Ru(II) complexes bearing 2,6-bis(benzimidazole-2-yl)pyridine ligands: A new class of catalysts for efficient dehydrogenation of primary alcohols to carboxylic acids and H2in the alcohol/CsOH system

Article abstract of DOI:10.1016/j.jorganchem.2016.11.038

Mono-cationic Ru(II)-complexes [Ru(L)X(CH₃CN)₂]?X 1～4 (1, L = 2,6-bis(benzimidazol-2-yl) pyridine (L1), X = Cl; 2, L = L1, X = OTf; 3, L = 2-(N-benzyl-benzimidazole-2-yl)-6-(benzimidazole-2-yl)pyridine (L2), X = Cl; 4, L = 2,6-bis(N-benzyl-benzimidazole-2-yl)pyridine (L3), X = Cl) were prepared and fully characterized. The two acetonitrile ligands of each complex are coordinated to the metal center cis to each other. Complex 2 was also structurally characterized by X-ray crystallography. It was found that complexes 1～4 can catalyze the acceptorless dehydrogenation of primary alcohols to corresponding carboxylic acids and H₂in the basic aqueous solution, and the reactivity follows the order 1 = 2 > 4 > 3. Furthermore, complexes 1 or 2 can efficiently catalyze the conversion of various primary alcohols to carboxylic acid in good yields (72%–98%) and high selectivity in an alcohol/CsOH system (1/1, mol/mol). Using an excess amount of alcohol to CsOH results in the formation of the carboxylic acid in higher yield (up to 100%, based on CsOH) and higher turnover numbers (TON ～ 10000) accompanied by the H₂evolution. Complexes 1 and 2 can act as a new class of phosphine- and N-heterocycle carbene free Ru(II) complexes for efficient conversion of primary alcohols to carboxylic acids and H₂in a homogeneous system.

Full text of DOI:10.1016/j.jorganchem.2016.11.038

Accepted Manuscript
Ru(II) complexes bearing 2,6-bis(benzimidazole-2-yl)pyridine ligands: A new class of
catalysts for efficient dehydrogenation of primary alcohols to carboxylic acids and H
2
in the alcohol/CsOH system
Zengjin Dai, Qi Luo, Xianggao Meng, Renjie Li, Jing Zhang, Tianyou Peng
PII:
S0022-328X(16)30551-4
10.1016/j.jorganchem.2016.11.038
JOM 19723
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 22 August 2016
Revised Date: 28 November 2016
Accepted Date: 29 November 2016
Please cite this article as: Z. Dai, Q. Luo, X. Meng, R. Li, J. Zhang, T. Peng, Ru(II) complexes bearing
,6-bis(benzimidazole-2-yl)pyridine ligands: A new class of catalysts for efficient dehydrogenation of
2
primary alcohols to carboxylic acids and H in the alcohol/CsOH system, Journal of Organometallic
2
Chemistry (2017), doi: 10.1016/j.jorganchem.2016.11.038.
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ACCEPTED MANUSCRIPT

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Ru(II) Complexes Bearing 2,6-Bis(benzimidazole-2-yl)pyridine Ligands:
A New Class of Catalysts for Efficient Dehydrogenation of Primary
Alcohols to Carboxylic Acids and H in the Alcohol/CsOH System
2
a
a
b
a
a*
a*
Zengjin Dai, Qi Luo, Xianggao Meng, Renjie Li, Jing Zhang, and Tianyou Peng
a
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, P. R. China
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education; School of Chemistry, Central
China Normal University, Wuhan430079, P. R. China
b
*
Tel./Fax: +86 27 6875 6679. E-mail: jzhang03@whu.edu.cn (J. Zhang); typeng@whu.edu.cn (T. Peng)
ABSTRACT: Mono-cationic Ru(II)-complexes [Ru(L)X(CH CN) ]⋅X 1~4 (1, L=2,6-bis(benzimidazol-2-yl)
3
2
pyridine (L1), X=Cl; 2, L=L1, X=OTf; 3, L=2-(N-benzyl-benzimidazole-2-yl)-6-(benzimidazole-2-yl)pyridine
L2), X=Cl; 4, L = 2,6-bis(N-benzyl-benzimidazole-2-yl)pyridine (L3), X=Cl) were prepared and fully
(
characterized. The two acetonitrile ligands of each complex are coordinated to the metal center cis to each other.
Complex 2 was also structurally characterized by X-ray crystallography. It was found that complexes 1~4 can
catalyze the acceptorless dehydrogenation of primary alcohols to corresponding carboxylic acids and H in the
2
basic aqueous solution, and the reactivity follows the order 1 = 2 > 4 > 3. Furthermore, complexes 1 or 2 can
efficiently catalyze the conversion of various primary alcohols to carboxylic acid in good yields (72%~98%)
and high selectivity in an alcohol/CsOH system (1/1, mol/mol). Using an excess amount of alcohol to CsOH
results in the formation of the carboxylic acid in higher yield (up to 100%, based on CsOH) and higher turnover
numbers (TON ~ 10000) accompanied by the H evolution. Complexes 1 and 2 can act as a new class of
2
phosphine- and N-heterocycle carbene free Ru(II) complexes for efficient conversion of primary alcohols to
carboxylic acids and H in a homogeneous system.
2

