The catalytic activity of a cyclometalated ruthenium(III) complex for aerobic oxidative dehydrogenation of benzylamines

Article abstract of DOI:10.1016/j.jorganchem.2010.10.063

The ruthenium(III) complex bearing benzo[h]quinoline as a cyclometalated ligand was synthesized and characterized by ESI-MS, elemental analysis, cyclic voltammetry and crystallography. The complex serves as an efficient catalyst for the aerobic oxidative dehydrogenation of benzylamines to the corresponding benzonitriles under mild conditions.

Full text of DOI:10.1016/j.jorganchem.2010.10.063

Journal of Organometallic Chemistry 696 (2011) 1301e1304
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The catalytic activity of a cyclometalated ruthenium(III) complex for aerobic
oxidative dehydrogenation of benzylamines
a
a
a
b
a,
*
Shota Aiki , Ayako Taketoshi , Junpei Kuwabara , Take-aki Koizumi , Takaki Kanbara
a
Tsukuba Research Center for Interdisciplinary Materials Science (TIMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba 305-8573, Japan
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 September 2010
Received in revised form
The ruthenium(III) complex bearing benzo[h]quinoline as a cyclometalated ligand was synthesized and
characterized by ESI-MS, elemental analysis, cyclic voltammetry and crystallography. The complex serves
as an efficient catalyst for the aerobic oxidative dehydrogenation of benzylamines to the corresponding
benzonitriles under mild conditions.
27 October 2010
Accepted 28 October 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Cyclometalated compounds
Ruthenium(III)
Dehydrogenation
Amine
1. Introduction
6
(bhq)(tpy)][PF ] (1b), which can be considered to satisfy the above
molecular design. Bhq is known to form readily cyclometalated
transition metal complexes due to the stability of the five-membered
heterometallacycle [5]. The Ru(III) complex 1b has not been isolated,
although the preparation of the corresponding Ru(II) complex, [RuCl
(bhq)(tpy)], was reported [6]. We report herein preparation and
characterization of 1b. The catalytic activity of 1b for the aerobic
oxidative dehydrogenation of benzylamines is also described.
Development of catalytic systems using molecular oxygen as an
oxidant has attracted much attention from the consideration of
green chemistry [1]. In particular, the aerobic oxidation of amines is
a useful method to obtain nitriles which have a great utility in the
synthesis of pharmaceutical compounds and industrial materials [2].
We previously reported that the aerobic oxidative dehydrogenation
of imidazolines and benzylamines is catalyzed by a cyclometalated
ruthenium complex, [RuCl(ppy)(tpy)][PF
6
] (1a) (ppy ¼ 2-phenyl-
0
0
00
pyridine; tpy ¼ 2,2 :6 ,2 -terpyridine) [3]. In these reactions, the
essential steps are the coordination of substrate to the ruthenium
center and the aerobic oxidation of the ruthenium center. Therefore,
the key feature of the catalyst 1a is to have a Cl ligand and a cyclo-
metalated ligand. Since the Cl ligand easily dissociates, a substrate
2. Results and discussion
2.1. Synthesis of Ru(III) complex 1b
The cyclometalated ruthenium(III) complex, [RuCl(bhq)(tpy)]
can coordinate to the vacant position. The s-donor character of the
[PF
6
] (1b), was prepared by stirring [RuCl
methoxyethanol at 70 C in the presence of AgPF
of AgCl precipitation and the anion exchange with NH
3
(tpy)] and bhq in 2-
. After the removal
PF , 1b was
cyclometalated ppy ligand lowers the redox potential, which enables
the aerobic oxidation of the ruthenium center. This concept is
considered to differ from that of the common ruthenium-catalyzed
dehydrogenation reaction which includes the formation of ruthe-
nium hydride species [2aeg,4]. To investigate this aerobic dehy-
drogenation system, we focused on a ruthenium(III) complex
bearing benzo[h]quinoline (bhq) as a cyclometalated ligand, [RuCl
ꢀ
6
4
6
obtained as a green solid in 31% yield (Scheme 1). The ESI-MS
spectrum of 1b showed the parent peak at m/z ¼ 548 as a mono-
cation pattern.
The structure of complex 1b was confirmed by X-ray analysis.
The ORTEP drawing of 1b is shown in Fig. 1 with selected bond
lengths. Since the RueCl bond length of 1b (2.418(2) Å) is slightly
shorter than that of 1a (2.4431(13) Å) [7], it is considered that the
trans influence of the cyclometalated bhq ligand is smaller than
that of ppy ligand.
*
Corresponding author. Tel.: þ81 29 853 5066; fax: þ81 29 853 4490.
E-mail address: kanbara@ims.tsukuba.ac.jp (T. Kanbara).
0
022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.063

