Ni(ii)-N′NN′ pincer complexes catalyzed dehydrogenation of primary alcohols to carboxylic acids and H2 accompanied by alcohol etherification

Article abstract of DOI:10.1039/c7cy00432j

Acceptorless dehydrogenation of alcohols to carboxylic acid derivatives catalyzed by a transition metal complex is an important reaction in modern organic synthesis and catalysis, for which nickel complexes have rarely been developed. Herein we report three Ni(ii) complexes bearing pyridine-based N′NN′ type pincer ligands, which catalyze the acceptorless dehydrogenation of primary alcohols to carboxylic acids under anhyrous conditions. The complex [NiCl₂(L3)] 3 (L3 = 2,6-bis(diethylaminomethyl)pyridine) displays the best catalytic reactivity, catalyzing the primary alcohols to carboxylic acids and H₂ in good yields (40-90%). Further investigation reveals that an unexpected alcohol etherification occurs, which gives the second oxygen atom for the formation of the carboxylic acid. Our results give a thread for the design of new nickel complexes without phosphine and N-heterocycle carbene ligands for the acceptorless oxidation of alcohols.

Full text of DOI:10.1039/c7cy00432j

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NiIJII)–N′NN′ pincer complexes catalyzed
dehydrogenation of primary alcohols to carboxylic
acids and H2 accompanied by alcohol
etherification†‡
Cite this: DOI: 10.1039/c7cy00432j
Zengjin Dai, Qin Luo, Huan Jiang, Qi Luo, Hua Li,
*
*
Jing Zhang
and Tianyou Peng
Acceptorless dehydrogenation of alcohols to carboxylic acid derivatives catalyzed by a transition metal
complex is an important reaction in modern organic synthesis and catalysis, for which nickel complexes
have rarely been developed. Herein we report three NiIJII) complexes bearing pyridine-based N′NN′ type
pincer ligands, which catalyze the acceptorless dehydrogenation of primary alcohols to carboxylic acids
under anhyrous conditions. The complex [NiCl2IJL3)] 3 (L3 = 2,6-bisIJdiethylaminomethyl)pyridine) displays
the best catalytic reactivity, catalyzing the primary alcohols to carboxylic acids and H2 in good yields (40–
90%). Further investigation reveals that an unexpected alcohol etherification occurs, which gives the sec-
ond oxygen atom for the formation of the carboxylic acid. Our results give a thread for the design of new
nickel complexes without phosphine and N-heterocycle carbene ligands for the acceptorless oxidation of
alcohols.
Received 7th March 2017,
Accepted 3rd May 2017
DOI: 10.1039/c7cy00432j
rsc.li/catalysis
relatively scarce. Grützmacher and co-workers reported a cat-
ionic complex [RhIJtrop2NH)IJPPh3)]·(OTf) (trop2NH = bisIJ5-H-
Introduction
Carboxylic acids and salts are broadly used in the chemical
industry and in our daily life. The one-pot oxidation of pri-
mary alcohol to carboxylic acid is consistently highly pro-
moted to streamline synthetic routes,1 and a number of non-
catalytic2–6 and catalytic7–11 systems have been developed in
the past decade. However, the reported systems normally suf-
fer from their use of stoichiometric amounts of toxic oxi-
dants, undesirable side reactions, or they produce stoichio-
metric amounts of inorganic or organic wastes. The
acceptorless transformation of primary alcohol to carboxylic
acid catalyzed by precious metal (Rh, Ir, Ru, etc.) complexes
gives a much “greener” process, and several homogeneous
systems have been developed in a basic aqueous solution by
Milstein12,13 and other groups.14–20
dibenzoija,d]-cyclohepten-5-yl)-amide) for the efficient dehy-
drogenation of primary alcohols to carboxylic acids (yields
Up to now, most of the reported pincer type complexes are
essentially characterized by the use of ligands containing
strongly σ-donating phosphine or N-heterocycle carbene do-
nors. Homogeneous systems capable of such conversion
using phosphine- and N-heterocycle carbene free ligands are
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072,
P. R. China. E-mail: jzhang03@whu.edu.cn, typeng@whu.edu.cn
† The authors declare no competing financial interests.