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Keywords: N′NN′ pincer ligands; Ru(II) complex; acceptorless dehydrogenation; carboxylic acid; H2
production
1
. Introduction
In the past decade, Ru(II) complexes with “bulky” pincer ligands have been well demonstrated for catalytic
acceptorless dehydrogenation of primary alcohols to esters [1], amides [2], aldehydes [3] and imines [4], which
presents a “green” pathway for transforming alcohols to carbonyl derivatives. However, the pincer ligands in
these Ru(II) pincer systems reported normally contains at least one phosphine or N-heterocycle carbene donor,
which are essentially sensitive to O or the moisture. It is worthy to develop new metal pincer systems in which
2
the ligands contain only N or O donors and can be prepared through a simple and low energy-consuming
procedure. In 2013, Szymczak’s group [5] reported a NNN-amide Ru(II) hydride complex which can efficiently
dehydrogenate alcohols to ketones or esters and H , but this complex still contains two triphenylphosphine
2
ligands. Although a Ru(II) complex with a quadridentate bis(olefin)diazidiene ligand was used to efficiently
catalyzed the dehydrogenation of MeOH/H O to CO and H in a basic aqueous system, the conversion of other
2
2
2
primary alcohols to the corresponding carboxylic acids was not reported [6]. Up to now, the examples of
ruthenium(II) complex based on NNN-typed pincer ligand are scarce to the best of our knowledge.
Carboxylic acids and salts are important chemicals in both laboratory and industry. One of the promoting
synthetic processes of carboxylic acids is direct oxidation of primary alcohols in one step [7]. However, most of
the reported methods have been performed by using stoichiometric amounts of toxic oxidation reagents [8],
highly pressurized dioxygens [9], or additional sacrificial substrates as the hydrogen acceptors [10]. So far only
three Ru(II) systems have been developed for efficient dehydrogenation of primary alcohols to carboxylic acid
and H in a basic aqueous solution[11-14] (Scheme 1). It is noted that an excess amount of water is necessary in
2
these systems in order to increase the selectivity of carboxylic acid. Milstein and co-workers[11] firstly
presented a Ru(II)-PNN′ typed complex (Scheme 1), which acceptorlessly converts various primary alcohols to

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carboxylic acid salts in a basic aqueous solution. The yields are 61~91% when the ratio of substate/catalyst is
5
00/1 (mol/mol)). Similarly, Beller and co-workers[12a] reported a Ru(II)-PNP complex (Scheme 1), which can
convert ethanol to acetic acid and H in a NaOH-water solution with high turnover numbers (TON ~80000)
2
when the reaction was performed for 96 h. However, the amount of by-products (a mixture of 1-butanol and
ethyl acetate) increased to 30% yield when the ratio of water/ethanol was 1/9 (v/v), while the sole product of
acetic acid was obtained in a 1/3 (v/v) ratio of water/ethanol solution. Herein, we report four Ru(II) complexes
based on 2,6-bis(benzimidazole-2-yl)pyridine (N′NN′) ligands, which efficiently catalyze the acceptorless
dehydrogenation of primary alcohols to carboxylic acids and H in a homogeneous alcohol-CsOH system. The
2
reactivity of our complexes can be compared to the reported Ru(II) complexes bearing the ligands with
phosphine- or N-heterocycle carbene donors [11~14]. Although several Ru(II) complexes with
2
,6-bis(benzimidazole-2-yl)pyridine or 2,6-bis(imidazole-2-yl-)pyridine ligands have been found uses in
transfer hydrogenation of ketones[15], anion-sensor[16], aerobic oxidation of alcohols to imine [17], and C-H
bond activation [18]. None of them has been applied in the acceptorless alcohol dehydrogenation to the best of
our knowledge.

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Scheme 1. Catalytic dehydrogenation of primary alcohols to carboxylic acid salts and H2
2
. Results and Discussion
2.1 Synthesis and Characterization of Ru(II) Complexes 1~4:
N′NN′ pincer ligand 2,6-bis(benzimidazole-2-yl)pyridine (L1) was prepared from pyridine-2,6-dicarboxylic
acid
,6-bis(N-benzyl-benzimidazole-2-yl)pyridine (L3) was synthesized from L1 with an excess of benzyl bromide
15b]. The new ligand 2-(N-benzylbenzimidazole-2-yl)-6-(benzimidazole-2-yl)pyridine (L2) was synthesized
and
benzene-1,2-diamine
according
to
the
known
method[19].
2
[
from L1 with one equivalent of benzyl bromide in a modest yield (57%) by using a slightly modified procedure
for the synthesis of L3. When L1 and 0.5 equivalent of [RuCl (p-cymene)] were heated in acetonitrile for 12h,
2
2
1
complex [RuCl(L1)(MeCN) ]⋅Cl (1) was obtained in good yield (88%). The H NMR spectrum of 1 shows two
2
singlet peaks with the integration of three protons each at 2.06 and 3.17 ppm, which can be assigned to the CH3
groups of two acetonitrile ligands. This observation indicates the two acetonitrile ligands are coordinated to the
1
3
metal center in the axial and equatorial positions, respectively [20], which is further confirmed by the C NMR
and HSQC spectra (see supporting information). A neutral complex [RuCl (L1)(MeCN)] was previously
2
reported by Dayan et al through a procedure similar to ours [15c].
Complex [Ru(OTf)(L1)(MeCN) ]⋅(OTf) (2) was prepared by the extraction of two chlorides from 1 with
2
1
two equivalents of AgOTf in methanol. The H NMR spectrum of 2 exhibits a singlet peak at 14.95 ppm for the
two magnetic equivalent N-H moieties of L1, presenting an upfield shift of 0.22 ppm relative to that of 1 (δ
1
1
5.17 ppm). The two magnetically nonequivalent acetonitrile ligands of 2 are also confirmed by its H NMR
spectrum which shows two singlet signals with equal integration of three protons at 2.09 and 3.20 ppm,
respectively. For further characterization of 2, single crystals suitable for X-ray diffraction study were obtained
by slow diffusion of diethyl ether into a concentrated 2,2,2- trifluoroethanol solution of 2. Although the crystal

