Ruthenium complexes with N-functionalized secondary amino ligands: a new class of catalysts toward efficient hydrogenation of esters

Article abstract of DOI:10.1039/C8DT04957B

A series of ruthenium complexes (o-PPh₂C₆H₄NHR)₂RuCl₂ (R = Me, 3; Et, 4; CH₂Ph, 5) and (o-PPh₂C₆H₄NH₂)[(CH₂NHR)₂]RuCl₂ (R = Me, 7; Et, 8; iPr, 9) modulated with mono-N-functionalized secondary amino ligands were synthesized and demonstrated as efficient catalysts in the hydrogenation of esters into alcohols. The catalytic performances of these new complexes are much better than their corresponding primary amino ligand-constituted complexes (o-PPh₂C₆H₄NH₂)₂RuCl₂ (2) and (o-PPh₂C₆H₄NH₂)[(CH₂NH₂)₂]RuCl₂ (6). The significant improvement is attributed to the increased electron density of the secondary amino ligand in comparison with that of the primary amino ligand.

Full text of DOI:10.1039/C8DT04957B

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DOI: 10.1039/C8DT04957B
Journal Name
COMMUNICATION
Ruthenium Complexes with N-Functionalized Secondary Amino
Ligands: A New Class of Catalysts toward Efficient Hydrogenation
of Esters‡
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaolong Fang,*ab Bin Li,†a Jianwei Zheng,a Xiaoping Wang,a Hongping Zhu,*a and Youzhu Yuan*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of ruthenium complexes (o-PPh2C6H4NHR)2RuCl2 (R = Me,
3; Et, 4; CH2Ph, 5) and (o-PPh2C6H4NH2)[(CH2NHR)2]RuCl2 (R = Me,
7; Et, 8; iPr, 9) modulated with mono-N-functionalized secondary
amino ligand were synthesized and demonstrated as efficient
catalysts in hydrogenation of esters into alcohols. The catalytic
performances of these new complexes are much better than their
corresponding primary amino ligand constituted complexes (o-
PPh2C6H4NH2)2RuCl2 (2) and (o-PPh2C6H4NH2)[(CH2NH2)2]RuCl2 (6).
The significant improvement is attributed to the increased
electron density of the secondary amino ligand in comparison with
that of the primary amino ligand.
H
H
M
H
Ph2
P
Ph2
P
H
Ru
H
H
Ru
H



N
N

M
Base
H-Base
P
Ph2
P
Ph2
N
H2
N
H2
THF-d8
M = K+ or Li+
Scheme 1 Substitution of amino hydrogen by alkali metal cation.
responsible for the improved hydrogenation activity.5
In 2013, Bergens et al. proposed that the amino hydrogen
was substituted by alkali metal cation to form the
intermediate of catalyst trans-[((R)-BINAP)((R,R)-dpen)RuH2] in
hydrogenation of amide and imide carbonyls at low
temperatures (Scheme 1).6 The metal hydride in the in-situ
formed active intermediate has higher nucleophilicity to
activate the C=O bond of carbonyl derivatives, resulting in
promoting the catalytic activity. Recent mechanistic studies
also indicated the positive correlation between the catalytic
activity of metal-NH catalyst and the nucleophilicity of the
metal hydride.5a,e,7 The nucleophilicity of the metal hydride can
be strengthened by increasing the electron density of the
central metal, which has been well demonstrated in systems of
Ru-MACHO complex [NH(CH2CH2PPh2)2]RuHCl(CO) and
bipyridine-based pincer complex (6-CH2PtBu2-bipy)RuHCl(CO).
