Hydrogenation of single and multiple N-N or N-O bonds by Ru(II) catalysts in homogeneous phase

Article abstract of DOI:10.1016/j.jorganchem.2005.04.045

The ruthenium(II) complexes RuH₂(CO)₂(P ⁿBu₃)₂, RuH₂(CO) ₂(PPh₃)₂, and RuH₂(PPh ₃)₄ are catalytically active in the hydrogenation of organic substrates containing a NN, N(O)N or NO₂ group. The reduction of the first two groups leads to hydrazine as intermediate and amine as the final product, while reducing a NO₂ group the corresponding amine is selectively formed. A complete conversion was reached, depending on temperature, catalyst and substrate concentration. The catalysts are also active in the hydrogenolysis of an N-N group giving the corresponding amine with a 97.3% conversion using RuH₂(PPh₃)₄ as catalyst. A first-order reaction rate with respect to substrate, catalyst or hydrogen pressure was detected in all cases. Finally, the activation parameters and the kinetic constants of these reactions were calculated. In the hydrogenation of azobenzene, the rate determining step involves an associative or a dissociative step depending on the catalyst employed while in the hydrogenation of all other substrates an associative rate determining step is always involved. A catalytic cycle is suggested for the hydrogenation of azobenzene, taking into account the intermediate complexes identified in the reaction medium.

Full text of DOI:10.1016/j.jorganchem.2005.04.045

Journal of Organometallic Chemistry 690 (2005) 3641–3651
www.elsevier.com/locate/jorganchem
Hydrogenation of single and multiple N–N or N–O bonds
by Ru(II) catalysts in homogeneous phase
*
Alessandro Toti, Piero Frediani , Antonella Salvini, Luca Rosi, Carlo Giolli
Department of Organic Chemistry, University of Florence, Via, della Lastruccia, 13, 50019 Sesto Fiorentino, Italy
Received 17 November 2004; received in revised form 8 April 2005; accepted 18 April 2005
Available online 22 June 2005
Abstract
The ruthenium(II) complexes RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2, and RuH2(PPh3)4 are catalytically active in the hydro-
genation of organic substrates containing a N@N, N(O)@N or NO2 group. The reduction of the first two groups leads to hydrazine
as intermediate and amine as the final product, while reducing a NO2 group the corresponding amine is selectively formed. A com-
plete conversion was reached, depending on temperature, catalyst and substrate concentration. The catalysts are also active in the
hydrogenolysis of an N–N group giving the corresponding amine with a 97.3% conversion using RuH2(PPh3)4 as catalyst. A first-
order reaction rate with respect to substrate, catalyst or hydrogen pressure was detected in all cases. Finally, the activation para-
meters and the kinetic constants of these reactions were calculated. In the hydrogenation of azobenzene, the rate determining step
involves an associative or a dissociative step depending on the catalyst employed while in the hydrogenation of all other substrates
an associative rate determining step is always involved. A catalytic cycle is suggested for the hydrogenation of azobenzene, taking
into account the intermediate complexes identified in the reaction medium.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Hydrogenation; Ruthenium; Azobenzene; Diphenylhydrazine; Azoxybenzene; Nitrobenzene
1. Introduction
process in homogeneous phase. Particular relevance
was given to the heterogeneous hydrogenation in the
presence of Pd or other transition metals supported on
carbon, alumina or silica [12]. The products formed in
the heterogeneous or homogeneous process are the
same: primary amines were formed from the hydrogena-
tion of nitro- [6] or azo-compounds [9] while a mixture
of primary, secondary and tertiary amines were ob-
tained from nitriles [7,8].
Amines play an important role in the synthesis of or-
ganic chemicals since they are intermediates in the pro-
duction of agrochemicals [1], dyes [2], fibers [3],
pharmaceuticals [4] and others [5]. The hydrogenation
of a CN, N@N or NO2 group may be an important
route to produce a variety of amines such as aniline,
benzylamine, ecc.
The catalytic hydrogenation of nitro- [6] nitrile- [7,8]
and azo-compounds [9] under a high pressure of hydro-
gen has been realised either in the presence of supported
catalysts [10] or in homogeneous phase [11]. In contrast
to the abundance of processes based on heterogeneous
hydrogenation, very few examples are reported for the
Finally, the studies on the activation of multiple
nitrogen-nitrogen bonds play a relevant role to under-
stand the mechanism involved in the reduction of nitro-
gen to produce ammonia [13]. The industrial process
requires high temperature and pressure, while the trans-
formation of atmospheric nitrogen into ammonia or its
derivatives is obtained by natural processes in very mild
conditions [14]. Transition metal are however involved
both in the industrial and biological processes.
*
Corresponding author. Tel.: +390554573522; fax: +390554573531.
E-mail address: piero.frediani@unifi.it (P. Frediani).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.04.045

3642
A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
In this paper, we report some recent achievement on
Hydrogenations were carried out in a Parr Model
4759 stainless steel autoclave (150 mL) electrically
heated and equipped with a magnetic drive stirrer or
in a home made stainless steel high pressure vessel
(150 mL) heated and rocked in a thermostated oil bath.