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data are not very satisfied, the structure of 2 still clearly displays the Ru(II) center adopt a distorted octahedral
coordination geometry with the coordination plane defined by three N atoms of L1 and one N atom of
acetonitrile, while another acetonitrile ligand and one water ligand occupy the axial positions (Fig. 1). The
water is probably from the solvents since the 2,2,2-trifluoroethanol was simply treated by the distillation from
commercially available reagent. The bond distance of Ru(1)-N(7) (2.053(8) Å) is larger than that of Ru(1)-N(6)
(1.994(9) Å), which is consistent with the larger trans effect of pyridyl N atom relative to that of water O atom.
The bond distances of Ru(II)-N(pyridyl) and Ru(II)-N(benzimidazol-2-yl) match well with other reported
Ru(II)-L1 and Ru(II)-terpyridines complexes[15, 21]. The bond angle of N(6)-Ru(1)-N(7) (94.1(3) Å) confirms
the two acetonitrile ligands are coordinated to the metal center cis to each other, which is consistent with the
1
13
observations in its H NMR and C NMR spectra.
Figure 1. The structure of 2·H O with thermal ellipsoids drawn at the 30% probability level. All hydrogen
2
atoms are omitted for clarity. Selected bond lengths(Å): Ru1-N1, 1.982(8); Ru1-N2, 2.080(8); Ru1-N4,
2
.076(8); Ru1-N6, 1.994(9); Ru1-N7, 2.053(8); Ru1-O1, 2.090(7); Selected bond angles(
°): N1-Ru1-N6,
9
0.0(3); N1-Ru1-N7, 175.9(3); N6-Ru1-N7, 94.1(3); O1-Ru1-N6, 177.8(3).
In comparison with 1, complexes [Ru(L2)Cl(CH CN) ]⋅Cl 3 and [Ru(L3)Cl(CH CN) ]⋅Cl 4 were
3
2
3
2

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synthesized from [RuCl (p-cymene)] with L2 and L3 by using the procedure similar to the one used for 1,
2
2
1
respectively (Scheme 2). The structures of 3 and 4 are similar to that of 1. The H NMR spectrum of 3 shows
one singlet peak with integration of two protons at 6.35 ppm, which is assigned to the benzyllic -CH - of L2. In
2
1
addition, the two acetonitrile ligands of 3 with different chemical environments are confirmed by its HNMR
spectrum, which exhibits two singlet signals at 2.07 and 3.19 ppm with equal integration of three protons,
respectively. Similarly, the two acetonitrile ligands of 4 attached to the metal center cis to each other is also
1
approved by its H NMR (two singlet signals with integration of three protons each at 2.09 and 3.22 ppm for
1
3
the –CH moiety, respectively) and its C NMR (two singlet peaks at 3.48 and 4.50 ppm for the –CH moiety,
3
3
1
respectively) spectra. No signal is observed at 9~16 ppm in the H NMR spectrum indicates the absence of N-H
of 4.
Scheme 2. The structures of complexes 3 and 4.
2.2. Catalytic dehydrogenation of primary alcohols
In preliminary studies, benzyl alcohol was chosen as the substrate for the acceptorless dehydrogenation. A
typical reaction was carried out under argon atmosphere by using 2 as the catalyst precursor (0.2 mol% to the
alcohol) in the basic aqueous solution (bi-phase) with the procedure reported by Milstein et al [11]. When KOH
was used as the base, benzoic acid was obtained in 19% yield (TON 95) after 24h (Table 1, entry 1). After
optimizing the effect of various bases, we found that the highest yield of benzoic acid was obtained (37%, TON
~
185) with the use of CsOH (Table 1, entry 5). Using weaker bases (LiOH or Na CO ) afforded relatively
2 3
lower yields (6% and 4%, respectively) (Table 1, entry 3 and 4). As expected, almost no benzoic acid was

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observed without the base under the same condition (Table 1, entry 6).
Table 1. Base screening for the catalytic reaction in alcohol-water system
a, b
Entry
Base
KOH
NaOH
Yield (%)
1
1
3
4
5
6
17
21
6
LiOH·H O
2
Na CO
4
2
3
CsOH·H O
37
˂ 2
2
none
a
Reaction condition: benzyl alcohol (5mmol), base (5.5mmol), catalyst 2 (10ꢀmol), H O (2mL) were refluxed
2
b
in an oil bath under an argon atmosphere. Isolated yield of benzoic acid=n (acid)/n (alcohol). Benzoic acid
was obtained by acid treatment of the salts.
When a solution of benzyl alcohol and 1.5 equivalents of CsOH⋅H O with 0.2 mol% of 2 was refluxed for
2
2
4h, benzoic acid was obtained in 44% yield (Table 2, entry 2), suggesting the use of an excess amount of base
just afforded slightly higher yield. As expected, prolonging the reaction time to 72h resulted in the formation of
benzoic acid in the higher yield (82%) (Table 2, entry 3). Complex 1 shows almost the same catalytic activity
with 2 under the same condition (Table 2 entry 1 and 2), probably because of the same intermediates formed in
the catalytic cycle by extraction of chloride or triflate anions with hydroxide anions from 1 and 2, respectively.
However, when complex 3 or 4 was tested instead of 2, benzoic acid was obtained in the relatively low yield
(10% and 28%, respectively), indicating the two N-H groups of L1 play an important role in the catalytic cycle.

