Chiral-at-Tungsten Dioxo Complexes─A Computational Study on Inhibiting Racemization.

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Catalysis

Background:

• Chiral tungsten complexes are of interest for asymmetric catalysis.
• Understanding racemization pathways is crucial for designing stable chiral catalysts.

Purpose of the Study:

• To investigate the racemization pathways of chiral cis-WO2(acac)2 and cis-WO2(nacnac)2 complexes.
• To assess the impact of steric hindrance on racemization barriers.
• To identify preferred racemization mechanisms for potential catalytic applications.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• The ωB97X-D/6-311+G(2d,f) level of theory with LANL2DZ for tungsten was used.
• Acetonitrile solvent effects were included in the calculations.

Main Results:

• Four racemization pathways were identified for the studied tungsten complexes.
• Steric congestion from N-Me and N-Ph substitutions significantly increased racemization barriers.
• The Dhimba-Muller-Lammertsma (DML) twist mechanism was found to be preferred over Conte-Hippler (CH), Bailar (B), and Ray-Dutt (RD) twists.
• A high ΔG barrier of 25.7 kcal/mol was calculated for the favored DML pathway in WO2(nacnac)2Ph4.

Conclusions:

• The DML twist is the dominant racemization pathway for these chiral tungsten complexes.
• Substituted cis-WO2(nacnac)2 complexes, particularly WO2(nacnac)2Ph4, exhibit high racemization barriers.
• These findings suggest that chiral tungsten complexes with appropriate steric bulk are viable candidates for asymmetric catalysis.