Fluoro-functionalized polymeric N-heterocyclic carbene-zinc complexes: Efficient catalyst for formylation and methylation of amines with CO2 as a C1-building block

Article abstract of DOI:10.1039/c5ra00380f

A fluoro-functionalized polymeric N-heterocyclic carbene (NHC)-Zn complex (F-PNHC-Zn) was designed and synthesized by taking fluorous imidazolium salts as precursors through a two-step alkylation. The resultant F-PNHC-Zn was applied in catalyzing the formylation and methylation of amines using CO₂ as a C1 building block in the presence of organosilane, which showed much higher activity than the corresponding non-fluorous PNHC-Zn under identical conditions. N-Methylanilines with both electron-withdrawing and electron-donating groups all could be converted to the corresponding formamides and methylamines in >90% conversion. Quantitative conversion of N-methylaniline was obtained even under very low CO₂ pressure (0.05 MPa diluted by N₂). Moreover, F-PNHC-Zn was highly stable and easily recyclable for these reactions. This journal is

Full text of DOI:10.1039/c5ra00380f

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Fluoro-functionalized polymeric N-heterocyclic
carbene-zinc complexes: efficient catalyst for
formylation and methylation of amines with CO2 as
a C1-building block†
Cite this: RSC Adv., 2015, 5, 19613
*
Zhen-Zhen Yang, Bo Yu, Hongye Zhang, Yanfei Zhao, Guipeng Ji and Zhimin Liu
A fluoro-functionalized polymeric N-heterocyclic carbene (NHC)-Zn complex (F-PNHC-Zn) was designed
and synthesized by taking fluorous imidazolium salts as precursors through a two-step alkylation. The
resultant F-PNHC-Zn was applied in catalyzing the formylation and methylation of amines using CO2 as
a C1 building block in the presence of organosilane, which showed much higher activity than the
corresponding non-fluorous PNHC-Zn under identical conditions. N-Methylanilines with both electron-
withdrawing and electron-donating groups all could be converted to the corresponding formamides and
methylamines in >90% conversion. Quantitative conversion of N-methylaniline was obtained even under
very low CO2 pressure (0.05 MPa diluted by N2). Moreover, F-PNHC-Zn was highly stable and easily
recyclable for these reactions.
Received 8th January 2015
Accepted 9th February 2015
DOI: 10.1039/c5ra00380f
www.rsc.org/advances
Another driving force to ensure efficient transformation of
CO2 was to employ high-energy starting materials including
epoxides/aziridines, alkenes/alkynes, and organometallics.15 In
1. Introduction
As an abundant, nontoxic, nonammable, renewable and easily
available synthon in organic synthesis, CO2 is appealing as a
typical feedstock for manufacturing commodity chemicals,
fuels and materials.1 The major challenge is the lack of effective
catalysts to facilitate its activation and subsequent trans-
formation, owing to the thermodynamic and kinetic stability of
CO2. One of the most powerful ways to ensure conversion of the
inert CO2 molecule was through efficient activation of CO2
using transition mental complexes,2 amine-containing species,3
frustrated Lewis pairs,4 and especially N-heterocyclic carbene
(NHC) complexes,5 which showed remarkable activity in various
transformations of CO2 such as the carboxylation of organo-
boron compounds,6 aromatic heterocyclic compounds7 and
alkynes,8 carboxylative cyclization of CO2 with propargylic
alcohols,9 reduction of CO2 into CO,10 hydrosilylation or
hydroboration of CO2 into formats.11 Recently, Cantat's group
reported the formylation of N–H bonds using CO2 and orga-
nosilane catalyzed by NHCs (IPr).12 Subsequently, taking NHC-
Zn complex (IPrZnCl2) as catalyst, further hydrosilylation of
formamide to the corresponding methylamine was realized by
the same group.13 Formamides and methylamines are both
basic reagents in nitrogen chemistry, which are generally used
as platform chemicals or solvents, etc.14
this respect, cheap and nontoxic organosilanes are appealing
reductants because they have mild reducing potentials (e.g., 92
kcal molꢀ1 in SiH4 vs. 104 kcal molꢀ1 in H2),16 and are thus an
attractive energy source to reduce CO2-derived systems without
wasting energy.17 Formylation of amines using CO2 as a C1
building block in the presence of hydrosilanes was an attractive
route to formamides, for which efficient homogeneous catalytic
systems have been developed, including nitrogen bases (e.g.
