A dimeric titanium-containing polyoxometalate. Synthesis, characterization, and catalysis of H2O2-based thioether oxidation

The previously unknown titanium(IV)-containing mu-hydroxo dimeric heteropol ytungstate (Bu4N)(7)[(PTiW11O39)(2)-OH] (TBA salt of H1) has been synthesiz ed, starting from H5PTiW11O40, and characterized by elemental analysis, mul tinuclear (P-31, O-17, W-183) NMR, IR, FAB-MS, cyclic voltammetry, and pote ntiometric titration. P-31 NMR reveals that H1 (delta - 12.76) readily form s in MeCN from the Keggin monomer (POM), PTiW11O405- (2, delta - 13.34), up on the addition of 1.5 equiv of H+, via the protonated species, P(TiOH)W11O 394- (H-2, delta -13.44). The ratio of H1, 2, and H2, which are present in equilibrium in MeCN solution at 25 degrees C, depends on the concentration of both H+ and H2O. The Ti-O-Ti linkage readily reacts with nucleophilic re agents, such as H2O and ROH, to yield monomeric Keggin derivatives. mu-Hydr oxo dimer H1 shows higher catalytic activity than 2 for thioether oxidation by hydrogen peroxide in acetonitrile. The reaction proceeds readily at roo m temperature and affords the corresponding sulfoxide and sulfone in ca. qu antitative yield. The addition of H2O2 to H1 or H2 results in the formation of a peroxo complex, most likely the hydroperoxo complex P(TiOOH)W11O394- (I), which has P-31 NMR resonance at -12.43 ppm. The rate of the formation of I is higher from Ht than from Ht. When Ht. is used as a catalyst precurs or, the rates of the thioether oxidation and peroxo complex formation incre ase with increasing H2O concentration, which favors the cleavage of H1 to H 2. H2O2 in MeCN slowly converts 2 to another peroxotitanium complex, P(TiO2 )W11O395- (II), which has P-31 NhlR resonance at - 12.98 ppm. Peroxo comple xes I and II differ in their protonation state and interconvert fast on the P-31 NMR time scale. Addition of 1 equiv of H1 completely converts II to I , while 1 equiv of OH- completely converts I to II. P-31 NMR confirms that I is stable under turnover conditions (thioether, H2O2, MeCN). Contrary to two-phase systems such as dichloroethane/aqueous H2O2, no products resultin g from the destruction of the Keggin POM were detected in MeCN in the prese nce of H2O2 (a 500-fold molar excess). The reactivity of I, generated in si tu from II by adding 1 equiv of H1, toward organic sulfides under stoichiom etric conditions was confirmed using both P-31 NMR and UV-vis spectroscopy. This is a rare demonstration of the direct stoichiometric oxidation of an organic substrate by a titanium peroxo complex.