When the CO ligand is replaced with more electron-rich
monodentate N-heterocyclic carbene and hemilable Et2N
group, respectively, these complexes behaves much more
active in catalysis.8
Catalytic hydrogenation of esters into alcohols is one of the
most important chemical processes in organic synthesis, and
has been attracting extensive attentions.1 Typically, Metal-NH
ligand bifunctional catalysis in homogeneous hydrogenation
has achieved significant progress,2 since the discovery
reported by Noyori and co-workers in 1995.3 The authors
discovered that the catalytic activity of phosphine-Ru(II) in
aromatic ketones hydrogenation was significantly improved
with the addition of 1 equiv. of 1,2-diamino ligand into the
system, which they owed to the interaction between amino
hydrogen and metal hydride. Recently, we reported that the o-
PPh2C6H4NH2 ligand coordinated ruthenium complexes
[(PPh3)(o-PPh2C6H4NH2)RuCl2]2 (1) and (o-PPh2C6H4NH2)2RuCl2
(2) could effectively catalyze the selective hydrogenation of
dimethyl oxalate (DMO) into methyl glycolate (MG).4 The
mechanistic and dynamic studies indicate that the cooperation
between the ruthenium hydride and the amino hydrogen is
Inspired by these, we synthesize Ru-H catalysts coordinated
with N-functionalized secondary amino ligand, aiming to
enhance the nulecophilicity by introducing electron donating
substituent
bis(aminophosphine)-ruthenium
on
proximal
N
atoms.
Herein,
containing
a. State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative
Innovation Center of Chemistry for Energy Materials (iChEM), National
Engineering Laboratory for Green Chemical Productions of Alcohols–Ethers–
Esters, College of Chemistry and Chemical Engineering, Xiamen University,
Xiamen, 361005, China. E-mail: xlfang@stu.xmu.edu.cn; hpzhu@xmu.edu.cn;
yzyuan@xmu.edu.cn
complexes
secondary amino groups were designed based on our previous
work. In addition, N,N’-substituted 1,2-diamino ligand was also
introduced in this system, since diamino ligands were well
known in Noyori-type catalysts.9 Although 1,2-diamino ligand
constituted ruthenium catalysts have been widely used in the
catalytic hydrogenation of ketones and aldehydes, but the
application in esters hydrogenation is rarely studied, with the
b. School of Chemistry and Materials Engineering, Chizhou University, Chizhou,
247000, China
† Bin Li is the Co-first author.
‡Electronic Supplementary Information (ESI) available: Synthesis, crystallographic
details, activity test, and NMR data. CCDC 1884111 & 1884112. For ESI and
crystallographic data in CIF see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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COMMUNICATION
Journal Name
Ph
Ph
Ph
Cl
View Article Online
Cl
Ph
Ph
Ph
P
Cl
PPh2
NHR
P
P
N
NHR
NHR
DOI: 10.1039/C8DT04957B
(PPh3)3RuCl2
toluene
1
1/2 [(PPh3)(o-PPh2C6H4NH2)RuCl2]2 ( )
H
Ru
Ru
Cl
R
N
N
N
H
toluene or THF or 1,4-dioxane
R
H
65 - 100 o
C
H
N
R
R
H
70 - 130 o
C
H
6
7
, R = H (88%)
, R = Me (45%)
R = Et (83%)
R = H
3
4,
, R = Me (90%)
R = Et (68%)
R = Me
R = Et
R = CH2Ph
R = Me
R = Et
R = iPr
8,
9
5
, R = CH2Ph (93%)
, R = iPr (59%)
Scheme 2 Synthesis of ruthenium complexes 3-5.
Scheme 3 Synthesis of ruthenium complexes 6-9.
only example reported by Kuriyama et al. in hydrogenation of
optically active esters into alcohols.10 Delightedly, these new
species exhibit excellent activity in hydrogenation of esters.
