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DOI: 10.1039/C7CC09036F
Journal Name
COMMUNICATION
2
4
concentrated HCl). Crystals of pure
collected from this extract (see full details in the ESI and CHN
analyses of fresh and recycled 40] in Table S1).
Figure 3 compares the powder XRD patterns obtained from
recycled (after one use as a hydrogenation
mediator) and after second use as
4 40
H [SiW12O ] were then
4
H [SiW12O
4 40
H [SiW12O ]
H
4
[SiW12
O
40
]
a
a
hydrogenation mediator. Both species give similar peak
patterns. The starred peaks agree with previous literature
2
5
measurements of
of each recovery cycle was calculated to be ~70%.
4 40
To show the reusability of the recovered H [SiW12O ], the
4 40
H [SiW12O ] using powder XRD. The yield
recycled compound was reduced electrochemically by two
electrons as before and the hydrogenation procedure was
repeated using nitrobenzene. Figure 2 shows a comparison of
the starting material, an authentic sample of the anticipated
product as a standard and the products obtained in the two
hydrogenation cycles. There is no distinct variation between
the product obtained from electrochemically reduced fresh
H [SiW O ] and the electrochemically reduced recycled
Figure 3: Powder XRD patterns for fresh and the recycled H
4
[SiW12
[SiW12
40] after one use as a hydrogenation mediator = Red, H
two uses as a hydrogenation mediator = dark blue
O
40
]
(* = peaks
25
confirmed as H
Black, H [SiW12
4
[SiW12
O40] from the literature) Colour code – H
4
O40] (fresh) =
4
12 40
4
O
4
[SiW12O40] after
H [SiW O ]. A 97% yield was obtained using the recycled
4
12 40
H [SiW O ] and nitrobenzene. Indeed, the same sample of
12 40
4
relates to the solubility of the starting materials. This appears
to dictate the yield of hydrogenated product obtained, with
the more water-soluble compounds (nitrobenzene, ethyl 2-
nitrobenzene and 2’-nitroacteophenone) showing markedly
higher yields that the insoluble organics (4-nitroanisole and 4-
nitrobenzoic acid). However, it is notable that even starting
materials with very low solubility in water still showed
appreciable conversion to the hydrogenated species. For
example, 4-nitroanisole has a solubility of 5.9 mg per 10 mL
H [SiW O ] could be recycled up to four times (with at least 70%
4
12 40
recovery of the H [SiW O ] after each step) and subsequently re-
4
12 40
used in the reduction of nitrobenzene (delivering a yield of aniline
of at least 92% in each cycle). The fact that the recovery of the
H [SiW O ] after each step is less than 100% is due to its solubility
4
12 40
in water and hence the difficulty in obtaining a 100% recovery from
its recrystallization.
In addition, we also performed the reduction of ethyl 2-
nitrobenzoate to ethyl 2-aminobenzoate using recycled
2
0
water, yet an isolated yield of between 51 and 65% was
obtained when 0.2 g of 4-nitroanisole was present in 10 mL of
H [SiW O ] (yield
4
= 89%) and the reduction of 2’-
12 40
nitroacetophenone to 2’-aminoacetophenone using recycled
H [SiW O ] (yield = 96%). These yields compare well with
0.5 M (reduced) silicotungstic acid. Hence it seems that even
4
12 40
largely insoluble species can be effectively hydrogenated by
this procedure. An alternative explanation for the lower yield
in the case of 4-aminobenzoic acid is the fact that the product
is rather water-soluble and hence may not be fully recovered
during the extraction process. Likewise, if the methyl group of
those obtained using fresh H [SiW O ] (see Table 1). Taken
4
12 40
together, these results suggest that the H [SiW O ] can be
4
12 40
recovered and re-used multiple times for mediated
electroreduction of various nitroarene substrates.
In conclusion, we have shown that an electrochemically
reduced polyoxometalate, in the form of silicotungstic acid,
can selectively hydrogenate the nitro group of a number of
nitroarenes in the presence of other functional groups. The
procedure is simple and sustainable in that it requires no
pressurised hydrogen atmospheres, elevated temperatures,
co-catalysts or sacrificial reagents. The mediator is recoverable
and recyclable using well known procedures and commonly-
available chemicals. Moreover, the recycled mediator can be
electrochemically re-reduced and can perform the
hydrogenation reaction without an appreciable drop in
4
-ansidine is cleaved under these hydrogenation-like
conditions, then that would lead to p-nitrophenol, which again
would be somewhat water-soluble.
The
4
H [SiW12O40] used in the hydrogenation process could be
recycled by employing the cycle shown in Figure 1. When the pH
of the reaction medium is raised (to allow extraction of the
aniline products), the polyoxometalate is subject to partial
disassembly into smaller metal-oxo building blocks. However,
upon re-acidification and exposure to oxygen, it is possible to
re-form
4
H [SiW12O40]. Hence the (dark blue) aqueous layers
from the extraction process (typical pH ≈ 5.5) were treated
with concentrated HCl followed by heating to 95 °C and left to
stir overnight. During this time the dark blue solution
performance compared with fresh
4 40
H [SiW12O ].
We thank the following funders: EPSRC (Grant Nos
EP/H024107/1, EP/J015156/1, EP/K023004/1, EP/L023652/1),
the EC (318671 MICREAGENTS), ERC (project 670467 SMART-
POM). The authors would like to thank the BBSRC, Dr
Guillaume Marie and Prof Richard Cogdell for their support.
MDS thanks the Royal Society for a University Research
Fellowship.
(indicating reduced mediator) would turn yellow (indicating a
fully oxidised species). This solution was then filtered through
2
1–23
activated carbon, producing a clear, colourless filtrate.
Following literature procedures, was then
isolated from this solution by extraction with diethyl ether and
subsequent addition of mineral acid (in this case
4 40
H [SiW12O ]
a
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