Catalytic activity of dihydride ruthenium complexes in the hydrogenation of nitrogen containing heterocycles

Article abstract of DOI:10.1016/j.ica.2005.06.025

The catalytic activity of the dihydride ruthenium complexes, RuH ₂(CO)₂(PⁿBu₃)₂, RuH ₂(CO)₂(PPh₃)₂ and RuH ₂(PPh₃)₄, in the hydrogenation of nitrogen containing heterocycles has been tested by analyzing the influence of reaction parameters such as temperature, hydrogen pressure, catalyst concentration, on the rate and regioselectivity of the reaction. RuH₂(PPh ₃)₄ shows a better catalytic activity with an 86.7% conversion of quinoline after 24 h at 100°C under a hydrogen pressure of 25 bar, while RuH₂(CO)₂(PPh₃)₂ and RuH₂(CO)₂(PⁿBu₃)₂ in the same conditions give a conversion of 37.1% and 35.6%, respectively. These results are confirmed by the reaction rate of the hydrogenation of quinoline, since the K_c in the presence of RuH₂(PPh₃) ₄ (1.46 × 10^-5 s^-1) is higher than others (6.37 × 10^-6 s^-1 for RuH₂(CO) ₂(PPh₃)₂ and 6.36 × 10^-6 s^-1 for RuH₂(CO)₂(PⁿBu ₃)₂). Noteworthy is the selectivity of these catalytic systems in the hydrogenation of quinoline: in all tests the three catalysts lead to 1,2,3,4-tetrahydroquinoline as the major product, furthermore this compound is the only formed in the presence of RuH₂(CO)₂(PPh ₃)₂. The selectivity is affected by the presence of an acid (CH₃COOH) or a base (NⁿBu₃) in the reaction media. The complex RuH₂(PPh₃)₄ is catalytically active, even if in a minor extent, in the hydrogenation of isoquinoline, pyridine and 2-methylpyridine. The basicity of the substrate and steric hindrance around the nitrogen atom show a great influence on the conversion. The results obtained suggest that the catalytic system activates a heterocyclic ring through the coordination of the heteroatom to the metal centre of the complexes.

Full text of DOI:10.1016/j.ica.2005.06.025

Inorganica Chimica Acta 359 (2006) 917–925
www.elsevier.com/locate/ica
Catalytic activity of dihydride ruthenium complexes in
the hydrogenation of nitrogen containing heterocycles
*
Piero Frediani , Valentina Pistolesi, Marco Frediani, Luca Rosi
Department of Organic Chemistry, University of Florence, via della Lastruccia, 13 – 50019 Sesto Fiorentino (Florence), Italy
Received 13 May 2005; accepted 13 June 2005
Available online 28 July 2005
Ruthenium and Osmium Chemistry Topical Issue
Abstract
The catalytic activity of the dihydride ruthenium complexes, RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2 and RuH2(PPh3)4, in the
hydrogenation of nitrogen containing heterocycles has been tested by analyzing the influence of reaction parameters such as tem-
perature, hydrogen pressure, catalyst concentration, on the rate and regioselectivity of the reaction.
RuH2(PPh3)4 shows a better catalytic activity with an 86.7% conversion of quinoline after 24 h at 100 ꢀC under a hydrogen pres-
sure of 25 bar, while RuH2(CO)2(PPh3)2 and RuH2(CO)2(PnBu3)2 in the same conditions give a conversion of 37.1% and 35.6%,
respectively. These results are confirmed by the reaction rate of the hydrogenation of quinoline, since the Kc in the presence of
RuH2(PPh3)4 (1.46 · 10À5 sÀ1
)
is higher than others (6.37 · 10À6 sÀ1 for RuH2(CO)2(PPh3)2 and 6.36 · 10À6 sÀ1 for
RuH2(CO)2(PnBu3)2).
Noteworthy is the selectivity of these catalytic systems in the hydrogenation of quinoline: in all tests the three catalysts lead to
1,2,3,4-tetrahydroquinoline as the major product, furthermore this compound is the only formed in the presence of RuH2
(CO)2(PPh3)2. The selectivity is affected by the presence of an acid (CH3COOH) or a base (NnBu3) in the reaction media.
The complex RuH2(PPh3)4 is catalytically active, even if in a minor extent, in the hydrogenation of isoquinoline, pyridine and 2-
methylpyridine.
The basicity of the substrate and steric hindrance around the nitrogen atom show a great influence on the conversion.
The results obtained suggest that the catalytic system activates a heterocyclic ring through the coordination of the heteroatom to
the metal centre of the complexes.
ꢁ 2005 Elsevier B.V. All rights reserved.
Keywords: Homogeneous catalysis; Ruthenium hydride; Hydrogenation; Quinoline; Pyridine
1. Introduction
used as model substrates in hydrodenitrogenation
(HDN), a reaction of relevant importance in the fossil
fuels industries. Therefore, increasing efforts are being
paid to set up selective processes to hydrogenate several
nitrogen containing heteroaromatic compounds.
Many examples of homogeneous catalytic complexes,
mostly based on Rh, Ru, Fe and Co, have been em-
ployed after the first application of Jardine and McQuil-
lin for the selective reduction of pyridine to piperidine
[1] in the presence of the Rh(py)3Cl3/NaBH4 system.