Tetrahydrofuran (THF) was dried and deoxygenated
by refluxing and distilling over sodium/potassium amal-
gam under nitrogen atmosphere.
The following ruthenium complexes were prepared
according to the literature: Ru(CO)2(CH3COO)2-
(PnBu3)2 [16], Ru(CO)2(CH3COO)2(PPh3)2 [17], RuH2-
(CO)2(PnBu3)2 (1) [18], RuH2(CO)2(PPh3)2 (2) [19],
RuH2(PPh3)4 (3) [20], RuH2[P(OEt)3]4 (4) [21]. The spec-
troscopic characteristics of these complexes were in
agreement with the data reported.
the catalytic activity of Ru(II) dihydride complexes in
the homogeneous hydrogenation of a R–N@N–R, R–
N(O)@N–R, or RNO2 group and in the hydrogenolysis
of a R–NH–NH–R compound (R = Ph). The RuH2-
(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2), RuH2(PPh3)4
(3), and RuH2[P(OEt)3]4 (4) complexes were chosen in
consideration of their well-known catalytic activity in
the hydrogenation of unsaturated substrates such as alk-
enes and carbonylic compounds [15].
Furthermore, the influence of various reaction
parameters and ligands coordinated to ruthenium(II)
complexes, affecting the electronic or steric properties
of catalysts were investigated.
The activation parameters and the kinetic constants
of these reactions were also calculated to collect infor-
mations on the reaction mechanism.
Azobenzene was prepared according to the Nystrom
and Brown method [22].
N,N0-Diphenylhydrazine was a commercial product,
purified following the method reported by Vogel [23].
Azoxybenzene was synthesised according to the pro-
cedure reported by Vogel [23].
2. Experimental
2.1. Instruments and materials
Nitrobenzene was a commercial product, purified by
distillation.
Quantitative analyses were performed using a Shi-
madzu GC14 chromatograph equipped with two FID
detectors, using 2 m packed columns filled with FFAP
(free fatty acid phase) or CW 20 M + KOH as station-
ary phase. Quantitative GC analyses were performed
using p-xylene as internal standard. The response fac-
tors of reagents and products versus p-xylene were de-
tected. The identity of the products was confirmed by
2.2. Hydrogenation experiments
Two typical hydrogenation experiments were re-
ported: the same procedure was employed for all sub-
strates and the data collected on the influence of
temperature, hydrogen pressure, catalyst concentration
and reaction time are reported in Tables 1–8.
GC–MS using
a
Shimadzu apparatus (GCMS-
QP5050A) equipped with a capillary column SPTM-1
(length 30 m, diameter 0.25 mm, film thickness 0.1
lm).
HPLC analyses were performed using a Gilson appa-
ratus: two pumps Gilson 305 and 306 having a 10 ll/min
head, autosampling Gilson 231 with dilution Gilson 401,
UV detector Gilson 116. A Perkin–Elmer Analytical C18
column having dimensions 250 · 4.6 mm, operating in a
reverse phase was employed.
2.2.1. Hydrogenation in the presence of RuH2(CO)2-
(PnBu3)2 (1) or RuH2(CO)2(PPh3)2 (2)
The catalyst was prepared immediately before its use
from the corresponding acetato complex, following the
procedure reported by Salvini et al. [18] for 1 or Frediani
et al. [19] for 2.
In a glass vial inserted in an autoclave Ru(CO)2(CH3-
COO)2(PR3)2 (R = nBu, Ph) (1.32 · 10ꢀ5 mol), 4 mL of
anhydrous THF and Na2CO3 (5 mmol) were intro-
duced, then hydrogen up to 100 bar was added. The ves-
sel was heated at 100 ꢁC for 14 h in a thermostated oil
bath. The reactor was cooled, the gas vented out and
the yellow solution filtered and transferred in a Schlenk
tube containing azobenzene (1.32 mmol) and p-xylene
(160 ll) as internal standard. A further amount of
THF was added up to a total volume of 20 mL. The
solution was introduced in a high pressure vessel under
nitrogen atmosphere, then hydrogen was added up to
the pressure required. The vessel was rocked in an oil
bath heated at the prefixed temperature, for the time
required.
Elemental analyses were performed with a Perkin–
Elmer Analyser Model 2400 Series II CHNS/O.
IR spectra were recorded with a Perkin–Elmer Model
1760 FT-IR spectrometer.
1H, 13C and 31P NMR spectra were recorded using a
Varian VXR300 spectrometer operating at 299.987
1
MHz for H, at 75.429 MHz for 13C and at 121.421
MHz for 31P NMR, using solutions in deuterated sol-
1
vents. SiMe4 was used as external standard for H and
13C NMR, H3PO4 (85%) for 31P NMR (signals reported
as positive downfield to the standard). 13C and 31P
NMR spectra were acquired using a broad band
decoupler.
At the end of the reaction the vessel was cooled, the
gas vented out and the solution analysed by GC or
HPLC and GC–MS techniques.
All manipulations were routinely carried out under a
nitrogen atmosphere using standard Schlenk techniques.

