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Insights into the Relationship between CO2-Switchability and Basicity: Examples of Melamine and its Derivatives Hongyao Yin, Yujun Feng, Hanbin Liu, Meng Mu, and Chenghong Fei Langmuir, Just Accepted Manuscript • DOI: 10.1021/la501461n • Publication Date (Web): 04 Aug 2014 Downloaded from http://pubs.acs.org on August 12, 2014
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1
Insights into the Relationship between CO2-
2
Switchability and Basicity: Examples of Melamine
3
and its Derivatives
4 † †§ †§ †§ Hongyao Yin, ,§ Yujun Feng,*,†,‡ Hanbin Liu, , Meng Mu, , and Chenhong Fei ,
5
6 7
†
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR
8 9
China. ‡
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan
10 11
University, Chengdu 610065, PR China. §
University of the Chinese Academy of Sciences, Beijing 100049, PR China.
12 13
Keywords: CO2-switchability; melamine derivatives; basicity; protonation–deprotonation
14
CO2-switchability of melamine and its derivatives
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Abstract
2
Owing to its wide availability, non-toxicity, and low-cost, CO2 working as a trigger to reversibly
3
switch material properties, including polarity, ionic strength, hydrophilicity, viscosity, surface
4
charge, and degree of polymerization or crosslinking, has attracted an increasing attention in
5
recent years. However, quantitative correlation between basicity of these materials and their
6
CO2-switchability has been less documented though it is of great importance for fabricating
7
switchable system. In this work, the “switch-on” and “switch-off” abilities of melamine and its
8
amino-substituted derivatives by introducing and removing CO2 are studied, and then their
9
quantitative relationship with basicity is established, so that performances of other organobases
10
can be quantitatively predicted. These findings are beneficial for forecasting the CO2-stimuli
11
responsive behavior of other organobases and the design of CO2-switchable materials.
12
Introduction
13
Since the pioneering work of Jessop team1 on CO2-switchable solvents, CO2, as a novel and
14
environmentally-benign trigger, has been playing an active role in fabricating switchable
15
surfactants,2,3 solutes,4 polymers,5−7 ionic liquids,8−11organogels,12,13 wormlike micelles,14−16 as
16
well as nano hybrids,17 all of which possess at least one basic group mainly including
17
amidine,1,2,5,18 guanidine3,19 or amine20−23 to react with CO2 in the presence of water. Among
18
these organobases, guanidine is relatively less used because of its “superbase” character which
19
makes it hard to be “switched-off” once it is reacted with CO2.3 Although their basicity is weaker
20
than that of guanidine, amidine-containing compounds are normally difficult to be synthesized
21
and prone to be hydrolytically instable,24,25 and amidine-based monomers display poor
22
polymerization activity, which seriously limits its application in polymers.5 On the contrary, as a CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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mild, stable, and readily available organobase, amines are the most commonly used CO2-
2
switchable functional groups.14,16,26,27 Generally, their protonated products, especially those
3
generated from the reaction of tertiary amines with CO2 in a wet or aqueous environment, are
4
unstable and easy to release CO2 to return to the original state even at room temperature,16
5
indicative of better reversibility and environmentally-benign nature.3 Several earlier
6
studies3,5,25,28,29 described the significant influence that the intrinsic basicity of organobases
7
exerts on their CO2-switchability. And Jessop group,3,28 reported that bicarbonate salts from less
8
basic compounds show better reversibility than those from more basic compounds. However, the
9
quantitative correlation between basicity and switchability has not been established possibly for
10
the lack of homologous compounds with a wide range of basicity, yet it is of great importance
11
for fabricating switchable systems. Furthermore, most of the amines in previous studies are
12
linear compounds that usually bear one single amino group. And cyclic polyamines containing
13
several or even more amino groups are expected to show high sensitivity and effective CO2-
14
switchability, but few studies have been documented so far.20
15
As a typical cyclic organobase, melamine, bearing an s-triazine ring along which three primary
16
amines are evenly distributed on the side-chains,30 is expected to enjoy strong CO2-sensitivity.
