Ruthenium NNN complexes with a 2-hydroxypyridylmethylene fragment for transfer hydrogenation of ketones

Article abstract of DOI:10.1002/aoc.4100

Four NNN tridentate ligands L₁–L₄ containing 2-methoxypyridylmethene or 2-hydroxypyridylmethene fragment were synthesized and introduced to ruthenium centers. When (HOC₅H₃NCH₂C₅H₃NC₅H₇N₂) (L₂) and (HOC₅H₃NCH₂C₅H₃NC₆H₆N₃) (L₄) reacted with RuCl₂(PPh₃)₃, two ruthenium chloride products Ru(L₂)(PPh₃)Cl₂ (1) and Ru(L₄)(PPh₃)Cl₂ (2) were isolated, respectively. Reactions of (MeOC₅H₃NCH₂C₅H₃NC₅H₇N₂) (L₁) and (MeOC₅H₃NCH₂C₅H₃NC₆H₆N₃) (L₃) with RuCl₂(PPh₃)₃ in the presence of NH₄PF₆ generated two dicationic complexes [Ru(L₁)₂][PF₆]₂ (3) and [Ru(L₃)₂][PF₆]₂ (4), respectively. Complex 1 reacted with CO to afford product [Ru(L₂)(PPh₃)(CO)Cl][Cl]. The catalytic activity for transfer hydrogenation of ketones was investigated. Complex 1 showed the highest activity, with a turnover frequency value of 1.44?×?10³?h^?1 for acetophenone, while complexes 3 and 4 were not active.

Full text of DOI:10.1002/aoc.4100

Received: 17 July 2017
DOI: 10.1002/aoc.4100
Revised: 16 August 2017
Accepted: 17 August 2017
F U L L P A P E R
Ruthenium NNN complexes with a 2‐
hydroxypyridylmethylene fragment for transfer
hydrogenation of ketones
Jing Shi | Shu Shang | Bowen Hu | Dafa Chen
MIIT Key Laboratory of Critical Materials
Four NNN tridentate ligands L1–L4 containing 2‐methoxypyridylmethene or
Technology for New Energy Conversion
and Storage, School of Chemistry and
Chemical Engineering, Harbin Institute of
Technology, Harbin 150001, China
2‐hydroxypyridylmethene fragment were synthesized and introduced to
ruthenium centers. When (HOC5H3NCH2C5H3NC5H7N2) (L2) and
(HOC5H3NCH2C5H3NC6H6N3) (L4) reacted with RuCl2(PPh3)3, two ruthe-
nium chloride products Ru(L2)(PPh3)Cl2 (1) and Ru(L4)(PPh3)Cl2 (2) were
isolated, respectively. Reactions of (MeOC5H3NCH2C5H3NC5H7N2) (L1) and
(MeOC5H3NCH2C5H3NC6H6N3) (L3) with RuCl2(PPh3)3 in the presence of
NH4PF6 generated two dicationic complexes [Ru(L1)2][PF6]2 (3) and
[Ru(L3)2][PF6]2 (4), respectively. Complex 1 reacted with CO to afford
product [Ru(L2)(PPh3)(CO)Cl][Cl]. The catalytic activity for transfer hydroge-
nation of ketones was investigated. Complex 1 showed the highest activity,
with a turnover frequency value of 1.44 × 103 h−1 for acetophenone, while
complexes 3 and 4 were not active.
Correspondence
Bowen Hu or Dafa Chen, MIIT Key
Laboratory of Critical Materials
Technology for New Energy Conversion
and Storage, School of Chemistry and
Chemical Engineering, Harbin Institute of
Technology, Harbin 150001, China.
Email: bowenhu@hit.edu.cn;
dafachen@hit.edu.cn
Funding information
China Postdoctoral Science Foundation,
Grant/Award Number: 2016M591519 and
2016 M591519; Heilongjiang Province
Postdoctoral Science Foundation, Grant/
Award Number: LBH‐Z16069; National
Natural Science Foundation of China,
Grant/Award Number: 21302028 and
21672045
KEYWORDS
[Fe]‐hydrogenase, 2‐hydroxypyridyl, metal–ligand cooperation, ruthenium, transfer hydrogenation
1 | INTRODUCTION
Transfer hydrogenation (TH) is an important and
attractive method for reducing ketones to alcohols.[24–29]
Metal–ligand cooperative catalysts have been developed
rapidly in recent years, due to their effective and signifi-
cant applications for molecular transformation in organic
reactions.[1–8] Ligand design is central for the develop-
ment of these bifunctional catalysts, because the systems
mainly depend on the cooperation of ligand with metal
center, leading to reversible structural changes of the
ligand in the catalytic cycle. A metal–ligand cooperative
complex might contain a ligand with a proton‐sensitive
site, because the ligand's structure could be changed by
a base. Following this strategy, many transition metal
complexes with 2‐hydroxypyridyl fragment have been
developed and applied in organic synthesis.[3,9–23]
Recently, Szymczak's group reported a ruthenium catalyst
based on the ligand 6,6′‐dihydroxyterpyridine (dhtp) for
TH of a series of ketones, reaching turnover frequencies
(TOFs) up to 100 h−1 (Figure 1a).[30] In comparison with
similar ruthenium complexes with 4,4′‐dhtp or terpyridine,
this complex shows higher catalytic efficiency.[31] Subse-
quently, Kundu and co‐workers developed another
ruthenium complex bearing 2‐(2‐pyridyl‐2‐ol)‐1,10‐
phenanthroline as the ligand, displaying higher activity
in TH of acetophenone (TOF of 2.40 × 103 h−1
)
(Figure 1b).[32] Notably, among the reported ruthenium
TH catalysts with a 2‐hydroxypyridyl fragment, this
complex is the most active one.
