Vanadium Thiolate Complexes for Efficient and Selective Sulfoxidation Catalysis: A Mechanistic Investigation

The structural and electronic properties as well as the catalytic activity toward sulfoxidation of two new vanadium complexes have been investigated. They both possess in their coordination sphere two alkyl thiolate ligands: a dioxido V<sup>V</sup> complex [VO<sub>2</sub>L<sup>NS2</sup>]­(HNEt<sub>3</sub>) (<b>1</b>) (L<sup>NS2</sup> = 2,2′-(pyridine-2,6-diyl)­bis­(1,1′-diphenylethanethiol)) and an oxido V<sup>IV</sup> complex [VOL<sup>N2S2</sup>] (<b>2</b>) (L<sup>N2S2</sup> = 2,2′-(2,2′-bipyridine-6,6′-diyl)­bis­(1,1′-diphenylethanethiol)). The X-ray structure of <b>1</b> has revealed that the V<sup>V</sup> metal ion is at the center of a distorted trigonal bipyramid. The optimized structure of <b>2</b> obtained by DFT calculations displays a square-pyramidal geometry, consistent with its EPR spectrum characterized by an axial <i>S</i> = 1/2 signal (<i>g</i><sub>⊥</sub> = 1.988, <i>g</i><sub>∥</sub> = 1.966, <i>A</i><sub><i>x</i>(V)</sub> = 45 × 10<sup>–4</sup> cm<sup>–1</sup>, <i>A</i><sub><i>y</i>(V)</sub> = 42 × 10<sup>–4</sup> cm<sup>–1</sup>, <i>A</i><sub><i>z</i>(V)</sub> = 135 × 10<sup>–4</sup> cm<sup>–1</sup>). DFT calculations have shown that the HOMO (highest occupied molecular orbital) of <b>1</b> is notably localized on the two thiolate sulfur atoms (56% and 22%, respectively), consistent with the expected covalent character of the V<sup>V</sup>–S bond. On the other hand, the SOMO (singly occupied molecular orbital) of <b>2</b> is exclusively localized at the V<sup>IV</sup> ion (92%). Complexes <b>1</b> and <b>2</b> have shown an ability to catalytically oxidize sulfide into sulfoxide. The oxidation reactions have been carried out with thioanisole as substrate and hydrogen peroxide as oxidant. Yields of 80% and 75% have been obtained in 10 and 15 min for <b>1</b> and <b>2</b>, respectively. However, in terms of conversion, <b>1</b> is more efficient than <b>2</b> (81% and 44%, respectively). More importantly, the reaction is completely selective with no trace of sulfone produced. While <b>1</b> displays a poor stability, catalyst <b>2</b> shows the same efficiency after five successive additions of oxidant and substrate. The difference in reactivity and stability between both complexes has been rationalized through a mechanism study performed by means of experimental data (<sup>51</sup>V NMR and EPR spectroscopy) combined with theoretical calculations. It has been shown that the structure of the <i>cis</i>-oxo peroxo V<sup>V</sup> intermediate species, which is related to its stability, can partly explain these discrepancies.