Photochemical reduction of carbon dioxide catalyzed by a ruthenium substituted polyoxometalate

Alexander M. Khenkin, Irena Efremenko, Lev Weiner, Jan M.L. Martin, Ronny Neumann

Research output: Contribution to journalArticle

109 Citations (Scopus)

Abstract

A polyoxometalate of the Keggin structure substituted with Ru III, 6Q5[RuIIIH2O) SiW11O39] in which 6Q = (C6H 13)4N+, catalyzed the photoreduction of CO 2 to CO with tertiary amines, preferentially Et3N, as reducing agents. A study of the coordination of CO2 to 6Q5[RuIIIH2O)SiW11O 39] 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, gy = 2.100, and gz = 1.935), and 3) the 13CNMR 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[Ru IIICO2)SiW11O39]. Electrochemical measurements showed the reduction of RuIII to RuIIin 6Q5[RuIII- (CO2)SiW 11O39] at -0.31 V versus SCE, but no such reduction was observed for 6Q5[RuIIIH2O)SiW 11O39]. DFT-calculated geometries optimized at the M06/PC1//PBE/AUG-PC1//PBE/ PCl-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-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 CO 2 by [RuIII- (H2O)SiW11O 39]5-. Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO2 and Et 3N. A mechanistic pathway for photoreduction of CO2 is suggested based on the experimental and computed results.

Original languageEnglish
Pages (from-to)1356-1364
Number of pages9
JournalChemistry - A European Journal
Volume16
Issue number4
DOIs
Publication statusPublished - Feb 18 2010

Keywords

  • Carbon dioxide
  • Photochemistry
  • Polyoxometalates
  • Reduction
  • Ruthenium

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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