Stable carbamate pathway towards organic-inorganic hybrid perovskites and aromatic imines

Article abstract of DOI:10.1039/d0ra07814j

Methyl ammonium methyl carbamate (MAC), formulated as CH3NH3+CH3NHCO2-, was synthesized by reacting liquid methylamine with supercritical CO2, and its structure was refined by single-crystal X-ray diffraction. MAC is a white crystalline salt and is as reactive as methylamine, and is a very efficient alternative to toxic methylamine. We were able to produce hybrid perovskite MAPbI3 (MA = methyl ammonium) by grinding MAC with PbI2 and I2 at room temperature, followed by storing the mixed powder. Moreover, this one-pot method is easily scalable for the large-scale synthesis of MAPbI3 in a small vessel. We have also investigated the reactivity of MAC towards aromatic aldehydes in the absence of solvent. The solventless reactions afforded imines as exclusive products with over 97% yield, which show higher selectivity than the methylamine-based synthesis. Complete conversions were typically accomplished within 3 h at 25 °C. The results of this study emphasize the importance of solid carbamates such as MAC to develop an environmentally friendly process for the synthesis of various amine-based materials on the industrial scale.

Full text of DOI:10.1039/d0ra07814j

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PAPER
Stable carbamate pathway towards organic–
inorganic hybrid perovskites and aromatic imines†
Cite this: RSC Adv., 2020, 10, 38055
Kyu Hyung Lee,a Sun Joo Kim,a Hee Sun Park,a Byung Wook Lim,a Byeongno Lee,a
b
a
Young Jun Park,b Wonwoo Nam
and Nam Hwi Hur
*
Methyl ammonium methyl carbamate (MAC), formulated as CH3NH3+CH3NHCO2ꢀ, was synthesized by
reacting liquid methylamine with supercritical CO2, and its structure was refined by single-crystal X-ray
diffraction. MAC is a white crystalline salt and is as reactive as methylamine, and is a very efficient
alternative to toxic methylamine. We were able to produce hybrid perovskite MAPbI3 (MA ¼ methyl
ammonium) by grinding MAC with PbI2 and I2 at room temperature, followed by storing the mixed
powder. Moreover, this one-pot method is easily scalable for the large-scale synthesis of MAPbI3 in
a small vessel. We have also investigated the reactivity of MAC towards aromatic aldehydes in the
absence of solvent. The solventless reactions afforded imines as exclusive products with over 97% yield,
which show higher selectivity than the methylamine-based synthesis. Complete conversions were
typically accomplished within 3 h at 25 ꢁC. The results of this study emphasize the importance of solid
carbamates such as MAC to develop an environmentally friendly process for the synthesis of various
amine-based materials on the industrial scale.
Received 12th September 2020
Accepted 7th October 2020
DOI: 10.1039/d0ra07814j
rsc.li/rsc-advances
methyl carbamate (MAC, CH3NH3 CH3NHCO2ꢀ) salt. A couple
of groups reported computational and experimental studies on
the interaction between CH3NH2 and CO2,18,19 but isolation and
+
Introduction
Methylamine is the simplest primary amine with the formula
CH3NH2, and is widely used as a precursor for the synthesis of
pharmaceuticals, pesticides, and organic–inorganic hybrid
perovskites.1–5 It is a poisonous and intractable gas, which
makes it challenging to handle. Accordingly, methylamine is
used as a solution diluted with various solvents in the labora-
tory. However, the vapor of methylamine dissolved in a solution
is still hazardous, which limits its extensive use. This disad-
vantage can be resolved by replacing methylamine with a solid
carbamate salt that contains the methylamine moiety.
structural characterization of the MAC had not been reported
yet. For the rst time, we isolated the MAC from the supercrit-
ical CO2 condition as a crystalline solid. The structure of MAC,
determined using single-crystal X-ray diffraction, is composed
+
of the cation (CH3NH3 ) and anion (CH3NHCO2ꢀ) units, which
are connected by weak inter-ionic hydrogen bonds. MAC has
shown excellent reactivity towards inorganic compounds and
organic substrates.