17
More importantly, modification by introducing different groups onto the amino nitrogen is
18
readily done to obtain a series of amino-substituted derivatives with different basicities.31 The
19
homologous structure is helpful to correlate their basicity with CO2-switchability. Although
20
melamine and its derivatives can be protonated by such a common acid as halogen acid and the
21
protonated species are widely employed as self-assembly,32 molecular recognition materials,33
22
flame retardants,34 and drugs,35 there are no reports to date, to our knowledge, focusing on
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protonation of these cyclic organobases using CO2. If CO2-switchability is imparted on these
2
compounds, “smart” response of these materials can be envisaged.
3
Herein, our primary goal is to evaluate the CO2-switchability of melamine and its amino-
4
substituted derivatives (Scheme 1) by examining the protonation form, protonation degree,
5
reversibility, and next, to establish the relationship between basicity and CO2-switchability for
6
this series of specifically-tailored amine compounds. To this end, methyl group was used to
7
replace one of the hydrogen atoms in the amino nitrogen of the melamine to give N2-methyl-
8
1,3,5-triazine-2,4,6-triamine (2), and then more complicated dimethylamino alkyl groups were
9
used as the substituents to afford N2-(2-(dimethylamino)ethyl)-1,3,5-triazine-2,4,6-triamine (4)
10
and N2-(3-(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (5), respectively. Furthermore,
11
tri-substituted derivatives, N2,N4,N6-trimethyl-1,3,5-triazine-2,4,6-triamine (3), N2,N4,N6-tris(2-
12
(dimethylamino)ethyl)-1,3,5-triazine-2,4,6-triamine
13
(dimethylamino)propyl)-1,3,5-triazine-2,4,6-triamine (7), were prepared for comparison.
14
Different substituents are expected to endow the derivatives with different basicities, thus allow
15
the correlation with their CO2-switchability.
(6)
and
16 17
1
2
18
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N2,N4,N6-tris(3-
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4
5
2 3
6
7
4
Scheme 1. Molecular structures of melamine and the series of specifically-tailored amino-
5
substituted derivatives used in this work.
6
Experimental Details
7
Materials
8
2-Chloro-4,6-diamino-1,3,5-trazine
(99%),
cyanuric
chloride
(99%),
2-
9
dimethylaminethylamine (99%), 3-dimethylaminopropylamine (99%), methylamine (40wt% in
10
water), were all purchased from Sigma–Aldrich and used without further treatment. Water that
11
was triply distilled by a quartz water purification system was used throughout this study. CO2
12
(≥99.998%) and N2 (99.998%) were used as received. The other reagents and solvents were
13
obtained from Shanghai Chemical Reagent Co., Ltd., and the solvents used in the reactions were
14
distilled prior to use.
15
Synthesis
16
The compounds 2, 4 and 5 were prepared by using 2-chloro-4,6-diamino-1,3,5-trazine to react
17
with methylamine, 2-dimethylaminethylamine and 3-dimethylaminopropylamine, respectively; 3,
18
6 and 7 were obtained in a similar manner but with cyanuric chloride as the starting material
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instead. Detailed synthesis and characterizations of these six compounds are given in the Figures
2
S1–S26 of Supporting Information (SI).
3
Characterizations
4
1
H and
13
C NMR spectra were recorded at 25 °C on a Bruker AV300 NMR spectrometer at
5
300 and 75 MHz, respectively. Chemical shifts (δ) were reported in parts per million (ppm) with
6
reference to the internal standard protons of tetramethylsilane (TMS). The NMR spectra of the
7
CO2-treated compounds were obtained by directly bubbling CO2 into an NMR tube with about 6
8
mg compound in the 0.6 mL solvent.
9 10
ESI–HRMS spectra were obtained with the Bruker Daltonics Data Analysis 3.2 system. Infrared spectra were registered on a Nicolet MX-1E FTIR spectrometer in the scanning range
11
of 4000–400 cm–1 using KBr pellet method.
12
Monitoring of pH Value
13
The variation of pH value of the base aqueous solutions (20 mL, 11.9 mM; all the bases in this
14
work can be fully dissolved in water and display obvious CO2-responsiveness at this
15
concentration) under CO2 bubbling was continuously monitored at 25 °C with a Sartorius basic
16
pH-meter PB–10 (± 0.01) calibrated with standard buffer solutions. The CO2 flow rate was fixed
17
at 3 mL⋅min–1.