‡ Electronic supplementary information (ESI) available: Hydrogen gas detection,
spectral data and CIF file for complex 1. CCDC 1504880. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c7cy00432j
Scheme 1 Typical transition metal complexes for the acceptorless
dehydrogenation of primary alcohols to carboxylic acids.
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41–99%, turnover numbers (TONs) <1000), but hydrogen ac-
ceptors (1-hexene or N2O) were necessary.21,22 We previously
reported
a series of RuIJII) complexes based on 2,6-
bisIJbenzimidazole-2-yl)pyridine (L1) which efficiently catalyze
the conversion of primary alcohols to corresponding carbox-
ylic acids with the liberation of H2 in the homogeneous alco-
hol/CsOH system.23 Nevertheless, it is still a challenge to de-
velop new catalyst systems with metal complexes containing
both cheap metal centers and ligands prepared with compo-
nents consuming little energy.24 Thus the first row transition
metal complexes analogous to Ru, Rh or Pd with the ligands
containing N or O donors should be good candidates. Al-
though Jones and coworkers reported a NiIJII) complex with
2-hydroxypyridine for the acceptorless dehydrogenation of al-
cohols to ketones or esters,25 no nickel complex has been
reported for the acceptorless conversion of primary alcohol to
carboxylic acid to the best of our knowledge. Herein we re-
port three NiIJII) complexes with N′NN′ type pincer ligands
(Scheme 1) as efficient catalyst precursors for the transforma-
tion of primary alcohol to the corresponding carboxylic acid
and H2 accompanied by alcohol etherification under anhy-
drous conditions.
Fig. 1 The structure of 1·2DMF with thermal ellipsoids drawn at the
30% probability level. One chloride anion and one water molecule are
omitted for clarity. Selected bond lengths (Å): Ni1–N1, 2.050(2); Ni1–
Cl1, 2.338(1); Ni1–N3, 2.102(2); Ni1–N5, 2.121(2); Ni1–O1, 2.112(2); Ni1–
O2, 2.108(2). Selected bond angles (°): Cl1–Ni1–N1, 178.90(8); Cl1–Ni1–
O1, 94.25(6); N1–Ni1–O1, 85.20(9); O1–Ni1–O2, 170.93(8).
benzyl alcohol (20 mmol) at 150 °C for 24 h under an argon
atmosphere. The benzoic acid was obtained in 10% isolated
yield (based on the CsOH, Table 1, entry 1). Upon using other
alkali hydroxides, or adding solvents (toluene, H2O, 1,4-diox-
ane etc.), the benzoic acid was obtained in poor yields (less
than 10%, see the ESI‡). Since water can be produced from
the reaction of alkali hydroxide and alcohol, which can com-
pete with the alcohol coordinated to the Ni center, we think
that reducing the amount of the water in the system might
favor the dehydrogenation of the alcohol. Thus further exper-
iments were performed under anhydrous conditions by using
tBuOK or sodium benzyloxide as the base. To our delight,
when potassium tert-butoxide or sodium benzyloxide were
used, benzoic acid was obtained in 20% and 21% isolated
yields, respectively, under the same conditions (Table 1, en-
tries 3 and 4). To provide a comparison with complex 1,
complexes [NiCl2IJL2)] (2) and [NiCl2IJL3)] (3) were also used
for the dehydrogenation of benzyl alcohol. Heating a solution
of sodium benzyloxide with 0.5 mol% of complex 3 in an ex-
cess amount of benzyl alcohol at 150 °C for 24 h, benzoic
acid was obtained in 28% yield, presenting a better catalytic
reactivity than that of complex 1 or complex 2. As expected,
almost no benzoic acid was observed without a NiIJII) complex
or base (Table 1, entries 7, 8).
