Catalytic N-H Bond Formation Promoted by a Ruthenium Hydride Complex Bearing a Redox-Active Pyrimidine-Imine Ligand

Area of Science:

• Organometallic Chemistry
• Catalysis
• Hydrogenation Reactions

Background:

• Deleterious C(sp2)-H reductive coupling and elimination deactivate related rhodium and iridium catalysts.
• A neutral, redox-active supporting ligand (PmIm) was designed to prevent catalyst deactivation pathways.

Purpose of the Study:

• To synthesize a piano-stool ruthenium hydride for catalytic dihydrogen activation and hydrogen atom transfer.
• To enable the formation of weak chemical bonds using H2 as a stoichiometric reductant.
• To investigate the catalytic activity and stability of the novel ruthenium complex.

Main Methods:

• Synthesis of a diruthenium complex [(η5-C5Me5)Ru(PmIm)]2 via reaction of [(η5-C5Me5)RuCl2]2 with PmIm, zinc, and sodium methoxide.
• Formation of the target ruthenium hydride [(η5-C5Me5)Ru(PmIm)H] through H2 addition to the dimer under thermal or photolytic conditions.
• Characterization of the ruthenium hydride, its metalloradical intermediate, and determination of Ru-H bond dissociation free energy (BDFE) via DFT calculations and equilibration studies.

Main Results:

• The ruthenium hydride [(η5-C5Me5)Ru(PmIm)H] was successfully synthesized and characterized, exhibiting a weak Ru-H BDFE (47.9 kcal/mol DFT, 50.8 kcal/mol experimental).
• The complex acts as an effective catalyst for the hydrogenation of TEMPO, acridine, and a cobalt-imido complex.
• Selective reduction of azobenzene to diphenylhydrazine was achieved, demonstrating the utility in weak bond formation.

Conclusions:

• The novel piano-stool ruthenium hydride is a stable and effective catalyst for hydrogenation and selective reductions.
• The PmIm ligand successfully prevents catalyst deactivation pathways observed in related systems.
• This work highlights the potential of ruthenium complexes in catalytic weak bond formation using H2.