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Table 2. Catalyst screening for the catalytic reaction
a
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
1
2
44
44
b
2
82
3
10
28
˂1
4
none
a
Reaction condition: benzyl alcohol (5mmol), CsOH·H O (7.5mmol), catalyst (10ꢀmol), H O (2mL) were
2
2
b
refluxed in an oil bath under an argon atmosphere for 24h, isolated yield of benzoic acid=n (acid)/n (alcohol).
The reaction time is 72h.
-
Since OH can be used as the oxygen source for the conversion of alcohol to carboxylic acid, and the
existence of large amount of water reduces the concentration of the base, we think reducing the amount of
water might benefit the alcohol coordinated to the Ru(II) center, followed by the β-H elimination of alkyloxide
occurring to produce the aldehyde as the initial step of the catalytic cycle. So, further investigations were
carried out in the homogeneous alcohol-CsOH system. To our delight, when benzyl alcohol and one equivalent
of CsOH⋅H O with 0.2 mol% of 2 were heated at 150°C (oil bath) under an argon atmosphere for 7 minutes, the
2
evolution of H gas (confirmed by the GC, see supporting information) was observed from the initial
2
homogeneous solution, accompanied by the formation of cesium benzoate as a white solid. The benzoic acid
was achieved in 75% yield after 4h (Table 3, entry 1), presenting the reaction rate is much faster than that in
water-alcohol bi-phase system (72h, 82%). However, only 80% yield of benzoic acid was obtained when the

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reaction time was 24h, probably because of the formation of benzoate salt solidifying the reaction solution. The
unreacted benzyl alcohol was recovered in 17% yield, and the small amounts of benzaldehyde and benzyl
1
benzoate (total lower than 2%) were detected by GC-MS and H NMR spectrum (see supporting information).
Under such condition, using an excess amount of CsOH afforded a lower yield (75%, 24h) (Table 3, entry 3).
Under the optimized reaction conditions, other primary alcohols can also be dehydrogenated to the
corresponding carboxylic acids in good yields by using complex 2 as the catalyst (Table 3). When
4
4
-methoxybenzyl alcohol and one equivalent of CsOH⋅H O with 0.2 mol% of 2 were heated at 150°C for 24h,
2
-methyloxy-benzoic acid was obtained in 84% yield. While 3-trifluoromethylbenzyl alcohol was used as a
substrate, 3-trifluoromethyl benzoic acid was obtained in 57% yield (Table 3, entry 6 and 7), probably because
of the electron-withdrawing group of the aromatic alcohol reducing the nucleophilicity of the alcohol to the
Ru(II) center, which is important in the dehydrogenation. Various diols can be converted to a mixture of
monocarboxylic acids and di-carboxylic acids in good yields (75~98%) under the similar condition (Table 3,
entry 8-11). As expected, a longer reaction time resulted in the formation of di-carboxylic acid in the higher
yield (Table 3, entry 12). Aliphatic alcohols can also be efficiently dehydrogenated to carboxylic acid in good
yields. For example, the hexylic acid was obtained in 77% isolated yield when 1-hexanol was loaded (Table 3,
entry 4). In all cases, the unreacted alcohols can be recovered within 3% lose.
Table 3. Dehydrogenation of alcohols to carboxylic acid in a 1:1 (mol/mol) ratio of alcohol/CsOH
a
Base
Yield
(%)
Entry
1
Alcohol
Products
(
equiv.)
b
1.0
75

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2
1.0
80
3
4
5
1.5
1.0
1.0
75
77
72
OH
6
7
1.0
1.0
84
57
O
c
5
2
8
9
1.0
1.0
2.0
c
c
2
3
0
8
c
8
c
3
9
1
0
c
c
5
3
0
0
1
1
2.0
2.0
c
c
5
1
6
0
d
1
2

