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

Abstract

Abstract A polyoxometalate of the Keggin structure substituted with Ru III , 6 Q 5 [Ru III (H 2 O)SiW 11 O 39 ] in which 6 Q=(C 6 H 13 ) 4 N + , catalyzed the photoreduction of CO 2 to CO with tertiary amines, preferentially Et 3 N, as reducing agents. A study of the coordination of CO 2 to 6 Q 5 [Ru III (H 2 O)SiW 11 O 39 ] showed that 1) upon addition of CO 2 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 13 C NMR spectrum had a broadened peak of bound CO 2 at 105.78 ppm (Δ 1/2 =122 Hz). It was concluded that CO 2 coordinates to the Ru III active site in both the presence and absence of Et 3 N to yield 6 Q 5 [Ru III (CO 2 )SiW 11 O 39 ]. Electrochemical measurements showed the reduction of Ru III to Ru II in 6 Q 5 [Ru III (CO 2 )SiW 11 O 39 ] at −0.31 V versus SCE, but no such reduction was observed for 6 Q 5 [Ru III (H 2 O)SiW 11 O 39 ]. DFT‐calculated geometries optimized at the M06/PC1//PBE/AUG‐PC1//PBE/PC1‐DF level of theory showed that CO 2 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 CO 2 to an oxygen atom of the polyoxometalate. The end‐on CO 2 bonding to Ru III is energetically less favorable but CO 2 is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp 2 hybridization state. Formation of a O 2 C–NMe 3 zwitterion, in turn, causes bending of CO 2 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 CO 2 by [Ru III (H 2 O)SiW 11 O 39 ] 5− . Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO 2 and Et 3 N. A mechanistic pathway for photoreduction of CO 2 is suggested based on the experimental and computed results.