Appl Organometal Chem. 2017;e4100.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4100

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SHI ET AL.
+
+
+
OH
N
PPh3
OH
PPh3
Cl
N
N
PPh3
N
N
N
-
-
-
PF6
Cl
N
PF6
OH
Ru
Ru
PF6
Ru Cl
R
PPh3
N
OH
PPh3
PPh3 N
(b)
(a)
(c)
+
+
PPh3
PPh3
N
N
N
Cl
N
Cl
Ru
Ru
Cl-
Cl-
R
N
N
PPh3
PPh3
R
N
N
N
FIGURE 1 (a) Szymczak's complex; (b)
Kundu's complex; (c) our complexes; (d, e)
Yu's complexes
N
N
H
R
R
H
(d)
(e)
[Fe]‐hydrogenase also contains a 2‐hydroxypyridyl
ligand, which is crucial for H2 activation,[33–35] and many
model complexes have been synthesized.[36–44] Inspired
by the structure of the [Fe]‐hydrogenase active site, we
have developed several NNN ruthenium complexes with
a 2‐hydroxypyridylmethylene fragment for ketone TH
(Figure 1c).[45,46] Although these complexes show high
activity, they are not as good as the catalyst of Kunda's
group.[32] Yu's group developed various excellent NNN
ruthenium catalysts with a pyrazole or benzotriazole
heterocyclic ring (Figure 1d,e), with TOF values up to
The mixture was heated slowly to reflux for 24 h and then
quenched with water (15 ml). After extraction with ethyl
acetate (3 × 100 ml), the combined organic phase was
dried with Na2SO4 and concentrated. The crude product
was purified by column chromatography (eluent: petro-
leum ether–ethyl acetate, 20:1 v/v), affording the product
L1 as a slightly yellow oil (9.2 g, 60%). HR‐MS (ESI): calcd
1
for C17H18N4O + H, 295.1559; found, 295.1553. H NMR
(400 MHz, CDCl3, ppm): 7.70–7.65 (m, 2H), 7.47 (t,
J = 8 Hz, 1H), 7.13 (d, J = 7 Hz, 1H), 6.79 (d, J = 8 Hz,
1H), 6.57 (d, J = 8 Hz, 1H), 5.94 (s, 1H), 4.20 (s, 2H),
3.89 (s, 3H), 2.51 (s, 3H), 2.28 (s, 3H). 13C NMR
(100 MHz, CDCl3, ppm): 163.7, 157.9, 157.0, 153.0, 149.6,
141.6, 138.9, 138.6, 120.3, 116.2, 113.1, 108.8, 108.0, 53.3,
46.5, 14.6, 13.7.
3.5 × 105 h−1 [47,48]
.
In the work reported in this paper, pyrazole or benzo-
triazole ring and 2‐hydroxypyridylmethylene derivative
were combined, and two NNN tridentate ligands L2 and
L4 were synthesized and introduced to ruthenium centers.
2.3 | Synthesis of L2
2 | EXPERIMENTAL
Slightly yellow oil L1 (7.5 g, 26 mmol) was added into HBr
(80 ml, 40% in water) and the reaction was heated to
reflux for 5 h. Then the solution was cooled with ice and
neutralized with an aqueous solution of NaOH. A white
solid precipitated out and the precipitate was filtered to
afford L2 (5.7 g, 80%). M.p. 150–155 °C. HR‐MS (ESI):
2.1 | Materials and methods
All solvents were distilled under nitrogen before use
through standard Schlenk techniques. The starting
materials
of
RuCl2(PPh3)3,[49]
2‐bromo‐6‐(3,5‐
dimethylpyrazol‐1‐yl)pyridine[50] and 1‐(6‐bromopyridin‐
2‐yl)‐1H‐benzo[d][1,2,3]triazole[48] were prepared as
calcd for C16H16N4O + H, 281.1402; found, 281.1399. H
1
NMR (400 MHz, CD3OD, ppm): 7.86 (t, J = 8 Hz, 1H),
7.61 (d, J = 8 Hz, 1H), 7.52 (m, 1H), 7.26 (d, J = 4 Hz,
1H), 6.40 (d, J = 12 Hz, 1H), 6.29 (d, J = 8 Hz, 1H), 6.01
(s, 1H), 4.11 (s, 2H), 2.38 (s, 3H), 2.23 (s, 3H). 13C NMR
(100 MHz, CD3OD, ppm): 156.0, 155.7, 152.6, 150.0,
146.5, 142.4, 141.9, 139.4, 120.4, 116.7, 113.7, 108.6,
107.5, 40.0, 13.1, 12.0.
previously described. The 13C NMR, H NMR and 31P
1
NMR spectra of as‐synthesized complexes were recorded
with a Bruker Avance 400 spectrometer.