18
Determination of pKaH value
19
The pKaH (pKa of the protonated species) values of the seven compounds were determined by
20
titrating 20 mL of 11.9 mM aqueous solutions with 100 mM hydrochloric acid, and the pH was
21
continuously monitored at 25 °C with the same pH meter mentioned above. The pKaH values
22
were obtained by taking the pH readings at the mid-point between two pH jumps.14 CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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Conductivity measurement
2
The conductivity (κ) of 20 mL 11.9 mM base aqueous solution was recorded with an EF30
3
conductometer (Mettler Toledo, USA) at 25 °C while bubbling CO2 or N2 through a syringe
4
needle with inner diameter of 1.5 mm. The large CO2 flow rate leads the switch “on” process too
5
fast and the experimental data are hard to be well recorded, whereas the small N2 flow rate
6
makes the switch “off” process too slow and takes too much time to measure; thus we fixed the
7
CO2 and N2 at different rates, 3 mL⋅min–1 for CO2 and 15 mL⋅min–1 for N2. The conductivity of
8
pure water under the same condition was also monitored as a reference.
9
UV–vis spectroscopy
10
The absorbance of base aqueous solutions with concentration of 1.2×10–5 M was recorded on a
11
double-beam UV–vis spectrophotometer (UV–4802, Unico, USA) at 25 °C, controlled by a
12
water circulating bath, over the wavelength range 190–450 nm.
13
Deprotonation test
14
The deprotonation ability of protonated amines was demonstrated by both of half time (t1/2, the
15
time when protonation degree δ is lowered to half of the maximum value upon the CO2 treatment)
16
and basicity. 20 mL 11.9 mM base aqueous solutions were prepared in 25-mL glass vials without
17
cap, followed by CO2 bubbling until the pH value of solutions remained unchanged. Then N2 at
18
the flow rate of 15 mL⋅min–1 was bubbled into the solutions via a syringe needle with inner
19
diameter of 1.5 mm at the set temperature. The pH was measured at set intervals when the
20
solutions were cooled down to 25 oC and then the protonation degree was calculated according to
21
Eqs. (1) and (2) which appear later.
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Results and Discussion
2
Protonation forms of melamine and its derivatives
3
To study the CO2-switchability of melamine and its derivatives, their protonation forms were
4
investigated firstly. An earlier study36 showed that melamine forms positive ions by assuming a
5
structure with one exocyclic double bond to an =NH2+ group when it is protonated, and the
6
amino-substituted derivatives act in the same manner. Meanwhile, the new conjugated structure
7
of protonated melamine will cause a new absorption band at around 240 nm in UV–vis spectra.37
8
Hence, it is easy to investigate the protonation forms of these melamine derivatives via
9
measuring their UV–vis absorbance upon treatment with CO2. The spectra of compound 1 and 6
10
in aqueous solution (Figure 1) before and after bubbling of CO2 are shown here as an example,
11
and the others can be found in Figure S27 of SI.
12 13
Figure 1. UV–vis spectra of melamine (1) and its derivative (6) in aqueous solution before and
14
after introducing CO2.
15
A new absorbance around 240 nm after bubbling CO2 was observed for the amines except 6
16
and 7, and the initial absorbance was still in existence, implying their melamine structure is CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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partially protonated by carbonic acid. Then the NMR spectroscopy characterization was
2
employed to further confirm the protonation forms. Since the protonated structure of melamine is
3
clear and its amino-substituted derivatives follow the same pattern, the protonation forms of 2, 3
4
as well as the mother structure of 4, 5 are deduced and illustrated in Scheme 2. However, no
5
obvious signal changes were observed in both 1H and 13C NMR spectra of 1, 2 and 3 between the
6
original and those treated by CO2 in wet DMSO-d6. To uncover the cause, the 1H and 13C NMR
7
spectra of compounds 1, 2 and 3 with one equivalent of added HCl (mono-protonation on
8
average) were measured and compared with the initial spectra (Figures S28-30, SI). The signals
9
of these mono-protonated samples experienced an obvious shift in both the 1H and
13
C NMR
10
spectra, demonstrating that the lack of an evident change of NMR spectra upon CO2 addition
11
results from the small protonation degree.