The preparation of secondary amino ligand constituted
complexes 3-5 and 6-9 is outlined in Schemes 2 and 3,
respectively. Similar to the preparation of 2,4 complexes 3-5
were successfully isolated by reaction of RuCl2(PPh3)3 with two
equivalents of o-PPh2C6H4NHR in toluene at 65 to 100 °C. The
31P{1H} NMR spectra of 3, 4, and 5 exhibit two singlets at
57.61, 61.86; 56.67, 60.03; and 57.66, 59.30 ppm, respectively,
revealing that there are two conformational isomers in
solution state for each species, and the two o-PPh2C6H4NHR
ligands bear same chemical environments. This is different
from the corresponding primary amine complex 2, which
exhibits only one configuration in solution. Interestingly, the
solid state 31P NMR spectrum of 3 merely exhibits one broad
peak (Fig. S7‡), suggesting that this complex transformed into
one configuration during precipitation. Similar phenomena
were also observed in 4 and 5. Complex 3 has been
crystallographically characterized and the molecular structure
is presented in Fig. 1.
The ruthenium complexes 6-9, in which the metal centers
were simultaneously coordinated with a o-PPh2C6H4NH2 ligand
and a 1,2-diamino ligand, were prepared by using complex 1 as
synthetic precursor. As shown in Scheme 3, the reaction of 1
with two equivalents of (CH2NHR)2 (R = H, Me, Et, iPr)
produced complexes 6-9 with yields of 45-88%, respectively.
The 31P{1H} NMR spectra of 7, 8, and 9 exhibit singlets at 71.35,
70.75, and 66.45 ppm, respectively, demonstrating that no
PPh3 ligands coordinated to the ruthenium center. Due to the
insolubility of 6, solid state 31P NMR was measured and only
one peak at 69.41 ppm was observed, suggesting the similar
structure with 7 (or 8, 9). The geometry structure of 8 was
established by X-ray diffraction analysis, which reveals the
coordination interaction between diamino ligand and the
ruthenium center (Fig. 1b). In addition, the two Cl atoms are
located in a cis-configuration.
To study the catalytic activity of these ruthenium species
armed with different secondary amino ligands, we evaluated
the performance of complexes 2-9 in hydrogenation of DMO
into MG under 100 °C with 0.05 mol% ruthenium catalyst, 0.5
mol% NaOMe, and 50 bar H2 for 1 h. As shown in Fig. 2, to
various extents, the activities of complexes 3-5 were improved
in comparison with 2, among which 3 gave the best result with
a DMO conversion of 77%. The corresponding turnover
frequency (TOF) value is 1520 h-1, much higher than that of 2
(160 h-1). In addition, complexes 4 and 5 also showed better
activities than 2, while worse than 3 under the same reaction
conditions, which realized DMO conversions of 27% (TOF = 540
h-1) and 24% (TOF = 480 h-1), respectively.
Interestingly, the diamino ligand constituted complexes
performed differently in DMO hydrogenation. Complex 6
binding to ethylenediamine showed no catalytic activity. In
contrast, N,N'-dimethyl-1,2-ethanediamine and N,N'-diethyl-
1,2-ethanediamine coordinated complexes 7 and 8 behaved
distinctly different activities, which achieved DMO conversions
of 36% and 100%, respectively. In addition, small amount of
ethylene glycol (EG) (1% yield) was concomitantly produced
over 8. Actually, almost quantitive conversion of DMO has
been realized by 8 within 0.5 h (vide infra, entry 2, Table 1),
and the TOF value is 3920 h-1. This value is nearly 27 and 24
times higher than those of 1 and 2, respectively. To the best of
our knowledge, this is the best result in selective
hydrogenation of DMO into MG so far. Because of the
relatively lower electron-donating ability of methyl group in
comparison with ethyl group, the activity of 7 is inferior to 8.
According to the catalytic activities of 6-8, a positive
correlation between the hydrogenation activity of
Conv.