The organic–inorganic hybrid perovskites, such as methyl
ammonium lead iodide (MAPbI3), were typically synthesized
from the reaction between lead iodide (PbI2) and methyl
ammonium iodide (MAI) in the presence of organic solvents.2–5
In a solution-based process, the low solubility of PbI2 interferes
with the precise control of the desired stoichiometry and the
large scale production of perovskite powders. Recently, several
groups reported mechanically induced solid-state reaction
methods for the synthesis of MAPbI3 from MAI and PbI2
precursors.20–22 However, high-speed ball milling is required to
produce the perovskite MAPbI3 because of the low reactivity of
the precursors. Reported herein is the superior reactivity of
MAC towards PbI2 and iodine (I2) for the preparation of MAPbI3,
which is accomplished in the absence of solvents. The solid-
state reaction relies on the use of MAC, which facilitates the
rapid and selective formation of MAPbI3 at ambient conditions.
This solid-state process involves the evolution of CO2 gas, which
leads to a forward reaction in the direction of perovskite
Primary amines (RNH2, R ¼ alkyl), such as methylamine, are
known for their strong interaction with carbon dioxide (CO2).
Typically, two equivalents of RNH2 react with CO2 to form an
alkyl ammonium carbamate salt (RNH3+RNHCO2 ).6–17 The
ꢀ
carbamate dissociates into the parent amines and CO2 by
external stimuli. Due to this reversible interaction between
amines and CO2, the solid carbamate was considered as
a synthetic equivalent to liquid amine for the preparation of
organic imines.10–17 Like other amines, the reaction of methyl-
amine (CH3NH2) with CO2 can produce a methyl ammonium
aDepartment of Chemistry, Sogang University, Seoul 04107, Korea. E-mail: nhhur@
sogang.ac.kr
bDepartment of Bioinspired Science, Department of Chemistry and Nano Science, Ewha
Womans University, Seoul 03760, Korea
† Electronic supplementary information (ESI) available. CCDC 2023499 and
2023500. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d0ra07814j
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formation. The release of methylamine enables us to synthesize manner, we prepared MAPbI3 samples by storing the ground
ꢁ
the MAPbI3 powder by grinding the mixture alone, followed by powder at 50 and 100 C for 1 h, respectively.
storing the mixed powder at the desired temperature. We also
report our studies of the solid-state reactivity of MAC toward
aromatic aldehydes for the synthesis of aromatic imines.23,24
The solvent-free reactions were performed as a one-pot
process, producing the imine derivatives as exclusive products
in over 97% yields at ambient conditions. This novel method
using MAC allows us to avoid the use of volatile and toxic
methylamine. It provides a novel sustainable approach for the
synthesis of inorganic materials as well as organic compounds.
Fabrication of perovskite MAPbI3 lms
Perovskite MAPbI3 lms were fabricated according to the spin
coating method previously reported.2–5,25 In short, a solution of
HPbI3 was prepared by dissolving PbI2 (1.0 mmol) in an acidi-
ed solution of DMF (1 mL) with hydroiodic acid (1.0 mmol),
followed by stirring for 1 h. The resulting solution was used as
the precursor solution. The HPbI3 solution was deposited on
the FTO/TiO2 substrate with a size of 2 ꢂ 2.5 cm2 by spin-
coating at 4000 rpm for 20 s. The HPbI3 lm was dried on
ꢁ
a hotplate at 100 C for 30 min. The dried lm was stored in
Experimental
Chemicals
a vial and then kept in an autoclave reactor lled with 0.16 g of
MAC. Finally, heating the reactor at 100 ꢁC for 2 h yielded
perovskite MAPbI3 lm. The thickness of the perovskite lm
was controlled by the HPbI3 concentration.
All the chemicals were obtained from commercial suppliers and
were used without further purication. Lead iodide (99.99%, PbI2)
and iodine ($99.99%, I2) were purchased from Alfa Aesar. N,N-
Dimethyl formamide (DMF) and hydroiodic acid (55% w/w, HI)
were purchased from Fisher Scientic. Benzaldehyde, 2-methox-
ybenzaldehyde, 4-chlorobenzaldehyde, 4-bromobenzaldehyde, 3-
nitrobenzaldehyde, cinnamaldehyde, 2-methoxycinnamaldehyde,
salicylaldehyde, 2-nitrocinnamaldehyde, benzophenone, and
methylamine solution (33 wt% in ethanol) were purchased from
Sigma-Aldrich.