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S. Aiki et al. / Journal of Organometallic Chemistry 696 (2011) 1301e1304
N
N
Ru
Cl
AgPF6
CH OC H OH, 70 °C, 12 h
N
N
N
N
+
Ru
N
PF6
N
3
2 4
Cl
Cl
Cl
1b
Scheme 1. Synthesis of Ru(III) complex 1b.
Cyclic voltammetry was performed on a DMF solution of 1b with
.1 M [ Bu N][PF ] as a supporting electrolyte. Three reversible
4 6
Next, the catalytic activity of 1b for the aerobic oxidative
dehydrogenation of various benzylamines 2ae2g was investigated;
the results are summarized in Table 2. The corresponding benzo-
nitriles 3ae3g were obtained in every case. When the substance
has an electron-withdrawing group, the overreaction forming by-
products such as 4-trifluoromethylbenzamide caused a decrease in
the yield of the nitrile [8].
n
0
redox waves were observed at E1/2 ¼ ꢁ0.16, ꢁ2.11, and ꢁ2.31 V vs.
þ
Fc /Fc. The former one is assigned to the Ru(III)/Ru(II) redox couple,
and latter two are based on the redox processes of terpyridine and
benzo[h]quinoline, respectively (Fig. 2). The redox potential of Ru
(
III)/(II) in 1b (E1/2 ¼ þ0.48 V vs. NHE) lies in a similar range
observed for 1a (E1/2 ¼ þ0.46 V) [7], and is expected to be suffi-
To probe the reaction pathway, similar ruthenium complexes,
0
ciently low for the aerobic oxidation of the ruthenium center.
[Ru(bhq)(bpy)
tpy)][PF
2
][PF
6
] 1c (bpy ¼ 2,2 -bipyridine) and [RuCl(phen)
(
6
] 1d (phen ¼ 1,10-phenanthroline), were employed. In
both cases, negligible catalytic activities were observed (Table 3,
entries 1 and 2). The inactivity of 1c shows the coordination of 2a to
the metal center is indispensable. The inactivity of 1d indicates that
the redox potential of Ru(III)/Ru(II) (E1/2 ¼ þ1.02 V vs. NHE [9]) is
not enough low for the aerobic oxidation of the ruthenium center.
These results show that the reaction is facilitated by the coordi-
nation of the substrate to the metal center and the control of the
redox potential at the metal center. Consequently, we conclude that
the reaction pathway for 1b is the same as that of 1a (Scheme B1,
see Appendix B) [3,10]. However, the reaction using 1b was slower
than that of 1a (Table 3, entries 3 and 4). This result is consistent
with the data of X-ray analysis and cyclic voltammetry; the cyclo-
metalated complex with a longer RueCl bond length and a lower
redox potential serves as a superior catalyst for this aerobic dehy-
drogenation system.
2
.2. Application to aerobic oxidative dehydrogenation
First, oxidative dehydrogenation of 4-methylbenzylamine (2a)
using 1b as a catalyst was carried out in methanol under reflux in
1
air. The reaction was monitored by H NMR spectroscopy using
mesitylene as an internal standard. The desired 4-methylbenzoni-
trile (3a) was obtained in good yield after 14 h (Table 1, entry 1). The
reaction proceeded much faster using molecular oxygen (1 atm),
the same reaction under a nitrogen atmosphere gave only a trace
amount of product (entries 2 and 3). These results indicate that
molecular oxygen participates as the oxidant in the catalytic reac-
tion. The addition of bases accelerated the reaction, K
2 3
CO and
Cs CO were especially effective (entries 2, 4e7). It is considered
2
3
that the base induces deprotonation of the NH group in the coor-
dinated substrate. The reaction proceeded even at room tempera-
ture; 3a was obtained in 80% yield (entry 8).
3. Conclusions
We have prepared a cyclometalated ruthenium(III) complex 1b
whose structure was confirmed by a single-crystal X-ray diffraction
study. The complex 1b was found to be an effective catalyst for the
aerobic oxidative dehydrogenation of benzylamines. It was revealed
-0.04
5
A
-1.99
-
2.18
-0.29
-2.23
-2.43
-
3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
+
ꢁ
Fig. 1. ORTEP drawing of Ru complex 1b at 30% ellipsoidal level. Hydrogen atoms, PF
6
Potential / V vs. Fc /Fc
anion and solvated acetone molecules are omitted for simplicity. Selected bond lengths
Å): Ru(1)eC(1), 1.981(9); Ru(1)eN(1), 2.137(9); Ru(1)eN(2), 2.065(9); Ru(1)eN(3),
.942(9); Ru(1)eN(4), 2.062(9); Ru(1)eCl(1), 2.418(2).
n
(
1
Fig. 2. Cyclic voltammogram of 1b in DMF containing [ Bu
rate ¼ 100 mV/s.
4 6
N][PF ] (0.1 M). Sweep