100
MG
EG
80
(a)
(b)
Cl1
60
40
20
0
Cl1
H3(B)
N1
Ru1
N2
P1
H1
H1
P2
H2
N2
Ru1
P1
N3
H3(A)
H2
Cl2
N1
2
3
4
5
6
7
8
9
Cl2
Fig. 2 Catalytic performance of complexes 2-9 for hydrogenation of DMO into MG
(and/or EG). Reaction condition: 11.35 mmol DMO, 0.05 mol% ruthenium, 0.5 mol%
NaOMe, 10 mL THF, 50 bar H2, 100 °C, 1 h. The conversion of DMO and yield of alcohols
were analyzed by gas chromatograph (GC) using p-xylene as an internal standard.
Fig. 1 X-ray crystal structures of (a) 3 and (b) 8 with thermal ellipsoids at 30%
probability level. Most hydrogen atoms have been omitted for clarity.
2 | J. Name., 2012, 00, 1-3
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Journal Name
Table 1 Hydrogenation of DMO into MG (and/or EG) by 8 under different reaction conditions.
COMMUNICATION
View Article Online
DOI: 10.1039/C8DT04957B
Entry
Ru
(%)
NaOMe
/Rua
T
(°C)
p(H2)
(bar)
time
(h)
Conv.
(%)b
Yield of
MG (%)b
Yield of
EG (%)b
TOF (h-1)
1
0.05
0.05
0.05
0.05
0.05
0.2
5
100
100
100
100
100
100
RT
50
50
50
50
50
10
50
50
10
50
50
50
50
1
0.5
1
1
1
1.5
20
2
87
86
98
98
86
67
99
95
99
94
83
0
0
1720
3920
2000
1720
1340
330
24
247
29
144
242
350
99
2
10
10
15
20
10
10
10
10
10
10
10
10
99
0
3
100
87
68
100
96
100
95
100
100
100
100
1
4
0
5
0
6
0
7
0.2
0
8
0.2
40
0
9
0.2
40
16
4
0
10
11
12
13
0.2
100
120
100
100
16
97
75
99
0.2
4
0.5
1
25
0
0.5
4
[a] Molar ratio. [b] Analyzed by GC using p-xylene as an internal standard.
ruthenium-NH catalyst and the electron density of the amino lead to lower catalytic activity. Gratifyingly, complex 8
ligand was provided. Surprisingly, although isopropyl group has smoothly catalyzed the hydrogenation of DMO into MG at
better electron-donating ability than methyl and ethyl groups, lower H2 pressure and/or reaction temperature (entries 6-9).
the activity of N,N'-diisopropyl-1,2-ethanediamine coordinated Especially, 95% yield of MG was afforded at room temperature
complex 9 is significantly lower than 7 and 8, merely (RT) after an extended reaction time (20 h, entry 7). The TOF is
completed a DMO conversion of 6%, which is probably 24 h-1, much higher than that of Ru-Macho catalyst RuH(η1-
attributed to the steric hindrance.
BH4)(CO)[NH(CH2CH2PPh2)2] in hydrogenation of diethyl
Obviously, the prominent catalytic performances of 3 and 8 oxalate under similar reaction conditions.11a
in DMO hydrogenation verify the feasibility of improving the
After the mono hydrogenation of DMO into MG, we also
activity of ruthenium-NH catalyst through replacing the investigated the catalytic hydrogenation of DMO into EG by 8.
primary amine with a more electron-donating secondary Under the conditions of 100 °C and 50 bar H2 with 0.2 mol%
amine. We also evaluated the catalytic performances of 2-9 in ruthenium for 4 h, MG and EG were obtained by yield of 83%
hydrogenation of acetophenone into 1-phenylethanol and and 16%, respectively (entry 10). EG was formed via
benzaldehyde into benzyl alcohol, which are the typical hydrogenation of the mono-hydrogenated product MG.15 Due
examples of ketone and aldehyde. As shown in Fig. S3‡ and S4 to the electron-withdrawing character of ester moiety, DMO is
‡, the activity trend is similar to that in DMO hydrogenation, more active than MG to be hydrogenated.16 The
indicating that suitable alkyl group substitution could hydrogenation of DMO into EG always requires harsher
remarkably improve the catalytic efficiency. These results reaction conditions than that of MG.16b As expected, EG was
clearly show the significant role of the electronic effect in produced as the main product with 97% (entry 11) and 99%
determining the catalytic efficiency. In addition, the steric yields (entry 13), respectively, after raising the reaction
effect can also inflict the catalytic activity.11 Recently, we have temperature to 120 °C or increasing the amount of 8 to 0.5
systemically elaborated this effect in determining the mol%.
