Synthesis, Characterization and Catalytic Application of Pyridine-Bridged N-Heterocyclic Carbene–Ruthenium Complexes in the Hydrogenation of Carbonates

Article abstract of DOI:10.1002/asia.201701303

A series of bulky pyridine-bridged NHC–Ru complexes have been rationally designed and synthesized; these exhibited very high catalytic activity in the hydrogenation of cyclic and linear carbonates under mild reaction conditions. In the presence of catalytic amounts of a weak base, a broad range of substrates with different ring size and steric bulk were well tolerated, providing methanol and the corresponding diols in excellent yields with a catalyst loading as low as 0.5 mol %.

Full text of DOI:10.1002/asia.201701303

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Synthesis, Characterization and Catalytic Application of Pyridine-
bridged N-Heterocyclic Carbene Ruthenium Complexes in
Hydrogenation of Carbonates
Authors: Tao Tu, Jiangbo Chen, Haibo Zhu, Jinjin Chen, and Zhang-
Gao Le
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To be cited as: Chem. Asian J. 10.1002/asia.201701303
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10.1002/asia.201701303
Chemistry - An Asian Journal
COMMUNICATION
Synthesis, Characterization and Catalytic Application of Pyridine-
bridged N-Heterocyclic Carbene Ruthenium Complexes in
Hydrogenation of Carbonates
Jiangbo Chen,[a] Haibo Zhu,[a,b] Jinjin Chen,*[c] Zhang-Gao Le*[b] and Tao Tu*[a]
Dedication ((optional))
Abstract:
A
series of novel bulky pyridine-bridged NHC-Ru
carbonates.[10]
complexes have been rational designed and synthesized, which
exhibited extremely high catalytic activity in the hydrogenation of
cyclic and linear carbonates under mild reaction conditions. In the
presence of catalytic amount of weak base, a broad range of
substrates with different ring size and steric bulkiness were well
tolerated providing methanol and corresponding diols in excellent
yields with the catalyst loading as low as 0.5 mol %.
As a consequence of global climate change, the increasing
emission of CO2 has attracted many attentions.[1] Therefore, a
variety of research focuses on the transformation of CO2, a
sustainable C1 source,[2] to value-added chemicals.[3] Among
them, methanol constitute one of the most attractive products,
because it can be directly used as a drop-in liquid fuel for
internal combustion engines and fuel cells.[4] As an efficient
hydrogen storage medium (12.5 wt% H2), methanol is also a
convenient chemical feedstock to produce a myriad of crucial
fine chemicals and important platform molecules.[5] However, the
direct transformation of CO2 to methanol has been somewhat
challenging at current stage, due to the chemical inertness of
CO2. Alternatively, the hydrogenation of formates, carbonates
and carbamates, readily accessed from CO2 in a large scale, is
much more attractive and provides an indirect approach to
produce methanol. Two strategies including “carbon capture and
sequestration” (Scheme 1a) and “carbon capture and recycling”
(Scheme 1b) are usually applied in these approaches, in which
amines,[6] vicinal diols[7] or aminoalcohols[8] are utilized as CO2
trapping agents. The resulting trapping intermediates, such as
cyclic carbonates, dimethylcarbamates, oxazolidinones, are
subsequently hydrogenated to produce desired methanol along
with regeneration of trapping agents. By using PNN-Ru pincer
complex 1 (Scheme 1c), Milstein and coworkers accomplished
the hydrogenation of carbonates, carbamates and formates to
efficiently produce methanol.[9] Subsequently, Ding and
coworkers found that PNP-Ru pincer complex 2 (Scheme 1c)
exhibited the highest activity in the hydrogenation of cyclic
Scheme 1. Indirect catalytic transformation of CO2 to methanol and
corresponding privileged catalysts.
According to previous study,[11] the plausible catalytic
mechanism of pyridine-bridged Ru complex catalyzed
hydrogenation of cyclic carbonates was proposed (Scheme 2).
