Synthesis, Structure and Catalytic Activity of ... - Jacobs University

Ch.1 Introduction
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[PW11(TiO2)O39] 5- while the protonated TBA-salt readily converted in a few
hours to a colored peroxo complex II [PW11(TiOOH)O39] 4- upon addition of 30-
fold molar excess of 77% H2O2; as distinguished from the 31 P-NMR signals
taken for both. The oxidation of the thioether proceeded more quickly when II
is formed.
In a later study 45 in 2000, the pH-dependent interconversion in MeCN
between 1 and H2 [(PTiW11O39)2OH] 7- the corresponding protonated form of
the dimer (Fig. 1.17) was investigated thoroughly. The dimer tends to
hydrolyze to give the monomer when the amount of water is sufficient, and
thus the relative concentrations of 1, its conjugate acid H1 [P(TiOH)W11O39] 4-
,H2 and its conjugate base 2 [(PTiW11O39)2O] 8- depend on both the
concentration of H + and H2O. Furthermore, the presence of water is highly
crucial in the case of the dimeric species since they need to hydrolyze first
before they form the hydro-peroxo active complex. The catalytic activity of
these Ti-POM species was found to be in this sequence: H2>H1>2>1. It was
confirmed again that the rate of thioether oxidation with H2O2 correlates
with the rate of H2O2 decomposition by the catalyst or in other words the rate
of formation of the peroxo-titanium complex that has to be protonated in
order to show activity.
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