Ru3(CO)12 in Acidic Media. Intermediates of the Acid-Cocatalyzed Water-Gas Shift Reaction (WGSR)

Article abstract of DOI:10.1021/ic960630v

The elucidation of the WGSR promoted by ruthenium carbonyls in acidic media started with the detection of the Ru(0), Ru(I), and Ru(II) intermediate complexes, namely Ru₃(CO)₁₂, Ru₂[μ-η²-OC(CF₃)O]₂(CO) ₆, and fac-[Ru(CF₃-COO)₃(CO)₃]^-, which accumulate when CF₃COOH is employed as an acid cocatalyst. Under catalytic conditions, the three were found to interconvert through elementary steps which produce CO₂ and H₂. In fact, Ru(0) is oxidized by H⁺ to Ru(I) and half the hydrogen of the catalytic cycle is supplied by this reaction. On the other hand, Ru(I) disproportionates to Ru(0) and Ru(II), and this latter species undergoes nucleophilic attack by H₂O. The decomposition of the metallacarboxylic acid intermediate gives back Ru(I), while H₂ and CO₂ are produced in a 1/2 molar ratio. The two alternating pathways for dihydrogen formation, namely Ru(0) oxidation by H⁺ and the decomposition of a metallacarboxylic acid intermediate, involve H₂ reductive elimination from the same RuHCF₃COO(CO)₂L₂ intermediate (L = H₂O, ethers). These findings define an acid-cocatalyzed WGSR whose distinctive features are (i) the intervention of a disproportionation reaction to generate a Ru(II) electron poor complex, whose CO ligands can undergo nucleophilic attack by water, (ii) the generation of the hydrido intermediate for dihydrogen production through two distinct reaction patways, and (iii) the reductive elimination of H₂ from the hydrido intermediate without involving H⁺ from the medium.