The regioselective hydrogenation of quinoline and its
derivatives is involved in the manufacture of intermedi-
ates of considerable industrial interest such as petro-
chemicals, fine chemicals and pharmaceuticals;
furthermore, quinoline and its derivatives are commonly
*
Corresponding author. Fax: +390554573531.
E-mail address: piero.frediani@unifi.it (P. Frediani).
0020-1693/$ - see front matter ꢁ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.06.025

918
P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
The activity and selectivity of several rhodium and
+
ruthenium complexes in different reaction conditions
have been studied by various authors [2] and extensive
studies on homogeneous hydrogenation of nitrogen con-
taining heterocycles by use of Rh(I) and Ru(II) com-
plexes, such as Rh(PPh3)3Cl, Ru(PPh3)3HCl and
[RhCp*(MeCN)3]2+, are due to Fish and co-workers,
who also proposed a mechanism for such reactions.
In this paper, we report our studies concerning the
hydrogenation of quinoline and some analogous sub-
strates, using the dihydride ruthenium complexes
RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2 and RuH2-
(PPh3)4. We have tried better conditions to perform
the selective reduction of quinoline to 1,2,3,4-THQ
(1,2,3,4-tetrahydroquinoline).
N
N
H
N
H
N
5,6,7,8-THQ
Q
DHQ
1,2,3,4-THQ
Scheme 1.
2.2.1. Reaction time
The influence of reaction time has been evaluated in
the range among 3–72 h, while the other reaction
parameters are kept constant. The conversion of the
substrate and the composition of the reaction mixture
are reported in Table 1.
In all cases, the conversion increases as the reaction
time improves. The RuH2(PPh3)4 (3) presents a greater
catalytic activity with respect to (1) and (2): just after
3 h the conversion in the presence of (3) is 54.4% (entry
11), while it is 5.4% in the presence of (1) (entry 1) and
7.5% in the presence of (2) (entry 6). After a reaction
time of 24 h, the conversion rises up to 86.7% in the
presence of (3) (entry 14).
The
complexes
RuH2(CO)2(PnBu3)2,
RuH2
(CO)2(PPh3)2 have been chosen in consideration of their
good catalytic activity in the hydrogenation of ketones,
a,b-unsaturated ketones, alkenes, alkynes, and unsatu-
rated nitrogen containing compounds [3,4], while the
specie RuH2(PPh3)4 provides an excellent catalytic activ-
ity toward the hydrogenation of nitrogen containing
compounds [3].
It is worthy to note that the hydrogenation is regio-
selective in the presence of (1) and (2), since 1,2,3,4-
THQ is the sole product even with a high conversion
of quinoline (82.1% and 68.2%, respectively, entries 5
and 10).
2. Results and discussion
2.1. Synthesis of ruthenium complexes: RuH2(PPh3)4,
RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2
A lower regioselectivity is obtained in the presence of
(3): 1,2,3,4-THQ is the major product while 5,6,7,8-
THQ and DHQ are formed in lower amounts. By
increasing the reaction time, the composition of the
reaction mixture changes: basically on increasing the
time from 3 to 15 h an increase of 1,2,3,4-THQ and
5,6,7,8-THQ is observed, while the percentage of DHQ
remains almost steady (entries 11–13). When the reac-
tion time is prolonged to 24 h, an appreciable increase
of 1,2,3,4-THQ and DHQ is observed (entry 14) to-
gether with a decrease of 5,6,7,8-THQ, suggesting that
DHQ is mainly formed through the reduction of
5,6,7,8-THQ. After a longer reaction time, until 96 h
(entries 15–17) the conversion and reaction composition
remain constant. These data suggest a deactivation of
the catalytically active species due to products formed.
It is reasonable to suppose a competition among quino-
line and the hydrogenated quinolines for the coordina-
tion of the catalytic species to the metal centre,
hampering the complete conversion of the substrate.
This hypothesis is confirmed by the data obtained from
the reaction performed in the presence of a Lewis base
(see Section 2.3).
The catalysts RuH2(CO)2(PnBu3)2 (1) [3], RuH2
(CO)2(PPh3)2 (2) [5] and RuH2(PPh3)4 (3) [6] have been
prepared according to the procedure described in the lit-
erature and their spectroscopic characteristics were in
agreement with the reported data.
While (1) and (2) are stable in solution under a nitrogen
atmosphere, the complex RuH2(PPh3)4 (3) is in equilib-
rium with two species: RuH2(PPh3)3 (4) and
RuH2(N2)(PPh3)3 (5), due to a phosphine dissociation
and coordination of a nitrogen molecule. The species (4)
and (5) are not present in the solid state. The spectroscopic
characteristics of the species present in solution are in
agreement with the data reported in the literature [3,5,6].
2.2. Catalytic activity of dihydride ruthenium complexes,
RuH2(CO)2(PnBu3)2, RuH2(CO)2(PPh3)2 and
RuH2(PPh3)4, in the hydrogenation of quinoline
The catalytic activity of (1), (2) and (3) has been
tested by evaluating the influence of reaction parameters
on the rate and regioselectivity. In a first step, 1,2,3,4-
tetrahydroquinoline (1,2,3,4-THQ) and 5,6,7,8-tetrahy-
droquinoline (5,6,7,8-THQ) are formed and in a second
step decahydroquinoline (DHQ) (Scheme 1) is formed.