2.2 | Synthesis of L1
Colorless liquid 2‐methyl‐6‐methoxypyridine (12.6 g,
103 mmol) was dissolved in 100 ml of dry tetrahydrofuran
(THF) and the mixture was cooled to −78 °C. n‐
Butyllithium (41 ml, 103 mmol, 2.5 M in pentane) was
added dropwise into the reaction within 10 min and
stirred for 60 min at −78 °C. Then Pd(PPh3)4 (1.8 g,
1.6 mmol) and 2‐bromo‐6‐(3,5‐dimethylpyrazol‐1‐yl)pyri-
dine (13 g, 52 mmol) were added quickly into the reaction.
2.4 | Synthesis of L3
The procedure was similar to that described for L1. Color-
less liquid 2‐methyl‐6‐methoxypyridine (6.2 g, 50 mmol)
was dissolved in dry THF (80 ml) and the mixture was
cooled to −78 °C. n‐Butyllithium (20 ml, 50 mmol,
2.5 M in pentane) was added dropwise into the reaction

SHI ET AL.
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within 10 min and stirred for 60 min at −78 °C. Then
Pd(PPh3)4 (1.3 mmol, 1.5 g) and 1‐(6‐bromopyridin‐2‐yl)‐
1H‐benzo[d][1,2,3]triazole (6.5 g, 25 mmol) were added
quickly into the reaction. The mixture was heated slowly
to reflux for 48 h and quenched with water (15 ml). After
extraction with CH2Cl2 (100 ml), the combined organic
phase were dried with Na2SO4 and concentrated. The
mixture was purified by column chromatography (eluent:
petroleum ether–ethyl acetate, 10:1 v/v), affording L3 as a
white solid (2.4 g, 30%). M.p. 116–120 °C. HR‐MS (ESI):
2.96 (s, 3H), 1.66 (s, 3H). 31P NMR (162 MHz, CD3OD,
ppm): 56.5.
2.7 | Synthesis of Ru(L4)(PPh3)Cl2 (2)
Using a similar procedure as described above, a mixture of
L4 (0.19 g, 0.63 mmol) and RuCl2(PPh3)3 (0.6 g, 0.63 mmol)
in dry methanol (300 ml) was refluxed for 24 h. After
drying under vacuum, the crude product was purified by
column chromatography on silica gel (eluent: ethyl
acetate–methanol, 1:1 v:v) to afford 2 (0.23 g, 49%) as a
red powder. Anal. Calcd for C35H28Cl2N5OPRu (%): C,
1
calcd for C18H15N5O + H, 318.1355; found, 318.1346. H
NMR (400 MHz, CDCl3, ppm): 8.48 (d, J = 8 Hz, 1H),
8.11–8.08 (m, 2H), 7.86 (t, J = 8 Hz, 1H), 7.56–7.49 (m,
2H), 7.43 (t, J = 8 Hz, 1H), 7.32 (d, J = 8 Hz, 1H), 6.90
(d, J = 8 Hz, 1H), 6.64 (d, J = 8 Hz, 1H), 4.35 (s, 2H),
3.92 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm): 163.9,
159.1, 156.5, 151.2, 146.7, 139.2, 139.0, 131.5, 128.5,
124.9, 122.0, 119.6, 116.2, 115.1, 111.8, 108.3, 53.0, 46.3.
1
57.0; H, 3.8; N, 9.5. Found (%): C, 57.2; H, 3.8; N, 9.7. H
NMR (400 MHz, CD3COCD3, ppm): 8.19 (d, J = 8 Hz,
1H), 8.15–8.10 (m, 2H), 7.98 (t, J = 8 Hz, 1H), 7.69–7.57
(m, 3H), 7.47 (d, J = 8 Hz, 1H), 7.32–7.08 (m, 15H), 6.27
(d, J = 7 Hz, 1H), 6.17 (d, J = 8 Hz, 1H), 4.04 (d,
J = 14 Hz, 1H), 3.57 (d, J = 14 Hz, 1H). 31P NMR
(162 MHz, CD3COCD3, ppm): 47.5.
2.5 | Synthesis of L4
2.8 | Synthesis of [Ru(L1)2][PF6]2 (3)
The procedure was similar to that described for L2. White
solid L3 (4.0 g, 12.6 mmol) was added into HBr (80 ml,
40% in water) and the reaction was heated to reflux for
5 h. Then the solution was cooled with ice and neutralized
with an aqueous solution of NaOH. A white solid precip-
itated out and the precipitate was filtered. The crude
product was purified by column chromatography (eluent:
ethyl acetate–methanol, 8:1 v/v), affording L4 as a white
solid (2.0 g, 53%). M.p. 193–196 °C. HR‐MS (ESI): calcd
A mixture of L1 (0.74 g, 2.5 mmol) and RuCl2(PPh3)3
(1.2 g, 1.25 mmol) was added into dry methanol
(200 ml). Then the reaction was stirred at reflux for
24 h. The solution was cooled to room temperature and
NH4PF6 (2 g, 12.5 mmol) was added into the reaction.