A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
3643
Table 1
Hydrogenation of azobenzene (A) in the presence of RuH2(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2), and RuH2(PPh3)4 (3)
Entry
Category code
T (K)
H2 (atm)
r.t. (h)
[Catalyst] (mM)
Conversion (%)
Reaction mixture
composition (%)
A
B
C
1
2
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
363
383
393
423
393
393
393
393
393
393
393
373
383
393
393
393
393
393
393
393
393
363
373
383
393
393
393
393
393
393
393
393
393
50
50
50
50
50
50
50
25
75
50
50
50
50
50
50
50
50
25
35
50
50
50
50
50
50
50
50
50
25
75
100
50
50
3
3
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.23
1.42
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.23
1.42
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.23
1.42
1.4
2.8
98.1
96.5
92.3
82.8
88.8
77.9
57.4
97.3
78.2
95.4
72.2
96.9
87.8
66.5
59.4
46.3
28.6
92.4
82.5
83.4
48.3
87.4
73.8
58.9
40.1
13.1
6.9
0.8
2.0
1.8
1.8
2.8
5.7
4.1
0.9
1.7
1.4
2.5
0.7
3.7
3.7
1.1
1.6
1.0
1.0
0.9
2.3
1.4
5.6
2.1
1.8
3.3
2.8
5.8
3.7
2.7
0.0
2.9
1.1
3.4
1.1
1.5
3
3
4.9
5.9
4
3
10.3
7.3
15.4
8.4
5
6
6
9
15.1
28.9
1.8
16.4
38.5
1.8
7
24
3
8
9
3
13.1
3.0
20.1
3.2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
3
3
17.3
1.9
25.3
2.4
3
3
8.3
8.5
3
21.9
26.0
37.4
55.9
4.4
29.8
39.5
52.1
70.4
6.6
6
9
24
3
3
10.0
10.2
35.5
9.4
16.6
14.3
50.3
7.0
3
3
3
3
16.1
26.7
44.1
77.5
87.8
99.8
23.6
49.8
65.4
13.5
59.4
24.1
39.3
56.6
84.1
87.3
96.2
33.8
66.5
75.6
21.9
70.2
3
3
6
9
24
3
0.1
63.5
33.5
21.5
77.0
26.4
3
3
3
3
Substrate: 0.066 M; volume of the solution: 20 mL; A: azobenzene; B: N,N-diphenylhydrazine; C: aniline.
Table 2
Kinetic and thermodynamic data for the hydrogenation of azobenzene (A) in the presence of RuH2(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2), and
RuH2(PPh3)4 (3)
Catalyst
Kc · 106
(sꢀ1
R2
Kp · 107
R2
Kcat · 104
R2
DH**
DS**
R2
)
(sꢀ1 atmꢀ1
)
(sꢀ1 mmolꢀ1 L)
(kJ molꢀ1
)
(J molꢀ1 Kꢀ1
)
RuH2(CO)2(PnBu3)2 (1)
RuH2(CO)2(PPh3)2 (2)
RuH2(PPh3)4 (3)
3.88
7.59
0.98
0.98
1.00
2.25
7.55
9.22
0.96
0.99
0.93
1.30
2.55
5.64
0.94
0.99
0.94
41.2
151.7
66.7
ꢀ246
+50
1.00
0.98
1.00
74.7
ꢀ159
Data from Table 1.
Table 3
Hydrogenation of azobenzene (A) in the presence of RuH2(PPh3)4 (3) and a variable amount of free triphenylphosphine
Entry
Free PPh3 (mM)
Conversion (%) after a reaction time (s) of
3600
7200
10800
21600
32400
86400
34
35
36
0
5.1
1.4
1.3
10.4
2.9
11.4
1.7
33.0
3.7
28.7
6.4
97.8
34.7
32.1
1.9
11.5
2.3
3.9
9.8
9.6
T = 353 K; p(H2) = 50 atm; RuH2(PPh3)4 = 0.66 mM. Volume of the solution 20 mL.

3644
A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
Table 4
Hydrogenolysis of N,N0-diphenylhydrazine (B) in the presence of RuH2(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2), and RuH2(PPh3)4 (3)
Entry
Category code
T (K)
P atm
r.t. (h)
[Catalyst] (mM)
Conversion (%)
Reaction
mixture
composition (%)
B
C
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
373
383
393
393
393
393
393
393
393
393
373
383
393
393
393
393
393
393
393
393
373
383
393
393
393
393
393
393
393
393
50
50
50
50
50
50
25
75
50
50
50
50
50
50
50
50
25
75
50
50
50
50
50
50
50
50
25
75
50
50
3
3
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.23
1.42
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.23
1.42
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.23
1.42
16.6
19.4
24.3
28.7
34.8
70.1
8.3
71.5
67.5
60.9
55.4
48.4
17.6
84.7
49.8
72.2
49.8
41.3
39.9
37.7
34.4
30.9
19.5
69.4
19.8
41.2
30.6
50.0
41.9
36.5
19.7
5.0
28.5
32.5
39.1
44.6
51.6
82.4
15.3
50.2
27.8
50.2
58.7
60.1
62.3
65.6
69.1
80.5
30.6
80.2
58.8
69.4
50.0
58.1
63.5
80.3
95.0
98.7
54.7
70.7
31.6
80.5
3
6
9
24
3
3
33.5
16.1
33.5
41.5
43.0
45.2
48.8
52.8
67.4
18.1
66.9
41.6
53.1
33.3
40.9
46.5
67.1
90.5
97.4
37.6
54.7
18.8
67.4
3
3
3
3
3
6
9
24
3
3
3
3
3
3
3
6
9
24
3
1.3
45.3
29.3
68.4
19.5
3
3
3
Substrate: 0.066 M; volume of the solution 20 mL; B: N,N-diphenylhydrazine; C: aniline.