General procedure for the preparation of imines from the
solventless reactions between MAC and aromatic aldehydes
A typical procedure is as follows: a round-bottom ask was charged
with 0.54 g of MAC (5.0 mmol) and an aromatic aldehyde (10.0
mmol) under the nitrogen atmosphere. The mixture was stirred at
ambient temperature until the reaction was completed. The reac-
tion progress was carefully monitored using 1H NMR spectroscopy.
Complete conversion to imine typically took about 3 h at room
temperature. No by-product, besides the imine, was identied
from the reaction mixture. The yields of immines based on the
MAC used were typically over 97%. The products were character-
ized by 1H and 13C NMR spectroscopy.
Synthesis of methyl ammonium methyl carbamate (MAC)
The reaction was conducted in a stainless steel autoclave reactor
with a glass liner (Parr 4760, 300 mL). A methylamine solution
(33 wt% in ethanol, 100.0 mmol) was introduced into a glass liner
charged with dry ice (50 g). Aer attaching the gauge and gauge
block assembly, the autoclave was placed in an oil bath on a hot
plate and was heated at 80 ꢁC. The CO2 pressure was maintained in
the range of 90 to 110 bar. Aer reacting for 10 h, the autoclave was
removed from the oil bath and was then cooled to room temper-
ature. The white crystalline powders formed in the glass liner were
ltered in air, followed by washing with cold methanol, cold
diethyl ether, and pentane. The resulting solid was dried under
vacuum for 5 h, which was conrmed as a methyl ammonium
methyl carbamate (MAC) salt. The yield of MAC based on the
methylamine used was 95.2%.
Growth of 3-nitrobenzylidene methanamine single crystals
A solution was prepared by dissolving 3-nitrobenzylidene
methanamine (0.1 g) in a 5 mL of pentane. The solution was
stored in a tightly locked bottle for 2 days. White crystals of 3-
nitrobenzylidene methanamine were slowly grown from the
solution, which were recovered by vacuum ltration.
Characterization
Powder X-ray diffraction (XRD) measurements were performed
on a Rigaku DMAX 2500 diffractometer (Rigaku, Japan) with Cu
Precaution: MAC could be harmful for health due to its
sublimation character. The equipment of adequate ventilation
is highly recommended for handling the MAC to avoid vapor
inhalation.
˚
Ka radiation (l ¼ 1.5406 A) operated at 40 kV and 150 mA. The
UV-visible absorption spectra were recorded using a Lambda
950 spectrophotometer (PerkinElmer, USA). High-resolution
scanning electron microscopy analyses were carried out using
a Hitachi S-5500 microscope (Hitachi, Japan). The operating
acceleration voltage is 15 kV. Thermogravimetric analysis was
Synthesis of methyl ammonium lead iodide (MAPbI3)
A typical procedure is as follows: lead iodide (0.461 g, 10 mmol) carried out using a TGA 2050 instrument (TA Instruments, USA).
and iodine (0.254 g, 10 mmol) were mixed with MAC (0.106 g, The sample was placed on a platinum pan for each run and
5.0 mmol), which was then ground using a pestle and mortar in analyzed in air or oxygen from 25 to 900 ꢁC at a heating rate of
an argon-lled glove box. The ground powder was stored in 5 C minꢀ1. GC/MS data were recorded on Agilent 5973N, and
ꢁ
a vial (10 mL) placed in a Parr reactor and allowed to react at elemental analyses were obtained using a Carlo Erba EA1180 at
room temperature for 16 h. The resulting product was analyzed the Organic Chemistry Research Center at Sogang University.
by X-ray diffraction and solid UV spectroscopy. The yield of 1H NMR and 13C NMR spectra in solution were recorded on
MAPbI3 based on the MAC used was over 97.2%. In a similar a Varian 400 MHz Gemini operating at 400 MHz for 1H and 100
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MHz for 13C, respectively. All chemical shis were referenced to network structure of MAC in the solid-state is due presumably
tetramethylsilane. Single-crystal X-ray diffraction data were to the inter-ionic hydrogen bonds between the cation and anion
collected using a Bruker SMART AXS diffractometer equipped species. Detailed crystallographic data are presented in Tables
ꢀ
with a monochromator with a Mo Ka (l ¼ 0.71073 A) incident S1 and S2 in the ESI.†
beam.