Upon increasing the loading of complex 3 to 1 mol% rela-
tive to the sodium benzyloxide in benzyl alcohol at 150 °C for
24 h, benzoic acid was obtained in a higher yield (55%) ac-
companied with the evolution of H2 (confirmed by GC, see
ESI‡) (Table 2, entry 1). However, only 66% of benzoic acid
was achieved when the reaction was carried out for 48 h
(Table 2, entry 2), probably because the formation of the ben-
zoate salt solidified the reaction mixture. When mesitylene
was added to the catalytic system to make stirring easier un-
der the same conditions, benzoic acid was obtained in 83%
yield, which is higher than that without the mesitylene
(Table 2, entry 3). With 1 mol% complex 3 as the catalyst
Results and discussion
Synthesis and characterization of NiIJII) complexes 1–3
The N′NN′ type pincer ligands 2,6-bisIJbenzimidazole-2-
yl)pyridine (L1), 2,6-bisIJisopropylaminomethyl)pyridine (L2)
and 2,6-bisIJdiethylaminomethyl)pyridine (L3) were prepared
according to the literature,26–28 and the complexes [NiCl2IJL2)]
(2) and [NiCl2IJL3)] (3) were synthesized through the proce-
dure reported by our group.29 Complex 1 was prepared
according to the reported procedure30 with slight modifica-
tion (yield 88%). There is no sharp peak observed in
the1HNMR spectrum (δ 0–16 ppm) of complex 1, indicating
that complex 1 is paramagnetic. For further characterization
of complex 1, single crystals suitable for X-ray diffraction
analysis were obtained by slow diffusion of diethyl ether into
a concentrated DMF solution of complex 1. The structure of
1·2DMF is shown in Fig. 1, in which the NiIJII) center adopts a
distorted octahedral coordination geometry with the coordi-
nation plane defined by three N atoms of L1 and one Cl
atom, while two DMF ligands occupy the axial positions
(Fig. 1), respectively. The bond distance of Ni–N1IJpyridyl)
[2.050(2) Å] is shorter than that of Ni–NIJbenzimidazole)
(2.102(2) and 2.121(2) Å), indicating that the coordination
ability of the pyridine N atom is stronger than that of the
benzimidazole N atom,31c which is consistent with other
reported NiIJII)–L1 complexes.31 The bond distance of Ni1–
Cl1IJ2.338IJ1) Å) in complex
1 is longer than that of
[NiIJL3)Cl]IJBF4)] (2.164(1) Å) and complex 3 (2.303(1) Å).29
Catalytic dehydrogenation of primary alcohol
Benzyl alcohol and complex 1 were selected for optimizing
the reaction conditions. A preliminary experiment was car-
ried out by using complex 1 (25 μmol) and CsOH (5 mmol) in
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Table 1 Base and catalyst screening for the catalytic dehydrogenation of
(Table 2). For instance, 3-methylbenzoic acid was obtained in
88% yield when sodium 3-methylbenzyloxide (5 mmol) with 1
mol% of complex 3 in 3-methylbenzyl alcohol (20 mmol) were
heated at 150 °C under argon for 48 h (Table 3, entry 4). Using
an alcohol with a strong electron-withdrawing group afforded
a relatively lower yield of the carboxylic acid (Table 2, entry 9),
indicating that the nucleophilic effect of the alcohol to the Ni
center plays an important role in the formation of carboxylic
acids. The aliphatic alcohols gave relatively lower yields of
acids under the same conditions (Table 2, entries 10–11). For
example, only a 42% yield of hexanoic acid was obtained
when 1-hexanol was used (Table 2, entry 10).
As expected, prolonging the reaction time accompanied by
a lower loading of complex 3 resulted in the formation of car-
boxylic acids with higher turnover numbers (TONs) (Table 3).
For example, on heating a solution of sodium benzyloxide
with 0.05 mol% of complex 3 in benzyl alcohol at 150 °C for
72 h, benzoic acid was obtained in 22.5% yield with TONs ∼
450 (Table 3, entry 3). When cyclohexylmethanol or 1-hexanol
was loaded, the corresponding carboxylic acids were achieved
in yields of 15.5% (TONs ∼ 290) and 14.6% (TONs ∼ 310), re-
spectively (Table 3, entries 2, 3). The catalytic efficiency of
complex 3 could be comparable to several reported RuIJII)
complexes for the acceptorless transformation of primary al-
cohols to carboxylic acids.16,17,19,20
benzyl alcohol to benzoic acida
Entry
Catalyst
Base
Yieldb (%)
1
2
3
4
5
6
7
8
1
1
1
1
2
3
CsOH
10c
6
NaOH
tBuOK
20
21
25
28
<2
0
PhCH2ONa
PhCH2ONa
PhCH2ONa
PhCH2ONa
None
None
3
a
Reaction conditions: benzyl alcohol (20 mmol), base (5 mmol),
b
catalysts (25 μmol), 150 °C, Ar, 24 h. Isolated yield of benzoic acid
= nIJacid)/nIJbase). Carboxylic acid was obtained by acid treatment of
c
the salt. Only a trace amount of benzoic acid was obtained when 1
mL degassed H2O was added.