hydrogenation activity of 2 in terms of density functional
Finally, the substrate scope of catalytic hydrogenation with 8
theory calculations and molecular dynamic simulations.12 was further explored (Table 2). Lactones with different
Similarly, in this work, due to the steric hindrance of the two member rings and substituent groups were employed and all
isopropyl groups, complex 9 exhibited poor performances in smoothly converted into the corresponding diols in yields of
DMO catalytic hydrogenation (Fig. 2). To some extent, it can 56-96% (entries 1-5). Similarly, MG and methyl lactate both
also explain the difference between the activities of 3 and 4 (or containing hydroxyl group at α–position were also
5).
hydrogenated into the diols EG and 1,2-propylene glycol in
Furthermore, the most prominent complex 8 was employed good yields of 99% and 95%, respectively (entries 6 and 7).
to investigate the effect of reaction conditions in the DMO However, when aromatic substituents were incorporated, the
hydrogenation, and representative results were tabulated in catalytic activity was obviously inhibited. The hydrogenation of
Table 1. According to the amount of base, the reaction the aliphatic ester methyl phenylacetate yielded a phenethyl
efficiency evolved in a volcanic trend (entries 1-5), passing alcohol of 67% (entry 8). In the case of methyl benzoate, the
through the summit by 10 equivalent of NaOMe over 8. performance became worse with merely 11% yield of benzyl
Although this trend is comparable with the reported catalyst alcohol (entry 9). The negative effect of phenyl substituents on
systems,7b,13 but the required amount of base for satisfactory the reduction of the adjacent ester moiety is probably due to
result was significantly decreased.9d,14 In a general trend, the steric hindrance between the catalyst and the ester
decreasing the H2 pressure or reaction temperature would substrate.
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Table 2 Scope of the hydrogenation of esters into alcohols by 8.
Chinese Universities (No. IRT_14R31), and the Starting Grants
View Article Online
for Young Teachers of Chizhou UniversityD(ONIo: 1.02.1001389/YCJ8RDCT000419)5.7B
Conv.
(Yield) (%)
Entry
Ester
Alcohol
Notes and references
OH
OH
1
2
86 (85)
HO
HO
O
O
O
1
(a) H. Adkins and K. Folkers, J. Am. Chem. Soc., 1931, 53, 1095-
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82 (80)
97 (96)
O
3
4
OH OH
O
O
O
2
71 (71)
OH OH
O
O
OH
O
OH
OH
5
6
93 (56)
3
4
5
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HO
100 (99)
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14 (11)
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were analyzed by GC using p-xylene as an internal standard.
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9
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Conclusions
In summary, a series of ruthenium complexes 3-5 and 7-9
bearing mono-N-functionalized secondary amino ligands o-
PPh2C6H4NHR or (CH2NHR)2 were synthesized, which exhibit
good catalytic activities in hydrogenation of DMO into MG/EG.
Remarkable improvement in hydrogenation activity was
achieved in comparison with the corresponding primary amino
ligand constituted complexes 2 and 6. Complexes 3 and 8 give
the optimal catalytic results by achieving TOF as high as 1520
h-1 and 3920 h-1, respectively. Moreover, complex 8 also
displays satisfactory activities in the hydrogenation of some
other aliphatic esters and lactones. This paves a new route to
the design of efficient homogeneous hydrogenation catalysts.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We acknowledge the financial support from the Natural
Science Foundation of China (Nos. 21473145, 91545115,
21802010), the Program for Innovative Research Team in
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