Other intermediates were not evidenced in the reaction
medium.
Kinetic analysis of the data reported in Table 1
(entries 1–17), following the procedure reported by
Frediani et al. [8], shows a first partial order with respect
to reaction time with all the catalysts tested. The kinetic
rates (Kc) are in the following order: (3) ꢀ (2) @ (1)
(14.60, 6.37 and 6.36 · 10À6 sÀ1, respectively) (Table 2).

P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
919
Table 1
Hydrogenation of quinoline: influence of reaction timea
Entry
Catalyst code
Reaction time (h)
Conversion (%)
Reaction mixture composition (%)
Q
1,2,3,4-THQ
5,6,7,8-THQ
DHQ
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
3
3
3
6
24
48
72
3
6
9
24
48
3
5.4
7.8
35.6
65.1
82.1
7.5
94.6
92.2
64.4
34.9
17.9
92.5
91.0
80.2
62.9
31.8
45.6
31.3
26.1
13.3
15.1
14.2
11.2
5.4
7.8
35.6
65.1
82.1
7.5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
9.0
9.0
19.8
37.1
68.2
54.4
68.7
73.9
86.7
84.9
85.8
88.8
19.8
37.1
68.2
48.0
56.0
59.1
75.2
73.2
74.6
76.4
0
0
0
3.5
9.8
12.4
3.4
3.2
3.4
3.2
2.9
2.9
2.4
8.1
8.5
7.8
9.2
6
15
24
48
72
96
[Cat]: 0.66 mM; [Substrate]: 0.066 M; T: 373 K; P(H2) 25 bar; THF 20 ml.
a
Q: quinoline; 1,2,3,4-THQ: 1,2,3,4-tetrahydroquinoline; 5,6,7,8-THQ: 5,6,7,8-tetrahydroquinoline; DHQ: decahydroquinoline.
Table 2
Kinetic and thermodynamic data for the reduction of quinoline in the presence of RuH2(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2), and RuH2(PPh3)4
(3)a
Catalyst
Code
Kc · 106
(sÀ1
R2
Kp · 107
R2
Kcat · 101
R2
DH**
R2
DS**
R2
)
(sÀ1 barÀ1
)
(sÀ1 MÀ1
)
(KJ molÀ1
)
(J molÀ1 KÀ1
)
RuH2(CO)2(PnBu3)2
RuH2(CO)2(PPh3)2
RuH2(PPh3)4
a
1
2
3
6.36
6.37
14.60
0.99
0.99
0.94
1.92
1.80
–
1.00
0.99
–
–
–
1.02
–
–
0.97
1.0
47.1
32.1
1.00
0.90
1.00
À334.0
À215.0
À240.0
1.00
0.90
1.00
Data from Tables 1, 3 and 4.
2.2.2. Hydrogen pressure
with 25 bar of hydrogen up to 18.9% with 100 bar of
hydrogen (entries 1 and 19). It is interesting to point
out that working under 25 bar of hydrogen the reaction
gives 1,2,3,4-THQ as the sole product (entry 1) even
when the conversion is 82.1% (entry 5). However, work-
ing with 50 or 100 bar of hydrogen 1,2,3,4-THQ is the
main product but 5,6,7,8-THQ and DHQ are also
formed, although in a low amount (entries 18 and 19).
The catalytic activity of the complexes (1)–(3) has
been tested in conditions giving low conversions to evi-
dence how the hydrogen pressure affects the conversion.
The catalytic activity is remarkably affected by the
hydrogen pressure (Table 3) with an increment of con-
version as the hydrogen pressure increases. For instance,
the conversion in the presence of (1) rises from 5.4%
Table 3
Hydrogenation of quinoline: influence of hydrogen pressurea
Entry
Catalyst code
Catalyst/substrate (molar ratio)
P(H2) (bar)
Conversion (%)
Reaction mixture composition (%)
Q
1,2,3,4-THQ
5,6,7,8-THQ
DHQ
1
18
19
6
20
21
22
23
24
25
1
1
1
2
2
2
3
3
3
3
1/100
1/100
1/100
1/100
1/100
1/100
1/200
1/200
1/200
1/200
25
50
100
25
50
100
25
50
75
100
5.4
9.3
18.9
7.5
10.5
19.8
40.1
56.7
33.6
36.2
94.6
90.7
81.1
92.5
89.5
80.2
59.9
43.3
66.4
63.8
5.4
8.5
17.3
7.5
10.5
19.8
36.6
51.8
29.6
26.9
0
0
0.4
0.9
0
0
0
1.8
3.3
3.4
9.1
0.4
0.7
0
0
0
1.7
1.6
0.6
0.2
[Substrate]: 0.066 M; reaction time: 3 h; THF: 20 ml; T: 373 K.
a
Q: quinoline; 1,2,3,4-THQ: 1,2,3,4-tetrahydroquinoline; 5,6,7,8-THQ: 5,6,7,8-tetrahydroquinoline; DHQ: decahydroquinoline.

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P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
Almost the same behaviour is shown in the presence of
(2) with a slightly greater catalytic activity with respect
to (1) but, noteworthy, the reaction always gives
1,2,3,4-THQ as the sole product.
