of the polyoxometalate. The end-on CO2 bonding to Ru-III is energetically less favorable but CO2 is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp(2) hybridization state. Formation of a O2C-NMe3 zwitterion, in turn, causes bending of CO2 and enhances the carbon sp(2) hybridization. The synergetic effect of these two interactions stabilizes both Ru-O and C-N interactions and probably determines the promotional effect of an amine on the activation of CO2 by [Ru-III(H2O)SiW11O39](5-). Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO2 and Et3N. A mechanistic pathway for photoreduction of CO2 is suggested based on the experimental and computed results.