Table 5
Kinetic and thermodynamic data from the hydrogenolysis of N,N0-diphenylhydrazine (B) in the presence of RuH2(CO)2(PnBu3)2 (1),
RuH2(CO)2(PPh3)2 (2), and RuH2(PPh3)4 (3)
Ru(II) complexes
Kc · 105
(sꢀ1
R2
Kp · 107
R2
Kcat · 104
R2
DH**
DS**
R2
)
(sꢀ1 atmꢀ1
)
(sꢀ1 mmolꢀ1 L)
(kJ molꢀ1
)
(J molꢀ1 Kꢀ1
)
RuH2(CO)2(PnBu3)2 (1)
RuH2(CO)2(PPh3)2 (2)
RuH2(PPh3)4 (3)
1.28
0.69
3.86
0.98
1.00
0.90
5.96
16.8
5.91
0.99
1.00
1.00
3.00
1.73
6.97
0.99
0.99
0.99
22.9
3.8
ꢀ277
ꢀ319
ꢀ269
1.00
1.00
1.00
23.2
Data from Table 3.
2.2.2. Hydrogenation in the presence of RuH2(PPh3)4
(3) or RuH2[P(OEt3)]4 (4)
In a Schlenk tube RuH2(PR3)4 (R = Ph, OEt)
(1.32 · 10ꢀ5mol), substrate (1.32 mmol) and p-xylene
(160 ll) as internal standard were dissolved in THF
(20 mL).
2.3. Synthesis of new complexes
2.3.1. Reaction of RuH2(CO)2(PnBu3)2 with PhN@NPh
A 1:1 molar solution of RuH2(CO)2(PnBu3)2 and
PhN@NPh in C6D6 was introduced in a NMR sample
1
tube and monitored by H and 31P NMR. The sample
The solution was transferred in a Parr autoclave by
suction and, after pressurisation with hydrogen up to
the pressure required, the reactor was stirred for the pre-
fixed time at the established temperature.
At the end of the reaction the vessel was cooled, the
gas vented out and the solution analysed by GC or
HPLC and GC–MS techniques.
was heated at 373 K for 24 h obtaining a 96% conver-
sion with formation of two new complexes identified
as RuH(CO)2(PhN–NHPh)(PnBu3)2 (51) 61% and Ru-
(CO)2(PhNH–NHPh)(PnBu3)2 (61) 35%. The following
characteristic resonances were attributed to the complex
1
51: H NMR (300 MHz, C6D6, d) ꢀ10.80 (t, 1H, RuH,
JHP 25.1 Hz), 4.82 (s, 1H, NH) ppm; 31P NMR (121

A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
3645
Table 6
Hydrogenation of azoxybenzene (D) in the presence of RuH2(CO)2(PPh3)2 (2) and RuH2(PPh3)4 (3)
Entry
Category code
T (K)
H2 (atm)
r.t. (h)
[Catalyst] (mM)
Conversion (%)
Reaction mixture composition
(%)
A
B
C
(D)
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
393
403
413
393
393
393
393
393
393
393
373
383
393
373
373
373
373
373
373
50
50
50
50
50
50
50
25
75
100
50
50
50
50
50
50
10
25
75
9
9
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
53.3
65.1
70.8
42.6
55.1
65.6
100.0
21.4
53.6
63.6
55.1
63.4
69.3
44.7
62.0
100.0
39.3
49.2
64.7
23.3
5.0
17.1
45.5
49.1
15.9
32.0
35.6
37.3
7.0
16.8
17.7
19.9
13.2
23.2
31.5
62.7
0.0
42.8
31.8
26.3
53.6
39.7
29.0
0.0
9
4.7
3
17.3
5.1
15
24
48
9
3.9
0.0
14.4
11.9
1.1
78.6
39.8
29.7
43.3
31.2
25.3
55.3
36.6
0.0
9
19.7
32.3
33.7
31.6
32.3
20.8
43.9
44.2
22.1
37.2
33.0
28.6
36.9
7.1
9
6
15.9
7.7
6
29.5
35.0
0.0
6
7.4
3
23.9
12.0
0.0
9
7.5
24
6
55.8
0.0
17.2
8.8
60.7
49.7
30.0
6
4.3
30.2
6
6.8
Substrate: 0.065 M; volume of the solution: 20 mL; A: azobenzene; B: diphenylhydrazine; C: aniline; D: azoxybenzene.