Table 2 Complex 3 catalyzed the dehydrogenation of alcohols to car-
a
boxylic acid with H2
Entry Alcohol
1
Base
Time Product
24 h
Yieldb (%)
55
Preliminary mechanism
2
3
4
5
6
48 h
48 h
48 h
48 h
48 h
66
Since the carboxylate salt precipitates directly from the reaction
solution accompanied by the evolution of two equivalents of H2
relative to the carboxylic acid (see ESI‡), and O2 has been ruled
out from the reaction system, it is interesting to find out how
the carboxylate obtains the second oxygen atom. In Milstein's
system,12,13 the water/OH− offers the oxygen atom to form the
carboxylic acid; in our previously reported RuIJII)–L1 pincer sys-
tems,23 the second oxygen atom of the carboxylate comes from
the CsOH. With regard to the present NiIJII)–N′NN′ system, the
free water of the initial reaction solution should have been
ruled out since the sodium alkoxide and the alcohol are freshly
83c
88
69
89c
7
48 h
48 h
48 h
48 h
70
90c
60
42
8
9
tBuOK
Table 3 Acceptorless dehydrogenation of primary alcohols to carboxylic
acids with lower loading of complex 3a
10
11
48 h
40
Entry
Alcohol
Yieldb (%)
TONsc
a
Reaction conditions: for entries 1–8, 10–11, sodium alkoxide (5
mmol), alcohol (20 mmol), complex 3 (19.0 mg, 50 μmol), 150 °C,
1
2
15.5
14.6
310
290
t
argon; for entry 9, 4-nitrobenzyl alcohol (20 mmol), BuOK (0.56 g, 5
mmol), and complex 3 (19.0 mg, 50 μmol) at 150 °C under argon
with vigorous stirring. Isolated yield of acid = nIJacid)/nIJbase).
Carboxylic acids were obtained by acid treatment of the salts. 1 mL
b
c
3
22.5
450
mesitylene was added.
a
Reaction conditions: sodium alkoxide (25.0 mmol), alcohol (100.0
b
mmol), complex 3 (4.8 mg, 0.0125 mmol), 150 °C, argon. Isolated
precursor, aromatic primary alcohols were converted to the
corresponding carboxylic acids with H2 in 60–90% yields
yield of acid = nIJacid)/nIJbase). Carboxylic acids were obtained by the
acid treatment of the salts. c TON = nIJacid)/nIJcatalyst).
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prepared or dried with standard methods, and the oxygen atom
migration should occur among the alcohols. Since several
transition metal complexes have been reported for alcohol de-
hydration32 and aldehyde reductive coulping33 to release water,
it is important to investigate whether dialkyl ether is formed in
our catalytic system. When 1 mol% complex 3 and sodium
4-methylbenzyloxide (5 mmol) in an excess amount of
4-methylbenzyl alcohol (20 mmol) were heated at 150 °C for
48 h under an argon atmosphere, 4-methylbenzoic acid was
obtained in 69% isolated yield (3.45 mmol, the yield based on
Scheme 3 Plausible reaction route of the catalytic reaction.
the sodium 4-mehylbenzyloxide) accompanied by the forma- Conclusions
1
tion of (4-Me-C6H4CH2)2O (2.15 mmol, confirmed by HNMR
In summary, we have demonstrated a new NiIJII)–N′NN′ pincer
system for the acceptorless dehydrogenation of primary alco-
hol/sodium alkoxide to carboxylic acid with the liberation of
two equivalents of H2. The second oxygen atom of the carbox-
ylate comes from the water which is produced from the ether-
ification of the alcohol. The present work gives a new exam-
ple of NiIJII)–N′NN′ type pincer complexes as an alternative
to precious metals (Ru, Rh, Ir etc.) with the phosphine
containing ligands for the acceptorless alcohol dehydrogena-
tion. An investigation of the detailed catalytic mechanism is
currently underway.
and GC-MS), while 4-methylbenzaldehyde (1.10 mmol) and 1,4-
dimethylbenzene (1.05 mmol) were also isolated from the or-
ganic phase (Scheme 2).