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c,
8
8
a
Reaction condition: alcohol (entries 1-7, 5mmol; entries 8-12, 2.5mmol), CsOH·H O (entries 1-2, 4-7 and
2
1
0-12, 5mmol; entries 8-9, 2.5mmol; entry 3, 7.5mmol), catalyst (entries 1-7, 10ꢀmol; entries 8-12, 5ꢀmol)
b
c
were heated under 150°C. The reaction time is 4h. The ratio of mono-carboxylic acid and di-carboxylic acid
1
was determined by the H NMR of the mixed acid isolated from the reaction mixture. Carboxylic acids were
d
obtained by acid treatment of the salts. The reaction time is 48h.
In many cases, the large amounts of carboxylic acid salts precipitate from the reaction solution, and the
magnetic bar stop stirring, so the dehydrogenation rate decreases quickly. This problem might be overcome by
adding proper solvents to the reaction system. However, the addition of normal solvents such as THF, 1,
4
-dioxane, toluene etc did not afford the higher yield of carboxylic acid. We think using an excess of alcohol
may promote the reaction. When a solution of benzyl alcohol and 0.5 equivalent of CsOH⋅H O with 0.1 mol%
2
of 1 was heated at 150°C for 24h under an argon atmosphere, benzoic acid was obtained in 95% yield (based on
the CsOH) (Table 4, entry 1), presenting the higher yield and higher TON (~475) than that in a 1/1 (mol/mol)
ratio of benzyl alcohol/CsOH⋅H O system (80% yield). Upon increasing the ratio of benzyl alcohol /CsOH to
2
5
/1 (mol/mol), the reaction completed after 2.5 h and the benzoic acid was obtained in 94% isolated yield
(
Table 4, entry 2), which is consistent with the H production test (table 5). Moreover, with a lower catalyst
2
loading, the carboxylic acids were also obtained in high yields (Table 4, entries 3~5). For example, when 0.01
mol% of 1 and CsOH⋅H O (25 mmol) in the excess of benzyl alcohol (125 mmol) were heated at 150°C for 24h,
2
benzoic acid was obtained in quantitative yield with the TON ~ 10000 (Table 4 entry 4). As expected, using
anhydrous NaOH as a base resulted in the formation of benzoic acid in only 33% yield (TON 3300) under the
same condition (table4, entry 6). For reasons that remain to be determined CsOH gives much better results than
NaOH, we think the concentration of alkyloxide anion play an important role in our reaction system. Since

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CsOH is a stronger base than NaOH or KOH in the alcohol solution, the use of CsOH results in a higher
-
+
concentration of alkyloxide anion according to the equilibrium: ROH + MOH
RO + M +H O. In
2
addition, CsOH may neutralize the carboxylic acid more efficiently than other metal hydroxides in the alcohol
solvent, which is favor to the dehydrogenation of alcohol [11]. In the case of benzyl alcohol used, the solid
benzoate salt can be easily isolated by filtration, and the remaining solution is subjected to the second catalytic
cycle without obviously catalytic efficiency decrease (see supporting information). For example, the total yield
of benzoic acid was 98% with TON ~ 19600 in the two round cycles. When cyclohexylmethanol was used as
the substrate under the similar condition, cyclohexanecarboxylic acid was obtained in 83% yield with TON ~
8
300 after 24h (table 4, entry 5), which is higher than that in a 1/1 ratio of cyclohexylmethanol/CsOH system
(72%, TON ~ 360).
Table 4. Catalytic dehydrogenation with excess alcohol as the solvent
a, b
Catalyst
Reaction
time(h)
Yield
c
Entry
Alcohol
n(alcohol/base)
TON
(mol%)
(%)
1
2
3
4
5
2:1
5:1
5:1
5:1
5:1
0.2
24
2.5
24
24
24
95
94
475
470
0.2
0.02
0.01
0.01
100
100
83
5000
10000
8300

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d
6
5:1
0.01
24
33
3300
a
Reaction condition: For entries 1-2. Alcohol (entry 1, 10mmol; entry 2, 25mmol), CsOH·H O (5mmol),
2
catalysts (10ꢀmol) were stirred and heated under argon; for entries 3-6. Alcohol (125mmol), CsOH·H O
2
b
(
25mmol), catalyst (entry 3, 5ꢀmol; entries 4-6, 2.5ꢀmol) were stirred and heated under argon. Isolated
c
d
yield=n (acid)/n (CsOH)); TON=n (acid)/n (catalyst); NaOH was used as the base.
2
.3. The H Production from catalytic dehydrogenation of benzyl alcohol
2
The formation of H from the catalytic system was confirmed by GC analysis (see supporting information)
2
and quantified by the gravity drainage method according to the reported literature [12]. The typical reaction
was carried out under an argon atmosphere by using 0.2 mol% of 1 and CsOH⋅H O (5 mmol) in an excess of
2
benzyl alcohol (25 mmol). The results are listed in table 5. It is noted that the H gas was observed in 74.2%
2
(
based on CsOH) after 1h and the catalytic reaction completed after 2.5h. The total amount of H was double
2
mol to that of CsOH, which is consistent with the isolated yield of benzoic acid (94%).
Table 5. The H production during the catalytic reaction
2
Hydrogen Gas
TON
(H2)
157
338
742
920
962
-1
Time (h)
n (mmol)
TOF (h )
Volume (mL)
0.25
38
82
1.57
3.38
7.42
9.20
9.62
628
676
742
614
480
0
.5
.0
.5
.0
1
180
223
233
1
2