Table 7
Kinetic and thermodynamic data for the hydrogenation of azoxybenzene (D) in the presence of RuH2(CO)2(PPh3)2 (2) and RuH2(PPh3)4 (3)
Catalyst
T (K) Kc · 106 (sꢀ1
6.37
55.3
)
R2
T (K) Kp · 107 (sꢀ1 atmꢀ1
)
R2
DH** (kJ molꢀ1
)
DS** (J molꢀ1 Kꢀ1
)
R2
RuH2(CO)2(PPh3)2 (2) 393
0.96 393
1.00 373
3.18
3.61
1.00 29.0
0.98 20.5
ꢀ262
ꢀ276
1.00
1.00
RuH2(PPh3)4 (3)
373
Data from Table 5.
MHz, C6D6, d) 34.9 ppm. An IR spectrum of the solu-
tion shows an absorption at 2931 (mNH) cmꢀ1. The fol-
lowing resonances were attributed to the complex 61:
1H NMR (300 MHz, C6D6, d) 4.43 (s, 1H, NH) ppm;
31P NMR (121 MHz, C6D6, d) 33.2 ppm.
3. Results and discussion
3.1. Hydrogenation of azobenzene (A)
The ruthenium dihydrides 1–4 are catalytically active
in the hydrogenation of azobenzene (A) with molecular
hydrogen giving N,N0-diphenylhydrazine (B) as interme-
diate and aniline (C) as the final hydrogenation product
(Scheme 1). No hydrogenation of the aromatic ring was
shown.
2.3.2. Reaction of RuH2(CO)2(PPh3)2 with PhN@NPh
A 1:1 molar solution of RuH2(CO)2(PPh3)2 and
PhN@NPh in C6D6 was introduced in a NMR sample
1
tube and monitored by H and 31P NMR. The sample
was heated at 363 K for 24 h obtaining a 97% conver-
sion with formation of two new complexes identified as
RuH(CO)2(PhN–NHPh)(PPh3)2 (52) 78% and Ru-
(CO)2(PhNH–NHPh)(PPh3)2 (62) 19%. The following
characteristic resonances were attributed to the com-
3.1.1. Influence of reaction parameters
The influence of several reaction parameters were
tested and the results are reported in Table 1.
The complex 3 shows a low activity in the reduction
of azobenzene with a 9.4% conversion at 363 K (entry
22) that increases up to 44.1% if the temperature rises
to 393 K (entries 22–25) and an almost complete conver-
sion) was reached after 24 h (99.8%, entry 28). Also an
increse of hydrogen pressure show a beneficial influence
on the conversion which increases from 23.6% up to
65.4% using 25 and 100 atm of hydrogen, respectively
(entries 29 and 31). When the catalyst concentration is
1
plex 52: H NMR (300 MHz, C6D6, d) ꢀ9.70 (t, 1H,
RuH, JHP 23.7 Hz) 4.72 (s, 1H, NH) ppm; 31P
NMR (121 MHz, C6D6, d) 56.9 ppm. An IR spectrum
of the solution shows an absorption at 2931 (mNH
)
cmꢀ1. The following resonances were attributed to
the complex 62: 1H NMR (300 MHz, C6D6, d) 4.40
(s, 1H, NH) ppm; 31P NMR (121 MHz, C6D6, d)
52.4 ppm.

3646
A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
Table 8
Hydrogenation of nitrobenzene (E) in the presence of RuH2(CO)2(PPh3)2 (2) and RuH2(PPh3)4 (3)
Entry
Category code
T (K)
H2 (atm)
r.t. (h)
[Catalyst] (mM)
Conversion (%)
Reaction mixture
composition (%)
C
E
86
87
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
373
383
393
393
393
393
393
393
393
393
393
363
373
383
393
373
373
373
373
373
373
373
50
50
50
50
50
50
50
10
25
75
100
50
50
50
50
50
50
50
50
25
75
100
6
6
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
0.66
20.5
25.1
29.7
27.0
33.2
43.1
100.0
24.3
28.3
51.2
59.9
18.1
24.0
50.7
63.1
20.3
27.1
31.6
100.0
19.7
28.1
33.4
20.5
79.5
74.9
70.3
73.0
66.8
56.9
0.0
25.1
29.7
27.0
33.2
43.1
100.0
24.3
28.3
51.2
59.9
18.1
24.0
50.7
63.1
20.3
27.1
31.6
100.0
19.7
28.1
33.4
88
89
6
3
90
91
8
15
48
6
92
93
75.7
71.7
48.8
40.1
81.9
76.0
49.3
36.9
79.7
72.9
68.4
0.0
94
95
6
6
96
97
6
6
98
99
6
6
100
101
102
103
104
105
106
107
6
3
9
15
48
6
80.3
71.9
66.6
6
6
Substrate: 0.065 M; volume of the solution: 20 mL; C: aniline; E: nitrobenzene.