Experimental section
General information
Scheme 2 Products in organic phase using 4-methylbenzyl alcohol as
All experiments with metal complexes were carried out under
purified argon atmosphere using the standard Schlenk tech-
niques. 1-Hexanol, cyclohexyl-methanol and benzyl alcohol
were purified according to standard procedures under argon
atmosphere. Other reagents were used as received. Solvents
were distilled and degassed with argon and kept in a spheri-
cal reservoir bottle with a 4 Å molecular sieve. The 1H and
13C NMR spectra were recorded on a 300 MHz/75 MHz Varian
the substrate.
Hauser and co-worker reported that the treatment of
dibenzyl ether with a strong base (KNH2) resulted in the for-
mation of benzaldehyde (28%) and toluene (22%) by the β-H
elimination of the intermediate carbanion (C6H5C(K)HOCH2-
C6H5).34 It is possible that there exists an equilibrium: (aryl-
CH2)2O + NaOCH2-aryl ↔ aryl-CHIJNa)OCH2-aryl + arylCH2-
OH, since the pKa values of arylCH2OH and (aryl-CHH)2O are
similar (∼30 in DMSO).35,36 In the case of 4-methylbenzyl al-
cohol as the substrate, the total amount of ether and p-xylene
is 3.20 mmol, which is consistent with the amount of carbox-
ylate (3.45 mmol). This observation indicates that the second
oxygen of carboxylate comes from the water which is pro-
duced by the etherification of the alcohol.
1
Gemini 300 spectrometer. The H NMR chemical shifts were
referenced to the residual hydrogen signals of the deuterated
solvent or TMS, and the 13C NMR chemical shifts were
referenced to the 13C signals of the deuterated solvent. L1,26
L2,27 L3 (ref. 28) were synthesized according to the published
literature. Complex 1 was prepared according to the reported
procedure30 with slight modification. Complexes 2 and 3 were
prepared through the procedures in our previous publication.29
Although more evidence is needed for the migration of an
oxygen atom from the alcohol to the intermediate aldehyde
(the formation of free water cannot be ruled out in our sys-
tem), a plausible reaction route is outlined in Scheme 3.
Firstly, the alcohol is dehydrogenated to the aldehyde and
H2, accompanied by the etherification of the alcohol to trans-
fer a water to the aldehyde; then, the diol (b) formed from
the aldehyde and water is sequentially dehydrogenated to the
carboxylate (c) and H2, which is similar to that of the Ru-PNP
catalyzed formation of carboxylate from alcohol reported by
Milstein.12 Finally, the formation of carbonion (d) occurs,
followed by the slow β-H elimination to form the aldehyde
and toluene derivatives.34
Synthesis and characterization of NiIJII) complex 1
To a solution of nickel chloride (0.082 g, 0.64 mmol) in metha-
nol (10 mL) was added ligand L1 (0.20 g, 0.64 mmol), and the
mixture was refluxed for 12 h under argon. After cooling to
room temperature, the mixture was filtered and the green
powder was washed with methanol (3 × 5 mL) and dried under
vacuum overnight to give the green powder of complex 1 (0.24
1
g, 88%). There is no clear signal observed in its HNMR spec-
trum, indicating that complex 1 is paramagnetic. A green sin-
gle crystal of 1·2DMF suitable for X-ray diffraction analysis was
obtained by slow diffusion of diethyl ether into the concen-
trated DMF solution of 1 at room temperature for one week.
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General procedure for catalytic reactions
the standard samples, and weighed to calculate the yield (see
ESI‡ for details).
Method A.23 Benzyl alcohol (2.16 g, 20 mmol), base (5.0
mmol) and catalyst (25 μmol) were charged in a 25 mL
Schlenk tube and the reaction mixture was heated at 150 °C
for 24 h under a flow of argon. After cooling to room temper-
ature, degassed water (10 mL) was added and the mixture
was extracted with diethyl ether (3 × 10 mL). The aqueous
phase was acidified with 6 M HCl and extracted with ethyl ac-
etate (5 × 20 mL). The combined organic phase was washed
with brine (25 mL), dried over anhydrous Na2SO4, and evapo-
rated to dryness under reduced pressure. The white solid of
the pure benzoic acid was obtained and weighed to calculate
the yield.
