sulfuric acid. When regarding each step separately in pathway
A, one can observe that each step has a limited efficiency.
Nevertheless, the whole process has a better AE than route B.
Table 1 Green metrics summary for the two processes
Pathway A
Pathway B
30
The E factors were calculated for both pathways. Calcula-
Yield (overall %)
36
7.2
31
tions for a multistep synthesis can be made using a tree analysis.
Atom economy (%)
14.99
106.06
19.1
4.66
8.23
336.36
3.6
-1
E factor (g g )
EcoScale
In the present case, another protocol was used. Namely, the
E factor was calculated for each step, reported in the next
one and so on till the end of the process. By applying this
method, the E factor was found equal to 106.6 g waste per g
final product for pathway A. Estimation for pathway B was
-1
Cost (€ g )
5.61
route A. All the results for the green metrics are summarized in
Table 1.
11
made on the basis of amounts reported in litterature. When
doing calculations, two problems appeared with pathway B.
The first one is the Pd catalyst. Of course it can be reused
for multiple experiments and this fact should be taken into
account in calculations. The second problem relies in that the
starting 4-ethylpyridine is not entirely consumed and therefore
its excess could be recovered and reused. Since there is no data
dealing with these two points, two scenarios were envisioned.
The first one assumes no reclaiming of both catalysts and 4-
ethylpyridine. Obviously the latter is the worst case possible
and is not suitable for comparison. The second scenario is
based on twenty uses of catalyst (average 0.2 g per experiment)
and full recovery of unused 4-ethylpyridine. The amount of
the latter was calculated by taking into account the amount
of intermediate 5 that is formed, but neglect the bipyridine
by-product (no data available concerning quantities of this
molecule). As a consequence, the amount of 4-ethylpyridine that
could be recovered is overestimated and real E factor should be
lower than the calculated value. So the E factor for pathway B
was found to be 336.36 g waste per g of final product, according
to the “best” scenario. Values for both pathways are high, but the
new route has an E factor that is approximately one third of that
of the original process, which is a great improvement. This can
be explained by the use of less solvent in work-up procedures
and less reagents in the oxidation step conjugated to a better
overall yield.
In conclusion, we have developed a new process for the prepa-
ration of the ligand 4,4¢,4¢¢-tricarboxy-2,2¢:6¢,2¢¢-terpyridine.
The principles of green chemistry are fully applied since this
new route generates less waste and avoids the use of dangerous
chromium salts. It also employs a reagent (furfural) that could
be obtained from renewable sources. Furthermore, the obtained
compound can be used for the preparation of sensitizers useful
in the field of solar cells with the final aim of producing “green”
electricity. Future work will focus on further characterization
of intermediate 3, and on preparing new sensitizers that include
ligand 1 in order to enhance performance of DSSCs. In addition,
owing to recent literature, it should be possible to further
improve the synthesis of 1, for example by modifying the
34,35
oxidation of the alkyl chains.
Acknowledgements
Ville de Besan c¸ on is gratefully acknowledged for a Ph. D. Grant
JD).
(
Notes and references
‡
3
A SciFinder search performed on January 2011 indicates more than
00 substances including this ligand.
While not directly related to environmental impact, price for
the product is also an important parameter. In fact, if one
wants solar energy to develop further, it is necessary to keep
DSSC’s cost as low as possible to be competitive. This implies
price control for all fabrication stages, including dye synthesis.
1 A. Hagfeldt, G. Boschloo, L. C. Sun, L. Kloo and H. Pettersson,
Chem. Rev., 2010, 110, 6595.
2
3
K. Kalyanasundaram, Dye-Sensitized Solar Cells, Presses Polytech-
niques et Universitaires Romandes, Lausanne, 2010.
M. Gr a¨ tzel, J. Photochem. Photobiol., C, 2003, 4, 145.
4 B. O’Regan and M. Gr a¨ tzel, Nature, 1991, 353, 737.
-
1
5 R.
M.
El-Shishtaury,
Int.
J.
Photoenergy,
2009,
The price for the final product was estimated to be 4.66 € g
DOI: 10.1155/2009/434897.
-
1
using pathway A and 5.61 € g with pathway B (in the best
case). It is interesting to note that electrical consumption was
not included in cost calculation. Nonetheless, the first step in
6
B. Zheng, H. J. Niu and X. D. Bai, Progress in Chemistry, 2008, 20,
828.
7 A. S. Polo, M. K. Itokazu and N. Y. M. Iha, Coord. Chem. Rev., 2004,
◦
248, 1343.
pathway B requires 9 days heating at 170 C, which is much more
8
9
M. Gr a¨ tzel, M. K. Nazeeruddin, P. Pechy, WO pat. 19980507, 1998.
G. C. Vougioukalakis, T. Stergiopoulos, G. Kantonis, A. G. Kontos
Kyriakos Papadopoulos, A. Stubllab, P. G. Potvin and P. Falaras, J.
Photochem. Photobiol., A, 2010, 214, 22.
than the total heating time in pathway A (including work-up
and purification). Therefore the new route is clearly less energy
greedy than the original one.
32
10 M. K. Nazeeruddin, P. P e´ chy and M. Gr a¨ tzel, Chem. Commun., 1997,
18), 1705.
Finally, EcoScale parameter was used as an additional tool
for environmental impact assessment. EcoScale is interesting
because it includes points such as toxicity and hazards of
reagents and solvents, price, setup and purification. The higher
the EcoScale is, the better the process is. Pathway A has a
value of 19,1 while it was estimated to be 3,6 for pathway B.
again the new route appears to be better. This could be partly
explained by the use of a toxic and carcinogenic Cr(VI) salt for
(
1
1 M. K. Nazeeruddin, P. P e´ chy, T. Renouard, S. M. Zakeeruddin, R.
Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover,
L. Spiccia, G. B. Deacon, C. A. Bignozzi and M. Gr a¨ tzel, J. Am.
Chem. Soc., 2001, 123, 1613.
2 E. C.-H. Kwok, M.-Y. Chan, K. M.-C. Wong, W. H. Lam and V.
W.-W. Yam, Chem.–Eur. J., 2010, 16, 12244.
13 U. S. Schubert, H. Hofmeier and G. R. Newkome, Modern Terpyri-
dine Chemistry, Wiley-VCH, Weinheim, 2006.
4 M. Heller and U. S. Schubert, Eur. J. Org. Chem., 2003, 6, 947.
5 R. A. Fallahpour, Synthesis, 2003, 2, 155.
16 F. Kr o¨ hnke, Synthesis, 1976, 1, 1.
1
1
1
33
the oxidation that is avoided with the new route. In addition,
the overall yield for pathway B is much lower compared to that of
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Green Chem., 2011, 13, 3337–3340 | 3339