At first, dihydrogen is added to the dearomatic complex B to
form the aromatic active catalytic species C. Then
decoordination of the “hemilabile site (L) provides the blank site
for ketone coordination. The resulting intermediate D could be
further converted to E after intramoleculer hydride transfer and
the second H2 addition. After ester C-O bond cleavage,
formaldehyde coordinated intermediate G is formed along with
diols generation. Further hydrogenation of formaldehyde via
similar intramoleculer hydride transfer and the H2 addition leads
complex H formation, which is further liberating the final product
MeOH and regenerating the dearomatc species B.
[a]
J. Chen, H. Zhu and Prof. Dr. T. Tu
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Department of Chemistry, Fudan University
220 Handan Road, Shanghai, 200433, China.
E-mail: taotu@fudan.edu.cn
[b]
[c]
Prof. Dr. Z.-G. Le
School of Chemistry, Biology and Material Science, East China
University of Technology, Nanchang, 330013, P. R. China
Prof. Dr. J. Chen
Department of Children Healthcare, Shanghai Children Hospital
affiliated to Shanghai Jiaotong University, No.355, Rd.Luding,
shanghai, 20062, China
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
Scheme 2. Plausible catalytic mechanism of hydrogenation of cyclic
carbonates with pyridine-bridged Ru catalysts (X = Cl, PPh3 and etc.).
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From the plausible mechanism, three issues are found
crucial for this catalytic cycle to produce methanol from cyclic
carbonates. The first is the σ-donor ability of L’. When ligand
with strong σ-donor and weak π-acceptor properties, the active
dihydride intermediate C is readily formed. The second is the
hemilabile property of the “arm” (L), which is favor to coordinate
to the metal center and is readily dissociated in the presence of
suitable substrate. The third is the bulkiness around the metal
center. Bulky enviroment helps the elimination steps to generate
biols and methanol. Therefore, the rational design of ligands and
corresponding catalysts may not only help us to further
increasing the catalytic efficiency for methanol production, but
also support the plausible mechanism. In comparison with air-
sensitive phosphines, robust N-heterocyclic carbenes (NHCs)
with strong σ-donor and weak π-acceptor properties represent a
class of privileged ligands,[12] which exhibit promising potential
activity in the hydrogenation reactions.[13] In addition, the
coordination topologies NHCs are bulky than phosphine, due to
the substituents on NHCs readily forming a pocket surrounding
to the metal center. Furthermore, imidazolines with similar
skeletons of NHCs may function as a new type of hemilabile
ligands, in which the bulkiness and coordination ability may be
easily tunning. To our best knowledge, the combination of NHCs
and imidazolines to fabricate new pyridine-bridged ligands is
rarely studied, especially, in the hydrogenation reactions.
Following our recent research interests in exploring novel NHC
metal complexes and their potential applications,[14] herein, new
(see the supporting information). After deprotonation by lithium
hexamethyldisilazide (LiHDMS) in dry THF, the respective free
carbenes further react with RuHCl(CO)(PPh3)3 to produce the
desired pyridine-bridged NHC-Ru complexes 3a-d in moderate
to good yields, which were further fully characterized by NMR,
MS, and IR studies. For instance, in the case of NHC-Ru
complex 3a, a singlet signal at 22.96 ppm in the 31P NMR
spectrum and a doublet signal originated from the hydride ligand
at -7.97 ppm (JPH = 104 Hz) in the 1H NMR spectrum were
observed. In addition, NHC carbon at 185.21 ppm was a broad
singlet, the carbonyl carbon at 206.92 ppm was a doublet signal
(JPC = 6.2 Hz) in the 13C NMR spectra. All aforementioned
information clearly indicates that PPh3 coordinates to the Ru(II)
center. The C-O stretch at 1910 cm-1 is observed in the IR
spectrum, which was slightly greater than what observed for
pincer complex 1 (1906 cm-1)[11]. The rest NHC-Ru complexes
3b-d all exhibited similar spectra and properties (see the
supporting information). To our delight, cubic crystals, which are
suitable for X-ray diffraction analysis, could be slowly formed
from the solution of complex 3a in the mixture of MTBE (methyl
tert-butyl ether) and MeOH. However, PPh3 ligand is not
observed. Instead, a neutral dichlorides pincer complex 7 was
obtained (Scheme 4), which probably caused by the congested
environment around the Ru center formed by the bulky pocket
NHC ligand with both Ad- and i-Pr groups. Therefore, the PPh3
ligand was easily dissociated from Ru center even under
ambient condition. The X-ray structure of complex 7 revealed a
distorted octahedral geometry around the Ru center, in which
the CO ligand coordinated trans to the nitrogen atom of pyridine
ring. The distances of Ru-CNHC and Ru-CCO are 1.846 and 1.981
Å, respectively, which clearly indicate the strong coordination of
NHC to Ru(II) center. In addition, the two important coordination-
bonds Ru-N3 and Ru-N4 are 2.144 Å and 2.107 Å, and the
difference is 0.037Å, which supported our strategy for ligand
design by using strong coordination benzimidazoline as the
hemilabile arm to construct the pincer ligands.
