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)(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 (Delta(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.