12
In contrast, the changes in both 1H and 13C NMR signals for 4, 5, 6, and 7 are quite obvious.
13
Here take compound 4 as an example for analysis. The peak at 2.08 ppm in the 1H NMR
14
spectrum of 4 (Figure 2a) is attributed to the gemini terminal –CH3 groups (Figure 2b). Two
15
triplets were observed at 2.35 and 3.23 ppm, which were assigned to the carbons 2a and 3a in the
16
methylene group, respectively. After bubbling CO2, obvious change in the chemical shift of 1H
17
NMR spectrum was observed, where the peaks of 1a and 2a have shifted downfield by around
18
0.7 ppm while the peak of 3a has shifted downfield only by 0.2 ppm. For the purpose of
19
comparison, the 1H NMR and
20
examined (Figures S31, SI). The signal of carbons 1a, 2a and 3a in 1H NMR spectrum
21
experienced a more obvious shift than that when CO2 was bubbled, indicating that CO2 cannot
22
fully protonate these tertiary amino groups in the mixed solvent of DMSO-d6 and D2O.
13
C NMR spectra with one equivalent of added HCl were
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Meanwhile, it also demonstrates the protonation only on the tertiary amino group by comparing
2
the 13C NMR spectra. a) 5a
1a
3a N N
1a
3
N
2a H 4a
HCO3 -
NH2 CO2 , H 2O
N N
NH2
5b
1b NH2
5a
3b N NH
1b
N
2b H
N N
4b 5b
NH2
4
5 6
Figure 2. Comparison of NMR spectra of compound 4 in the mixed solvent of DMSO-d6 and
7
D2O (5:1, v/v) with and without CO2 treatment. (a) Numerical assignment of 4 and
8
corresponding protonation form; (b) 1H NMR spectrum; and (c) 13C NMR spectrum.
9
Additionally, a new peak around 160.0 ppm is evidenced in
13
C NMR spectrum (Figure 2c),
10
implying HCO3– was formed. In short, the chemical shifts in both 1H and 13C spectra distinctly
11
demonstrate that tertiary amino moiety in melamine derivatives can react with CO2 in the
12
presence of water to generate corresponding ammonium bicarbonate salt.15,27 CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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The NMR spectra of compounds 5, 6 and 7 also demonstrate that the tertiary amino group can
2
be protonated by carbonic acid and converted to corresponding bicarbonate salts. Their
3
comparative NMR spectra with and without CO2 were listed in Figures S32–S34 of SI. The
4
protonated structure of them are shown in Scheme 2.
5 6
1
2
3
7 8
4
5
9 10
6
7
11
Scheme 2. Protonation forms of melamine and its derivatives.
12
Basicity of melamine and its derivatives
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The key to establishing the quantitative relationship between basicity of organobases and their
2
CO2-switchability is to obtain homologous compounds with gradually varied basicity. Therefore,
3
the basicity of different amines used in this study was determined.
4
Generally, the basicity of organobases is characterized by pKaH value, and the larger the pKaH,
5
the stronger the basicity is. For instance, the pKaH of most small molecule amines, such as N,N-
6
dimethyloctylamine, benzylamine and 4-aminopyridine, is usually centered at 9,3,20,38 while the
7
pKaH value of short-chain alkyl-substituted guanidine derivatives approaches 14 because they are
8
widely recognized as super organobases.3,25,39 The pKaH values of the seven bases and the
9
corresponding reaction sites are shown in Scheme 2. The original titration curves are depicted in
10
Figure S35 of SI.
11
The pKaH values of these amines range from 4.1 to 9.4. It is worth noting that the three tertiary
12
amino groups of compounds 6 and 7 should have three different pKaH values; however, the
13
method used in this work was unable to resolve them, so that they present a merged value.
14
Moreover, the compounds 1, 2, and 3 just have a single pKaH value, implying that only one type
15
of amino group is protonated in this case, while the compounds 4, and 5 have two pKaH values,
16
indicating that two kinds of amino groups are protonated.
