3650
A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
Table 9
Kinetic and thermodynamic data for the hydrogenation of nitrobenzene (E) in the presence of RuH2(CO)2(PPh3)2 (2) and RuH2(PPh3)4 (3)
Catalyst
T (K) Kc · 106 (sꢀ1
)
R2
T (K) Kp · 106 (sꢀ1 atmꢀ1
)
R2
DH** (kJ molꢀ1
)
DS** (J molꢀ1 Kꢀ1
)
R2
RuH2(CO)2(PPh3)2 (2) 393
RuH2(PPh3)4 (3)
5.94
3.50
0.99 393
0.99 373
3.39
1.23
0.90 23.0
0.92 30.5
ꢀ280
ꢀ254
1.00
0.95
373
Data from Table 7.
The complex 3 shows a higher activity than 2 giving a
31.6% conversion after 15 h at 373 K and 50 atm of
hydrogen (entry 103) and a complete conversion after
48 h (entry 104). A positive influence on the conversion
is shown by an increase of hydrogen pressure and cata-
lyst concentration.
The complex 4 confirms its low catalytic activity in
these reductions giving a very poor conversion (2.9%)
in the hydrogenation of E in the same condition of entry
89.
azobenzene (A), azoxybenzene (D), nitrobenzene (E)
and in the hydrogenolysis of diphenylhydrazine (B) in
consideration of the analogous kinetic and thermody-
namic data collected.
Finally, the data collected on the hydrogenation of A
and B suggest that the Ru(N,N0-diphenylhydrazine)
complex 6 formed in the hydrogenation of azobenzene
(A) or azoxybenzene(D) is further reduced toanilinewith-
out dissociation of the coordinated diphenylhydrazine.
In all hydrogenations a complete chemoselectivity to-
wards aniline (C) is obtained; the possible intermediates
nitrosobenzene (F) and N-phenylhydroxylamine (G)
were never detected. Furthermore, no hydrogenation
of the aromatic ring was shown. These data suggest that
F and G are easily hydrogenated than E in the presence
of 2 or 3 as catalyst.
The hydrogenation of nitrobenzene in the presence of
catalysts 2 and 3 show a first partial order with respect
to substrate concentration and hydrogen pressure. The
catalyst 3 is more active than 2 in the hydrogenation
of nitrobenzene, in line with their specific rates (Table
9). In fact almost the same reaction rate is obtained at
373 K in the presence of 3 and at 393 K employing 2
as catalyst. Also the Kp values are in the following order
3 > 2.
Acknowledgements
The authors thank the University of Florence, and
`
the Ministero della Industria, Universita e Ricerca
(MIUR), Programmi di Ricerca Scientifica di Notevole
Interesse Nazionale, Cofinanziamento MIUR 2005–06,
for financial support, and the Ente Cassa di Risparmio
– Firenze for the gift to acquire an NMR instrument.
We also thank Maurizio Passaponti and Brunella
Innocenti, Department of Organic Chemistry, Univer-
sity of Florence for elemental and HPLC analyses.
References
The activation parameters DG**, DS** and DH**
evaluated using the Gibbs equation [24] show a negative
values of DS** (ꢀ280 J molꢀ1 Kꢀ1 for 2 and ꢀ254 J
[1] G. Heilen, H.J. Mercker, D. Frank, R.A. Reck, R. Jackh, in: B.
Elvers, S. Hawkins, M. Ravenscroft, G. Shulz (Eds.), UllmannÕs
Encyclopedia of Industrial Chemistry, fifth ed., vol. A2, VCH,
Weinheim, 1985, p. 1.
molꢀ1
K
ꢀ1, for 3), suggesting an associative rate deter-
[2] G. Booth, H. Zollinger, K. McLaren, W.G. Sharples, A. Westwell,
in: B. Elvers, S. Hawkins, M. Ravenscroft, G. Shulz (Eds.),
UllmannÕs Encyclopedia of Industrial Chemistry, fifth ed., vol.
A9, VCH, Weinheim, 1987, p. 73.
mining step in all cases.
The data obtained are in agreement with the stability
of the ruthenium complexes, as above reported and a
mechanism analogous to that reported in Scheme 2
may be hypothesised.
[3] M.I. Kohan, in: B. Elvers, S. Hawkins, M. Ravenscroft, G. Shulz
(Eds.), UllmannÕs Encyclopedia of Industrial Chemistry, fifth ed.,
vol. A21, VCH, Weinheim, 1992, p. 179.
[4] W. Kiel, in: B. Elvers, S. Hawkins, M. Ravenscroft, G. Shulz
(Eds.), UllmannÕs Encyclopedia of Industrial Chemistry, fifth ed.,
vol. A4, VCH, Weinheim, 1985, p. 9.
4. Conclusion
[5] R.A. Smiley, in: B. Elvers, S. Hawkins, M. Ravenscroft, G. Shulz
(Eds.), UllmannÕs Encyclopedia of Industrial Chemistry, fifth ed.,
vol. A12, VCH, Weinheim, 1989, p. 629.
The catalysts tested are catalytically active in the
hydrogenation of azobenzene (A), azoxybenzene (C)
and nitrobenzene (D) and in the hydrogenolysis of diph-
enylhydrazine (B), even if different activities were
shown. The catalyst RuH2(PPh3)4 (3) shows a very high
activity as shown by the almost complete conversion ob-
tained with all the substrates tested.
[6] F.R. Lawrence, W.J. Marshall, in: B. Elvers, S. Hawkins, M.
Ravenscroft, G. Shulz (Eds.), UllmannÕs Encyclopedia of
Industrial Chemistry, fifth ed., vol. A2, VCH, Weinheim,
1985, p. 303.
[7] P.N. Rylander, in: B. Elvers, S. Hawkins, M. Ravenscroft, G.
Shulz (Eds.), UllmannÕs Encyclopedia of Industrial Chemistry,
fifth ed., vol. A13, VCH, Weinheim, 1992, p. 493.
A catalytic cycle, analogous to that reported in
Scheme 2, may be assumed for the hydrogenation of
[8] A.M. Joshi, K.S. MacFarlane, B.R. James, P. Frediani, in: J.R.
Kosak, T.A. Johnson (Eds.), M. Dekker, 1994, p. 497.