at 100 1C. We also found that it is hard to oxidize azobenzene to
azoxybenzene. Overall, azobenzene is the major product
between these transformations.
In conclusion, a sustainable environmental and industrial
friendly process for Aazos formation has been developed using
ultra-thin, unsupported Pt NW catalysts. The Pt NW catalysts
exhibited excellent activity and selectivity. The reaction mechanism
was proposed by time-dependent GC analysis and ReactIR which
was further demonstrated by DFT calculation. At a high pressure
2
of 4 bar initial H , the reaction speed is dramatically increased and
B90% Aazos can be obtained in 40 minutes. Kilogram scale of
Aazos can be achieved in 4 hours, which further demonstrates
the possibility of Pt NWs catalysts for industrial application. To the
best of our knowledge, this is the most effective way for the
synthesis of Aazos. We envision that this type of Pt catalysts will
be an important catalyst for the industrial synthesis of organic dyes,
food additives, radical reaction initiators, and therapeutic agents
and these unique nanomaterials might lead to the development of
novel heterogeneous catalyst systems for other reactions as well.
H.W.G. and L. H. acknowledge financial support from the
National Natural Science Foundation of China (No. 21003092),
the scientific innovation research of college graduate in Jiangsu
province (CXZZ11_0102), the Priority Academic Program
Development of Jiangsu Higher Education Institutions; X. Q. C.
thanks National Engineering Laboratory of Modern Silk; X.H.S.
acknowledges financial support from the National Basic Research
Program of China (973 Program) (Grant No. 2010CB934502).
Fig. 2 (A) Time-conversion plot for nitrobenzene hydrogenation at
0 1C; (B) proposed mechanism for the azobenzene formation.
6
simulation package (VASP), with projector augmented wave
PAW) functional. The formation of azobenzene undergoes
(
seven sequential steps (Fig. S9, ESIw). Initially, one activated
H atom transfers from Pt (111) to the vicinity of the –NO2
group to form a hydroxyl (step 1). Subsequently, another H
Notes and references
atom from NWs attaches to the hydroxyl to form a H O and
2
1
A. K. Singh, J. Das and N. Majumdar, J. Am. Chem. Soc., 1996,
18, 6185.
B. W. Gung and R. T. Taylor, J. Chem. Educ., 2004, 81, 1630;
K. Haghbeen and E. W. Tan, J. Org. Chem., 1998, 63, 4503.
desorbs, resulting in the formation of the intermediate RNO
1
(
step 2). RNO can be further hydrogenated to RNOH (step 3),
2
which are readily dimerized to form RN(OH)–N(OH)–R (step 4)
and spontaneously dehydrated to RNOQNR (step 5). After
that, the O on RNO–NR can be hydrogenated further to form a
hydroxyl product RN(OH )Q NR (step 6). Finally, an H atom
3 G. R. Srinivasa, K. Abiraj and D. C. Gowda, Synth. Commun.,
003, 33, 4221; K. Ohe, S. Uemura, N. Sugita, H. Masuda and
T. Taga, J. Org. Chem., 1989, 54, 4169.
2
4
C. Zhang and N. Jiao, Angew. Chem., Int. Ed., 2010, 49, 6174.
transfers to this hydroxyl to form a H O molecule to form the
2
5 A. Grirrane, A. Corma and H. Garcia, Science, 2008, 322, 1661.
6 L. Hu, X. Cao, L. Shi, F. Qi, Z. Guo, J. Lu and H. Gu, Org. Lett.,
azobenzene (step 7). In the whole process, the steps 1 and 6 with
relatively high reaction barriers are the rate-limiting steps (0.27
eV for step 1 and 0.56 eV for step 6), whereas the rest can take
place nearly barrierlessly. We divided the whole process into two
sequential and competing reactions including hydrogenation of
nitrobenzene to azoxybenzene (steps 1–5) and hydrogenation of
azoxybenzene to azobenzene (steps 6 and 7). Apparently, the
former reaction is preferred because of the lower reaction
barriers. That is, no azobenzene will be formed unless the former
reaction is completed although most of the nitrobenzene is
consumed. As a result, azoxybenzene is the only observable
intermediate via GC analysis since all other intermediates would
be spontaneously consumed.
2
A. Corma, P. Serna, P. Concepcion and J. J. Calvino, J. Am. Chem.
Soc., 2008, 130, 8748; Y. Li and G. A. Somorjai, Nano Lett., 2010,
10, 2289; Z. Rong, W. Du, Y. Wang and L. Lu, Chem. Commun.,
011, 13, 5640.
7
8
´
2
010, 46, 1559.
K. Yamamoto, T. Imaoka, W.-J. Chun, O. Enoki, H. Katoh,
M. Takeaga and A. Sonoi, Nat. Chem., 2009, 1, 397; S. Vajda,
M. J. Pellin, J. P. Greeley, C. L. Marshall, L. A. Curtiss,
G. A. Ballentine, J. W. Elam, S. Catillon-Mucherie, P. C. Redfern,
F. Mehmood and P. Zapol, Nat. Mater., 2009, 8, 213; C. Wang,
H. Daimon, Y. Lee, J. Kim and S. Sun, J. Am. Chem. Soc., 2007,
129, 6974; G. W. Qin, W. L. Pei, X. M. Ma, X. N. Xu, Y. P. Ren,
W. Sun and L. Zuo, J. Phys. Chem. C, 2010, 114, 6909.
H. C. Yao and P. H. Emmett, J. Am. Chem. Soc., 1961, 83, 796;
S. L. Karwa and R. A. Rajadhyaksha, Ind. Eng. Chem. Res., 1987,
9
2
6, 1746; S. L. Karwa and R. A. Rajadhyaksha, Ind. Eng. Chem.
Res., 1988, 27, 21.
10 E. A. Gelder, S. D. Jackson and C. M. Lok, Chem. Commun., 2005,
22; M. Boronat, P. Concepcion, A. Corma, S. Gonzalez, F. Illas
The reaction was remarkably selective to azobenzene. A serial
experiment of the transformations between azoxybenzene,
azobenzene and hydroazobenzene was performed (Table S5, ESIw).
Hydrogenation of azoxybenzene can form azobenzene with high
yield which can be continually reduced to form hydrazobenzene
while extending the reaction time. Hydrazobenzene can be
converted back to azobenzene while heating in air for 5 hours at
5
´
´
and P. Serna, J. Am. Chem. Soc., 2007, 129, 16230; B. Li and
Z. Xu, J. Am. Chem. Soc., 2009, 131, 16380.
11 C. Wang, Y. Hou, J. Kim and S. Sun, Angew. Chem., Int. Ed.,
2
007, 46, 6333.
1
2 G. A. Russell, E. J. Geels, F. J. Smentowski, K.-Y. Chang,
J. Reynolds and G. Kaupp, J. Am. Chem. Soc., 1967, 89, 3821;
P. Zuman and B. Shah, Chem. Rev., 1994, 94, 1621; M. G. Pizzolatti
and R. A. Yunes, J. Chem. Soc., Perkin Trans. 2, 1990, 759.
100 1C. Even in the absence of Pt catalysts, hydrazobenzene
could convert to azobenzene with a yield of 95.5% in 12 hours
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3445–3447 3447