Modular Pincer-type Pyridylidene Amide Ruthenium(II) Complexes for Efficient Transfer Hydrogenation Catalysis

A set of bench-stable ruthenium complexes with new N,N,N-tridentate coordinating pincer-type pyridyl-bis(pyridylideneamide) ligands was synthesized in excellent yields, with the pyridylidene amide in meta or in para position ( m-PYA and p-PYA, respectively). While complex [Ru( p-PYA)(MeCN)₃]²⁺ is catalytically silent in transfer hydrogenation, its meta isomer [Ru( m-PYA)(MeCN)₃]²⁺ shows considerable activity with turnover frequencies at 50% conversion TOF₅₀ = 100 h^-1. Spectroscopic, electrochemical, and crystallographic analyses suggest considerably stronger donor properties of the zwitterionic m-PYA ligand compared to the partially π-acidic p-PYA analogue, imparted by valence isomerization. Further catalyst optimization was achieved by exchanging the ancillary MeCN ligands with imines (4-picoline), amines (ethylenediamine), and phosphines (PPh₃, dppm, dppe). The most active catalyst was comprised of the m-PYA pincer ligand and PPh₃, complex [Ru( m-PYA)(PPh₃)(MeCN)₂]²⁺, which reached a TOF₅₀ of 430 h^-1 under aerobic conditions and up to 4000 h^-1 in the absence of oxygen. The presence of oxygen reversibly deactivates the catalytically active species, which compromises activity, but not longevity of the catalyst. Ligand exchange kinetic studies by NMR spectroscopy indicate that the strong trans effect of the phosphine is critical for high catalyst activity. Diaryl, aryl-alkyl, and dialkyl ketones were hydrogenated with high conversion, and α,β-unsaturated ketones produced selectively the saturated ketone as the only product due to exclusive C═C bond hydrogenation, a distinctly different selectivity from most other transfer hydrogenation catalysts.