2.2.3. Temperature
The influence of the temperature on the conversion
and selectivity of the reactions in the presence of RuH2
(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2) and RuH2
(PPh3)4 (3) has been investigated among 353 and
393 K. The results are reported in Table 4.
The complex (3) shows the best catalytic activity giv-
ing a 34.7% conversion at 353 K, with formation of
1,2,3,4-THQ as the main product and 5,6,7,8-THQ
and DHQ as secondary products (entry 32). An increase
of the temperature causes an appreciable increase of the
conversion from 34.7% at 353 K to 76.4% at 393 K (en-
try 33). In the presence of (3), 1,2,3,4-THQ is always the
main product. As the temperature increases the amount
of DHQ prevails on that of 5,6,7,8-THQ.
Concerning the other catalysts, complex (1) does not
show any catalytic activity at 353 K, under 100 bar of
hydrogen (entry 26) while (2) is slightly active with a
conversion of 3.9% (entry 29). Working at 393 K the
conversion increases up to 20.1% for (1) and 20.8% for
(2) (entries 28 and 31). The temperature does not affect
the selectivity of the reaction, since the catalyst (1) pro-
vides 1,2,3,4-THQ with a high regioselectivity while (2)
gives exclusively 1,2,3,4-THQ.
The activation parameters [9] have been evaluated
using the data reported in Table 3 and the Gibbs equa-
tion. Negative activation entropies were found for all
catalysts, indicating, in all cases, an associative process
in the rate-determining step of the reaction (Table 2).
The reaction in the presence of (3) shows a different
behaviour. An increase of the hydrogen pressure in the
range from 25 to 50 bar causes an increase of the conver-
sion from 40.1% to 56.7% (entries 22 and 23), as shown
in the presence of (1) or (2). In these reductions,
however, 1,2,3,4-THQ is the major product but
5,6,7,8-THQ and DHQ are present; the amount of
5,6,7,8-THQ is higher than DHQ. By increasing the
hydrogen pressure up to 75 bar, a surprising lower con-
version has been observed: 1,2,3,4-THQ is still the main
product but the relative amounts of 5,6,7,8-THQ and
DHQ are increased (entry 24). A further improvement
of the hydrogen pressure increases the amount of
5,6,7,8-THQ, while the yield of 1,2,3,4-THQ remains al-
most unchanged (entry 25). Such behaviour could be
rationalised suggesting the formation of a dihydrogen
complex which at high pressure of hydrogen is not able
to activate quinoline. A RuH2(H2)(PPh3)3 has been re-
ported in the literature [6,7]. Nevertheless, it must be
noted that RuH2(PPh3)4 (3) does not show an analogous
behaviour in the reduction of other substrates but this
behaviour may also be affected by the coordinating abil-
ity of the substrates itself. An alternative hypothesis may
be the formation of different catalytic species at different
hydrogen pressures and the species operating at pressure
higher than 50 bar should be less active. This hypothesis
is in agreement with the different regio- and chemoselec-
tivity observed in the experiments under different hydro-
gen pressures.
2.2.4. Catalyst concentration
An increase of the concentration of (3) shows a ben-
eficial effect on the conversion that changes from 12.9%
to 54.4% when the catalyst/Q ratio changes from 1/1000
to 1/100 (entries 24 and 34–35) (Table 5), while the selec-
tivity of the reaction seems basically unaffected. The ki-
netic analyses of the data reported in Table 4 show a
first partial order with respect to the concentration of
the catalyst, with a specific rate of 1.02 · 10À1 MÀ1 sÀ1
(Table 2).
Kinetic analysis of the data reported in Table 2 (en-
tries 1, 6, 18–21), following the procedure reported by
Frediani et al. [8], shows a first partial order with respect
to hydrogen pressure for (1) and (2). The specific rate
(Kp) is almost the same with values, respectively, of
1.92 · 10À7 (sÀ1 barÀ1
)
for (1) and 1.80 · 10À7
(sÀ1 barÀ1) for (2) (Table 2).
Table 4
Hydrogenation of quinoline: influence of reaction temperaturea
Entry
Catalyst code
T (K)
P(H2) (bar)
Conversion (%)
Reaction mixture composition (%)
Q
1,2,3,4-THQ
5,6,7,8-THQ
DHQ
26
27
28
29
30
31
32
11
33
1
1
1
2
2
2
3
3
3
353
373
393
353
373
393
353
373
393
100
100
100
100
100
100
25
0
100
81.1
0
0
0
18.9
20.1
3.9
19.8
20.8
34.7
54.4
76.4
17.3
18.4
3.9
19.8
20.8
26.1
48.0
70.6
0.9
1.0
0
0
0
6.7
3.5
2.4
0.7
0.7
0
0
0
1.9
2.9
3.5
79.9
96.1
80.2
79.2
65.3
45.6
23.6
25
25
[Cat]: 0.66 mM; [Substrate]: 0.066 M; THF 20 ml; reaction time: 3 h.
a
Q: quinoline; 1,2,3,4-THQ: 1,2,3,4-tetrahydroquinoline; 5,6,7,8-THQ: 5,6,7,8-tetrahydroquinoline; DHQ: decahydroquinoline.