TBD),17b,18 NHCs (IPr),12,18 Rh2(OAc)4/K2CO3,19 copper–diphos-
phine complexes20 and iron(II) salts/phosphine donors.21
Methylation of amines was realized through complete deoxy-
genation of CO2 via a 6-electron reduction pathway using
hydrosilanes as reductants catalyzed by NHC-Zn complex
(IPrZnCl2),13 iron(II) salts/phosphine donors21 and ruthenium/
phosphine complexes.22 However, in most cases, the reactions
were accomplished under homogeneous catalytic conditions. In
addition, for the NHC-catalyzed systems, inert atmosphere was
generally needed due to their sensitivity to air and moisture.
Therefore, it is still desirable to search for an efficient, easily
separating and recyclable heterogeneous catalytic system for
the formylation and methylation of amines using organosilanes
and CO2.
Due to the excellent s-donating properties, NHCs was
employed both as nucleophile catalysts and as ligands for a
wide range of transition metals, which led to many applications
of NHC-based catalysts in organic synthesis.23 Hence, signi-
cant efforts were also devoted towards developing
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. E-mail: liuzm@iccas.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra00380f
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Scheme 1 Synthetic route and structures of the poly(N-heterocyclic carbene)zinc ((F)-PNHC-Zn).
heterogeneous NHC organometallic catalysts through immo- Brucker WB Avance II 400 MHz spectrometer. The 13C CP/MAS
bilization,24 polymerization25 as side chain, and more effi- NMR spectra were recorded with a 4 mm double-resonance MAS
ciently, through preparation of poly-NHCs taking poly probe and with a sample spinning rate of 10.0 kHz; a contact
imidazolium salts as precursors.8,26 Previously, our group time of 2 ms (ramp 100) and pulse delay of 3 s were applied. ESI-
designed uoro-functionalized polymeric imidazolium ionic MS were recorded on a Thermo Finnigan LCQ Advantage
liquids (F-PILs), which showed three times higher activity for spectrometer in ESI mode with a spray voltage of 4.8 kV. FTIR
the cycloaddition reactions of CO2 with epoxides than the cor- spectra of the samples were collected on a TENSOR 27 FTIR at a
responding non-uorous PILs.27 Han et al. also reported a per- resolution of 2 cmꢀ1. The elemental analysis of the polymer was
uorinated covalent triazine-based framework (FCTF-1), which determined using a Flash EA1112 analyzer. The thermal prop-
exhibited high CO2 uptake capacity especially at low pressures.28 erties of the materials were evaluated using a thermogravi-
Fluorinated microporous organic polymers (F-MOPs) were also metric analysis (TGA) instrument (STA PT1600 Linseis) over the
designed by our group, showing twice higher CO2 adsorption temperature rangeꢁof 25 to 800 ꢁC under air atmosphere with a
capacity than the corresponding non-uorous MOPs. In addi- heating rate of 10 C minꢀ1. Field emission SEM observations
tion, F-MOP-Ag(I) displayed high efficiency for CO2 reacting ꢁwith were performed on a Hitachi S-4800 microscope operated at an
propargyl alcohols to a-alkylidene cyclic carbonates at 25 C.29 accelerating voltage of 10.0 kV. XPS analysis was performed on
The promotion of CO2 conversion and adsorption by the an ESCAL Lab 220i-XL spectrometer at a pressure of ꢂ3 ꢃ 10ꢀ9
strongly polar C–F bonds was presumably derived from the mbar (1 mbar ¼ 100 Pa) using Al Ka as the excitation source (hn
interaction between the uorine atom and the carbonyl carbon ¼ 1486.6 eV) and operated at 15 kV and 20 mA. The BEs were
atom of CO2, thus leading to activation of CO2 and facilitating referenced to the C1s line at 284.8 eV from adventitious carbon.
