Unexpected CNN-to-CC Ligand Rearrangement in Pincer–Ruthenium Precatalysts Leads to a Base-Free Catalyst for Ester Hydrogenation

We report the conversion of a series of CNN–pincer–ruthenium complexes Ru­(CNN)­HCl­(CO) to a CC-chelated form Ru­(CC)­(PR3)2H­(CO) on reaction with sodium tert-butoxide and monodentate phosphines. When the phosphine is triphenylphosphine, cis-phosphine complexes form at room temperature, which convert to the trans isomer at elevated temperatures. When the phosphine is tricyclohexylphosphine, only the trans-phosphine isomer is observed. The CC-chelated complexes are active catalysts for the hydrogenation of esters, without the need for added base. The ligand structure–activity relationship in the series of CC-chelated complexes mirrors that in the precursor CNN-Ru complexes, potentially indicating a common catalytic mechanism. Density functional theory calculations establish a plausible mechanism for the CNN-to-CC rearrangement and demonstrate that this rearrangement is potentially reversible under the conditions of ester hydrogenation catalysis.