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2
3
.5
.0
243
10.03
~1000
~1000
400
-
250
10.06
Reaction condition: Benzyl alcohol (25mmol), CsOH·H O (5mmol), and complex 1 (10ꢀmol) were heated
2
under argon. A blank experiment without catalyst was taken at the same time.
The standard atmospheric pressure P=101.325KPa; room temperature T=289.15K; the corresponding water
saturated vapor pressure under the temperature P =1.8185KPa. The approximate amount of H was calculated
0
2
-1
-1
by Van der Waals equation: (P-P )V=nRT (R=8.314 KPa·L·mol ·K ). TON=n(H )/n(catalyst); TOF=TON/t.
0
2
3
. Conclusion
We have demonstrated new Ru(II)-N′NN′ pincer complexes with high efficiency in dehydrogenation of
primary alcohols to corresponding carboxylic acids and H in the alcohol-base system. The highest yield was
2
obtained by using CsOH as the base. The higher catalytic reactivity was achieved in the homogeneous
alcohol-CsOH system than that in the alcohol-water bi-phase system, while only very small amounts of
aldehydes and esters (total lower than 2%) were observed as the by-products. Moreover, the much higher yields
(up to 100%) and high turnover number (TON ~ 10000) were obtained by using an excess of alcohol with a
lower catalyst loading. Complexes 1 and 2 present a new example of phosphine- and N-heterocycle carbene
free Ru(II) complexes with high efficiency for converting primary alcohols to carboxylic acids and H . Our
2
work may open a thread to design new transition metal complexes for acceptorless dehydrogenation of alcohol
to corresponding carbonyl derivatives. The investigation of the detail catalytic mechanism is underway.
4
. Experimental Section
4
.1. General Information

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All experiments with metal complexes were carried out under a purified argon atmosphere using the standard
schlenk techniques. Ethanol, n-butanol, 1-hexanol, cyclohexyl-methanol and benzyl alcohol were purified
according to standard procedures under argon atmosphere. Other reagents were used as received. Solvents were
1
degassed with argon or nitrogen and kept in the spherical reservoir bottle with 4Å molecular sieve. The H and
13
1
C spectra were recorded at 300MHz/75MHz Varian Gemini 300 spectrometer. The H NMR chemical shifts
1
3
were referenced to the residual hydrogen signals of the deuterated solvent or TMS, and the C NMR chemical
1
3
shifts were referenced to the C signals of the deuterated solvent. Compounds of L1 [19], [RuCl (p-cymene)]
2
2
[22], L3 [15b] were synthesized according to published procedures.
4.2. Synthesis and characterization of Ru(II) complexes (1~4)
4
.2.1. [Ru(L1)Cl(CH CN) ]·Cl (1) Ligand L1 (0.622 g, 2mmol) and [RuCl (p-cymene)] (0.616 g, 1mmol)
3
2
2
2
were dissolved in acetonitrile (50 mL) and the solution was refluxed for 12h under an argon atmosphere. After
cooling to room temperature, the red-brown precipitate was filtered, washed with diethyl ether (3×5 mL) and
1
then dried under vacuum for 12h. Complex 1 was obtained as a red-brown power (1.0 g, 88%). H NMR (300
MHz, DMSO-d ) δ (ppm): 2.06 (s, 3H); 3.17 (s, 3H); 7.49-7.58 (m, 4H); 7.80 (d, J=8.0 Hz, 2H); 8.01 (d, J=7.3
6
1
3
Hz, 2H); 8.22 (t, J=8.0 Hz, 1H); 8.57 (d, J=8.0 Hz, 2H); 15.17 (s, 2H, -NH). C NMR (75 MHz, DMSO-d ) δ
6
(ppm): 3.86; 4.71; 114.07; 118.64; 121.62; 122.44; 125.04; 126.11; 129.95; 134.57; 136.09; 143.29; 152.61;
1
1
53.45. Elemental Anal. Calcd. for C H N Cl Ru(%): C, 48.85; H, 3.39; N, 17.34. Found: C, 48.90, H, 3.60,
23 19 7 2
7.12.
.2.2. [Ru(L1)(CH CN) (OTf)]·OTf (2) Complex
4
1
(0.226 g, 0.400 mmol) and silver
3
2
trifluoromethanesulfonate (0.206 g, 0.800 mmol) were dissolved in methanol (20 mL) and the mixture solution
was stirred at room temperature for 4 hours in the dark under an argon atmosphere. After the white precipitate
of AgCl were filtered off through a celite pad, the red solution was evaporated to dryness under vacuum. The

