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

Ruthenium aqua complexes [(η⁶-C₆Me ₆)Ru^II(L)(OH₂)]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)(OH₂)]²⁺ {Cp* = η⁵-C₅Me₅, L = bpy (5) and 4,4′-OMe-bpy (6)} act as catalysts for hydrogenation of CO₂ into HCOOH at pH 3.0 in H₂O. The active hydride catalysts cannot be observed in the hydrogenation of CO₂ 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 CO₂ 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, [(η⁶-C₆Me₆)Ru^II(L)(H)] ⁺, to the reactions of [Cp*Ir^III(L)(H)]⁺ with CO₂, as indicated by the kinetic studies.