Synthesis of MAPbI3 by solid-state grinding
To explore the reactivity of MAC towards inorganic substances,
the reaction among MAC, PbI2, and I2 was investigated to make
perovskite MAPbI3. Specically, the precursors were ground
Results and discussion
Structural characterization of MAC
Like most primary amines, methylamine is known to react using a mortar and pestle, and the mixed powder was allowed to
readily with carbon dioxide to yield methyl ammonium methyl react at 25 ꢁC in the absence of intermediate grindings. A facile
carbamate (MAC). MAC is known, but its crystal structure has color change from white to black was noticed in the ground
not been determined. Single crystals of MAC were grown from powder, implying that the conversion of precursors to MAPbI3
a methylamine solution charged with pressurized CO2 gas. Aer occurs via the mechanically induced solid-state reaction. The
the reaction was completed, the reactor was cooled very slowly products were carefully analyzed by powder XRD data as
down to 0 ꢁC. The solid-state structure of MAC was determined a function of storage time at 25 ꢁC, which is illustrated in
by single-crystal X-ray diffraction. As illustrated in Fig. 1(a), the Fig. 2(a). Peak intensities of MAPbI3 increased with increasing
+
MAC is composed of the cation (CH3NH3 ) and anion (CH3- storage time. The XRD pattern collected aer storing the
NHCO2ꢀ) parts, which is structurally analogous to typical powder for 4 h clearly shows new peaks corresponding to the
carbamate salts.
MAPbI3 phase. On the other hand, intensities of main PbI2
There are three different C–N bonds of MAC: C1–N1, C2–N2, peaks are signicantly suppressed aer the 4 h reaction time.
˚
and C3–N2. The C1–N1 bond distance of 1.479 A in the methyl Complete conversion was achieved in a 16 h period and only
ammonium cation closely matches the C–N bond length of free MAPbI3 was obtained from the solid-state reactions. It is thus
18
˚
˚
CH3NH2 (1.471 A). The C3–N2 bond (1.452 A) in the anion is evident that MAC is a very effective reagent for the synthesis of
˚
slightly shorter than the C1–N1 one. The C2–N2 bond (1.359 A) MAPbI3 through the solid-state reaction alone. The yield of
connected to the CO2 group is much shorter than the C1–N1 MAPbI3 was estimated by the intensity value of (220) peak at
bond, indicating than the CO2 group is rmly bound to the 28.45ꢁ in MAPbI3 relative to (011) peak at 25.92ꢁ in unreacted
CH3NH2 moiety. The two C–O bond distances are virtually PbI2. Fig. 2(b) shows yields of MAPbI3 as a function of reaction
identical. The bond lengths of C2–O1 and C2–O2 are 1.269 and time, clearly demonstrating that yield increases with increasing
˚
1.279 A, respectively.
reaction time.
A notable feature is that there are strong inter-ionic inter-
To evaluate the reaction rate at elevated temperature, ground
actions ascribed to hydrogen bonds between the cation and powders were allowed to react in a vial at 50 and 100 ꢁC for 1 h,
anion units. Each methyl carbamate anion is hydrogen-bonded without any additional agitation. Powder XRD patterns of the
to three neighbouring methyl ammonium cations, which are products in Fig. 3 reveal that the formation of MAPbI3 is
illustrated in Fig. 2(b). The three O/H distances of O1–H4, O2– signicantly accelerated at high temperatures. Remarkably, the
H5, and O2–H6 are 1.838, 1.839, and 1.908 A, respectively. On complete conversion was accomplished within 1 h at 100 ꢁC. All
˚
the other hand, the H atom attached to the NH group of the the peaks from the product at 100 ꢁC are well matched with
anion weakly interacts with the O atoms. Their bond distances those of MAPbI3, and no PbI2 peaks are visible. These time- and
are much longer than those of the three O–H bonds. The stable temperature-dependent results suggest that the solid-state
Fig. 1 (a) ORTEP drawing and numbering scheme for the methyl ammonium cation and methyl carbamate anion in the formula unit of the
ꢀ
CH3NH3+CH3NHCO2 salt. All the atoms are shown as 30% probability displacement ellipsoids. Covalent bonds are linked by solid lines. Red,
black, blue, and pale pink ellipsoids represent O, C, N, and H atoms, respectively. (b) Illustration of hydrogen bonds in MAC. Blue dashed lines
represent the hydrogen bonds in which the O/H distances are shorter than 2.0 A˚. For clarity, other O/H distances longer than 2.0 A˚ are
omitted.
