Mechanism of Thioether Oxidation over Di‐ and Tetrameric Ti Centres: Kinetic and DFT Studies Based on Model Ti‐Containing Polyoxometalates. Issue 41 (13th August 2015)

• Record Type: Journal Article
• Authors: Skobelev, Igor Y.; Zalomaeva, Olga V.; Kholdeeva, Oxana A.; Poblet, Josep M.; Carbó, Jorge J.
• Is Part Of: Chemistry. Volume 21:Issue 41(2015)
• Page Start: 14496
• Page End: 14506
• Publication Date: 2015-08-13
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201501157
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The oxidation of thioethers by the green oxidant aqueous H<sub>2</sub>O<sub>2</sub> catalysed by the tetratitanium‐substituted Polyoxometalate (POM) (Bu<sub>4</sub>N)<sub>8</sub>[{γ‐SiTi<sub>2</sub>W<sub>10</sub>O<sub>36</sub>(OH)<sub>2</sub>}<sub>2</sub>(μ‐O)<sub>2</sub>], as a model catalyst comprising tetrameric titanium centres, was investigated by kinetic modelling and DFT calculations. Several mechanisms of sulfoxidation were evaluated by using methyl phenyl sulfide (PhSMe) as a model substrate in the experiments and dimethyl sulfide in the calculations. The first mechanism assumes that the active hydroperoxo species forms directly through interaction of the Ti<sub>2</sub>(μ‐OH)<sub>2</sub> group in [{γ‐SiTi<sub>2</sub>W<sub>10</sub>O<sub>36</sub>(OH)<sub>2</sub>}<sub>2</sub>(μ‐O)<sub>2</sub>]<sup>8−</sup> (<bold>1 D</bold>) with H<sub>2</sub>O<sub>2</sub>. The second mechanism includes hydrolysis of Ti‐O‐Ti bonds linking two γ‐Keggin units in structure <bold>1 D</bold> to produce the monomer [(γ‐SiW<sub>10</sub>Ti<sub>2</sub>O<sub>38</sub>H<sub>2</sub>)(OH)<sub>2</sub>]<sup>4−</sup> (<bold>1 M</bold>), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H<sub>2</sub>O<sub>2</sub>. Both kinetic modelling and DFT calculations support the mechanism through the monomeric species that involves the hydrolysis step. According to the DFT studies the activation of H<sub>2</sub>O<sub>2</sub> by compound <bold>1 M</bold> is preferred by 6.5 kcal mol<sup>−1</sup> with respect to anion <bold>1 D</bold> due to the more flexible Ti environment of the terminal Ti hydroxo group in <bold>1 M</bold>. The calculations also indicate that for the „monomeric" mechanism two pathways are operative: the mono‐ and the multinuclear pathway. In the mononuclear mechanism, the active group is the terminal TiOH group, whereas in the multinuclear path the active group is the bridging Ti<sub>2</sub>(μ‐OH) moiety. Moreover, unlike previous studies, the sulfoxidation is preferred through a β‐oxygen atom transfer from the Ti hydroperoxo group because the α‐oxygen atom transfer leads to an unfavourable seven‐fold coordinated Ti environment in the transition state. Finally, we have generalised these results to other Ti‐containing POMs: the Ti‐monosubstituted α‐Keggin ion [α‐PTi(OH)W<sub>11</sub>O<sub>39</sub>]<sup>4−</sup> and the dititanium‐substituted sandwich‐type ion [Ti<sub>2</sub>(OH)<sub>2</sub>As<sub>2</sub>W<sub>19</sub>O<sub>67</sub>]<sup>8−</sup>.</p> </abstract>