CO2 conversion with increasing activity.
The loading content of Zn in the catalysts was determined by
Herein an easily operating uoro-functionalized polymeric ICP-AES (VISTA-MPX).
NHC-Zn complex (F-PNHC-Zn) was designed and synthesized
via taking uorous imidazolium salts as precursors through a
2.2 Synthesis of the polymeric ionic liquids26
two-step alkylation (Scheme 1), which was characterized by
means of different techniques including Fourier transform
infrared (FTIR), elemental analysis, cross-polarization magic-
angle spinning (CP/MAS) 13C NMR, inductively coupled
plasma-optical emission spectroscopy (ICP-OES), eld-emission
scanning electron microscopy (SEM), and X-ray photoelectron
spectroscopy (XPS). The resultant F-PNHC-Zn was applied in
catalyzing the formylation and methylation of amines using
CO2 as a C1 building block in the presence of organosilane, and
its activity and stability were investigated.
NaH (60% in oil, 11 mmol) was added to a THF solution of
imidazole (10 mmol), and the resulting suspension was stirred
at room temperature for 2 h. 1,4-bis(chloromethyl)benzene
(for ImCH2PhCH2Im (A),
5 mmol)/1,4-bis(chloromethyl)-
tetrauorobenzene (for ImCH2PhF4CH2Im (A), 5 mmol) was
then added to the residue, and stirred at room temperature for
another 4 h. The product was extracted with dichloromethane,
and ImCH2PhCH2Im/ImCH2PhF4CH2Im was obtained in
quantitative yield aer removing the solvent under vacuum.
ImCH2PhCH2Im 1H NMR (D2O, 400 MHz) d 5.21 (s, 4H), 7.04
(s, 2H), 7.11 (s, 2H), 7.24 (s, 4H), 7.80 (s, 2H); ESI-MS found
[M + H]+: 239.1.
2. Experimental section
2.1 Instrumentation
1
ImCH2PhF4CH2Im H NMR (D2O, 400 MHz) d 5.66 (s, 4H),
Liquid NMR spectra were recorded on a Brucker 400 spec- 7.01–7.40 (m, 2H), 7.58 (s, 2H), 7.70–8.39 (m, 2H); ESI-MS found
trometer. Solid-state NMR experiments were performed on a [M + H]+: 311.2.
19614 | RSC Adv., 2015, 5, 19613–19619
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In a typical experiment to synthesize F-PIL, 2,4,6-tris-
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(bromomethyl)mesitylene (B) (2 mmol) and ImCH2PhF4CH2Im
(A) (6 mmol) dissolved in 100 mL of DMF were loaded in a
stainless steel autoclave, which was capped and placed in an
oven at 110 ꢁC for 24 h. A white solid product was precipitated
and ltered, washed with DMF and Et2O, followed by being
ꢁ
dried under vacuum at 80 C for 24 h. The yield of the white
powder (denoted as F-PIL) was about 80%.
Similarly, PIL was synthesized using 2,4,6-tris(bromomethyl)
mesitylene (B) and ImCH2PhCH2Im (A) as raw materials, a 85%
yield was reached.
2.3 Synthesis of polymeric NHC-Zn26
NaOtBu (0.2 mmol) was added to a DMF (5 mL) suspension of F-
PIL (0.02 g) in a reaction ask under N2. The reaction mixture was
stirred at room temperature for 4 h. Then ZnCl2 (0.2 mmol) was
added and the resulting mixture was stirred at 80 ꢁC for 10 h. The
solid product was ltered, washed with DMF and dried under
vacuum at 60 ꢁC for 24 h, which was denoted as F-PNHC-Zn.
Fig. 1 FTIR spectra of (F)-PIL and (F)-PNHC-Zn.