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Fig. 2 (a) Powder XRD patterns of products obtained from grinding a mixture of MAC, PbI2, and I2 at intervals of 1, 2, 4, 8, and 16 h at 25 ꢁC.
Theoretical diffraction data of MAPbI3 and PbI2 are provided for reference at the bottom of the XRD patterns. (b) Yields of MAPbI3 as a function of
time for the solid-state reactions performed at 25 ꢁC.
pathway using MAC not only provides high reactivity but also prepared by adding hydroiodic acid (HI) into PbI2. Moreover,
renders increased selectivity towards the formation of MAPbI3 the solution is well miscible with various solvents, which is
compared to solution-based methods. In contrast, most solu- advantageous for the production of high-quality lms without
tion methods involve a combination of well-soluble methyl agglomerates. The deposited lm was placed in an autoclave
iodide (CH3I) and less-soluble lead iodide (PbI2). This difference reactor lled with the MAC powder. Heating the reactor at
in solubility leads to the formation of MAPbI3 along with 100 ꢁC leads to the facile dissociation of MAC into methylamine
unreacted PbI2.
and CO2. The methylamine gas released spreads evenly over the
HPbI3 lm, which can accelerate the conversion of HPbI3 to
MAPbI3. In the present process, the lm thickness relies on the
concentration of HPbI3, which was controlled from 500 nm to
1000 nm. The MAC precursor enables the formation of highly
uniform perovskite MAPbI3 lms through the vapor to solid
reaction. Scanning electron microscopy (SEM) was employed to
examine the surface morphology of the MAPbI3 lms. As shown
in the top-view SEM image (Fig. S1†), the MAPbI3 lm has
Fabrication of MAPbI3 lm
The solid-state reaction route can be extended to the fabrication
of MAPbI3 lm applicable for perovskite solar cells and
modules. The MAPbI3 lm was grown on a TiO2/FTO substrate
using the MAC as a source of methylamine. As shown in Fig. 4,
the HPbI3 lm was rst deposited by a spin-coating method. An
important feature is that the HPbI3 solution can be easily
Fig. 3 (a) Powder XRD patterns of the products synthesized in solid-state reactions of MAC, PbI2, and I2 at 25 ꢁC, 50 ꢁC, and 100 ꢁC for 1 h. (b)
Yields of MAPbI3 obtained from the three reactions.
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recrystallization of the MAPbI3 material. This vapor-assisted
annealing is a convenient technique for the fabrication of
MAPbI3 lms. However, a drawback is that it is difficult to
control the amount of gas that requires complete conversion
accurately. In contrast, MAC has the advantage of being easy to
handle and being able to use accurately as needed.
The formation of MAPbI3 lm on a TiO2/FTO substrate by the
vapor-phase reaction with MAC was also evaluated by UV-visible
spectroscopy, which is shown in Fig. S2.† The UV-visible spec-
trum of the MAPbI3 lm shows an absorption edge at about
770 nm, which is the characteristic onset corresponding to the
MAPbI3 phase. The onset of absorption cannot be determined
accurately because the slope change is small near the absorp-
tion edge. However, the optical band gap, Eg, can be determined
from the extrapolation of the linear part of a Tauc plot, as shown
in the inset of Fig. S2.† The bandgap is 1.57 eV, which is similar
to that reported in other MAPbI3 lms.29,30 The spectrum shows
a gradual increase of absorbance in the range from 750 down to
350 nm, which is attributed to light scattering effects caused by
the large MAPbI3 grains.
Fig. 4 Schematic illustration of the formation of MAPbI3 film. Fabri-
cation of HPbI3 film on a TiO2/FTO substrate was achieved by the spin-
coating method (top panel). Conversion of HPbI3 film to MAPbI3 film
using MAC, where the vapor-phase reaction was performed in an
autoclave reactor filled with the MAC powder (bottom panel).
a dense surface morphology without pin-holes and appears to
mostly consist of grains larger than 1 mm.
