Ruthenium carbonyl compounds containing polypyridine ligands as catalysts in the reaction of N-benzylideneaniline hydrogenation

Article abstract of DOI:10.1016/j.inoche.2012.05.047

The synthesis and characterization of ruthenium complexes containing polypyridine ligands: Ru(dppz)(PPh₃)₂Cl₂, Ru(bpy)(PPh₃)₂Cl₂, Ru(phen)(PPh ₃)₂Cl₂, Ru(dppz-Cl)(PPh₃) ₂Cl₂, Ru(phen)(CO)₂Cl₂, Ru(bpy)(CO)₂Cl₂ and Ru(dppz)(CO)₂Cl₂ (where dppz: dipyrido[3,2-a:2′,3′-c]phenazine, dppz-Cl: 10-chlororodypirido[3,2-a:2′,3′-c]phenazine, phen: 1,10-phenanthroline and bipy: 2,2′-bipyridine) are reported. The ruthenium complexes show high activity as catalysts in the hydrogenation reaction of N-benzylideneaniline and the hydrogen transfer reaction. The products of the catalysis were obtained with conversions between 21 and 91% after 2 h of reaction. The Ru(phen)(CO)₂Cl₂ complex was the catalyst that showed the highest conversion (91%) for the hydrogenation of N-benzylideneaniline. The complexes Ru(dppz)(PPh₃)₂Cl ₂, Ru(bpy)(PPh₃)₂Cl₂ and Ru(dppz)(CO)₂Cl₂ showed 99% conversion in the hydrogen transfer reaction.

Full text of DOI:10.1016/j.inoche.2012.05.047

Inorganic Chemistry Communications 22 (2012) 146–148
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Ruthenium carbonyl compounds containing polypyridine ligands as catalysts in the
reaction of N-benzylideneaniline hydrogenation
a
a
a
b
b,
S.A. Moya a, , M. Vidal , K. Brown , C. Negrete-Vergara , G. Abarca , P. Aguirre
⁎
⁎
a
Universidad de Santiago de Chile, Facultad de Química y Biología, Av. Libertador Bernardo O'Higgins 3363, Casilla 40, Correo 33, Santiago, Chile
Universidad de Chile, Facultad de Ciencias Químicas y Farmacéuticas, Casilla 233, Santiago 1, Chile
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of ruthenium complexes containing polypyridine ligands: Ru(dppz)(PPh3)2Cl2,
Ru(bpy)(PPh3)2Cl2, Ru(phen)(PPh3)2Cl2, Ru(dppz-Cl)(PPh3)2Cl2, Ru(phen)(CO)2Cl2, Ru(bpy)(CO)2Cl2 and
Ru(dppz)(CO)2Cl2 (where dppz: dipyrido[3,2-a:2′,3′-c]phenazine, dppz-Cl: 10-chlororodypirido[3,2-a:2′,3′-c]
phenazine, phen: 1,10-phenanthroline and bipy: 2,2′-bipyridine) are reported. The ruthenium complexes show
high activity as catalysts in the hydrogenation reaction of N-benzylideneaniline and the hydrogen transfer reac-
tion. The products of the catalysis were obtained with conversions between 21 and 91% after 2 h of reaction.
The Ru(phen)(CO)2Cl2 complex was the catalyst that showed the highest conversion (91%) for the hydrogenation
of N-benzylideneaniline. The complexes Ru(dppz)(PPh3)2Cl2, Ru(bpy)(PPh3)2Cl2 and Ru(dppz)(CO)2Cl2 showed
99% conversion in the hydrogen transfer reaction.
Received 21 December 2011
Accepted 25 May 2012
Available online 31 May 2012
Keywords:
Ruthenium hydrogenation
Imine hydrogenation
Homogeneous catalysis
© 2012 Elsevier B.V. All rights reserved.
Introduction: The hydrogen transfer reaction of polar functional
groups has significantly contributed to the recent growth in the area
of organic synthesis [1]. The reaction is recommended for its simplic-
ity since no hydrogen gas is required. When propan-2-ol is used as
the hydrogen donor, the only by-product formed is acetone, which
is easily removed by distillation during workup. While the transfer
hydrogenation reaction of ketones [1–3] has been widely explored
during the last two decades, the corresponding reaction of imines
[1,4,5] has been less studied (Scheme 1). The hydrogenation of imines
using [RuCl2(PPh3)3], propan-2-ol, and K2CO3 as base was carried out
nearly a decade ago [4,5]. Noyori et al. reported the first asymmetric
transfer hydrogenation of imines by formic acid using a chiral ruthe-
nium catalyst with excellent yields and enantioselectivities. More re-
cently, Baker and Mao reported transfer hydrogenation of imines by
formic acid using chiral Noyori ligands on a Rhodium catalyst [5].