After stirring for 20 min, the solid precipitated out and
was filtered, and washed with copious amounts of diethyl
ether to afford 3 as a yellow powder (0.93 g, 81%). Anal.
Calcd for C34H36F12N8O2P2Ru (%): C, 41.7; H, 3.7; N,
11.4. Found (%): C, 41.5; H, 3.8; N, 11.6. 1H NMR
(400 MHz, CD3COCD3, ppm): 8.36 (t, J = 8 Hz, 2H),
8.25 (d, J = 8 Hz, 2H), 8.01 (d, J = 8 Hz, 2H), 7.95 (t,
J = 8 Hz, 2H), 7.58 (d, J = 8 Hz, 2H), 6.78 (d, J = 8 Hz,
2H), 6.18 (s, 2H), 5.00 (d, J = 14 Hz, 2H), 4.44 (d,
J = 14 Hz, 2H), 3.12 (s, 6H), 2.90 (s, 6H), 0.97 (s, 6H).
1
for C17H13N5O + H, 304.1198; found, 304.1195. H NMR
(400 MHz, CD3OD, ppm): 8.33 (d, J = 8 Hz, 1H), 8.20
(d, J = 8 Hz, 1H), 8.07–8.05 (m, 2H), 7.64–7.55 (m, 2H),
7.51–7.43 (m, 2H), 6.53 (d, J = 12 Hz, 1H), 6.43 (d,
J = 8 Hz, 1H), 4.31 (s, 2H). 13C NMR (100 MHz, CD3OD,
ppm): 165.1, 157.0, 151.2, 146.2, 142.5, 140.0, 131.3, 128.9,
125.1, 121.7, 118.7, 117.0, 114.6, 112.0, 107.6, 39.9.
2.6 | Synthesis of Ru(L2)(PPh3)Cl2 (1)
2.9 | Synthesis of [Ru(L3)2][PF6]2 (4)
A mixture of L2 (0.35 g, 1.23 mmol) and RuCl2(PPh3)3
(1.2 g, 1.23 mmol) was added into dry methanol
(300 ml). Then the reaction was stirred at reflux for
24 h. The solution was cooled to room temperature and
the solvent was removed under vacuum. The crude
product was washed with a little acetone to afford 1 as
an orange powder (0.35 g, 78%). Anal. Calcd for
C34H31Cl2N4OPRu (%): C, 57.2; H, 4.4; N, 7.8. Found
Using a similar procedure as described above, a mixture of
L3 (0.4 g, 1.23 mmol) and RuCl2(PPh3)3 (0.6 g, 0.63 mmol)
was added into dry methanol (200 ml). Then the reaction
was stirred at reflux for 24 h. The solution was cooled to
room temperature and NH4PF6 (1.2 g, 7.4 mmol) was
added into the reaction. After stirring for 20 min, the solid
precipitated out and was filtered, and washed with copi-
ous amounts of diethyl ether to afford 4 as a yellow pow-
der (0.43 g, 67%). Anal. Calcd for C36H30F12N10O2P2Ru
(%): C, 42.2; H, 3.0; N, 13.7. Found (%): C, 42.3; H, 3.1;
1
(%): C, 57.1; H, 4.4; N, 7.6. H NMR (400 MHz, CD3OD,
ppm): 7.96 (d, J = 8 Hz, 1H), 7.82 (t, J = 8 Hz, 1H), 7.71
(t, J = 8 Hz, 1H), 7.38–7.20 (m, 15H), 7.02 (d, J = 8 Hz,
1H), 6.98 (d, J = 8 Hz, 1H), 6.76 (d, J = 8 Hz, 1H), 6.38
(s, 1H), 3.97 (d, J = 14 Hz, 1H), 3.46 (d, J = 14 Hz, 1H),
1
N, 13.5. H NMR (400 MHz, CD3COCD3, ppm): 8.85 (d,
J = 8 Hz, 2H), 8.68 (t, J = 8 Hz, 2H), 8.61 (d, J = 8 Hz,
2H), 8.37 (d, J = 8 Hz, 2H), 8.04 (d, J = 8 Hz, 2H),

4 of 10
SHI ET AL.
7.86–7.80 (m, 4H), 7.66 (t, J = 8 Hz, 2H), 7.45 (d, J = 8 Hz,
2H), 6.59 (d, J = 8 Hz, 2H), 5.52 (d, J = 16 Hz, 2H), 5.13 (d,
J = 16 Hz, 2H), 3.13 (s, 6H).
reaction mixture was removed as a sample and immedi-
ately diluted with 0.5 ml of gelid 2‐propanol for GC
analysis. After the reaction was completed, the reaction
solution was removed under reduced pressure and
subjected to purification by flash silica gel column chro-
matography to afford the corresponding alcohol product,
which was identified by comparison with an authentic
sample through NMR and GC analyses.
2.10 | Synthesis of [Ru(L2)(PPh3)(CO)Cl]
[Cl] (5)
CO (1 atm) was bubbled into a solution of 1 (0.77 g,
1.08 mmol) in methanol (80 ml) with stirring for 24 h.