S. Aiki et al. / Journal of Organometallic Chemistry 696 (2011) 1301e1304
1303
Table 1
Oxidative dehydrogenation of 2a using 1b as a catalyst.
Table 3
a
a
Oxidative dehydrogenation of 2a using 1ae1d as a catalyst.
Yield (%)b
Yield (%)b
Entry
Ru catalyst
Time (h)
Entry
Conditions
Time (h)
1
2
3
1c
1d
1a
1b
1.5
1.5
1
Trace
0
87
1
2
3
4
5
6
7
K
K
K
2
2
2
CO
CO
CO
3
3
3
Air
14
78
89
Trace
10
23
88
31
80
O
2
1.5
1.5
1.5
1.5
1.5
1.5
24
c
N
2
4
1
77
e
O
2
O
2
O
2
O
2
O
2
Na
2
CO
CO
DBU
CO
3
a
The reaction was carried out in 1 mL CD
3
OD with 2a (0.15 mmol), Ru catalyst
Cs
2
3
ꢁ3
(
7.5 ꢂ 10 mmol) and a base (0.15 mmol).
b
c
Determined by 1H NMR spectroscopy using mesitylene as an internal standard.
The data are originated from ref. [3b].
c
8
K
2
3
a
The reaction was carried out in
3
1 mL CD OD with 2a (0.15 mmol), 1b
ꢁ3
(
7.5 ꢂ 10 mmol) and a base (0.15 mmol).
b
Determined by 1H NMR spectroscopy using mesitylene as an internal standard.
The reaction was performed at room temperature.
1
þ0.64 V vs. NHE. H NMR spectra were measured on a JEOL EX-270,
c
a JNM-ECS-400 and a Bruker AVANCE-400 NMR spectrometers.
that the catalytic activity correlates with the RueCl bond length and
the redox potential. The method outlined in this paper is expected to
contribute to the design of catalysts for aerobic oxidative dehydro-
genation. Further studies including an investigation of the reaction
mechanism are in progress.
4
.2. Synthesis of Ru(III) complex 1b
3
[RuCl (tpy)] (200 mg, 0.45 mmol), bhq (163 mg, 0.91 mmol) and
AgPF
6
(184 mg, 0.73 mmol) were dissolved in 2-methoxyethanol
ꢀ
(55 mL) and stirred at 70 C for 12 h. The solution was cooled to
ꢀ
ꢁ
20 C for 1 h and then filtered through Celite to remove the AgCl
precipitate. The filtrate was concentrated to ca. 1 mL. An aqueous
4
. Experimental
NH
4 6
PF solution was added to the concentrate. The resulting
precipitate was filtered off and purified by column chromatography
4
.1. General
(
grade III alumina, acidic, toluene/acetonitrile ¼ 2/1). The green band
was collected and acetonitrile was evaporated. The precipitate was
2
a, methanol-d
4
(Acros), AgPF
6
, 2c, 2d, 2f, 2g, 1,8-diazabicyclo
, Na CO , Cs CO , 2-methoxy-
collected by filtration to give 1b as a green solid (98 mg, 31%). ESI-
[5.4.0]undec-7-ene (Aldrich), K
2
CO
3
2
3
2
3
þ
MS: m/z ¼ 548{M ꢁ PF
O (C28 23ClF
H, 3.52; N, 7.44.
6
} . Anal. Calcd. for [RuCl(bhq)(tpy)][PF
6
]$
ethanol (Kanto Chemical), benzo[h]quinoline, 2b, 2e, and mesity-
lene (TCI) were commercially available and were used without any
further purification. Complexes [RuCl
2H
2
H
6 4 2
N O PRu): C, 46.13; H, 3.18; N, 7.69. Found C, 46.31;
3
(tpy)] [11], 1a [7], 1c [5e], and
1
d [9] were prepared in accordance with the previous literature
methods. Column chromatography was carried out by using
Aluminium oxide 90 active acidic (Merck).
4.3. X-ray crystal structure determination
Elemental analysis was carried out with a PerkineElmer 2400-
CHN instrument. ESI-Mass spectrum was recorded on an Applied
