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ACS Catalysis
similar ZnII or MnII substituted polyoxometalates that
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showed chemoselectivity strongly favoring epoxidation,
the BiIII substituted polyoxometalate catalyzed reactions
with minimal epoxide formation. In reactions with al-
kenes and dienes, ene type reactivity was observed, a
hallmark of singlet oxygen formation, that is explained by
an umpolung of a reactive peroxo intermediate; electro-
philic for substitution with MnII or ZnII to nucleophilic for
substitution with BiIII. The chemoselectivity observed in
the oxidation of α, β-unsaturated alcohols strongly favors
C-H bond activation rather than epoxidation and is ex-
plained by alkoxide formation at the Lewis basic BiIII fol-
lowed by an oxidative β-elimination.
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Experimental Part
Na4(Zn(H2O)2)2H6[α-BiIII2Zn2(ZnW9O34)2]•~26H2O
Na2WO4•2H2O (3 g, 9 mmol) of was dissolved in solution
of 25 mL water and 0.5 mL 6 M HNO3 and heated for 15
min at 90 °C. After cooling to room temperature, a solu-
tion of 0.60 g of Zn(NO3)2•6H2O (2 mmol) in 2mL of water
was added followed by the drop wise addition of a solu-
tion of 0.48 g of Bi(NO3)3•5H2O (1 mmol) dissolved in 1
mL of 6 M HNO3 with vigorous stirring. The pH of the
solution adjusted to 8 by adding aqueous ammonia and
then heated at 90 °C for 1 h. The mixture was filtered and
the filtrate was allowed to cool to room temperature.
Within 24 h colorless crystals of Na4(Zn(H2O)2)2H6[α-
BiIII2Zn2(ZnW9O34)2]•~26H2O were formed. These crystals
were used for analysis by X-ray diffraction. The yield was
1.65 g (50 %, based on W). IR – 918, 868, 765 cm–1. Ele-
mental Analysis: Calc (exp) Na 1.57 (1.63); Zn 6.75 (6.55);
Bi 7.16 (7.33); W 56.61 (55.98); H2O 9.24 (9.5).
Q6Zn2Na4[BiIII2Zn2(ZnW9O34)2]. Na4(Zn(H2O)2)2H6[α-
BiIII2Zn2(ZnW9O34)2]•~26H2O (0.200 g) were dissolved in 6
mL of distilled water (pH-8.5); 20 equivalents of tetrabu-
tylammonium bromide were added and the product was
precipitated by the addition of 2 ꢀL 6M H2SO4 (pH-7.4).
The crude precipitate, Q6 ZnxNayHz[BiIII2Zn2(ZnW9O34)2],
was collected by filtration, washed several times with wa-
ter to remove unreacted tetrabutylammonium salt and
dried under vacuum. The number of tetrabutyl ammoni-
um cations was determined by thermogravimetric analy-
sis, Figure S1 (see supporting information) and the pres-
ence of the intact [BiIII2Zn2(ZnW9O34)2] anion only along
with various combinations of cations was verified by neg-
ative anion MALDI-TOF MS in the reflector mode.
Figure 3. Conceivable or exemplary peroxygen species (not
crystal structures) formed upon reaction with the
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[BiIII2Zn2(ZnW9O34)2]14–
(left)
and
[WZn(ZnH2O)2(ZnW9O34)2]12- (right) anions. Black – W; red –
O; turquoise – Zn; Blue – Bi; green – O from H2O2; pink – H
from O from H2O2.
A consensus mechanism for the oxidation of alcohols by
peroxygen oxidants is that the reactions occur by coordi-
nation of both reacting species, H2O2 and RR'CHOH to
the same metal center to yield a HOO-M-OCHRR' inter-
mediate species that lead to formation of RR'C=O and
H2O.56 Such a peroxo-metal pathway with WVI, MoVI, VV,
and TiIV based catalysts are thought to be typical.56 How-
ever, when the substrates are allylic alcohols, typically
epoxidation products are obtained with high chemoselec-
tivity as is the case also with [WZn(ZnH2O)2(ZnW9O34)2]12-
48-49
.
Contrarily, QBiPOM catalyzes the oxidation of α, β-
unsaturated alcohols with a chemoselectivity strongly
favoring C-H bond activation and formation of carbonyl
products. In addition, there are some anomalies such as
the formation of 4-isopropylbenzyl alcohol (cuminol) by
aromatization from ((4-(prop-1-en-2-yl)cyclohex-1-en-1-
yl)methanol (perillyl alcohol), and the cis←→trans isom-
erization observed in the oxidation of geraniol and nerol.
One may reasonably suggest that the reactivity observed
in the oxidation of allylic alcohols can be best explained a
base assisted or promoted oxidation reaction where the
BiIII center leads to formation of an alkoxide intermediate
that is then oxidized by a tungsten-peroxo species by oxi-
dative β-elimination to yield the carbonyl product, as
commonly observed in PdII catalyzed oxidations of alco-
hols. In the case of perillyl alcohol in addition to the for-
mation of perillaldehyde, the formation of an alkoxide can
yield to a series of double bond isomerizations.57 Upon
formation of an endo-diene, oxidation will lead to for-
mation of the aromatic cuminol. Similarly, the isomeriza-
tion observed in the geraniol and nerol oxidations can
also be attribution to catalysis involving a base.58
Na~10H~4[BiIIIBiVO[Zn2(ZnW9O34)2]•~37H2O.
Na4(Zn(H2O)2)2H6[α-BiIII2Zn2(ZnW9O34)2]•~26H2O (0.2 g,
33 μmol) was dissolved in 2 mL distilled water by heating
at 70 °C for 10 min. Ozone (concentration - 25 mg/L, rate -
6.25 mg/min), prepared by an ozonator, was then bubbled
through this solution for 1 h at room temperature during
which time the solution became mildly yellowish. The
solution was let to stand under ambient conditions; light
Conclusion
Incorporation of the Lewis basic BiIII at the accessible
terminal position of a “sandwich” type polyoxometalate
led to a change in the chemoselectivity of catalytic oxida-
tion reactions of alkenes and α, β-unsaturated alcohols.
As opposed to the previously reported and structurally
yellowish
crystals
of
Na~10H~4[BiIIIBiVO[Zn2(ZnW9O34)2]•~37H2O were obtained
after ten days and analyzed by X-ray crystallography. El-
emental Analysis: Calc (exp) Na 3.84 (4.02); Zn 4.36 (4.51);
Bi 6.97 (7.10); W 55.22 (55.00); H2O 11.13 (11.5). Yield – 0.08
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