After the reaction was completed, the solution turned
from orange to yellow. The solvent was removed under
vacuum and the crude product was washed with acetone
(10 ml). The resulting yellow precipitate was filtered,
and then dried to afford 5 as a faint yellow powder
(0.69 g, 86%). Anal. Calcd for C35H31Cl2N4O2PRu (%): C,
56.6; H, 4.2; N, 7.5. Found (%): C, 58.5; H, 4.1; N, 7.4. IR
(in DMSO, νCO, cm−1): 1989 (s). 1H NMR (400 MHz,
DMSO, ppm): 8.15 (t, J = 8 Hz, 1H), 7.99 (d, J = 8 Hz,
1H), 7.69 (t, J = 8 Hz, 1H), 7.47–7.31 (m, 10H), 7.12 (m,
6H), 7.01 (d, J = 4 Hz, 1H), 6.92 (d, J = 8 Hz, 1H), 6.49
(s, 1H), 4.36 (d, J = 16 Hz, 1H), 3.36 (d, J = 16 Hz, 1H),
2.85 (s, 3H), 1.57 (s, 3H). 31P NMR (162 MHz, DMSO,
ppm): 46.1.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization of
ligands and corresponding Ru(II)
complexes
Reaction of the known complex 2‐bromo‐6‐(3,5‐
dimethylpyrazol‐1‐yl)pyridine with two equivalents of
the lithium salt of 2‐bromo‐6‐methoxypyridine in THF
afforded L1 in 60% yield. After treatment with HBr
(40% in water), L1 transformed to L2, with a 3,5‐
dimethylpyrazol ring and a 2‐hydroxypyridylmethylene
fragment (Scheme 1). L3 and L4 were synthesized simi-
larly to L1 and L2, respectively (Scheme 2).
When L2 was treated with an equimolar amount of
RuCl2(PPh3)3 in refluxing methanol, the orange product
1 was obtained (Scheme 3). The 31P NMR spectrum of 1
in CD3OD exhibits one singlet for the PPh3 group at
56.5 ppm, which is comparable to similar NNN ruthe-
nium complexes reported by us.[45,46] The 1H NMR
2.11 | General procedure for catalytic TH
of ketones
Complex 1 (71.4 mg, 0.10 mmol) was dissolved in 2‐
propanol (10.0 ml) as the catalyst solution and dodecane
(340 mg, 2.0 mmol) was also dissolved in 2‐propanol
(10.0 ml) as internal standard solution. Under a nitrogen
atmosphere, a mixture of ketone (2.0 mmol), 0.4 ml of
the catalyst solution (0.004 mmol), 1.0 ml of the internal
standard solution (0.2 mmol) and 2‐propanol (10.0 ml)
was stirred at reflux for 10 min. Then 0.2 ml of a 0.5 M
iPrOK (0.10 mmol) solution in 2‐propanol was added to
initiate the reaction. At the required time, 0.1 ml of the
OH
OH
N
N
PPh3
N
N
RuCl2(PPh3)3
Ru Cl
N
N
Cl
N
N
1
L2
SCHEME 3 Synthesis of complex 1
O
OH
N
N
N
Li
HBr
N
N
N
Br
O
N
N
N
N
N
N
SCHEME 1 Synthesis of ligands L1 and
L2
L2
L1
O
OH
N
Li
N
HBr
N
N
N
Br
O
N
N
N
N
N
N
N
N
N
N
SCHEME 2 Synthesis of ligands L3 and
L4
L4
L3

SHI ET AL.
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spectrum shows one singlet at 6.38 ppm for the CH proton
in the pyrazolyl ring, and two singlets at 2.96 and 1.66 ppm
for the two methyl groups. The two characteristic doublets
at 3.97 and 3.46 ppm for the ―CH2― group indicate that
the two protons are in different environments. From
the NMR data, the formula of 1 can be concluded as
Ru(L2)(PPh3)Cl2, with two Cl atoms in cis configuration.
The structure of complex 1 was further confirmed
using X‐ray crystallography (Figure 2). In the solid state,
the two chlorides are cis to each other, consistent with
OH
OH
PPh3
N
N
N
N
RuCl2(PPh3)3
Cl
Ru
N
N
N
Cl
N
N
N
L4
2
SCHEME 4 Synthesis of complex 2
1
2+
2PF6
the H NMR results. Strong hydrogen bonding between
O
N
N
N
N
-
the hydroxyl group and Cl(1) was evident from the angle
O(1)–H(1A)–Cl(1) (173.43°), the O(1)–Cl(1) distance
(2.871 Å) and the O(1)–H(1) distance (0.8200 Å).[30]
Similarly, complex 2 was generated by the reaction of
L4 with RuCl2(PPh3)3 (Scheme 4). The ―CH2― protons
N
OMe
RuCl2(PPh3)3
N
Ru
N
N
N
N
NH4PF6
N
N
OMe
L1
3
1
appear as two doublets at 4.04 and 3.57 ppm in its H
SCHEME 5 Synthesis of complex 3
NMR spectrum, and the 31P signal is present at
47.5 ppm as a singlet in its 31P NMR spectrum. The results
indicate that 2 has a structure similar to that of 1.
and three singlets at 3.12, 2.90 and 0.97 ppm for the
methyl groups. No signal of PPh3 group was observed,
indicating a coordination mode different from that of 1
or 2.
