Ruthenium(II) Complexes of a Xanthene-Spanned Dicarbene Ligand

Area of Science:

• Organometallic Chemistry
• Catalysis
• Electrochemistry

Background:

• Development of novel ancillary ligands is crucial for advancing homogeneous catalysis.
• Ruthenium complexes are widely explored for catalytic applications due to their versatile reactivity.
• Xanthene-based N-heterocyclic carbene (NHC) ligands offer unique structural and electronic properties.

Purpose of the Study:

• To synthesize and characterize novel octahedral ruthenium(II) complexes featuring a xanthene-di(N-heterocyclic carbene) ancillary ligand (XdC).
• To investigate the catalytic activity of these complexes in transfer hydrogenation of ketones and electrochemical reduction of carbon dioxide.
• To elucidate the mechanism of CO2 reduction using electrochemical and computational methods.

Main Methods:

• Synthesis and structural characterization of ruthenium(II)-XdC complexes.
• Catalytic testing for transfer hydrogenation of ketones.
• Electrochemical studies (including IR-SEC) and Density Functional Theory (DFT) calculations for CO2 reduction.
• Reaction of complex 1 with KBEt3H to form hydride dimers.

Main Results:

• Successfully prepared and characterized octahedral ruthenium(II) complexes with XdC ligands.
• Complexes demonstrated catalytic activity in transfer hydrogenation of ketones.
• Selective electrochemical reduction of CO2 to CO was achieved with a ruthenium complex at a low overpotential (0.40 V).
• DFT calculations indicated that the Ru-O(xanth) bond cleavage facilitates CO2 binding, with selectivity over protons.
• Hydride dimers were formed from the reaction of complex 1, showing stability against ligand reductive elimination.

Conclusions:

• Ruthenium(II) complexes bearing flexible and hemilabile xanthene-(NHC)2 ancillary ligands show significant promise in catalysis.
• The electronic properties of the XdC ligand, particularly the diffuse HOMO, contribute to the selectivity in CO2 reduction.
• The findings suggest potential for similar complexes in various catalytic transformations.