Mechanism of thioether oxidation over di- and tetrameric ti centres: kinetic and DFT studies based on model Ti-containing polyoxometalates

Abstract

The oxidation of thioethers by the green oxidant aqueous H2 O2 catalysed by the tetratitanium-substituted Polyoxometalate (POM) (Bu4 N)8 [{γ-SiTi2 W10 O36 (OH)2 }2 (μ-O)2 ], 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 Ti2 (μ-OH)2 group in [{γ-SiTi2 W10 O36 (OH)2 }2 (μ-O)2 ](8-) (1 D) with H2 O2 . The second mechanism includes hydrolysis of Ti-O-Ti bonds linking two γ-Keggin units in structure 1 D to produce the monomer [(γ-SiW10 Ti2 O38 H2 )(OH)2 ](4-) (1 M), followed by the formation of an active hydroperoxo species upon interaction of the Ti hydroxo group with H2 O2 . 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 H2 O2 by compound 1 M is preferred by 6.5 kcal mol(-1) with respect to anion 1 D due to the more flexible Ti environment of the terminal Ti hydroxo group in 1 M. 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 Ti2 (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)W11 O39 ](4-) and the dititanium-substituted sandwich-type ion [Ti2 (OH)2 As2 W19 O67 ](8-) .

Keywords: density functional calculations; homogenous catalysis; kinetic modeling; polyoxometalates; sulfoxidation; titanium.