Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Area of Science:

• Homogeneous catalysis
• Organometallic chemistry
• Reaction kinetics

Background:

• Enhancing catalytic activity via synergistic effects is crucial in homogeneous catalysis.
• Established cooperation mechanisms include metal-metal and metal-ligand interactions.
• Novel activation pathways beyond these are actively sought.

Purpose of the Study:

• To demonstrate substrate self-activation controlling catalytic activity.
• To investigate the regioselective cyclotrimerization of aryl acetylenes.
• To elucidate the mechanism of a two-coordinate iron complex.

Main Methods:

• Kinetic studies of aryl acetylene cyclotrimerization using a two-coordinate iron complex ([Fe(2,6-Xyl2C6H3)2]).
• Analysis of reaction kinetics under varying substrate-to-catalyst ratios (low and high regimes).
• Determination of kinetic parameters through sigmoidal response, Hill indices, Lineweaver-Burk plots, and Eyring analysis.

Main Results:

• Two distinct kinetic regimes (low and high substrate-to-catalyst ratios) were observed, both exhibiting sigmoidal kinetics and positive substrate cooperativity.
• Binuclear iron complexes are the active species, with the complex in the high regime being 6 times faster.
• Eyring analysis revealed two different catalytic cycles with distinct transition states and thermodynamic profiles.

Conclusions:

• The study demonstrates substrate self-activation by aryl acetylenes in the cyclotrimerization catalyzed by a two-coordinate iron complex.
• The reaction proceeds via two distinct catalytic cycles involving binuclear iron species, controlled by substrate concentration.
• This work expands the understanding of cooperative effects in homogeneous catalysis.