In a similar manner, when L1 was replaced by L3,
complex 4 could be produced in 67% yield (Scheme 6).
In contrast, if L1 was replaced by L2 or L4, no pure charac-
teristic product could be isolated. Similar to complex 3,
The reactivities of RuCl2(PPh3)3 with L1 and L3 were
also investigated. Reaction of RuCl2(PPh3)3 with two
equivalents of L1, followed by the addition of excess of
NH4PF6, generated product 3 in 81% yield (Scheme 5).
Even if only one equivalent of L1 was added, 3 was still
the only identified product. The 1H NMR spectrum shows
six signals between 8.36 and 6.78 ppm for the pyridyl
rings, one singlet at 6.18 ppm for the pyrazolyl rings,
two doublets at 5.00 and 4.44 ppm for the ―CH2― groups
1
the H NMR spectrum of 4 also does not exhibit any
PPh3 signal. X‐ray diffraction analysis shows 4 is a
dicationic complex with the ruthenium center
coordinating to two tridentate ligands L3 in a distorted
octahedral geometry (Figure 3). Both NNN ligands are
coordinated to the Ru(II) center in a meridional fashion,
which is analogous to the reported complex
[(CH3OC5H3NCH2C5H3NC5H4N)2Ru][PF6]2.[45]
Reactions of 1 and 2 with CO were also investigated.
In the presence of CO (1 atm) in CH3OH, complex 1 trans-
formed to complex 5 (Scheme 7). The protons of PPh3
1
group are evident as signals at 7.47–7.12 ppm in its H
NMR spectrum. The 31P NMR spectrum shows a singlet
at 46.1 ppm. The IR spectrum shows a strong CO absorp-
tion at 1989 cm−1. These data suggest the accurate
structure of 5 is as shown in Scheme 7, which is similar
to ruthenium complex [Ru(dhtp)(PPh3)(CO)Cl][PF6] of
FIGURE 2 Single‐crystal structure of 1. The thermal ellipsoids
are displayed at 30% probability. Hydrogen atoms not involved in
hydrogen bonding and phenyl rings on PPh3 ligand have been
omitted for clarity. Selected bond distances (Å) and angles (°):
Ru(1)–N(1), 2.074(5); Ru(1)–N(3), 2.044(4); Ru(1)–N(4), 2.188(5);
Ru(1)–P(1), 2.296(14); Ru(1)–Cl(1), 2.4755(14); Ru(1)–Cl(2),
2.536(12); C(14)–O(1), 1.312(7); O(1)–Cl(1), 2.871; N(3)–Ru(1)–
Cl(1), 174.56(13); P(1)–Ru(1)–Cl(2), 174.65(5); N(1)–Ru(1)–N(4),
160.67(18); C(8)–C(9)–C(10), 112.9(5); O(1)–H(1A)–Cl(1), 173.43
2+
O
N
N
N
N
N
N
-
2PF6
N
RuCl2(PPh3)3
NH4PF6
OMe
N
N
N
Ru
N
N
N
N
N
OMe
L3
4
SCHEME 6 Synthesis of complex 4

6 of 10
SHI ET AL.
TABLE 1 Optimization of reaction conditions for TH of
acetophenone catalyzed by Ru(II) complexes 1–5a
Time
(min)
Temp.
(°C)
Yield
(%)b
Entry
Catalyst
1
Base
iPrOK
1
5
82
82
82
82
82
82
82
30
82
82
82
82
82
58
96
54
0
2
1
2
3
4
5
5
1
1
iPrOK
iPrOK
iPrOK
iPrOK
iPrOK
iPrOK
iPrOK
iPrOK
iPrOK
20
20
20
20
20
60
120
20
20
20
5
3
4
5
0
6
37
91
84
42
0
7c
8c
9d
10
11
12e
13e
FIGURE 3 Solid‐state structure of 4. The thermal ellipsoids are
−
1
1
1
0
displayed at 30% probability. Hydrogen atoms, solvents and PF6
iPrOK
iPrOK
56
96
anion have been omitted for clarity. Selected bond distances (Å) and
angles (°): Ru(1)–N(4), 1.998(2); Ru(1)–N(9), 2.002(2); Ru(1)–N(7),
2.056(2); Ru(1)–N(2), 2.061(2); Ru(1)–N(6), 2.165(2); Ru(1)–N(1),
2.172(2); N(9)–Ru(1)–N(6), 169.14(9); N(7)–Ru(1)–N(2), 167.61(9);
N(1)–Ru(1)–N(4), 167.29(10); C(7)–C(6)–C(5), 117.5(3); C(25)–
C(24)–C(23), 118.7(3)
20
aGeneral conditions: ketone, 2.0 mmol (2 M in 50 ml of iPrOH); ketone/base/
catalyst = 500/25/1; N2 (0.1 MPa); 82 °C.
bGC yield.
cKetone/base/catalyst = 200/10/1.
dReaction carried out under air atmosphere.
e5 mol% of NaCl was added.