NH NH
N
N
NH2
H2
H2
2
[cat]
[cat]
C
A
B
Scheme 1. Hydrogenation of azobenzene.
reduced to 0.23 mM the conversion goes down to 13.5%
(entry 32) while it rises to 59.4% if the catalyst concen-
tration is 1.42 m (entry 33).
more, the complex containing two tributylphosphine 1
displays a higher stability than 2, containing two triphe-
nylphosphine, as shown by their IR spectra in the mCO
stretching region and consequently its catalytic activity
is lower. Also the substitution of the triphenylphosphine
with triethylphosphite as ligand increases the stability of
the complex but depress considerably the catalytic
activity.
In all hydrogenations the main product is always ani-
line (C) while the amount of the N,N0-diphenylhydrazine
(B) intermediate does not overcome 5.7%. These results
confirm that these catalysts hydrogenate A, as reported
in Scheme 1, giving B as intermediate, and C as the final
product.
The kinetic of azobenzene hydrogenation in the pres-
ence of catalysts 1–3, have been evaluated following the
procedure reported by Salvini et al. [15], using the data
reported in Table 1. The reaction rate shows a first par-
tial order with respect to substrate concentration,
hydrogen pressure and catalyst concentration. The spe-
cific rates are reported in Table 2. The catalyst 3 shows
the higher specific rate Kc in the hydrogenation of
The catalyst 1 shows a lower activity than 3 and 2,
giving at 363 K only a 1.4% conversion (entry 1, Table
1) that increases up to 10.3% when the temperature rises
up to 423 K (entries 1–4), or to 13.1% when the hydro-
gen pressure increases up to 75 atm (entries 8, 3 and 9)
or to 28.9% after 24 h (entry 7). Finally, the conversion
increases from 3.0% to 17.3% when the concentration of
the catalyst rises from 0.23 up to 1.42 mM (entries 10, 3
and 11).
The hydrogenation of A in the presence of 4, contain-
ing two P(OEt)3 ligands, has been performed with low
conversion: in the same conditions of entry 25, 4 gives
only a 1.4% conversion.
The catalytic activity of the complexes 1–4 is in agree-
ment with their stability. The substitution of two triphe-
nylphosphines with two carbonyl groups in the
coordination sphere of the ruthenium catalyst increases
the stability but decreases the catalytic activity as shown
by the data obtained using 3 or 2 as catalyst. Further-

A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
3647
azobenzene, an order higher than that obtained in the
presence of 2 or 1. Also the Kp and Kcat are in the fol-
lowing order 3 > 2 > 1.
tion of azobenzene into a Ru–H bond forming a new
complex containing
Ru–N bond RuHL2L02-
a
(PhNNHPh) (5). In the following step IV the complex
5 gives the complex RuL2L02(PhNHNHPh) (6) through
a hydrogen shift. This last complex reacts with hydrogen
giving N,N0-diphenylhydrazine (C) and restoring the
starting complex ready for another catalytic cycle. The
first-order reaction rate with respect to hydrogen pres-
sure is in agreement with this last step V.
According to this mechanism the presence of free li-
gand in the reaction medium have a negative effect on
step I, where a displacement of phosphine is required
to coordinate azobenzene.
The activation parameters DG**, DS** and DH**,
evaluated using the Gibbs equation [24] in the tempera-
ture range among 363 and 393 K, are reported in Table
2. The positive value of DS** (+50 J molꢀ1
K
ꢀ1) for 2
suggests a dissociative rate determining step. On the
contrary, in the presence of 1 and 3 a negative value
,
of DS** is obtained (ꢀ246 and ꢀ159 J molꢀ1 Kꢀ1
respectively), suggesting the formation of an intermedi-
ate having a higher steric encumbrance than the starting
ruthenium complex in the rate determining step.
We have collected evidences on the formation of the
complexes 5 and 6 reported in Scheme 4 working in an
NMR sample tube and employing the complexes 1 or 2.
The main spectroscopic data of the new complexes
are:
3.1.2. Mechanism
The catalytic activity is, as above reported, in the fol-
lowing order 3 > 2 ꢁ 1 ꢁ 4 in agreement with the apti-
tude of these complexes to dissociate a phosphinic
ligand, as previously reported [25]. This dissociation
may be required to form the activated substrate contain-
ing intermediate as reported in previous results on the
hydrogenation of benzonitrile. In that reduction the
RuH2(PhCN)(PPh3)3 intermediate was evidenced and
spectroscopically characterised [9]. In this hydrogena-
tion an analogous intermediate is not evidenced but
we have evaluated that the presence of free phosphine
drastically decreases the catalytic activity of 3 (Table
3). For instance working for 24 h at 353 K the hydroge-
nation of azobenzene goes down from 97.8% in the ab-
sence of free phosphine to 32.1% when the
concentration of free phosphine is 11.5 mM (Fig. 1).
Taking into account these data the following mecha-
nism is hypothesised (Scheme 2).
ꢂ For RuH(CO)2(PBu3)2(PhNNHPh) (51): 1H NMR
(d) ꢀ10.80 (t, 1H, RuH, JHP 25.1 Hz), 4.82 (s, 1H,
NH) ppm; 31P NMR (d) 34.9 ppm, IR (mNH) 2931
cmꢀ1
.