2.4 Typical procedures for the reaction of N-methylaniline
with CO2 and PhSiH3
To a stainless steel autoclave (25 mL inner volume), F-PNHC-Zn
(5 mol%), PhSiH3 (0.75 mmol) and N-methylaniline (0.25 mmol)
were added successively. CO2 was then charged into the reactor
up to a desired pressure (e.g., 1 MPa) at room temperature. The
autoclave was heated at 80 ꢁC for 24 h. Aer reaction, the
autoclave was cooled to room temperature, and then CO2 was
1
vented. The product yield was determined by H NMR (CDCl3,
400 MHz) using 1,4-bis(chloromethyl)-benzene (1 mmol) as an
internal standard. The products were also isolated by column
chromatography (eluent: petroleum and CH2Cl2) on silica gel
and identied by NMR spectra. For catalyst (F-PNHC-Zn) recy-
cling, the catalyst was separated by lꢁtration, washed with THF
and then dried under vacuum at 60 C for 24 h. The recycled
catalyst was reused for the next run without further
purication.
3. Results and discussion
3.1 Synthesis and characterization of F-PNHC-Zn
Fig. 2 (A) CP/MAS 13C NMR spectra of (F)-PIL. (B) TGA analysis of (F)-
PIL and (F)-PNHC-Zn under air up to 800 ꢁC at a ramping rate of 10 ꢁC
The F-PNHC-Zn particles were obtained from uorous poly-
imidazolium ionic liquids (F-PIL) and ZnCl2 (Scheme 1).26,27 For
comparison, nonuorous PNHC-Zn was also synthesized. The
formation of F-PIL and F-PNHC-Zn were characterized by FTIR,
CP/MAS, and 13C NMR. In the FTIR spectra (Fig. 1), the imida-
zolium structures were conrmed by the characteristic bands at
1160 (C–N+), 1560 (C]C), 1620 (C]N) cmꢀ1 and 2960 cmꢀ1
(–CH2). In addition, the band at 1035 cmꢀ1 was ascribed to C–F
stretching, which was absent in the spectra of PIL and PNHC-
Zn. In the CP/MAS 13C NMR spectrum of F-PIL (Fig. 2(A)), the
presence of chemical shis in the range of 114.1–142.6 ppm
belonged to the aromatic carbons, and those located at 17.8–
49.2 ppm to the methylene carbon in the backbone of the
polymers. The thermogravimetric analysis (TGA) results
(Fig. 2(B)) indicated that both the imidazolium F-PIL precursor
minꢀ1
.
and the F-PNHC-Zn catalysts were stable in air up to 250 ꢁC,
which met the demands for potential applications under the
relatively high temperature conditions. Elemental analysis
(Table 1) showed that the contents of C, H and N in F-PIL and F-
PNHC-Zn were slightly lower than that in the corresponding
non-uorous materials, indicating the substitution of some H
atoms by F atoms on the aromatic ring. The Zn content in F-
PNHC-Zn and PNHC-Zn was 4.55% and 4.44%, respectively, as
determined by ICP-OES analysis (Table 1). SEM images showed
that the samples were composed of tiny particles with irregular
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Table 1 Elemental analysis (C, H, N) and ICP-OES (Zn) analysis for the
resultant polymers
cations (BE at 401.12 eV) and NHC structure (BE at 399.37 eV)
existed in the skeleton.31 Notably, both F–C bond (BE at 687.37
eV) and F–Zn bond (BE at 684.67 eV) existed as shown in the XPS
spectra of F1s.32 The BE for Zn2p appeared at 1021.77 and
1044.77 eV for Zn2p3/2 and Zn2p1/2, respectively.
Polymer
C [wt%]
H [wt%]
N [wt%]
Zn [wt%]
PIL
PNHC-Zn
F-PIL
47.69
49.44
42.54
44.10
5.33
5.19
4.44
4.12
9.67
10.08
8.86
—
4.44
—
F-PNHC-Zn
9.15
4.55
3.2 Reactions of CO2 with N-methylanilines catalysed by F-
NHC-Zn
As shown in Table 2, the reaction of N-methylaniline (1a) and
CO2 in the presence of PhSiH3 proceeded catalyzed by F-NHC-
Zn, producing formamide (2a) and N,N-dimethylaniline (3a),
while no reaction occurred in the absence of any catalyst (entry
1). Notably, the catalytic efficiency was increased by introducing
uorine onto the skeleton of the polymers (entry 2 vs. 3). For
example, using PNHC-Zn as the catalyst, 70% conversion of 1a
together with 61% yield of 2a and 8% yield of 3a was obtained,
while in the presence F-PNHC-Zn under identical conditions,
100% conversion of 1a and the yields of 2a and 3a of 71% and
27%, respectively, were achieved. The catalytic activity of F-
NHC-Zn was greatly inuenced by the solvents. It was demon-
strated that the polar solvents were more favourable to this
Table 2 Solvent screening for the formylation and methylation of N-
methylaniline (1a) catalysed by F-PNHC-Zna
Fig. 3 SEM images of F-PIL and F-PNHC-Zn particles.