No asymmetric transfer hydrogenation of imines by propan-2-ol has
been reported so far. Some additional recent contributions of transfer
hydrogenation of imines include Ru(II) [6,7], Rh(III) [8,9], Ir(III) [10]
and Ni(0) [11] catalysts. The proposed mechanism for transfer
hydrogenation of ketones and imines with ruthenium complex in-
volves a step where the hydride and the proton are transferred
from the catalyst to the substrate in a concerted reaction without co-
ordination of the substrate.
We report the synthesis and characterization of ruthenium
complexes containing polypyridine ligands: Ru(dppz)(PPh3)2Cl2,
Ru(bpy)(PPh3)2Cl2, Ru(phen)(PPh3)2Cl2 Ru(dppz-Cl)(PPh3)2Cl2,
Ru(phen)(CO)2Cl2, Ru(dppz)(CO)2Cl2 and Ru(bpy)(CO)2Cl2 (where
dppz: dipyrido[3,2-a:2′,3′-c]phenazine, dppz-Cl:10-chlororodypirido
[3,2-a:2′,3′-c]phenazine, phen: 1,10-phenanthroline and bipy: 2,2′-
bipyridine). The complexes were used in homogeneous catalytic
processes to transfer hydrogen from 2-propanol towards N-
benzylideneaniline (Scheme 1). The compounds Ru(bpy)(PPh3)2Cl2,
Ru(dppz)(PPh3)2Cl2 and Ru(phen)(PPh3)2Cl2 were successfully syn-
thesized using the procedure for Ru(dppz-Cl)(PPh3)2Cl2. The prod-
ucts of the catalytic reaction were obtained in high yields with
conversions in the range of 21–91% after 2 h of reaction with N-
benzylideneaniline. The best conversion was obtained using the
Ru(phen)(CO)2Cl2 complex as catalyst (91%).
Experimental: 1H-NMR and 31P-NMR spectra were recorded on a
350 MHz Bruker spectrometer. The precursor, Ru(PPh3)3Cl2, was syn-
thesized by a procedure previously described [12] without modifica-
tions. The procedure of synthesis of the complexes Ru(bpy)(CO)2Cl2,
Ru(phen)(CO)2Cl2 and Ru(dppz)(CO)2Cl2 have been published previ-
ously [13]. The complexes trans-PPh3-[Ru(dppz)(PPh3)2Cl2 (1) and
trans-PPh3-[Ru(dppz-Cl)(PPh3)2Cl2 (2) trans-PPh3-[Ru(bpy)(PPh3)2Cl2
⁎
Corresponding authors. Fax: +56 2 7370567.
E-mail addresses: sergio.moya@usach.cl (S.A. Moya), paguirre@ciq.uchile.cl
(P. Aguirre).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.05.047

SA. Moya et al. / Inorganic Chemistry Communications 22 (2012) 146–148
147
Table 1
Catalytic activity in hydrogen transfer reaction of acetophenone.
Conversion (%); (TOF h−1
30 60 90
91; (1820) 96; (960) 98; (653) 99; (495)
Run Catalyst/time (minutes)
)
120
1
Ru(dppz)(PPh3)2Cl2 (1)
Ru(dppz-cl)(PPh3)2Cl2 (2) 56; (1120) 67; (670) 74; (493) 78; (390)
Scheme 1. Hydrogenation of N-benzylideneaniline by a new ruthenium (II) complex
derivated from polypyridine ligands.
2
3
4
5
6
7
8
Ru(phen)(PPh3)2Cl2 (3)
Ru(bpy)(PPh3)2Cl2 (4)
Ru(bpy)(CO)2Cl2 (5)
Ru(phen)(CO)2Cl2 (6)
Ru(dppz)(CO)2Cl2 (7)
Ru(PPh3)3Cl2 (8)
50; (1000) 56; (560) 59; (393) 63; (315)
96; (1920) 97; (970) 99; (660) 99; (495)
64; (1280) 90; (900) 96; (640) 97; (485)
74; (1480) 93; (930) 95; (633) 98; (490)
97; (1940) 98; (980) 99; (660) 99; (495)
(3) [14] trans-PPh3-[Ru(phen)(PPh3)2Cl2 [15] (4) were synthesized
from Ru(PPh3)3Cl2.1 The complex Ru(bpy)2Cl2 and Ru(phen)2Cl2 were
synthesized using the procedure reported in the literature[16]. The N-
benzylideneaniline hydrogenation was conducted using iso-propanol
in a basic medium with a ratio of substrate/catalyst=400 and a ratio
of KOH/Ru=20. A similar procedure was used in the transfer hydroge-
nation reaction of acetophenone with a ratio substrate/catalyst=1000/
1. All experiments were analyzed by GC‐chromatography2 using a HP-5
column.