OH
OH
PPh3
PPh3
N
N
N
N
CO
Cl
Cl
Ru Cl
Ru
respectively (entries 3 and 6). The higher activity of com-
plex 1 than 2 might be caused by the stronger electron
density of the ruthenium center in 1, which results from
the higher electron donating ability of L2 than L4. If com-
plex 5 was increased to 0.5 mol%, the yield could reach
91% within 60 min (entry 7). No reaction was observed
when using complex 3 or 4 as catalyst, because their
ruthenium centers cannot accept the coordination of
ketone or iPrOK (entries 4 and 5). Notably, if the amount
of catalyst 1 was increased to 0.5 mol%, the yield could
reach 84% even at 30 °C within 120 min (entry 8). Air
atmosphere reduced the catalytic activity, with a yield of
42% (entry 9). No reaction was observed in the absence
of base or catalyst (entries 10 and 11). If 5 mol% of NaCl
was added into the solution, the yields were similar to
those without NaCl addition (56 and 96% within 5 and
20 min, respectively) (entries 12 and 13). The results sug-
gest Cl− is not involved in the catalytic cycle.
CH3OH
N
N
Cl
CO
N
N
1
5
SCHEME 7 Synthesis of complex 5
Szymczak's group.[31] While unexpectedly, reaction of 2
with CO generated an unidentified mixture.
3.2 | Catalysis
TH of ketones was carried out to investigate the catalytic
activity of complexes 1–5 (Table 1). Acetophenone was
chosen as the substrate, 0.2 mol% of complex was chosen
as the catalyst, 5 mol% iPrOK was chosen as the base, and
the reactions were performed under nitrogen atmosphere
in refluxing 2‐propanol. According to the results (Table 1,
entries 1–6), complex 1 was the most effective catalyst,
lading to 58 and 96% yields within 5 and 20 min, respec-
tively (entries 1 and 2). Under the same conditions, the
catalytic activities of complexes 2 and 5 were much lower
than that of 1, leading to 54 and 37% yields within 20 min,
On the basis of the optimal conditions (Table 1, entry
2), various ketones were used to investigate the catalytic
activity for TH reactions (Table 2). Aryl ketones were
reduced to corresponding alcohols in 74–98% yields
within 20–180 min, with final TOFs up to 1.47 × 103 h−1

SHI ET AL.
7 of 10
TABLE 2 TH of ketone catalyzed by complex 1a
Entry
Ketone
Time (min)
GC yield (isolated yield) (%)
Final TOF (h−1 b
)
1
20
96 (90)
1.44 × 103
2
3
4
20
20
20
97 (91)
98 (93)
97 (88)
1.46 × 103
1.47 × 103
1.46 × 103
5
6
7
8
20
20
20
20
96 (90)
96 (89)
92 (85)
96 (90)
1.44 × 103
1.44 × 103
1.38 × 103
1.44 × 103
9
20
96 (91)
1.44 × 103
10
11
12
13
20
120
45
95 (90)
93 (90)
93 (85)
74 (69)
1.43 × 103
233
620
180
123
(Continues)

8 of 10
SHI ET AL.
TABLE 2 (Continued)
Entry
Ketone
Time (min)
GC yield (isolated yield) (%)
Final TOF (h−1 b
)
14
150
80 (72)
160
15
16
90
15
91 (84)
97
303
1.94 × 103
17
18
19
40
99
52c,d
90c
480
n.d.
n.d.
120
120
i
aGeneral conditions: ketone, 2.0 mmol (2 M in 50 ml of PrOH); ketone/base/catalyst = 500/25/1; N2 (0.1 MPa); 82 °C.
bTOF calculated with GC yield.
cYield determined by GC/MS; yield of carbonyl hydrogenation product. Note other products are observed (see text).
d1 mol% of PPh3 was added.
(Table 2, entries 1–15). 1‐Phenylethanol was afforded in
96% yield within 20 min when acetophenone was chosen
as the substrate (Table 2, entry 1). For fluoro‐, chloro‐,
bromo‐ and o‐methyl‐substituted acetophenones, complex
1 showed similar catalytic activity (92–98%; Table 2,
entries 2–10). In contrast, when electron‐donating group,
such as methyl or methoxy, was introduced to meta or
para position of acetophenone, a longer time was needed
for high conversion (Table 2, entries 11–13). Benzophe-
none converted into diphenylmethanol in 80% yield in
150 min, with a TOF value of 160 h−1 (Table 2, entry
14). 2‐Acetonaphthone was more reactive than benzophe-
none, and corresponding alcohol was generated in 91%
yield in 90 min (Table 2, entry 15). Two aliphatic ketones
were also used, with cyclohexanone showing higher
activity (TOF = 1.94 × 103 h−1; Table 2, entry 16) than
2‐heptanone (TOF = 0.48 × 103 h−1; Table 2, entry 17).
Unsaturated aliphatic ketones were also used to
investigate the selectivity and explore the mechanism.
When 5‐hexen‐2‐one was selected as the substrate, a high
conversion (93%) over 120 min was found (Table 2, entry
18). However, the products were a mixture of 5‐hexen‐2‐ol
(45%), 2‐heptanone (27%) and 2‐hexanol (21%), with a
ratio of 1:0.6:0.46. If 1 mol% PPh3 was added, the yields
of 5‐hexen‐2‐ol, 2‐heptanone and 2‐hexanol changed to
52, 26 and 15%, respectively, with a ratio of 1:0.5:0.29.