Solvent-free synthesis of aromatic imines using MAC
XRD measurements characterized the phase formation and
crystallinity of MAPbI3 lms prepared by the present method
using MAC. Two sharp and intense peaks appeared at 14.23ꢁ,
and 28.45ꢁ are indexed to (110) and (220) reections of MAPbI3,
respectively (see Fig. 5). The full-width at half-maximum
(FWHM) in the (110) plane is 0.24ꢁ, which is about the same
as those of typical MAPbI3 lms prepared by conventional
methods.26,27 The XRD data show not only a high degree of (110)
type preferential grain growth but also excellent crystallinity of
the MAPbI3 lm. It is worth mentioning that high-quality
perovskite lms with smooth and pinhole-free lms over spec-
ied areas can also be prepared by heat treatment of lead iodide
precursors in the presence of the CH3NH2 gas.28 Methylamine
To demonstrate the generality of the MAC molecule, we inves-
tigated the one-pot reactions of MAC with aromatic aldehydes.
The addition of MAC to the liquid aromatic aldehyde in the
absence of solvents yielded the corresponding imine as a sole
product. A distinctive color change was noticed upon the
addition of MAC. The solution was allowed to react in a vial
without any agitation. The condensation reaction proceeded to
form the C]N double bond with the simultaneous loss of
water, which appears to proceed via the dissociation of MAC
into methylamine and CO2. Nearly complete conversion of
aromatic aldehyde to imine was typically achieved even at 25 ꢁC
1
within 3 h, where the product was carefully analyzed using H
and 13C NMR spectroscopy. Neither by-products nor unreacted
aromatic aldehydes were identied from the reaction mixture,
indicating that the neat reaction is highly selective. Table 1
summarizes the results of the reactions between MAC and
aromatic aldehydes along with detailed reaction conditions.
For comparison, liquid methylamine dissolved in ethanol
was reacted with cinnamaldehyde to determine whether the
methylamine-based reaction also yields the imine selectively.
The reaction was performed in ethanol under the same condi-
tions, which produced not only the imine but also unknown
impurities (Table 2, entry 1). The selectivity towards the imine
was relatively low, which is presumably due to the presence of
ethanol. When MAC was used as a methylamine source, on the
other hand, the yield was well above 97%, and no by-products
gas has been shown to have
a signicant effect on
1
were identied by H NMR analysis. (see Table 2, entry 2). The
neat reaction between MAC and cinnamaldehyde clearly shows
the enhanced selectivity compared to the conventional
methylamine-based method. The reason for the high selectivity
is that methylamine and CO2 gases are rapidly released from
MAC. Methylamine gas is anhydrous and highly reactive. This
one-pot solvent-less synthesis of imine using MAC is unprece-
dented, which provides an environmentally benign approach
Fig. 5 XRD patterns of the MAPbI3 film converted from the HPbI3 film
via vapor-phase reaction using MAC. Asterisk marks indicate diffrac-
tion peaks of TiO2/FTO. The inset shows photographs of HPbI3 (left)
and MAPbI3 (right) films. The images were taken before (left) and after
(right) the reaction between HPbI3 and MAC.
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Table 1 Reactions of MAC with aromatic aldehydesa
Entry
Reactant
Product
Reaction time (h)
3
Yieldb (%)
>99
Remark
1
Yellow liquid
2
3
4
5
3
3
3
3
>99
>99
>99
>97
Yellow liquid
Pale yellow liquid
Pale yellow liquid
Pale yellow liquid
6
7
1
>99
>96
>99
Yellow liquid
Yellow liquid
Yellow liquid
0.5
3
8
a
Reaction conditions: 1 (5 mmol, 0.53 g), carbonyl reactant 2 (10 mmol), no solvent, in a 10 mL vial at 25 ꢁC. b Isolated yield based on MAC was
determined using 1H NMR.
for preparing various imines. Moreover, this reaction is easily a 50 mmol scale under identical conditions. Almost the same
scalable and can produce imine on a large scale in a small yield was obtained (see Table 2, entry 3).
container without separation and purication steps. As an
To convince the formation of an imine (–C]N–) bond, we
illustrative example, the same reaction was performed on grew single crystals of 3-nitrobenzylidene methanamine by the
Table 2 Reactions of cinnamaldehydes with MAC and methylamine in ethanol (33 wt%)a
Entry
Methyl amine (mmol)
CH3NH2 in ethanol (5.0 mmol)
MAC (5.0 mmol)
Product
Yield (%)
<84b
Remark
c
1
2
>97b
Yellow liquid
Yellow liquid
3
MAC (50.0 mmol)
>97c
a
b
Reaction conditions: cinnamaldehyde/methyl amine ¼ 2/1, no solvent, the precursors were reacted at 25 ꢁC for 0.5 h. Isolated yield based on
MAC was determined using 1H NMR. c Product(s) resulting from the reaction(s) at the olen group in cinnamaldehyde were identied by 1H NMR.