2; (40)
3; (30)
3; (20)
3; (15)
9
10
11
[Ru(CO)2Cl2]n (9)
Ru(phen)2Cl2 (10)
Ru(bpy)2Cl2 (11)
10 (200)
No active
No active
14 (140) 19; (126) 24; (120)
Substrate/ruthenium=1000; solvent=2-propanol; base/ruthenium=20.
have shown activities near 99%, however compound 3 shows 63%.
This is the consequence of an increased electron density on the
metal provided by the polypiridine ligand. On the other hand the com-
pounds Ru(bpy)2Cl2 and Ru(phen)2Cl2 were not active catalysts in the
hydrogen transfer of acetophenone maybe because the lower solubil-
ity of complexes or the steric effect did not allow the formation of the
hydride–ruthenium complex.
Table 2 shows the catalytic activity of compounds of ruthenium (II)
with polypyridine ligands in the hydrogenation of N-benzylideneaniline.
Conversions range from 56 to 86% after 30 min of reaction (entries
1–6). When comparing the catalytic activity of the compounds
Ru(dppz)(PPh3)2Cl2 and Ru(dppz-Cl)(PPh3)2Cl2 a reverse situation is ob-
served compared to that seen in the hydrogen transfer reaction of
acetophenone; in this case the activity of Ru(dppz-Cl)(PPh3)2Cl2 is better
than the one observed for Ru(dppz)(PPh3)2Cl2. These results seem to be
contradictory, as both catalytic processes of metal hydride formation are
fundamental to the reaction and this intermediate is strongly favored by
the presence of donor ligands. However, since the imine is basic it will
react more rapidly with the more acidic metal, in this case with the com-
plex containing the dppz-Cl ligand.
When the activities of complexes 1, 3 and 4 are compared, it can
be seen that the activity showed by complex 1 is better than that of
complex 3 and both have much better activity than complex 4. Even
though the three different nitrogen ligands provide electronic effects,
the differences observed in the activities cannot be rationalized on
the basis of the small differences of the electronic effects. Probably
the differences in the catalytic activities of these complexes are the
consequence of the size of the nitrogen ligands. The higher the size
of the ligands the less stable the formed complex becomes. Thus,
the ligands dppz and phen should not form a very stable bond in
RuCl2(PPh3)3 reacted with one equivalent of dppz, bpy and phen li-
gands, in dichloromethane at room temperature, with high yield. The
paternal signals in 1H-NMR and 31P-NMR confirm the structure pro-
posed for these compounds and for the complexes and they are in accor-
dance with the elemental analysis and the IR spectrum. The complexes
trans-PPh3-[Ru(bpy)(PPh3)2Cl2, trans-PPh3-[Ru(phen)(PPh3)2Cl2 trans-
PPh3-[Ru(dppz)(PPh3)2Cl2 and trans-PPh3-[Ru(dppz-Cl)(PPh3)2Cl2
were obtained in high yield. The compounds were obtained in an oc-
tahedral coordination with the phosphine group appearing in a trans-
configuration which was confirmed by 31P-NMR. The complexes
Ru(bpy)(CO)2Cl2 and Ru(phen)(CO)2Cl2 and Ru(dppz)(CO)2Cl2 used
in this work show the paternal signal similar to those reported previous-
ly by us [13]. The compounds were studied as catalysts in the transfer
hydrogenation reaction of imine and ketone.
Table 1 shows the catalytic activity in the hydrogen transfer reac-
tion of acetophenone catalyzed by compounds of ruthenium (II)
with polypyridine ligands. The activities displayed by these com-
pounds vary between 50 and 97 in 30 min of reaction (entries 1–7).
