Spin surface crossing in chromium-mediated olefin epoxidation with O(2)

Area of Science:

• Organometallic Chemistry
• Catalysis
• Computational Chemistry

Background:

• The reactivity of organochromium complexes with dioxygen was investigated.
• CpCr(O)Cl(2) was synthesized and structurally characterized.
• Initial catalytic studies showed limited olefin epoxidation activity.

Purpose of the Study:

• To understand the mechanism of olefin epoxidation by CpCr(O)Cl(2).
• To investigate the role of spin state changes in the reaction pathway using DFT calculations.
• To computationally predict and experimentally validate improved epoxidation catalysts.

Main Methods:

• Synthesis and structural characterization of CpCr(O)Cl(2).
• Catalytic oxidation reactions with PPh(3) and 1,4-cyclohexadiene.
• Density Functional Theory (DFT) calculations to explore the reaction potential energy surface for ethylene epoxidation.
• Experimental testing of predicted catalytic improvements.

Main Results:

• CpCr(O)Cl(2) was synthesized and characterized.
• The complex showed catalytic activity for some oxidations but not olefin epoxidation.
• DFT calculations indicated that spin-blocking was not a major barrier and predicted lower activation energy for a related species.
• The predicted compound, CpCr(O)Cl(2) (3), was synthesized and demonstrated stoichiometric olefin epoxidation.

Conclusions:

• Computational studies are valuable for predicting and optimizing catalytic activity in organometallic systems.
• CpCr(O)Cl(2) (3) represents a more effective reagent for olefin epoxidation compared to its precursor.
• The findings provide insights into the mechanism of chromium-mediated oxidation reactions.