pyridine-bridged
Ru
complexes
3
containing
bulky
benzimidazoline and NHC fragments have been successfully
developed and exhibited high catalytic activity towards the
hydrogenation of cyclic carbonates under mild reaction
conditions.
Scheme 3. Synthesis of pyridine-bridged NHC-Ru complexes 3a-d.
Scheme 4. Deprotonation and crystallization of complex 3a; and X-ray
structure of complex 7 (hydrogen atoms were omitted for clarity).
A
series of pyridine-bridged NHC-Ru complexes 3a-d
(Scheme 3) were designed according to our aforementioned
strategy, in which benzimidazoline with bulky adamantyl was
selected as hemilabile moiety. Due to the bulkiness of ligand
may also accelerate the elimination steps in the catalytic cycles,
several imidazoles and benzimidazoles 5a-d with bulky
substitutes groups (2,6-diisopropylphenyl: Dipp and 2,4,6-
trimethylphenyl: Mes) were selected for N-alkylation with benzyl
chloride 4 to synthesize desired (benz)imidazolium salts 6a-d
Deprotonation of complex 3a with LiHMDS in CDCl3 resulted
in formation of the dearomatized complex 8 (Scheme 4), which
was somewhat unstable at room temperature and hard to get
satisfactory 1H NMR data. However, it still tells us important
information from 31P NMR spectrum that a new singlet at -5.48
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ppm further confirmed the formation of complex 8 under the
basic condition (see the Supporting Information, Figure S21).
(Table 1, entry 7). When other solvents, such as acetonitrile,
dioxane, toluene and 1,2-dichloroethylane(DCE) were applied,
inferior outcomes were obtained (Table 1, entries 8-11). In
addition, when the catalyst loading of complex 3a was
decreased to 0.05 mol%, the conversion only suffered slightly
reducing (Table 1 entry 12). Prolonging reaction time to 45 h, a
full conversion could also be achieved (Table 1, entry 13).
Table 1. Hydrogenation of ethylene carbonate in the presence of 3a-d.[a]
Table 2: Hydrogenation of diverse cyclic carbonates with complex 3a.[a]
[Cat.]
Entry
Base
Solvent
Conv.[b]
Yield (%)[b]
(mol %)
EG
99
93
92
84
93
99
22
/
MeOH
1
3a (0.5)
3b (0.5)
3c (0.5)
3d (0.5)
3a (0.5)
3a (0.5)
3a (0.5)
3a (0.5)
3a (0.5)
3a (0.5)
3a (0.5)
3a (0.05)
3a (0.05)
K3PO4
K3PO4
K3PO4
K3PO4
KOtBu
K2CO3
KHMDS
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
THF
>99%
93%
92%
85%
>99%
>99%
25%
/
90
92
91
84
80
60
20
/
2
THF
3
THF
4
THF
5
THF
6
THF
7
THF
8
MeCN
Toluene
Dioxane
DCE
THF
9
15%
87%
/
12
80
/
10
62
/
10
11
12[c]
13[d]
90%
>99%
85
99
51
67
THF
[a] Reaction was carried out with 10 mmol ethylene carbonate in 10 mL
solvent and 1 mol% base in the presence of pyridine-bridged NHC-Ru
complexes 3; [b] The conversion and yield was determined by GC-MS with p-
xylene (50 μL) as the internal standard; [c] with 0.1 mol% base for 24 h; [d]
with 0.1 mol% base for 45 h.