When comparing the catalytic activity of these compounds with the
precursors Ru(PPh3)3Cl2 and [Ru(CO)2(Cl)2]n (entries 8–9) used for
the synthesis of the complexes, it was observed that the catalytic ac-
tivity increased significantly with the presence of a polypyridine li-
gand in the coordination sphere of the metal. The presence of the
polypyridine ligand stabilizes the metal in catalytic conditions thus
enabling the reaction that forms ruthenium hydride, which is the ac-
tive species for this reaction. The compounds Ru(dppz)(PPh3)2Cl2
and Ru(bpy)(PPh3)2Cl2 and Ru(dppz)(CO)2Cl2 were the most active
after 30 min of reaction. After 2 h of reactions, complexes 1, 4–7
1
Synthesis of the complexes. Trans-PPh3-[Ru(dppz)(PPh3)2Cl2 (1). Ru(PPh3)2Cl2
(0.1 g, 1.0 mmol) and dipyrido[3,2-a:2′,3′-c]phenazine (0.029 g, 1.11 mmol) dis-
solved in chloroform, (20 mL) were placed into a round bottom flask. The mixture
was stirred for 30 min. The solution was precipitated with ethyl ether; the solid
obtained was filtered and washed with hexane and ethyl ether. The product was rec-
rystallized (chloroform/ethyl ether). 1H-NMR (δ (ppm) CDCl3): 9.3 (s, 1H), 8.4 (dd,
6 Hz, 4H), 8.0 (dd, 6 Hz, 4H), 7.1–6.9 (m, PPh3). Anal. Calc. (%): for C54H40N4Cl2N4P2-
Ru: C, 66.26; H, 4.12; N, 5.72. Found (%) C, 66.33; H, 4.38; N, 4.98. Yield 65%. Trans-
PPh3-[Ru(dppz-Cl)(PPh3)2Cl2 (2). This compound was prepared by the same proce-
dure used for the synthesis of trans-PPh3-[Ru(dppz)(PPh3)2Cl2. 1H-NMR (δ (ppm)
CDCl3): 9.1 (s, 2H), 9.0 (d, 7.7 Hz, 2H), 8.4 (s, 1H), 8.3 (d, 9.1 Hz, 1H) 7.9 (d, 9.1 Hz,
1H) 7.4–7.0 (m, PPh3). Anal. Calc. (%): for C54H39Cl3N4P2Ru: C, 64.01; H, 3.88; N,
5.53. Found (%) C, 64.21; H, 3.83; N, 5.21. Yield 70%.
Table 2
Catalytic activity in the hydrogenation of N-benzylideneaniline.
Run Catalyst/time (minutes)
Conversion (%) (TOF h−1
30 60 90
60; (480) 72; (288) 76; (202) 83; (166)
Ru(dppz-cl)(PPh3)2Cl2 (2) 75; (600) 83; (332) 85; (226) 88; (176)
)
120
1
2
3
4
5
6
7
8
Ru(dppz)(PPh3)2Cl2 (1)
Ru(phen)(PPh3)2Cl2 (3)
Ru(bpy)(PPh3)2Cl2 (4)
Ru(bpy)(CO)2Cl2 (5)
Ru(phen)(CO)2Cl2 (6)
Ru(dppz)(CO)2Cl2 (7)
Ru(PPh3)3Cl2 (8)
56; (448) 60; (240) 64; (171) 69; (138)
–
–
–
b5a
65; (520) 77; (308) 83; (221) 88; (176)
86; (688) 88; (352) 90; (240) 91; (182)
2
Catalytic hydrogenation. The catalyst precursor (0.012 mmol) was dissolved in 2-
3; (24)
5; (20)
12; (32)
21; (42)b
propanol (8 mL), and the solution was refluxed. After 10 min, a distilled amine com-
pound (10 mmol) was added. After 10 min, the transfer hydrogenation reaction was
initiated by addition of sodium hydroxide (9.9 mg, 0.24 mmol) dissolved in 2-
propanol (1 mL). The progress of the reaction was monitored by gas chromatography
with periodic sampling every 10 min. Gas chromatographic analysis was carried out
with a Perkin Elmer 8500P instrument equipped with FID, using a Carbowax 20 M col-
umn and nitrogen as carrier gas. GC-Mass spectra were carried in order to confirm the
identity of products in a MAT 95 XP Thermo Electron.
0
0
0
0
0
0
0
0
0
0
0
0
0
9
10
11
[Ru(CO)2Cl2]n (9)
0
Ru(phen)2Cl2 (10)
Ru(bpy)2Cl2 (11)
b5
b5
Substrate/ruthenium=400; solvent=2-propanol; base/ruthenium=20;
a
The conversion increase after 10 h to 82%.
