Ruthenium polyhydrides supported by rigid PCP pincer ligands: dynamic behaviour and reactions with CO 2 .

Two rigid β-elimination immune PC carbene P 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 LRRuCl2 (R = H or NMe 2 ). These compounds were converted to hydrido chlorides by treatment with dihydrogen (H 2 ) and a base. By converting to tert -butoxide derivatives in situ under an atmosphere of H 2 , the poly hydride PC alkyl P complexes LHRRu(H)3 compounds were generated. In these complexes, H 2 has added across the RuC bond in the PC carbene P 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 CO 2 to give the PC carbene P formato hydride complexes LRRu(H)-κ2-O2CH. For R = H, the 1,2-hydride shift from the anchoring alkyl of the PC alkyl P carbon to the metal is reversible, but for R = NMe 2 it is irreversible. The CO 2 incorporated into the formato ligand of these compounds exchanges with free CO 2 via a bimolecular mechanism that is more rapid for R = NMe 2 than for R = H; plausible explanations for this observation are proffered. Experiments designed to evaluate the efficacy of the R = NMe 2 formato hydride complex as a catalyst precursor for CO 2 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-κ 2 -κ 1 -CO 3 carbonate complex that is not an active catalyst for the reaction.