shape and size (Fig. 3). The morphologies of the particles in F-
PIL remained unchanged aer coordinating with zinc
complexes. XPS measurement was performed to investigate the
oxidation state of the elements in F-PNHC-Zn (Fig. 4). In the C1s
spectrum, three peaks with binding energies (BEs) of 284.82,
286.32 and 288.62 eV were corresponded to aromatic carbon
(C]C), imidazolium carbon (C–N) and covalent C–F carbon,
respectively.30 N1s spectrum indicated that both imidazolium
Yieldb/%
Entry
Solvent
1a Conversionb/%
2a
3a
1
2
3
4
5
6
7
8
THFc
0
70
100
97
100
95
93
84
82
78
68
50
0
61
71
69
82
71
69
76
61
58
66
45
19
51
24
0
8
THFd
THF
27
26
18
24
23
8
20
19
2
5
0
18
5
THFe
DMSO
1,4-Dioxane
CH2Cl2
CH3CN
Acetyl acetate
Et2O
9
10
11
12
13
14
15
CHCl3
Toluene
Et3N
20
71
31
THFf
THFg
a
Reaction conditions: 1a 0.25 mmol, PhSiH3 0.75 mmol, F-PNHC-Zn 5
mol% relative to 1a, based on the content of Zn, solvent 1 mL, CO2
1
b
MPa, 80 ꢁC, 24 h. Conversion and yield was determined by 1H NMR
(CDCl3, 400 MHz) using 1,4-bis(chloromethyl)-benzene as an internal
c
d
standard. No catalyst was added. Non-uorous PNHC-Zn (5 mol%)
e
was added in stand of F-PNHC-Zn. Aer the catalyst was reused for
ve times. 0.5 mmol of PhSiH3 was added. 1.25 mmol of PhSiH3
f
g
was added.
Fig. 4 XPS spectra (C1s, N1s, F1s and Zn2p) of F-PNHC-Zn.
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reaction. For example, >90% conversion of 1a with 2a yield of 69% yield of 2a and 26% yield of 3a was obtained aer the
69–82% and 3a yield of 18–27% was obtained taking THF, catalyst was recycled for ve times (entry 4).
DMSO, 1,4-dioxane and CH2Cl2 as reaction media (entries 3, 5–
As shown in Fig. 5(A), quantitative conversion of 1a together
7). In addition, CO2 reductive functionalization to 2a and 3a with molar ratio of 2.6 for 2a/3a could be retained at a low CO2
also depended on the nature of the reductant and less reactive pressure range of 0.05–1 MPa. As previously reported, uorous
hydrosilanes, such as Ph2SiH2, Ph3SiH, Ph(Me)2SiH, Et3SiH, materials could activate CO2 and enhance CO2 adsorption espe-
Et2SiH2 and polymethylhydrosiloxane (PMHS) are unreactive, cially at low CO2 pressure. Minute quantity of CO2 could be
with no conversion of 1a. The optimal molar ratio of 1a:PhSiH3 “absorbed” through interaction between the uorine atom
was 1 : 3 (entry 3 vs. 14, 15), with quantitative conversion of 1a. (strongly electronegative) and the carbonyl carbon atom (elec-
Notably, the catalyst F-PNHC-Zn showed good reusability, tropositive) of CO2, thus leading to increased concentration of
conrmed by the fact that 97% conversion of 1a together with CO2 around the catalytic centre, thus facilitating CO2 conversion
with increasing activity.27,28,33 When CO2 pressure was further
increasing to 4 MPa, although 100% conversion of 1a was
obtained, the yield of formamide 2a was increased to 86% and
the yield of methylation product 3a was decreased to 14%.
