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Keywords:

  • carbon dioxide;
  • photochemistry;
  • polyoxometalates;
  • reduction;
  • ruthenium

Abstract

A polyoxometalate of the Keggin structure substituted with RuIII, 6Q5[RuIII(H2O)SiW11O39] in which 6Q=(C6H13)4N+, catalyzed the photoreduction of CO2 to CO with tertiary amines, preferentially Et3N, as reducing agents. A study of the coordination of CO2 to 6Q5[RuIII(H2O)SiW11O39] showed that 1) upon addition of CO2 the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (gx=2.146, gy=2.100, and gz=1.935), and 3) the 13C NMR spectrum had a broadened peak of bound CO2 at 105.78 ppm (Δ1/2=122 Hz). It was concluded that CO2 coordinates to the RuIII active site in both the presence and absence of Et3N to yield 6Q5[RuIII(CO2)SiW11O39]. Electrochemical measurements showed the reduction of RuIII to RuII in 6Q5[RuIII(CO2)SiW11O39] at −0.31 V versus SCE, but no such reduction was observed for 6Q5[RuIII(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 RuIII in the polyoxometalate through formation of a Ru[BOND]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 RuIII is energetically less favorable but CO2 is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp2 hybridization state. Formation of a O2C–NMe3 zwitterion, in turn, causes bending of CO2 and enhances the carbon sp2 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 [RuIII(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.