P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
921
Table 5
Hydrogenation of quinoline in the presence of RuH2(PPh3)4 (3): influence of catalyst concentrationa
Entry
Catalyst concentration (mM)
Conversion (%)
Reaction mixture composition (%)
Q
1,2,3,4-THQ
5,6,7,8-THQ
DHQ
34
24
35
0.0822
0.3300
0.6600
12.9
40.1
54.4
87.1
59.9
45.6
12.9
36.6
48.0
0
1.8
3.5
0
1.7
2.9
[Substrate]: 0.066 M; THF 20 ml; P(H2): 25 bar; reaction time: 3 h; T: 373 K.
a
Q: quinoline; 1,2,3,4-THQ: 1,2,3,4-tetrahydroquinoline; 5,6,7,8-THQ: 5,6,7,8-tetrahydroquinoline; DHQ: decahydroquinoline.
2.3. Hydrogenation of 1,2,3,4-tetrahydroquinoline and
5,6,7,8-tetrahydroquinoline in the presence of
RuH2(PPh3)4
2.4. Influence of an acid or a basic media
In the literature, several studies are reported on the
possibility to modify the regioselectivity of the catalytic
hydrogenation of quinoline using an acid or a basic
media. Vierhapper and Eliel [10] obtained the prevalent
hydrogenation of Q to 5,6,7,8-THQ working in the pres-
ence of HCl, in heterogeneous phase with PtO2 as
catalyst.
Campanati et al. [11] reported the use of a sterically
hindered Lewis base to favour the formation of DHQ
in the presence of heterogeneous catalytic systems
(Ru/Al2O3 5 wt.%). They report that the products of
partial hydrogenation of the heterocyclic ring of Q,
formed during the reaction, are able to coordinate
the active sites of the metal causing a progressive de-
crease of activity. This inhibition is a general behaviour
in many catalytic reactions in which the products are
stronger bases than reactants. The amine NnBu3, added
to the media, should compete for the active sites of the
catalytic system, hampering change in causing its
deactivation.
We have reported in Section 2.2.1 that RuH2(PPh3)4
provides an appreciable amount of DHQ in the hydro-
genation of Q. Its formation is attributed to the hydro-
genation of 5,6,7,8-THQ (Scheme 2). In order to verify
this hypothesis, we have tested the catalytic activity of
the complex (3) in the hydrogenation of 1,2,3,4-THQ
or 5,6,7,8-THQ, respectively. The reaction parameters
have been chosen in order to compare the data of these
reactions with those of the reduction of Q (Table 6).
The complex (3) shows a greater catalytic activity in
the hydrogenation of the carbocyclic ring rather than
the heterocyclic one: in fact there is no conversion of
1,2,3,4-THQ after 3 h (entry 36), while the conversion
of 5,6,7,8-THQ is 7.5% (entry 37). A 2.1% conversion
of 1,2,3,4-THQ may be obtained only after 24 (entry 38).
We may conclude that DHQ is formed from Q
through the hydrogenation of the 5,6,7,8-THQ interme-
diate. The different reactivity of 5,6,7,8-THQ with re-
spect to 1,2,3,4-THQ may be attributed to the
presence of a more electronegative nitrogen atom on
5,6,7,8-THQ favouring the attack of the reducing agent
on the heterocyclic ring.
On this basis, we have carried out some experiments
in the presence of bases or acids such as NaOH, NnBu3,
CH3COOH or HCl in order to evaluate their influence
on the rate and regioselectivity of the reaction.
RuH2(PPh3)4 (3) has been chosen as catalyst in consid-
eration of its better catalytic activity. Different ratios be-
tween catalyst and base or acid were employed. The
results are collected in Table 7.
N
The addition of NaOH to the reaction solution com-
pletely inhibits the Q conversion, in agreement to that
reported by Campanati et al. [11]. Also, the presence
of NnBu3 reduces the conversion from 54.4% (entry
11) to 41.7% (entry 40) and causes a change in the selec-
tivity: 1,2,3,4-THQ still remains the prevailing product
but the yield of DHQ significantly increases and over-
whelms that of 5,6,7,8-THQ. Therefore, the base has a
beneficial effect on the hydrogenation of Q to DHQ. It
may be likely that in the absence of an added base the
DHQ formed during the reaction competes with the sub-
strate for the active sites of the catalyst reducing its
activity. To confirm this hypothesis, it is worthy to note
that the hydrogenation of quinoline in the presence of
(3) carried out for 48 h, and longer (entries 15–17), does
not show any remarkable increase of the conversion
N
H
N
N
H
Scheme 2.

922
P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
Table 6
+
Hydrogenation of tetrahydroquinolines in the presence of
N
N
N
N
RuH2(PPh3)4 (3)a
H
5,6,7,8-THIQ
IQ
DHIQ
1,2,3,4-THIQ
Entry Substrate
Conversion Reaction Reaction mixture
(%)
time (h)
composition (%)
Scheme 3.
Substrate DHQ
36
37
38
1,2,3,4-THQ
0
5,6,7,8-THQ 7.5
1,2,3,4-THQ 2.1
3
3
24
100
92.5
97.9
0
7.5
2.1
N
R
N
R
[RuH2(PPh3)4]: 0.66 mM; [Substrate]: 0.066 M; THF: 20 ml; P(H2):
25 bar; T: 373 K.
a
1,2,3,4-THQ: 1,2,3,4-tetrahydroquinoline; 5,6,7,8-THQ: 5,6,7,8-
R = H, PY
R = H, PP
tetrahydroquinoline; DHQ: decahydroquinoline.