Method B. Alcohol (20 mmol), base (5.0 mmol) and com-
plex 3 (19.0 mg, 50 μmol) were charged in a 25 mL Schlenk
tube and 1 mL mesitylene was added. The reaction mixture
was heated at 150 °C for 48 h under a flow of argon. After
cooling to room temperature, degassed water (10 mL) was
added and the mixture was extracted with diethyl ether (3 ×
10 mL). The aqueous phase was acidified with 6 M HCl and
extracted with ethyl acetate (5 × 20 mL). The combined or-
ganic phase was washed with brine (25 mL), dried over
anhydrous Na2SO4, and evaporated to dryness under reduced
pressure. The pure carboxylic acids were obtained and
weighed to calculate the yield, and they were further charac-
terized by their 1H NMR with comparison to the standard
samples.
X-ray crystallography
A Bruker SMART APEX II diffractometer with graphite-
monochromated Mo-Kα (λ = 0.71073 Å) was employed to col-
lect the intensity data for the single crystal of complex 1. The
data was collected at about 100 K using ω-scan techniques.
The structure was solved by direct methods using SHELXL-
97.37 Multi-scan empirical absorption corrections were ap-
plied to the data set using the program SADABS.38 The struc-
ture was refined with SHELXL-97.37 Hydrogen atoms bound
to carbon were placed at calculated positions and refined
using a riding mode. All non-hydrogen atoms were refined by
full-matrix least squares on F2 using the SHELXTL program
package.39 Cell refinement, data collection, and reduction
were done by Bruker SAINT.40 The crystallographic data is
available in the ESI‡ as the CIF file.
Complex 1·2DMF: C25H27Cl2N7O3Ni, monoclinic, P21/c,
a = 16.1455IJ18) Å, b = 19.402(2) Å, c = 9.224(1) Å, α = 90°,
β = 103.732IJ2)°, γ = 90°, Z = 4, Mr = 603.13, FIJ000) = 1248.0,
Dx = 1.427 g cm−3, Mμ = 0.921 mm−1. The final cycle of refine-
ment based on F2 gave an agreement factor R1 = 0.0423 for
data with I > 2σ(I) and wR2 = 0.1092 for all data (5815 reflec-
tions) with a goodness-of-fit of 1.000. The X-ray crystal struc-
ture of complex 1·2DMF is deposited in CCDC number
1504880.
Method C. Alcohol (100.0 mmol), base (25.0 mmol) and
complex 3 (4.8 mg, 12.5 μmol) were charged in a 100 mL
Schlenk tube and the reaction mixture was heated at 150 °C
for 72 h under a flow of argon. After cooling to room temper-
ature, degassed water (30 mL) was added and the mixture
was extracted with diethyl ether (3 × 30 mL). The aqueous
phase was acidified with 6 M HCl and extracted with ethyl ac-
etate (5 × 30 mL). The combined organic phase was washed
with brine (50 mL), dried over anhydrous Na2SO4, and evapo-
rated to dryness under reduced pressure. The pure carboxylic
acids were obtained and weighed to calculate the yield, and
they were further characterized by their 1H NMR with com-
parison to the standard samples.
Method D. 4-Methylbenzyl alcohol (2.44 g, 20 mmol),
4-methylbenzyloxide (0.72 g, 5.0 mmol) and complex 3 (19.0
mg 50 μmol) were charged in a 25 mL Schlenk tube, and the
reaction mixture was heated at 150 °C for 48 h under a flow
of argon. After cooling to room temperature, degassed water
(10 mL) was added and the mixture was extracted with
diethyl ether (4 × 10 mL). The aqueous phase was acidified
with 6 M HCl and extracted with ethyl acetate (5 × 20 mL).
The combined organic phase was washed with brine (25 mL),
dried over anhydrous Na2SO4, and evaporated to dryness un-
der reduced pressure, and the pure 4-methylbenzoic acid
(0.47 g, 69%) was obtained. The products in the organic
phase were separated by column chromatography using silica
gel (elute: petroleum ether/ethyl acetate 40/1 (V/V)). The prod-
ucts were characterized by NMR analysis with comparison to
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21171134, 21573166), the Natural Sci-
ence Fund of Hubei Province (2011CDB441), and the Funds
for Creative Research Groups of Hubei Province (2014CFA007).
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