However, CO2 pressure higher than 4 MPa led to too much CO2
molecular around the catalytic centre due to the interaction of
CO2 with uorine atom on the backbone, which might retard the
interaction between 1a and the catalyst, and might cause a low
concentration of substrate in the vicinity of the catalytic centre,
Table 3 Formylation and methylation of N-methylanilines using CO2
and PhSiH3 catalyzed by F-PNHC-Zna
Yieldb/%
Entry
Substrate
Conv.b/%
100
2
3
1
2
71
65
27
35
100
3
4
91
95
61
72
29
22
5
6
100
91
66
81
34
8
Fig. 5 Influences of CO2 pressure (A), reaction temperature (B) and
reaction time (C) on the formylation and methylation of 1a using
PhSiH3 and CO2. Reaction conditions: 1a, 0.25 mmol; PhSiH3, 0.75
mmol; F-PNHC-Zn, 5 mol% relative to 1a; THF, 1 mL. (A) 80 ꢁC, 24 h;
CO2 pressure <0.5 MPa was obtained by diluting with N2. (B) CO2
pressure, 1 MPa; 24 h. (C) 80 ꢁC; CO2 pressure, 1 MPa. Conversion
and yield was determined by 1H NMR (CDCl3, 400 MHz) using
1,4-bis(chloromethyl)-benzene as an internal standard. Selectivity was
the molar ratio of 3a/2a.
7
100
85
15
a
Reaction conditions: N-methylaniline 0.25 mmol, PhSiH3 0.75 mmol,
F-PNHC-Zn 5 mol% relative to 1, based on the content of Zn, THF 1 mL,
CO2 1 MPa, 80 ꢁC, 24 h. Conversion and yield was determined by 1H
b
NMR (CDCl3, 400 MHz) using 1,4-bis(chloromethyl)-benzene as an
internal standard.
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precursors through a two-step alkylation, which showed high
efficiency for the reactions of amines with CO2 in the presence
of organosilane. A broad range of N-methylanilines could be
converted to the corresponding formamides and methylamines
catalyzed by F-PNHC-Zn. In addition, F-PNHC-Zn showed good
stability and easy recyclability. The as-synthesized F-PNHC-Zn
may nd promising applications in catalysis.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (Grants nos 21125314, 21321063,
21402208) and the Chinese Academy of Sciences.
Scheme 2 Proposed reaction mechanism for the F-PNHC-Zn-cata-
lyzed methylation and formylation of amines.
Notes and references
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thus resulting in low conversion of 1a.34 Both conversion of 1a
and yield of 2a, 3a were strongly affected by the reaction
temperature and optimal performances were achieved at 80 C
ꢁ
(Fig. 5(B)). Quantitative conversion of 1a was attained within 24 h,
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more N-methylanilines was examined and the results are
summarized in Table 3. It was indicated that N-methylanilines
with both electron-withdrawing and electron-donating groups
could be transformed to the corresponding formamides and
methylamines with >90% conversion of the substrates. In
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CH3–) on the meta- and para-positions of the benzene ring all
converted to formamides and methylamines (entries 4–7). N-
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to form formamides (yield of 2: 81–85%) (entries 6, 7) compared
with those substrates with electron-withdrawing groups (–F, –Cl
and –Br) (yield of 2: 61–72%) (entries 2–5). The possible reaction
mechanism for the F-PNHC-Zn-catalyzed methylation and for-
mylation of amines was in accordance with that reported in the
literature (Scheme 2).13 The reaction was proceeded through F-
PNHC-Zn-catalyzed formylation of the N–H bond using CO2 and
the hydrosilane, and then hydrosilylation of the formamide
gave the methylamine product. The reduction of formamide 2a
was carried out, forming N,N-dimethylaniline (3a) solely in 23%
yield in the presence of F-PNHC-Zn and PhSiH3 under the same
other conditions (Scheme 3).
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