R = Me, MPY
R = Me, MPP
Scheme 4.
with respect to the reaction carried out for 24 h (entry
14); this may be attributed to the inhibiting effect of
DHQ on the catalytic species.
2.5. Hydrogenation of isoquinoline, pyridine and
2-methylpyridine in the presence of RuH2(PPh3)4
Also, the presence of CH3COOH causes a decrease of
the conversion from 54.4% (entry 11) to 39.6% (entry
41), and affects the regioselectivity favouring the hydro-
genation of the carbocyclic ring of the quinoline: 1,2,3,4-
THQ and 5,6,7,8-THQ are obtained in similar amount,
while DHQ remains lower. This behaviour may be as-
cribed to a different co-ordination of protonated quino-
line, which can determinate a remarkable change in the
selectivity.
In order to evaluate the influence of different elec-
tronic and steric characteristics of the substrate on the
conversion and selectivity, the hydrogenation of iso-
quinoline (IQ) (Scheme 3) has been performed and the
results are reported in Table 8.
The complex (3) hydrogenates IQ with a lower cata-
lytic activity than Q: the conversion is 36.3% (entry
43) but surprisingly an opposite selectivity is obtained
since the reaction gives exclusively 5,6,7,8-tetrahydro-
quinoline (5,6,7,8-THIQ).
Using a strong acid, that is a THF solution saturated
with HCl, no reaction has been observed (entry 42).
Table 7
Hydrogenation of quinoline in the presence of RuH2(PPh3)4 (3): influence of the presence of a base or an acida
Entry
Acid or base
Concentration (mM)
Conversion (%)
Reaction mixture composition (%)
Q
1,2,3,4-THQ
5,6,7,8-THQ
DHQ
11
39
40
41
42
//
//
3.96
66.00
3.96
54.4
0
41.7
39.6
0
45.6
100
58.3
60.4
100
48.0
0
38.6
21.9
0
3.5
0
0.8
15.8
0
2.9
0
2.3
1.9
0
NaOH
NnBu3
CH3COOH
HCl
saturated sol.
[RuH2(PPh3)4]: 0.66 mM; [Substrate]: 0.066 M; THF: 20 ml; P(H2): 25 bar; T: 373 K; reaction time: 3 h.
a
Q: quinoline; 1,2,3,4-THQ: 1,2,3,4-tetrahydroquinoline; 5,6,7,8-THQ: 5,6,7,8-tetrahydroquinoline; DHQ: decahydroquinoline.
Table 8
Hydrogenation of the nitrogen containing heterocycles in the presence of RuH2(PPh3)4 (3)a
Entry
Substrate
Conversion (%)
Reaction mixture composition (%)
Substrate
1,2,3,4-TH
5,6,7,8-TH
DH
PP
MPP
11
43
44
45
Q
IQ
PY
MPY
54.4
36.3
36.3
4.1
45.6
63.7
63.7
95.9
48.0
0
–
3.5
36.3
–
2.9
0
–
–
–
36.3
–
–
–
–
4.1
–
–
–
[RuH2(PPh3)4]: 0.66 mM; [Substrate]: 0.066 M; THF 20 ml; P(H2): 25 bar; T: 373 K; reaction time: 3 h.
a
Q: quinoline; 1,2,3,4-TH: 1,2,3,4-tetrahydroquinoline or 1,2,3,4-tetrahydroisoquinoline; 5,6,7,8-TH: 5,6,7,8-tetrahydroquinoline or 5,6,7,8-tet-
rahydroisoquinoline; DH: decahydroquinoline or decahydroisoquinoline; PY: pyridine; MPY: 2-methylpyridine; PP: piperidine; MPP: 2-
methylpiperidine.

P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
923
Pyridine (PY) and 2-methylpyridine (MPY) are also
hydrogenated in the presence of RuH2(PPh3)4 (3) (Table
8) (Scheme 4). A lower catalytic activity with respect to
the hydrogenation of Q has been observed also in these
reductions with conversions, respectively, of 36.3% (en-
try 44) and 4.1% (entry 45). These data are in agreement
with those reported by Fish et al. [12], who noted a de-
crease of the conversion as the basicity and steric hin-
drance around the nitrogen atom increase.
steric and electronic factors, as reported by FishÕs stud-
ies [12–15].1
In conclusion, the three complexes (1)–(3) are catalyt-
ically active in the hydrogenation of nitrogen containing
heterocycles giving results comparable with those re-
ported in the literature for other ruthenium systems like
RuCl2(CO)2(PPh3)2 and Ru4H4(CO)12, but operating
under milder reaction temperatures (353–373 K against
453 K) and lower catalyst/substrate ratio (1/100 against
1/10) [15].
The data collected are not sufficient to propose a
mechanism of the reaction but the negative value of
the activation entropy indicates with all catalysts an
associative rate-determining step. Further studies are
in progress to collect new evidences on the mechanism
of these reactions.