Biosystems QStar Pulsar i spectrometer. Cyclic voltammograms
were recorded on a ALS/CH Instruments Electrochemical analyzer
C
29.5
H
22
N
4
F
6
PClO0.5Ru, M ¼ 722.01, monoclinic, space group C2/
m (No. 12), a ¼ 15.0252(8) Å, b ¼ 24.2483(12) Å, c ¼ 16.4057(9) Å,
ꢀ
ꢁ
3
b
¼ 97.9443(16) , V ¼ 5919.8(5) Å , T ¼ 88(1) K, Z ¼ 8,
3
ꢁ1
D
0
calcd ¼ 1.620 g cm
,
m
¼ 7.412 cm , F(000) ¼ 2888.00, crystal size
1
200A with a PFCE carbon working electrode, a Pt wire counter
electrode and a 0.10 M AgNO /Ag reference electrode in a DMF
N][PF ] as a supporting electrolyte
.30 ꢂ 0.10 ꢂ 0.05 mm. 28552 reflections collected, 6900 unique
3
(R
int ¼ 0.175), R
(I > 2s
(I)) ¼ 0.1089, R (All reflections) ¼ 0.1952, wR2
1
n
solution containing 0.10 M [ Bu
4
6
(
All reflections) ¼ 0.3525, Single crystals of 1b were obtained by the
þ
at room temperature. Fc /Fc ¼ þ0.060 V vs. 0.10 M AgNO
3
/Ag, and
slow diffusion of hexane into its solution in acetone. Intensity data
were collected on a Rigaku R-AXIS Rapid diffractometer with Mo-K
a
radiation. Crystals were mounted on a glass capillary tube. A full
Table 2
matrix least-squares refinement was used for non-hydrogen atoms
a
Oxidative dehydrogenation of 2ae2g using 1b as a catalyst.
ꢁ
except for PF
6
with anisotropic thermal parameters method by
SHELXL-97 program. Hydrogen atoms were refined using the riding
model.
ꢀ
Yield (%)b
4.4. Oxidative dehydrogenation of 4-methylbenzylamine (2a) to 4-
methylbenzonitrile (3a) using 1b as a catalyst (Table 1, entry 2)
Entry
R
Temp. ( C)
Time (h)
1
2
3
4
5
6
7
p-MeC
m-MeC
o-MeC
p-MeOC
Ph
6
H
H
6 4
4
a
b
c
d
e
f
Reflux
Reflux
Reflux
Reflux
Reflux
30
1.5
1.5
1.5
1.5
1.5
24
89
76
83
93
74
72
41
6
H
4
A mixture of 2a (19
m
L, 0.15 mmol), ruthenium complex 1b
CO (21 mg, 0.15 mmol), and mesi-
OD (1 mL) was stirred under
6
H
4
ꢁ3
(
5.2 mg, 7.5 ꢂ 10 mmol), K
2
3
ꢁ
2
tylene (10
m
L, 7.5 ꢂ 10 mmol) in CD
3
p-ClC
p-F CC
6 4
H
2
O (1 atm). After stirring under reflux for 1.5 h, the yield was
3
6
H
4
g
30
24
1
a
determined by H NMR using mesitylene as an internal standard
(89%). The spectral data of the obtained 3a were identical to the
previous literature [12].
The reaction was carried out in
1
mL CD
3
OD with 2 (0.15 mmol), 1b
ꢁ3
(
7.5 ꢂ 10 mmol) and a base (0.15 mmol).
b
Determined by 1H NMR spectroscopy using mesitylene as an internal standard.

1304
S. Aiki et al. / Journal of Organometallic Chemistry 696 (2011) 1301e1304
Acknowledgements
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(
4
(
The authors thank the Chemical Analysis Center of University of
Tsukuba for the measurements of X-ray analysis, elemental analysis
and NMR spectra.
(
[
[
3] (a) A. Taketoshi, A. Tsujimoto, S. Maeda, T. Koizumi, T. Kanbara, Chem-
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for complex 1b. These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif.
(
(
(
[
(
Appendix B. Supplementary data
(
Supplementary data associated with this article can be found, in
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