Cationic Cobalt(II) Bisphosphine Hydroformylation Catalysis: In Situ Spectroscopic and Reaction Studies

Area of Science:

• Organometallic Chemistry
• Catalysis
• Homogeneous Catalysis

Background:

• Hydroformylation is a key industrial process for converting alkenes to aldehydes.
• Cobalt-based catalysts are widely used but often struggle with selectivity for internal alkenes.
• Understanding the active catalytic species is crucial for catalyst design and optimization.

Purpose of the Study:

• To investigate the active species in the [HCo(CO)(bisphosphine)](BF4) hydroformylation catalyst system.
• To elucidate the mechanism of catalyst activation and deactivation.
• To determine the role of cationic cobalt complexes in hydroformylation.

Main Methods:

• In situ infrared (IR) spectroscopy
• Electron paramagnetic resonance (EPR) spectroscopy
• Nuclear magnetic resonance (NMR) spectroscopy
• Density functional theory (DFT) calculations

Main Results:

• The catalyst system, [HCo(CO)(bisphosphine)](BF4), demonstrates high activity for hydroformylation, especially of internal branched alkenes.
• IR spectroscopy revealed the formation of a dicationic, CO-bridged cobalt(I) dimer, [Co2(μ-CO)2(CO)(bisphosphine)2]2+, at lower temperatures via H2 elimination.
• EPR studies identified a high-spin (S = 3/2) cobalt(II) complex.
• DFT calculations supported a dimer structure with distinct cobalt coordination geometries.
• Reaction studies indicated that the cationic cobalt(II) bisphosphine complex is the primary catalytic species, downplaying the role of neutral cobalt(I) species.

Conclusions:

• The active hydroformylation catalyst is a cationic cobalt(II) bisphosphine species.
• A monomer-dimer equilibrium involving hydrogen elimination is proposed, analogous to known cobalt carbonyl systems.
• The findings provide critical insights into the mechanism of cobalt-catalyzed hydroformylation, favoring cationic species over neutral ones.