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yellow residue was then dissolved in ethanol (6 mL) and diethyl ether (30 mL) was added slowly to precipitate
the brown micro-crystals, which was filtered, washed with diethyl ether (3 ×5 mL), and then dried under
1
vacuum for 12h to yield 2 (0.275 g, 82%). H NMR (300 MHz, DMSO-d ) δ (ppm): 0.5 (t, J=7.0Hz, 3H), 2.09
6
(
s, 3H, CH -CN), 2.86 (p, 2H), 3.20 (s, 3H), 6.04 (t, J=4.8 Hz, 1H), 7.60-7.65 (m, J=7.7 Hz, 4H,), 7.89 (d,
3
13
J=6.7 Hz, 2H), 8.06 (dd, J=6.0, 2.5 Hz, 2H), 8.44 (dd, J=22.7, 6.9 Hz, 3H), 14.95 (s, 2H, -NH). C NMR (75
MHz, DMSO-d ) δ (ppm): 4.10 (s), 4.78 (s), 17.21 (s), 19.23 (s), 59.73 (s), 62.93 (s), 114.58 (s), 118.05 (d),
6
1
18.76 (d), 122.24 (s), 123.48 (s), 126.28 (d), 131.77 (s), 134.68 (s), 138.56 (s), 143.15 (s), 157.07 (s), 153.58
(
s). Elemental Anal. Calcd for C H O N F S Ru (Contain one molecule of ethanol) (%): C, 38.66; H, 3.00; N,
27
25
7
7 6 2
1
1.69 Found: C, 38.26, H, 2.89, 11.42. A brown single crystal of 2·H O suitable for X-ray diffraction analysis
2
was obtained by slow diffusion of diethyl ether to a concentrated 2,2,2-trifluoroethanol of 2 at room
temperature for several weeks.
4
.2.3. 2-((N-benzylbenzimidazole-2-yl)-6-benzimidazole-2-yl)pyridine (L2). Under an argon atmosphere,
t
a solution of L1 (1.00g ,3.2mmol) and BuOK (0.36g, 3.2mmol) in acetone (60mL) was stirred at room
temperature for 1h, then the benzyl bromide (0.55g, 0.32mmol) was added dropwisely and the mixture was
refluxed for another 12h. After cooling to room temperature, the solvent was evaporated, and the residue was
extracted with a mixed solvent of CH Cl / CH OH (10:1, V: V, 3×50 mL). The combined organic solution was
2
2
3
evaporated under reduced pressure and the crude product of L2 was purified by the column chromatography
1
using silica gel (elute: CH Cl /ethyl acetate, 3/1 (V/V)). Yield: 0.75g (58%). H NMR (300 MHz, DMSO-d ) δ
2
2
6
(ppm): 6.35 (s, 2H), 6.97 (d, J=7.3 Hz), 7.27 (ddd, J=45.0, 25.6, 6.9 Hz, 7H), 7.62 (d, J=7.9 Hz, 1H), 7.96-7.68
13
(
m, 3H), 8.16 (d, J=7.8 Hz, 1H), 8.29 (d, J=7.7 Hz, 1H), 8.40 (d, J=7.8 Hz, 1H), 12.54 (s, 1H). C NMR (75
MHz, CDCl3) δ (ppm): 49.16 (s), 110.53 (s), 111.55 (s), 120.31 (s), 120.67 (s), 121.86 (s), 122.90 (s), 123.55
s), 124.23 (d), 125.35 (s), 126.11 (s), 127.99 (s), 129.58 (s), 133.78 (s), 136.83 (s), 137.74 (s), 138.59 (s),
(

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1
43.01 (s), 144.40 (s), 147.66 (s), 149.29 (s). Elemental Anal. Calcd for C H N (%): C, 77.76; H, 4.47; N,
2
6
19
5
1
6.63. Found: C, 77.79; H, 4.77; N 16.44.
.2.4. Ru(L2)(CH CN) Cl]·Cl (3). Ligand L2 (0.15g, 0.373mmol) and [RuCl (p-cymene)] (0.115g, 0.187
4
3
2
2
2
mmol) were dissolved in acetonitrile (15mL) and the solution refluxed for 12h under an argon atmosphere.
After cooling to room temperature, the mixture was filtered, the crude products was further purified by the
column chromatography using silica gel (elute: ethanol) and dried under vacuum to afford complex 3 as a
1
red-brown solid (0.085g, 35%). H NMR (300 MHz, DMSO-d ) δ (ppm): 2.07 (s, 3H), 3.19 (s, 3H), 6.35 (s,
6
2
1
1
1
1
H), 7.23 (dd, J=67.2, 6.5Hz, 4H), 7.85-7.44 (m, 5H), 8.03 (d, J=13.7 Hz, 3H), 8.15 (t, 1H), 8.29 (d, J=8.2 Hz,
1
3
H), 8.48 (d, 1H), 15.03 (br). C NMR (75 MHz, DMSO-d6) δ (ppm): 48.11, 112.54, 113.90, 118.25, 121.78,
23.17, 124.85, 125.52, 126.15, 126.44, 128.31, 129.49, 130.00, 134.54, 135.53, 136.21, 136.36, 142.20,
42.88, 151.55, 152.27, 152.65, 153.70; Elemental Anal. Calcd for C H N Cl Ru (%): C, 54.97; H, 3.84; N,
3
0
25
7
2
4.96. Found: C, 55.22; H, 4.05; N, 14.68.
.2.5. Ru(L3)(CH CN) Cl]·Cl (4) Ligand L3 (0.15g, 0.305mmol) and [RuCl (p-cymene)] (0.094g, 0.153
4
3
2
2
2
mmol) were dissolved in a mixed solvent of acetonitrile (5mL) and ethanol (5mL) and then refluxed for 12h
under an argon atmosphere. After cooling to room temperature, the solution was taken to dryness under
vacuum, the residue was passed through the column chromatography using silica gel (elute: methanol) to afford
1
pure 4 as a red brown powder (0.077g, 34%). H NMR (300 MHz, DMSO-d ) δ (ppm): 2.09 (s, 3H), 3.22 (s,
6
3
2
1
H), 6.34 (s, 4H), 7.49-6.99 (m, 11H), 7.65 (m, 4H), 7.86 (t, J=8.2 Hz, 1H), 7.95 (d, 2H), 8.10 (d, 2H), 8.25 (d,
1
3
H). C NMR (75 MHz, DMSO-d ) δ (ppm): 3.48, 4.50, 48.12, 112.65, 118.77, 121.62, 122.75, 125.68,
6
26.12, 126.47, 128.26, 129.48, 130.57, 136.22, 136.45, 141.93, 151.40, 153.15. Elemental Anal. Calcd for
C H N Cl Ru: C, 59.60; H, 4.19; N, 13.15. Found: C, 59.83; H, 4.30; N, 12.97.
37
31
7
2
4.3. General procedure for catalytic reactions