The selectivity change suggests that dissociation of PPh3
is involved in the catalytic cycle. However, more informa-
tion is needed to elucidate the detailed mechanism.
For 6‐methyl‐5‐hepten‐2‐one,
a
much higher
chemoselectivity was obtained with a ratio of 1:0.02:0.02
1
OH
iPrOK
R1
R2
O
N
N
iPrOH
Ru
N
Cl
N
A
R1
R1
OH
OH
OiPr
N
N
N
O
Ru
Ru
N
N
N
N
N
Cl
Cl
D
B
OH
N
N
H
Ru
N
N
Cl
O
O
R1 R2
C
SCHEME 8 Proposed mechanism catalyzed by complex 1

SHI ET AL.
9 of 10
[2] B. Zhao, Z. Han, K. Ding, Angew. Chem. Int. Ed. 2013, 52, 4744.
for 6‐methyl‐5‐hepten‐2‐ol (90%), 6‐methylheptan‐2‐one
and 6‐methylheptan‐2‐ol (Table 2, entry 19). This means
that the chemoselectivity might be controlled by changing
the steric environment around the olefin.
[3] J. R. Khusnutdinova, D. Milstein, Angew. Chem. Int. Ed. 2015,
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[4] C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588.
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[6] O. R. Luca, R. H. Crabtree, Chem. Soc. Rev. 2012, 42, 1440.
A plausible TH mechanism catalyzed by 1 is shown in
Scheme 8. For clarity, the PPh3 moiety is omitted, because
it is not certain in which step its dissociation occurs. Ini-
tially, the precatalyst 1 transforms to an intermediate A
with an open site by extrusion of one molecule of HCl
in the presence of iPrOK. Next, the intermediate A reacts
[7] H. Li, B. Zheng, K. W. Huang, Coord. Chem. Rev. 2015, 293–294,
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i
with one molecule of PrOH to afford an alkoxy ruthe-
nium species B. The β‐hydrogen of the alkoxy ruthenium
species in B transfers to the Ru center to give a Ru–H
intermediate C, followed by the release of one molecule
of acetone. Intermediate C then reacts with ketone to
form a new Ru–alkoxy intermediate D. Finally, D releases
a molecule of alcohol, and A is regenerated. However,
further experimental studies are needed to support the
mechanism, since no intermediated has been isolated.
[10] J. B. Geri, N. K. Szymczak, J. Am. Chem. Soc. 2015, 137, 12808.
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4 | CONCLUSIONS
[15] K. Fujita, R. Kawahara, T. Aikawa, R. Yamaguchi, Angew.
Chem. Int. Ed. 2015, 54, 9057.
Five tridentate ruthenium complexes 1–5 with NNN
ligands containing 2‐hydroxypyridylmethene or 2‐
methoxypyridylmethene fragment have been synthesized.
The catalytic activity for TH reactions was investigated.
Among the five complexes, 1 is the most active catalyst,
with TOF value for acetophenone of 1.44 × 103 h−1 in
20 min. Although this complex exhibits a slightly lower
efficiency than Kundu's catalyst,[32] it is more active than
similar NNN ruthenium complexes with 2‐hydroxypyridyl
reported by Szymczak and co‐workers and us.[30,31,45,46]
Mechanistic studies suggest Cl− is not involved in cataly-
sis, while PPh3 dissociation occurs in the catalytic cycle.
Further experimental studies are ongoing to understand
the detailed mechanism.
[16] G. Zeng, S. Sakaki, K. Fujita, H. Sano, R. Yamaguchi, ACS
Catal. 2014, 4, 1010.
[17] W. H. Wang, J. F. Hull, J. T. Muckerman, E. Fujita, Y. Himeda,
Energy Environ. Sci. 2012, 5, 7923.
[18] W. H. Wang, J. T. Muckerman, E. Fujita, Y. Himeda, ACS
Catal. 2013, 3, 856.
[19] N. Onishi, S. Xu, Y. Manaka, Y. Suna, W. H. Wang, J. T.
Muckerman, E. Fujita, Y. Himeda, Inorg. Chem. 2015, 54, 5114.
[20] S. Chakraborty, P. E. Piszel, C. E. Hayes, R. T. Baker, W. D.
Jones, J. Am. Chem. Soc. 2015, 137, 14264.
[21] S. Chakraborty, P. E. Piszel, W. W. Brennessel, W. D. Jones,
Organometallics 2015, 34, 5203.
[22] R. Wang, J. Ma, F. Li, J. Org. Chem. 2015, 80, 10769.
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J. I. van der Vlugt, Organometallics 2017, 36, 1541.
ACKNOWLEDGEMENTS
[24] D. Wang, D. Astruc, Chem. Rev. 2015, 115, 6621.
This work was supported by the National Natural Science
Foundation of China (nos. 21672045 and 21302028), the
China Postdoctoral Science Foundation (2016 M591519)
and Heilongjiang Province Postdoctoral Science Founda-
tion (LBH‐Z16069).
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Dafa Chen http://orcid.org/0000-0002-7650-4024
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hydroxypyridylmethylene fragment for transfer
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