ꢂ For Ru(CO)2(PBu3)2(PhNHNHPh) (61): 1H NMR
(d) 4.43 (s, 1H, NH) ppm; 31P NMR (d) 33.2 ppm.
ꢂ For RuH(CO)2(PPh3)2(PhNNHPh) (52): 1H NMR
(d) ꢀ9.70 (t, 1H, RuH, JHP 23.7 Hz), 4.72 (s, 1H,
NH) ppm; 31P NMR (d) 56.9 ppm, IR (mNH) 2931
cmꢀ1
.
ꢂ For Ru(CO)2(PPh3)2(PhNHNHPh) (62): 1H NMR
(d) 4.40 (s, 1H, NH) ppm; 31P NMR (d) 52.4 ppm.
The activation parameters for the hydrogenation of A
using the three different catalysts (Table 2) suggest a dif-
ferent rate determining step involving the formation of a
sterically crowded specie for RuH2(PPh3)4 (3) and
RuH2(CO)2(PnBu3)2 (1) while a dissociative step is
involved for RuH2(CO)2(PPh3)2 (2).
In a first step I, a phosphine ligand is displaced by
azobenzene, according to the above reported catalytic
activity and to the first partial order of the reaction rate
with respect to substrate and catalyst concentration.
This reaction is followed (steps II and III) by the inser-
100
80
60
40
20
0
Free phosphine 0 mM
Free phosphine 1.9 mM
Free phosphine 11.5mM
0
5
10
15
20
25
reaction time (h)
Fig. 1. Hydrogenation of azobenzene (A) in the presence of RuH2(PPh3)4 (3) and a variable amount of free triphenylphosphine. T = 353 K;
p(H2) = 50 atm; RuH2(PPh3)4 0.66 mM. Volume of the solution 20 mL.

3648
A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
N
N
H
N
N
(A)
L
L
L
H
(B)
L'
L'
H
H
L'
L'
H
H
Ru
Ru
I
L
(1) - (3)
N N
H2
V
H
L
N
N
L'
L'
II
Ru
L
L
H
IV
#
L'
L'
H
L
Ru
L
H
(6)
L'
L'
H
III
N
N
Ru
H
L
N
N
(5)
(51) L = PnBu3; L’ = CO
(52) L = PPh3; L’ = CO
(53) L = L’ = PPh3;
(61) L = PnBu3; L’ = CO
(62) L = PPh3; L’ = CO
(63) L = L’ = PPh3;
Scheme 2. Suggested mechanism for azobenzene hydrogenation.
The dissociation of the phosphine (step I) is very fast,
The catalyst 3 shows a fairly good activity at 373 K
and after 3 h, a 33.3% conversion (entry 57) is shown
that increases when hydrogen pressure, temperature
and catalyst concentration increase. An almost total
conversion of B (97.4%) is obtained at 393 K, after 24
h using 50 atm of hydrogen (entry 62).
The catalyst 2 shows in this hydrogenolysis a higher
activity than 3 with a conversion of 41.5% at 373 K,
however a low increase of the conversion from 41.5%
to 45.2% is shown when the temperature rises from
373 to 393 K (entries 47–49), while a strong increases
is evidenced when the hydrogen pressure is improved
from 25 to 75 atm (conversions from 18.1% to 66.9%,
entries 53, 49 and 54) and almost the same conversion
(67.4%) was collected in the presence of 2, after 24 h
at 393 K using 50 atm of hydrogen (entry 52).
expecially when 3 is the catalyst employed, according to
the data reported on the reactivity of these complexes.
In the following step II a four-centre intermediate is
formed followed by the re-insertion of the previously
dissociated phosphine (step III) to give the complex 5.
In our opinion, in the presence of 1 or 3 as catalyst,
the step III involving the formation of the complex 5
is the rate determining step, because involves the forma-
tion of an intermediate having a high steric encum-
brance around the ruthenium. As a consequence a
positive activation entropy is required. However, when
2 is the starting ruthenium complex, the rate determin-
ing step may be ascribed to the reaction of 6 with hydro-
gen (step V) to give diphenylhydrazine and restore the
starting ruthenium complex.
In the presence of the catalyst 1, the conversion of B
is only 16.6% after 3 h at 373 K (entry 37). Increasing
the temperature from 373 to 393 K causes a fairly good
effect on the conversion that rises from 16.6 to 24.3%
(entries 37-39). An analogous beneficial effect has been
shown by an increment of hydrogen pressure (entries
43, 39 and 44), or catalyst concentration (entries 45,
39 and 46). Finally, also in the presence of 1 working
at 393 K for 24 h under 50 atm of hydrogen a 70.1%
conversion of B is reached (entry 42).
3.2. Hydrogenolysis of N,N0-diphenylhydrazine (B)
N,N0-Diphenylhydrazine is always present among the
products of the catalytic hydrogenation of azobenzene
(A) even if its concentration is low. These data prompted
us to study the hydrogenolysis of this product following
the procedure above reported for the hydrogenation of
azobenzene.
The complexes 1–3 are catalytically active in the
hydrogenolysis of B; the results are reported in Table 4.
Surprisingly, all the catalysts show a lower activity in
the hydrogenolysis of B than that detected in the hydro-
genation of A.
