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.