Ruthenium polyhydrides supported by rigid PCP pincer ligands: dynamic behaviour and reactions with CO2

Abstract

Two rigid β-elimination immune PC_carbeneP pincer ligands, differing in their electron donor properties by variation of the substitution pattern on the aromatic linker arms, were complexed to ruthenium to form the dichlorides L_RRuCl₂ (R = H or NMe₂). These compounds were converted to hydrido chlorides by treatment with dihydrogen (H₂) and a base. By converting to tert-butoxide derivatives in situ under an atmosphere of H₂, the poly hydride PC_alkylP complexes L_HRRu(H)₃ compounds were generated. In these complexes, H₂ has added across the RuC bond in the PC_carbeneP starting materials. The polyhydrides are dynamic in solution and extensive NMR studies helped to elucidate the speciation and fluxional processes operative in this dynamic system. The polyhydride complexes react rapidly with CO₂ to give the PC_carbeneP formato hydride complexes L_RRu(H)-κ²-O₂CH. For R = H, the 1,2-hydride shift from the anchoring alkyl of the PC_alkylP carbon to the metal is reversible, but for R = NMe₂ it is irreversible. The CO₂ incorporated into the formato ligand of these compounds exchanges with free CO₂via a bimolecular mechanism that is more rapid for R = NMe₂ than for R = H; plausible explanations for this observation are proffered. Experiments designed to evaluate the efficacy of the R = NMe₂ formato hydride complex as a catalyst precursor for CO₂ hydrogenation to formate salts reveal poor performance in comparison to state-of-the-art ruthenium-based catalysts. This is due primarily to the precipitation of a dimeric μ-κ²-κ¹-CO₃ carbonate complex that is not an active catalyst for the reaction.