3. Conclusion
The dihydride ruthenium complexes, RuH2(CO)2(Pn-
Bu3)2 (1), RuH2(CO)2(PPh3)2 (2) and RuH2(PPh3)4 (3)
are catalytically active in the hydrogenation of quinoline
(Q). The best performance has been obtained with
RuH2(PPh3)4 (3), which allows a 86.7% conversion at
373 K while RuH2(CO)2(PnBu3)2 (1) and RuH2
(CO)2(PPh3)2 (2) give lower conversions (35.6% and
37.1%, respectively) but higher selectivities.
4. Experimental
4.1. Instruments and materials
The catalytic activity is significantly affected by the
hydrogen pressure. The reaction in the presence of
RuH2(CO)2(PnBu3)2 (1) or RuH2(CO)2(PPh3)2 (2) shows
a first partial order with respect to hydrogen pressure.
The RuH2(PPh3)4 (3) catalyst shows a singular trend:
the conversion rises when increasing the hydrogen pres-
sure up to 50 bar, but at higher pressure (75 and
100 bar) the conversion decreases, and the selectivity
changes suggesting the presence of different catalytically
active species as the amount of hydrogen changes.
A first partial order with respect to the catalyst con-
centration has been detected in the presence of
RuH2(PPh3)4 (3).
The regioselectivity of the hydrogenation of Q is
complete using RuH2(CO)2(PPh3)2 (2) as catalyst,
while in the presence of RuH2(CO)2(PBu3)2 or
RuH2(PPh3)4 an appreciable amount of 5,6,7,8-THQ
and DHQ is formed. Also, the presence of a weak acid
or base in the reaction medium changes the selectivity.
It is interesting to note that in the presence of acetic
acid the products of partial reduction (1,2,3,4-THQ
and 5,6,7,8,-THQ) are formed in equal amount, there-
fore, the presence of acetic acid favours the hydrogena-
tion of the carbocyclic ring with respect to heterocyclic
one.
Quantitative analysis was performed by GC using a
Shimadzu GC-14A chromatograph coupled with a com-
puter Shimadzu C-R4A, equipped with a FID detector,
using 2-m packed columns filled with a CW 20
M + KOH (Chromosorb W) as stationary phase. p-Xy-
lene was used as internal standard and calibration
curves for quinoline (Q) and 1,2,3,4-tetrahydroquinoline
(1,2,3,4-THQ) were calculated 5,6,7,8-tetrahydroquino-
line (5,6,7,8-THQ), decahydroquinoline (DHQ), iso-
quinoline (IQ) and 5,6,7,8-tetrahydroisoquinoline
(5,6,7,8-THIQ) were assumed to have the same response
factor than 1,2,3,4-THQ. Pyridine (PY), 2-methylpyri-
dine (MPY), piperidine (PP) and 2-methylpiperidine
(MPP) were assumed to have the same response factors.
The identity of the products was confirmed by GC–
MS using a Shimadzu apparatus (GCMS-QP5050A)
equipped with a capillary column SPTM-1 (length
30 m, i.d. 0.25 mm, film thickness 0.1 lm).
Elemental analyses were performed with a Perkin–
Elmer Analyser model 2400 Series II CHNS/O. IR spec-
tra were recorded with a Perkin-Elmer mod. 1760 FTIR
spectrometer.
1H, 13C and 31P NMR spectra were recorded using a
Varian
VXR300
spectrometer
operating
at
The complex (3) is also catalytically active in the
hydrogenation of isoquinoline (IQ), although in a less
extent than quinoline (Q), with a complete and unusual
regioselectivity towards 5,6,7,8-THIQ [13]. A different
coordination mode of the substrate to the metal centre
may be claimed in agreement with the result obtained
for the hydrogenation of Q in acid media.
299.987 MHz for 1H, at 75.429 MHz for 13C and at
121.421 MHZ for 31P NMR, using solutions in appro-
priate solvents. SiMe4 was used as external standard
1
for H and 13C NMR, H3PO4 (85%) for 31P NMR (sig-
nals reported as positive downfield to the standard). 13
C
Pyridine (PY) and 2-methylpyridine (MPY) may be
hydrogenated in the presence of the complex (3). The
conversions are lower confirming a strong influence of
1
The pKa of the substrates are: Q 4.94, IQ 5.10, PYR 5.23, MePYR
5.97; The steric hindrance are in the following order:
PYR @ IQ < MePYR @ Q.

924
P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
and 31P NMR spectra were acquired using a broadband
decoupler.
4.2.2. Hydrogenation in the presence of RuH2(PPh3)4
(3)
All manipulations were carried out under a nitrogen
atmosphere using standard Schlenk techniques. Re-
agents and solvents were purified and dried as re-
ported. Tetrahydrofuran (J.T. Baker pure at 99%)
was dried and deoxygenated by refluxing and distilling
over sodium/potassium amalgam under nitrogen atmo-
sphere, immediately before its use (bp 66 ꢀC).
Ru3(CO)12 (Aldrich pure at 99%) was purified by crys-
tallisation from methanol under a 20 bar of CO pres-
sure. Pyridine (Carlo Erba pure at 99.6%) was
refluxed over KOH for 2 h, then distilled under nitro-
gen, immediately before its use (bp 115 ꢀC). 2-Methyl-
pyridine (Aldrich pure at 98%) was distilled at reduced
pressure, immediately before its use (bp 25 ꢀC/
5 mmHg). Other solvents and reagents were obtained
from commercial suppliers and employed without fur-
ther purification.
