Vanadium thiolate complexes for efficient and selective sulfoxidation catalysis: a mechanistic investigation

Area of Science:

• Coordination Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Vanadium complexes are explored for catalytic applications.
• Sulfoxidation reactions are crucial in organic synthesis.
• Understanding structure-activity relationships in vanadium catalysts is key.

Purpose of the Study:

• To investigate the structural, electronic, and catalytic properties of two novel vanadium complexes.
• To evaluate their efficacy in the catalytic oxidation of sulfides to sulfoxides.
• To rationalize differences in reactivity and stability between the complexes.

Main Methods:

• Synthesis and characterization of two vanadium complexes: a V(V) dioxido complex (1) and a V(IV) oxido complex (2).
• X-ray crystallography for structural determination of complex 1.
• Density Functional Theory (DFT) calculations for structure optimization and electronic property analysis (HOMO, SOMO).
• Electron Paramagnetic Resonance (EPR) spectroscopy for characterization of complex 2.
• Catalytic oxidation reactions using thioanisole as substrate and hydrogen peroxide as oxidant.
• Reaction mechanism study using experimental data ((51)V NMR, EPR) and theoretical calculations.

Main Results:

• Complex 1 (V(V)) exhibits a distorted trigonal bipyramidal geometry, while complex 2 (V(IV)) shows a square-pyramidal geometry.
• DFT calculations reveal distinct electronic localizations: HOMO in complex 1 is sulfur-localized, while SOMO in complex 2 is V(IV)-localized.
• Both complexes catalyze sulfide to sulfoxide oxidation with high yields (80% for 1, 75% for 2) and complete selectivity, avoiding sulfone formation.
• Complex 1 is more efficient in conversion (81%) but less stable, whereas complex 2 is less efficient (44%) but highly stable and reusable over five cycles.
• Mechanism study suggests the structure of a cis-oxo peroxo V(V) intermediate influences stability and reactivity differences.

Conclusions:

• The investigated vanadium complexes display promising catalytic activity and selectivity for sulfoxidation.
• Complex 2 offers superior stability and reusability, making it a potentially more practical catalyst.
• The observed differences in catalytic performance are attributed to variations in electronic structure, geometry, and intermediate species.