[a] Reaction was carried out with 10 mmol cyclic carbonate in 10 mL THF and
1 mol% K3PO4 in the presence of 0.5 mol % complex 3a, in which yields of diol
(D) and methanol (M) were determined by GC analysis using p-xylene (50 μL)
as the internal standard.
With the optimized reaction conditions in hand, we
subsequently focused on the substrate scope of the newly
developed catalytic system (Table 2). Pleasingly, all selected
cyclic carbonates are able to fully convert to the corresponding
diol and methnol under the optimized reaction conditions.
Remarkably the carbonates with mono- (10,13,15-17), di-
(11,12,18), and even tetra-(14) substituents are fully compatible,
up to quantitative yields for methanol and corresponding diol
were obtained.
With these novel pincer NHC-Ru complexes (3a-d) in hand,
their catalytic activity was then investigated by the
hydrogenation of cyclic carbonates, which are readily prepared
from CO2 and corresponding epoxides according to the previous
reports.[15] Initially, ethylene carbonate(9) was selected as the
model substrate to evaluate their performance (Table 1). Even in
the presence of 0.5 mol% catalyst loading, all these pincer
complexes exhibited high catalytic activity and resulted in
satisfactory to quantitative outcomes (Table 1, entries 1-4),
when the reaction was carried out in THF with K3PO4 under 50
atm of H2 at 140 oC. Among them, the most steric hindered
complex 3a gave out the best yields of ethylene glycol (EG)
(>99%) and methanol (90%) after 24 h (Table 1, entry 1).
Therefore, less-hindered complexes 3b-d gave slightly inferior
conversions (Table 1, entry 2-4), in which methyl formate was
detected by GC-MS analysis in the reaction process, leading to
the worse yield of methanol versus EG. In contrast with strong
base,[10] relatively weaker base K2CO3 also gave a good
methanol yield with full conversion of ethylene carbonate (Table
1, entry 6). However, when steric hindered strong base LiHMDS
was employed, unsatisfied conversion was observed, which
might due to the hard formation of dearomatized complex 8
Scheme 5. Hydrogenation of Dimethyl carbonate with complex 3a..
Besides the high catalytic activity observed with cyclic
substrates, the liner dimethyl carbonate is also considered as a
suitable substrate under the identical reaction conditions,
providing methanol in quantitative yield (Scheme 5). The good
compatibility of both cyclic and liner carbonates may highlight a
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[5]
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potential alternation approach of “carbon capture and recycling”
by using methanol directly.
In conclusion, a series of rational designed bulky pyridine-
bridged NHC-Ru complexes were successfully synthesized,
which exhibited extremely high catalytic activity in the
hydrogenation of various cyclic and liner carbonates under mild
reaction conditions. Even with low catalyst loading and catalytic
amount of weak base, a broad range of substrates with different
ring size and steric hindered cyclic carbonates were well
tolerated providing methanol and their corresponding diols in
excellent yields, which not only highlights our strategy efficiency
of catalyst design but also provide an indirect and practical
protocol to produce methanol from CO2.
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Financial support from the National Key R&D Program of China
(2016YFA0202902), the Shanghai International Cooperation
Program (14230710600), National Natural Science Foundation
of China (No. 21572036 and 81670810), the External
Cooperation Program of Jiangxi Province (20151BDH80045)
and Department of Chemistry, Fudan University is gratefully
acknowledged.
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2009, 109, 3445-3478.
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Keywords: carbonates, hydrogenation, methanol, N-
heterocyclic carbene, ruthenium pincer complex
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COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
In the presence of a series of rational
designed bulky pyridine-bridged NHC-
Ru complexe, the hydrogenation of
various cyclic and liner carbonates
has been successfully realized under
mild reaction conditions, which
provide an indirect and practical
protocol to produce methanol from
CO2.
Jiangbo Chen, Haibo Zhu, Jinjin Chen,*
Zhang-Gao Le* and Tao Tu*
Page No. – Page No.
Synthesis, Characterization and
Catalytic Application of Pyridine-
bridged N-Heterocyclic Carbene
Ruthenium Complexes in
Hydrogenation of Carbonates
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