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through the grant numbers (NRF-2016K1A4A3914691, NRF-
2019R1A2C1003666). We thank professor Wan In Lee for
providing the TiO2/FTO substrate.
Notes and references
1 D. R. Corbin, S. Schwarz and G. C. Sonnichsen, Catal. Today,
1997, 37, 71–102.
2 Z. Zhou, Z. Wang, Y. Zhou, S. Pang, D. Wang, H. Xu, Z. Liu,
N. P. Padture and G. Cui, Angew. Chem., Int. Ed., 2015, 54,
9705–9709.
3 S. R. Raga, Y. Jiang, L. K. Ono and Y. Qi, Energy Technol.,
2017, 5, 1750–1761.
4 M. Safdari, P. H. Svensson, M. T. Hoang, I. Oh, L. Kloo and
J. M. Gardner, J. Mater. Chem. A, 2016, 4, 15638–15646.
Fig. 6 ORTEP drawing and numbering scheme of 3-nitrobenzylidene
methanamine. Non-hydrogen atoms are shown as 30% probability
displacement ellipsoids. Red, black, blue, and pale pink ellipsoids
represent O, C, N, and H atoms, respectively.
¨
5 J.-H. Im, I.-H. Jang, N. Pellet, M. Gratzel and N.-G. Park, Nat.
Nanotechnol., 2014, 9, 927–932.
6 M. Aresta, Carbon Dioxide as Chemical Feedstock, Wiley-VCH
Verlag GmbH & Co. KGaA, Weinheim, 2010, pp. 121–167 and
references cited therein.
recrystallization of the initial yellow product in pentane. Its
solid-state structure was determined by single-crystal X-ray
diffraction. As illustrated in Fig. 6, the molecular structure is
composed of the methylamine and aromatic aldehyde units
7 N. Della Ca', B. Gabriele, G. Ruffolo, L. Veltri, T. Zanetta and
M. Costa, Adv. Synth. Catal., 2011, 353, 133–146.
8 M. Aresta, D. Ballivet-Tkatchenko, D. B. Dell'Amico,
M. C. Bonnet, D. Boschi, F. Calderazzo, R. Faure, L. Labella
and F. Marchetti, Chem. Commun., 2000, 1099–1100.
9 M. F. Garbauskas, R. P. Goehner and A. M. Davis, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 1983, 39, 1684–
1686.
10 D. B. Dell'Amico, F. Calderazzo, L. Labella, F. Marchetti and
G. Pampaloni, Chem. Rev., 2003, 103, 3857.
11 Y. Liu, Y. Tang, N. N. Barashkov, I. S. Irgibaeva, J. W. Y. Lam,
R. Hu, D. Birimzhanova, Y. Yu and B. Z. Tang, J. Am. Chem.
Soc., 2010, 132, 13951–13953.
12 D. J. Heldebrant, P. K. Koech, M. T. C. Ang, C. Liang,
J. E. Rainbolt, C. R. Yonkera and P. G. Jessop, Green Chem.,
2010, 12, 713–721.
13 T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007, 107,
2365–2387.
14 P. G. Jessop, D. J. Heldebrant, X. W. Li, C. A. Eckert and
C. L. Liotta, Nature, 2005, 436, 1102.
15 T. C. Drage, C. E. Snape, L. A. Stevens, J. Wood, J. Wang,
A. I. Cooper, R. Dawson, X. Guo, C. Satterley and R. Irons,
J. Mater. Chem., 2012, 22, 2815–2823 and references cited
therein.