In
a
Schlenk tube, RuH2(PPh3)4 (3) (1.32 ·
10À5 mol), substrate (1.32 · 10À3 mol), and p-xylene
(1.131 · 10À3 mol) as internal standard were dissolved
in THF (20 mL). The solution was transferred by suc-
tion in a Parr autoclave model 4759 (150 mL) electrically
heated, with a magnetic drive stirrer. After addition of
hydrogen up to the pressure required, the reactor was
stirred at the established temperature.
At the end of the reaction, the vessel was cooled
down, the gas vented out and the solution analysed by
GC and GC–MS techniques.
The same procedure was followed for the catalytic
hydrogenations in the presence of NaOH, CH3COOH,
and NnBu3. The hydrogenation in the presence of HCl
was carried out using a saturated HCl solution prepared
by bubbling gaseous HCl in THF.
The following catalysts were prepared according to
the literature and their spectroscopic characteristics
were in agreement with the data reported: Ru(CO)2-
(CH3COO)2(PnBu3)2 [16], Ru(CO)2(CH3COO)2(PPh3)2
[17], RuH2(CO)2(PnBu3)2 (1) [3], RuH2(CO)2(PPh3)2
(2) [5], and RuH2(PPh3)4 (3) [1].
Acknowledgements
The authors thank the University of Florence, and
the Ministero della Industria, Universita e Ricerca
`
(MIUR), Programmi di Ricerca Scientifica di Notev-
ole 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, Department of
Organic Chemistry, University of Florence for elemental
analyses.
4.2. Catalytic hydrogenation
A typical hydrogenation experiment was reported for
each ruthenium catalyst: the same procedure was em-
ployed for all substrates.
4.2.1. Hydrogenation in the presence of
RuH2(CO)2(PnBu3)2 (1), RuH2(CO)2(PPh3)2 (2)
Each catalyst was prepared immediately before its use
from the corresponding acetate complex, following the
procedure reported in the literature [3,5]. Ru(CO)2(CH3-
COO)2(PR3)2, (1.32 · 10À5 mol), 4 mL of dry THF, and
Na2CO3 (0.5 g) were introduced in a glass vial inserted
in a stainless steel autoclave, then hydrogen up to
100 bar was added. The vessel 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 fil-
tered and transferred in a Schlenk tube containing the
References
[1] I. Jardine, F.J. McQuillin, J. Chem. Soc. D (1970) 626.
[2] (a) R.A. Sanchez-Delgado, D. Rondon, A. Andriollo, W.
´
Herrera, G. Martin, B. Chaudret, Organometallics 12 (1993)
4291;
(b) Y. Alvarado, M. Busolo, F. Lopez-Linares, J. Mol. Catal.
A 142 (1999) 163;
(c) C. Bianchini, P. Barbaro, M. Macchi, A. Meli, F. Vizza,
Helv. Chim. Acta 84 (2001) 2895.
[3] A. Salvini, P. Frediani, M. Bianchi, F. Piacenti, Inorg. Chim.
Acta 227 (1994) 247.
[4] A. Toti, P. Frediani, A. Salvini, L. Rosi, C. Giolli, C. Giannelli,
C. R. Chimie 7 (2004) 769.
[5] P. Frediani, C. Faggi, S. Papaleo, A. Salvini, M. Bianchi, F.
Piacenti, S. Iannelli, M. Nardelli, J. Organomet. Chem. 123
(1997) 536.
[6] J.J. Levinson, S.D. Robinson, J. Chem. Soc. A (1970) 2947.
[7] D.E. Linn, J. Chem. Ed. 76 (1999) 70.
[8] A. Salvini, P. Frediani, S. Gallerini, Appl. Organometal. Chem.
14 (2000) 570.
[9] I. Klotz, Chemical Thermodynamic, W.A. Benjamin, Inc., New
York, 1964, p. 165.
[10] W.F. Vierhapper, E.L. Eliel, J. Am. Chem. Soc. 96 (1974) 7.
[11] M. Campanati, A. Vaccari, O. Piccolo, J. Mol. Catal. A: Chem.
179 (2002) 287.
substrate
(1.32 · 10À3 mol)
and
p-xylene
(1.131 · 10À3 mol) 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 tempera-
ture, for the time required.
At the end of the reaction, the vessel was cooled
down, the gas vented out and the solution analysed by
GC and GC–MS techniques.

P. Frediani et al. / Inorganica Chimica Acta 359 (2006) 917–925
925
[12] R.H. Fish, R.H. Fong, A. Than, E. Baralt, Organometallics 10
(1991) 1209.
[15] R.H. Fish, A.D. Thormodsen, G.A. Cremer, J. Am. Chem. Soc.
104 (1982) 5234.
[13] T.L. Gilchrist, Heterocyclic Chemistry, third ed., Longman,
London, 1997, p. 168.
[14] K. Schofield, Hetero-Aromatic Nitrogen Compounds, Plenum
Press, New York, 1967, p. 146.
[16] M. Bianchi, P. Frediani, U. Matteoli, G. Menchi, F. Piacenti, G.
Petrucci, J. Organomet. Chem. 259 (1983) 207.
[17] S.D. Robinson, M.F. Uttley, J. Chem. Soc. Dalton Trans. (1973)
1912.