Mechanistic investigation of CO2 hydrogenation by Ru(ii) and Ir(iii) aqua complexes under acidic conditions: two catalytic systems differing in the nature of the rate determining step

Abstract

Ruthenium aqua complexes [(eta(6)-C(6)Me(6))Ru(II)(L)(OH(2))](2+) {L = bpy (1) and 4,4'-OMe-bpy (2), bpy = 2,2'-bipyridine, 4,4'-OMe-bpy = 4,4'-dimethoxy-2,2'-bipyridine} and iridium aqua complexes [Cp*Ir(III)(L)(OH(2))](2+) {Cp* = eta(5)-C(5)Me(5), L = bpy (5) and 4,4'-OMe-bpy (6)} act as catalysts for hydrogenation of CO(2) into HCOOH at pH 3.0 in H(2)O. The active hydride catalysts cannot be observed in the hydrogenation of CO(2) with the ruthenium complexes, whereas the active hydride catalysts, [Cp*Ir(III)(L)(H)](+) {L = bpy (7) and 4,4'-OMe-bpy (8)}, have successfully been isolated after the hydrogenation of CO(2) with the iridium complexes. The key to the success of the isolation of the active hydride catalysts is the change in the rate-determining step in the catalytic hydrogenation of CO(2) from the formation of the active hydride catalysts, [(eta(6)-C(6)Me(6))Ru(II)(L)(H)](+), to the reactions of [Cp*Ir(III)(L)(H)](+) with CO(2), as indicated by the kinetic studies.

Arene ruthenium oxinato complexes: Synthesis, molecular structure and catalytic activity for the hydrogenation of carbon dioxide in aqueous solution

“…7). In accordance with observations by Ogo and Fukuzumi for (g 6 -arene)Ru(bipyridine) complexes [35,36], we believe the catalytic cycle for carbon dioxide hydrogenation by (g 6 -arene)Ru(oxinato) complexes to involve a hydrido complex formed in situ from the corresponding aqua or chloro complexes (Scheme 2). The hydrido complex may insert CO 2 to give the corresponding formyl complex, which will then react with hydroxide to give the formate anion and a hydroxo complex that can be converted with molecular hydrogen into the catalytically active hydrido complex, see Scheme 2.…”

“…Based on the studies of Himeda [34], Ogo and Fukuzumi [35][36][37] on the use of the water-soluble arene ruthenium complexes containing bipyridine (bipy) as ligand for the hydrogenation of carbon dioxide to give formic acid in aqueous solution, we checked the catalytic potential of the g 2 -oxinato complexes 1, 4, 9, 10 and 12 [BF 4 ] for this reaction. They do indeed catalyze this reaction in alkaline aqueous solution to give formate, the highest activity being observed for 12[BF 4 ] ( Table 6).…”

“…Himeda carefully studied the influence of the pH on this reaction and designed rhodium, iridium and ruthenium complexes containing chelating N,N-ligands that can be deprotonated and that are highly active in basic solution [34]. The aqueous hydrogenation of carbon dioxide under acidic conditions, catalyzed by water-soluble arene ruthenium or pentamethylcyclopentadienyl iridium complexes, was pioneered by Ogo and Fukuzumi [35][36][37].…”

H-atom bombardment of CO2, HCOOH, and CH3CHO containing ices

“…Indeed, in the chemical physics literature CO 2 is found to hydrogenate to HCOOH on ruthenium and iridium catalysts through formate complexes with the catalyst (Ogo et al 2006). The chemically bonded formate species subsequently reacts with H 3 O + to form HCOOH.…”

Recent Advances in Transition Metal-Catalysed Homogeneous Hydrogenation of Carbon Dioxide in Aqueous Media

“…In 2006, Ogo et al determined that the different rate-determining step for bpybased Ru and Ir complexes by the observation of the saturation behaviour of the TON with increasing PH2 and PCO2 respectively. (Ogo et al, 2006) The rate-determining step of [( 6 -C6Me6)Ru(bpy)(OH2)](SO4) and [( 6 -C6Me6)Ru(4,4'-OMe-bpy)(OH2)](SO4) was suggested to be the reaction of aqua complexes with H2. In contrast, the Ir analogous was supposed to be the CO2 insertion into the iridium hydride complexes which were isolated and characterized by NMR, ESI-MS, and IR.…”