˚
linked by a N]C double bond. The C2–N1 bond length (1.263 A)
of the molecule is considerably shorter than that of free
methylamine, which is an intermediate between single (C–N)
and triple (C^N) bonds. It is quite similar to those of typical
C]N double bonds.31–34 Accordingly, the formation of imine
group is evident. Details of the crystal structure of 3-nitro-
benzylidene methanamine including bond lengths and angles
are given in Tables S3 and S4 in the ESI.†
Conclusion
We have demonstrated the excellent reactivity of a newly
synthesized methyl ammonium methyl carbamate (MAC) solid
towards inorganic compounds for the preparation of hybrid
perovskite MAPbI3 and also organic compounds for the
synthesis of imines. MAC turned out to be a highly effective
alternative to toxic methylamine. A remarkable benet of MAC
is that it enables us to use the precise amount of methylamine
as well as it can be used safely in a laboratory. This one-pot
solvent-less process provides an environmentally benign
synthesis of various methylamine-based materials under rela-
tively mild conditions, which surpasses gas-based or liquid-
based methods in environmental and economic aspects. Also,
this approach renders the scalable production of methylamine-
based materials on a large scale.
16 N. Du, H. B. Park, G. P. Robertson, M. M. Dal-Cin, T. Visser,
L. Scoles and M. D. Guiver, Nat. Mater., 2011, 10, 372–375.
17 G. T. Rochelle, Science, 2009, 325, 1652–1654.
18 H. Kayi, R. I. Kaiser and J. D. Head, Phys. Chem. Chem. Phys.,
2011, 13, 11083–11098.
19 W. Buijs and S. d. Flart, Ind. Eng. Chem. Res., 2017, 56,
12297–12304.
20 K. V. Manukyan, A. V. Yeghishyan, D. O. Moskovskikh,
J. Kapaldo, A. Mintairov and A. S. Mukasyan, J. Mater. Sci.,
2016, 51, 9123–9130.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
ˇ
´
21 D. Prochowicz, M. Franckevicius, A. M. Cieslak,
¨
´
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIT)
S. M. Zakeeruddin, M. Gratzel and J. Lewinski, J. Mater.
Chem. A, 2015, 3, 20772–20777.
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RSC Advances
Paper
22 A. Karmakar, M. A. Abdelrahman, M. G. Guy, V. T. Victor, 29 D. Bogachuk, L. Wagner, S. Mastroianni, M. Daub,
H. Michelle, P. Sahil, S. Karthik and K. M. Vladimir, Chem.
Mater., 2018, 30, 2309–2321.
H. Hillebrecht and A. Hinsch, J. Mater. Chem. A, 2020, 8,
9788–9796.
´
23 O. Dolotko, J. W. Wiench, K. W. Dennis, V. K. Pecharsky and 30 C. A. Lopez, C. Abia, J. E. Rodrigues, S.-S. Federico,
´ ´
N. M. Nemes, J. L. Martınez, M. T. Fernandez-Dıaz,
V. P. Balema, New J. Chem., 2010, 34, 25–28.
24 B. Lee, K. H. Lee, B. W. Lim, J. Cho, W. Nam and N. H. Hur,
Adv. Synth. Catal., 2013, 355, 389–394.
ˇ
´
N. Biskup, C. Alvarez-Galvan, F. Carrascoso, A. Castellanos-
Gomez and J. A. Alonso, Sci. Rep., 2020, 10, 11228.
25 I. S. Yang, S. Lee, J. Choi, M. T. Jung, J. Kim and W. I. Lee, J. 31 N. Arai, N. Utsumi, Y. Matsumoto, K. Murata, K. Tsutsumi
Mater. Chem. A, 2019, 7, 6028–6037. and T. Ohkuma, Adv. Synth. Catal., 2012, 354, 2089–2095.
26 M. Long, T. Zhang, H. Zhu, G. Li, F. Wang, W. Guo, Y. Chai, 32 J. Waser, B. Gaspar, H. Nambu and E. M. Carreira, J. Am.
W. Chen, Q. Li, K. S. Wong, J. Xu and K. Yan, Nano Energy,
2017, 33, 485–496.
27 D. L. Jacobs and L. Zang, Chem. Commun., 2016, 52, 10743–
10746.
28 L. K. Ono, R. R. Sonia, M. Remeika, A. J. Winchester,
G. Atushi and Y. Qi, J. Mater. Chem. A, 2015, 3, 15451–15456.
Chem. Soc., 2006, 128, 11693–11712.
33 J. F. Larrow, E. N. Jacobsen, Y. Gao, Y. Hong, X. Nie and
C. M. Zepp, J. Org. Chem., 1994, 59, 1939–1942.
34 H.-K. Fun, H. Kargar and R. Kia, Acta Crystallogr., Sect. E:
Struct. Rep. Online, 2008, 64, O1894.
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