The data obtained in the hydrogenolysis of B are an
indication that this substrate is not easily coordinated
by the catalysts 1–3. In fact the yields of C using A as
substrate are higher than those obtained when the

A. Toti et al. / Journal of Organometallic Chemistry 690 (2005) 3641–3651
3649
intermediate B is the starting material. These results sug-
gest that A is reduced to give B coordinated to ruthenium
and this complex, in a large extent, is immediately re-
duced to C while only a small amount of B is de-coordi-
nated from the catalyst and, in a following step, it may be
slowly activated and transformed into C.
conversion from 21.4% to 63.6% at 393 K (entries 74–
76) and a complete conversion of D is obtained, using
2 as catalyst after 48 h at 393 K (entry 73).
The data obtained confirm that the catalysts 2 and 3
hydrogenate D giving A and B as intermediate, and C as
the final product (Scheme 3). Aniline (C) is the main
product when a complete conversion of D is reached.
On the contrary when the conversion of D is low, C is
not present.
The kinetic of the hydrogenation of azoxybenzene in
the presence of catalysts 2–3, shows a first partial order
with respect to substrate concentration, and hydrogen
pressure. The higher activity of 3 in the hydrogenation
of D, is in line with the specific rates reported in Table 7.
The activation parameters DG**, DS** and DH**
evaluated using the Gibbs equation [24] (Table 6) show
a negative value of DS**, suggesting an associative rate
determining step in all cases.
The kinetic parameters evaluated using the data re-
ported in Table 4 show a first partial order rate with re-
spect to the concentration of substrate, catalyst and
hydrogen pressure (Table 5). Surprisingly, the specific
rates with respect to substrate and catalyst concentra-
tion are higher for the complex 3 (Kc = 3.86 · 10ꢀ5
s
ꢀ1) than for 2 while the conversions are on the con-
trary. These data suggest that the starting catalyst 2 is
transformed in the course of the reaction in a new cata-
lyst having a low activity. The Kp are almost the same
when the catalyst 1 or 3 are employed while a higher va-
lue is obtained in the presence of 2.
The activation parameters evaluated for the three cat-
alysts (Table 5) show negative values of DS** suggesting
an associative rate determining step in all cases.
A mechanism analogous to that reported in Scheme
2, may be hypothesised for this hydrogenolysis.
The substitution of two carbonyl groups with two tri-
phenylphosphines in the coordination sphere of the
ruthenium catalyst increases the catalytic activity as
shown by the data obtained in the presence of 2 and 3.
3.4. Hydrogenation of nitrobenzene
3.3. Hydrogenation of azoxybenzene
The ruthenium dihydrides 2–4 have been also tested
as catalysts in the hydrogenation of nitrobenzene (E).
The complex 1 was not tested due to its low activity in
the hydrogenation of A. The reaction may give in a first
step nitrosobenzene (F), then N-phenylhydroxylamine
(G) and in the last step aniline (C) (Scheme 4) [26].
The influence of reaction parameters such as tempera-
ture, reaction time, and hydrogen pressure are reported
in Table 8.
In the presence of 2 an increment of conversion from
20.5% to 29.7% is shown when the temperature rises
from 373 up to 393 K (entries 86–88), while increasing
hydrogen pressure from 10 up to 100 atm (entries 93–
96) at 393 K the conversion rises from 24.3 to 59.9%.
The conversion rises also if the reaction time is increased
and a complete conversion is reached after 48 h (entry
92).
The catalytic activity of the ruthenium dihydrides 2
and 3, has been also tested in the hydrogenation of
azoxybenzene (D), a possible intermediate in the synthe-
sis of azobenzene from nitrobenzene. The catalyst 1 is
not tested due to its low ability to hydrogenate A.
The reaction gives in a first step azobenzene (A) then,
according to the data above reported (Section 3.1).
N,N0-Diphenylhydrazine (B) and in the last step aniline
(C) (Scheme 3).
The influence of several reaction parameters are re-
ported in Table 6.
The complex 3 shows a high activity in the reduction
of D with a conversion of 62.0% at 373 K after 9 h (entry
81) that rises to a 100% conversion after 24 h (entry 82).
Furthermore, 3 is already active with a low pressure of
hydrogen (10 atm) giving a conversion of 39.3% after
6 h (entry 83). This value increases up to 64.7% when
the hydrogen pressure is 75 atm (entry 85).
The catalyst 2 shows a lower activity than 3 with a
conversion of 53.3% working at 393 K (entry 67) that
rises to 70.8% at 413 K (entry 69). An increase of the
hydrogen pressure from 25 up to 100 atm rises the
H2
H2
H2
PhNO2
PhNHOH
PhNH2
PhNO
[Cat]
[Cat]
[Cat]
E
F
G
C
Scheme 4. Hydrogenation of nitrobenzene.
O-
N+ Ph
H2
H2
H2
Ph
N
PhN NPh
PhNH NHPh
2 Ph NH2
[Cat]
[Cat]
[Cat]
A
C
B
D
Scheme 3. Hydrogenation of azoxybenzene.

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-
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