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4
.3.1. Method A [11]: Benzyl alcohol (0.54g, 5mmol), 2 (8.3mg, 0.01mmol), base (5.5mmol) and 2mL
degassed water were mixed in a 25mL schlenk tube and the reaction mixture was refluxed for 24h in an open
system under argon. After cooling to room temperature, the degassed water (5mL) was added and the mixture
was extracted with diethyl ether (3 ×10 mL). The aqueous phase was acidified with 6M HCl and extracted with
ethyl acetate (5 × 20mL). The combined organic phase were washed with brine (25mL), dried over anhydrous
Na SO , and evaporated to dryness under reduced pressure, the pure benzoic acid was obtained and further
2
4
1
characterized by HNMR which is consist with the standard sample.
.3.2. Method B: To a schlenk tube (25mL) benzyl alcohol (0.54g, 5mmol), complex 1 (or 2, 3, 4)
0.01mmol), CsOH·H O (1.26g, 7.5mmol) and 2mL deionized water were added and the reaction mixture was
4
(
2
refluxed for 24h in an open system under argon. After cooling to room temperature, the degassed water (5mL)
was added and the mixture was extracted with diethyl ether (3 ×10mL). The aqueous phase was acidified with
6
M HCl and extracted with ethyl acetate (5 ×20mL). The combined organic phase was washed with brine
(
25mL), dried over anhydrous Na SO , and evaporated under vacuum, the pure benzoic acid was obtained and
2
4
1
weighed for calculating the yield, which was further characterized by its HNMR which is consist with the
standard sample.
4
.3.3. Method C: Complex 2 (8.3 mg, 10ꢀmol), CsOH·H O (0.84 g, 5 mmol), alcohol (5 mmol) was added
2
to a 25 mL schlenk tube and the solution was heated at 150°C (oil bath) for 24h in an open system under argon.
After cooling to room temperature, the degassed water (5mL) was added and the mixture was extracted with
diethyl ether (3 ×10mL). A sample of ether phase was subjected to the GC-MS analysis and the residual
solution was evaporated, then subjected to the NMR analysis. The aqueous phase was acidified with 6M HCl
and extracted with ethyl acetate (5 ×20mL). The combined organic phase was washed with brine (25mL), dried
over anhydrous Na SO , and evaporated under reduced pressure, the pure carboxylic acid was collected and
2
4

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1
weighed for calculating the yield, which was further characterized by its H NMR which is consist with the
standard sample.
4
.3.4. Method D: For substrates of benzyl alcohol and cyclohexylmethanol: To a 100 mL schlenk tube
complex 1 (1.4mg, 2.5ꢀmol), CsOH·H O (4.2g, 25mmol), and alcohol (125mmol) were added and then the
2
solution was heated at 150°C (oil bath) for 24h in an open system under argon. After cooling to room
temperature, the degassed water (20mL) was added and the mixture was extracted with diethyl ether (3 ×20mL).
The ether phase was evaporated under vacuum and the residue was subjected to the GC-MS and the NMR
analysis. The aqueous phase was acidified with 6M HCl and extracted with ethyl acetate (5 ×30mL). The
combined organic solution was washed with brine (50mL), dried over anhydrous Na SO , and evaporated
2
4
under vacuum, the pure acid was obtained and weighed for calculating the yield, which was further
1
characterized by its H NMR which is consist with the standard sample.
4
.4. Procedure for H gas production
2
Under an argon atmosphere, benzyl alcohol (2.70g, 25mmol), CsOH·H O (0.84g, 5mmol), and complex 1
2
(5.7mg, 0.01mmol) were added to a 25 mL schlenk tube which is connected with a gas collection instrument
through gravity drainage method. The reaction mixture was heated at 150°C (oil bath). Over a period of time,
the volume of the gas was recorded. A blank experiment without catalyst was taken at the same condition.
4.5. X-ray crystallography
A Bruker SMART APEX II diffractometer with graphite-monochromated Mo-Kα (λ= 0.71073 Å) was
employed to collect the intensity data for the single crystal of 2. The data was collected at about 100K using
ω-scan techniques. The structure was solved by direct methods using SHELXL-97 [23]. Multi-scan empirical
absorption corrections were applied to the data set using the program SADABS [24]. The structure was refined

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with SHELXL-97[23]. Hydrogen atoms bound to carbon were placed at calculated positions and refined using
2
a riding mode. All non-hydrogen atoms were refined by full-matrix least squares on F using the SHELXTL
grogram package [25]. Cell refinement, data collection, and reduction were done by Bruker SAINT[26]. The
crystallographic data is available in the SI as CIF file.
Acknowledgements
This work was supported by the Natural Science Foundation of China (21171134, 21573166), the Natural
Science Fund of Hubei Province (2011CDB441), and the Funds for Creative Research Groups of Hubei
Province (2014CFA007), China. We also thank Prof. Xianggao Meng for the support of X-ray diffraction
analysis of 2 (CCDC 1449878) and Prof. Xiaojun Wu for NMR supporting and analysis.
Notes
The authors declare no competing financial interests.
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[
[
[
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Synthesis of ruthenium(II) complexes bearing rigid N′NN′-typed pincer ligands;
Catalytic dehydrogenation of primary alcohols to carboxylic acids and H2;
Higher yields and higher selectivity obtained in homogeneous alcohol-CsOH system
Quantitative yield of benzoic acid and TON~10000 achieved in 24h.