Versatile Rh- and Ir-Based Catalysts for CO₂ Hydrogenation, Formic Acid Dehydrogenation, and Transfer Hydrogenation...

Considering the interest in processes related to hydrogen storage such as CO₂ hydrogenation and formic acid (FA) decomposition, we have synthesized a set of Ir, Rh, or Ru complexes to be tested as versatile precatalysts in these reactions. In relation with the formation of H₂ from FA, the possible applicability of these complexes in the transfer hydrogenation (TH) of challenging substrates as quinoline derivatives using FA/formate as hydrogen donor has also been addressed. Bearing in mind the importance of secondary coordination sphere interactions, N,N' ligands containing NH₂ groups, coordinated or not to the metal center, were used. The general formula of the new complexes are [( p-cymene)RuCl(N,N')]X, X = Cl^-, BF₄^- and [Cp*MCl(N,N')]Cl, M = Rh, Ir, where the N,N' ligands are 8-aminoquinoline (HL1), 6-pyridyl-2,4-diamine-1,3,5-triazine (L2) and 5-amino-1,10-phenanthroline (L3). Some complexes are not active or catalyze only one process. However, the complexes [Cp*MCl(HL1)]Cl with M = Rh, Ir are versatile catalysts that are active in hydrogenation of quinolines, FA decomposition, and also in CO₂ hydrogenation with the iridium derivative being more active and robust. The CO₂ hydrogenation takes place in mild conditions using only 5 bar of pressure of each gas (CO₂ and H₂). The behavior of some precatalysts in D₂O and after the addition of 9 equiv of HCO₂Na (pseudocatalytic conditions) has been studied in detail and mechanisms for the FA decomposition and the hydrogenation of CO₂ have been proposed. For the Ru, Ir, or Rh complexes with ligand HL1, the amido species with the deprotonated ligand are observed. In the case of ruthenium, the formate complex is also detected. For the iridium derivative, both the amido intermediate and the hydrido species have been observed. This hydrido complex undergoes a process of umpolung D⁺↔ Ir-D. All in all, the results of this work reflect the active role of -NH₂ in the transfer of H⁺.