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-) .

Mechanistic Investigation into Olefin Epoxidation with H2O2 Catalyzed by Aqua‐Coordinated Sandwich‐Type Polyoxometalates: Role of the Noble Metal and Active Oxygen Position

“…It can be seen that step 2 is typical beta‐oxygen transfer from the bridging hydroperoxo moiety to the olefin. Recently, Poblet and co‐workers have proposed that the mechanism of Ti−OOH through beta‐oxygen has a lower energy barrier for dititanium‐substituted sandwich‐type POMs . As could be expected, this process will proceed with a slightly lower energy barrier; the calculated energy barrier was 16.6 (Δ E ) or 29.7 kcal mol −1 (Δ G ) at the located transition state, TS3Rh (Figure ).…”

“…According to previous reports on the epoxidation by POMs, the catalytic reaction proceeded through a two‐step mechanism: 1) activation of H 2 O 2 and 2) oxygen transfer; in addition, three distinct H 2 O 2 activation mechanism have been reported . Musaev and co‐workers reported a divanadium‐substituted POM, [γ‐1,2‐H 2 SiV 2 W 10 O 40 ] 4− , which contained a core of [OV−(η‐OH) 2 −VO] .…”

“…For the second type, several computational studies have reported the lacunary‐substituted POM, [γ‐H 2 SiW 10 O 36 ] 4− , as well as mono‐ and dititanium‐substituted POMs [PTi(OH)W 11 O 39 ] 4− and [Ti 2 (OH) 2 As 2 W 19 O 67 (H 2 O)] 8− . A two‐step mechanism has been proposed.…”

“… Epoxidation mechanism based on the mono‐ and ditransition‐metal‐substituted polyoxometlates, [PTM(OH)W 11 O 39 ] 4− and [TM 2 (OH) 2 As 2 W 19 O 67 (H 2 O)] 8− , TM=Ti, V, Zr, Nb, Mo, W and Re …”

“…[10,11] Polyoxometalates (POMs), as abundant molecular-basedi norganic materials, are potentially interesting catalysts for the epoxidation of olefins. Particular attention has been paid to transition-metal-substituted polyoxotungstates,w hich have showns ignificant catalytic activity and selectivity,s uch as Venturello-type anions, {PO 4 [WO(O 2 ) 2 ] 4 } 3À , [15][16][17] Keggin-type [18][19][20][21][22][23][24][25][26][27][28][29] and To urnØ sandwich-typepolyoxotungstates. [30][31][32][33][34][35][36][37][38][39][40] In the literature, the low-valent noble transition-metal-substituted sandwich-typep olyoxotungstates, {[WZnTM 2 (H 2 O) 2 ](ZnW 9 O 34 ) 2 } nÀ (TM=Rh III ,P d II ,a nd Pt II ), referredt oa s1Rh, 1Pd,a nd 1Pt (Figure 1), have been reported to be highly active and selective catalysts with ag ood solvolytic stability for olefin epoxidation with H 2 O 2 .…”