The conversion increase after 10 h to 78%.
b

148
SA. Moya et al. / Inorganic Chemistry Communications 22 (2012) 146–148
Complex (1)
transfer reaction, which allows working in glass reactors without ex-
Complex (2)
Complex (3)
Complex (4)
Complex (5)
Complex (6)
Complex (7)
treme security measures. Using a hydrogen transfer reaction, Morris
reported zero activity for the hydrogenation of N-benzylideneaniline
catalyzed by ruthenium compounds. We report better activities than
to those recently reported by us [18] and by Jia et al. [19], using the
same substrate.
100
80
60
40
20
0
Fig. 1 shows the conversion and reaction time for the hydrogen
transfer reaction. It is possible to observe that the rate for the reaction
corresponding to the best systems studied decreased sharply after
30 min, but in all cases conversion exceeds 70%.
Graph 2 shows the conversion versus time for the hydrogenation
reaction of N-benzylideneaniline using the hydrogen transfer reac-
tion as a source of hydrogen. The reaction shows, that after 30 min,
high conversions with high speed have been achieved. The activities
of the catalysts show small differences, probably due to the different
ability to coordinate the substrate, which could reduce the concentra-
tion of active metal in the reaction (Fig. 2).
0
20
40
60
80
100 120 140
Time (min)
Fig. 1. Hydrogen transfer reaction using ruthenium complexes containing polypyridine
ligands.
the product, Ru-benzylideneanilne, allowing the catalytic reaction to
continue. Instead, the bond formed by the bipy ligand in the product,
Ru-benzylidenaniline must be stable enough to stop and kill the cat-
alytic process.
Conclusions: Compounds of ruthenium (II) containing polypyridine
and carbonyl ligands are active catalysts in the transfer hydrogenation
reaction of N-benzylideneaniline by hydrogen transfer. The activities
observed for short reaction times show high selectivity. Upon compar-
ing the activity reported for different compounds it was observed that
the compounds Ru(phen)(CO)2Cl2 and Ru(dppz-Cl)(PPh3)2Cl2 and
Ru(bpy)(CO)2Cl2 show better activity in the hydrogenation reaction of
N-benzylideneaniline than those catalysts without nitrogen ligands.
On the other hand complexes Ru(dppz)(PPh3)2Cl2, Ru(bpy)(PPh3)2Cl2
and Ru(dppz)(CO)2Cl2 showed 99% conversion in the hydrogen transfer
reaction after 2 h reaction.
The catalytic activity in the hydrogenation of N-benzylideneaniline
of polypyridine carbonyl compounds (see Table 2, entries 5–7). It is
well known that the ligand CO stabilizes low oxidation states of the
metal complexes of ruthenium. Considering this fact, we think that
the addition of a CO ligand in the structure of the polypyridine com-
plex could increase the catalytic potential of the system, by favoring
the formation of the active species Ru–H. However the experimental
data obtained for complexes of the type Ru(polypyridine)(CO)2Cl2
where the ligands polypyridine and CO coexist, do not show that the
systems improve their catalytic activity with the incorporation of CO.
When the results obtained in this study for the complexes of the
type Ru(L)(PPh3)2Cl2 (with L=dppz, bpy, phen) are compared with
the compounds Ru(L)(CO)2Cl2 (with L=bpy, phen, dppz) it becomes
evident that for compounds Ru(bpy)(L)2Cl2 (L=CO or PPh3) the ac-
tivity is increased by replacing the phosphine ligand by a carbonyl li-
gand (entries 4 and 5). Similar behavior is obtained for the compound
Ru(phen)(CO)2Cl2 (entries 3 and 6). An opposite behavior is observed
with the compound Ru(dppz)(CO)2Cl2, (entries 1 and 7). Although
the compound, Ru(dppz)(CO)2Cl2 exhibits low solubility in the reac-
tion medium, its catalytic activity shows that our rationalization that
the presence of carbonyl in the complex could help the formation of
the species Ru–H increasing the catalytic activity is not conclusive
and is currently under study.
Acknowledgments
The authors thank Fondecyt — Chile (projects 1120149 and 1120685).
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Complex (1)
Complex (2)
Complex (3)
Complex (5)
Complex (6)
100
80
Complex (7)
60
40
20
0
[19] W. Jia, X. Chen, R. Guo, C. Sui-Seng, D. Amoroso, A.J. Lough, K. Abdur-Rashid,
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0
20
40
60
80
100 120 140
Time (min)
Fig. 2. Hydrogenation of N-benzylideneaniline using ruthenium complexes which con-
tain polypyrdine ligands.