17
As a typical weak base, melamine shows pKaH value of about 5.0, which is in good line with
18
the previously reported results.37,40 The pKaH value of the compound 2, 5.3, is also in good
19
agreement with the previously reported data.36,41 According to the protonation forms, the single
20
smaller pKaH value of compounds 1, 2 and 3 in this work is attributed to the protonation of the
21
melamine structure while the larger pKaH value of compound 6 results from the protonated
22
tertiary amines of the three branches. Consequently, it can be concluded that the larger pKaH
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values of 4, 5 and 7 are produced from the protonation of tertiary amines, whereas the smaller
2
pKaH values are stemmed from the protonation of melamine ring as shown in Scheme 2.
3
Based on the above results, the basicity of amino-substituted derivatives increases with the
4
incorporation of electron-donating group such as methyl group on the melamine structure. In
5
comparison of 2 and 3 with 1, the increase of basicity appears to be directly related to the
6
number of electron-donating substituents. On the other hand, the basicity of melamine ring
7
decreases with the connection of electron-withdrawing groups, for example, the protonated
8
dimethylamino alkyl group used here, with amino nitrogen in melamine, and the reduction is also
9
directly related to the number of electron-withdrawing substituents. This can be demonstrated by
10
the pKaH values of 1, 4, 5, 6 and 7, because once the tertiary amine is protonated, it converts into
11
an electron-withdrawing group that exerts a negative effect on the mother structure. By
12
comparing 4, 5, 6 and 7, it is evident that the basicity of melamine ring follows the order 5 (4.7)
13
> 4 (4.5) and 7 (4.1) > 6 (too weak to be measured), which can be explained by the fact that the
14
alkyl chains of 4 and 6 are shorter than those in 5 and 7, so the protonated tertiary amine poses
15
larger negative effect on the melamine structure and leads to the smaller pKaH value.
16
Protonation degree of melamine and its derivatives
17
To uncover their CO2-responsive capability in water, the protonation degree of these bases was
18
determined by titration. Once the dissociation constant (Ka) of conjugated acid and pH value of
19
solution are obtained, the protonation degree (δ) of organobase can be calculated according to
20
Eqs. (1) and (2):5
21
Ka =
[B][H + ] [BH + ]
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(1)
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1
(2)
2
where [B] and [BH+] are the concentration of non-protonated base and protonated base,
3
respectively.
4
The pH value of 11.9 mM aqueous solutions was continuously monitored at 25 °C during the
5
CO2 bubbling process until saturation, i.e., the pH value remains stable. The calculated
6
protonation degrees are shown in Table 1. It should be noted that the concentration of
7
organobases and the temperature can exert effects on the pH and corresponding protonation
8
degree, thus, the pH and protonation degree discussed below are only valid at the set
9
concentration and temperature mentioned above. The variation of pH with time (the volume of
10
CO2 bubbled) is detailed in Figure S36 of SI.
11
Table 1. Protonation degree of 11.9 mM solutions of melamine and its derivatives at 25 °C Amine
1
2
3
4
5
6
7
pKaH
5.0
5.3
5.8
8.9; 4.5
9.4; 4.7
8.2
8.7; 4.1
pH
5.64
5.70
5.80
6.15
6.24
6.30
6.30
δ (%)
18.6
28.5
50.0
99.8; 2.2
99.9; 2.8
98.7
99.6; 0.6
12 13
Combined with the basicity of these amines, the protonation degree is found to be highly
14
dependent on the basicity. A clear relation is that the larger the basicity, the higher the
15
protonation degree, which is in good line with the Henderson-Hasselbalch equation.42 More
16
interestingly, different basicity of organobases has different protonation degree by CO2,
17
suggesting that a proper base can be chosen to meet the requirement of precise protonation for
18
particular applications. For example, Yuan and co-workers5 reported a diblock copolymer CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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composed of a poly(ethylene oxide) block and a polyacrylamide one bearing an amidine side
2
group (PAD) can be used to fabricate CO2-switchable vesicle with biommimetic “breathing”
3
feature. When dissolved in water, it can self-assemble into vesicles, and cause 37% of the PAD
4
units to be charged upon the bubbling of CO2, which forces the vesicles to swell, whereas the
5
uncharged PAD species still serve as the inner layer to lock the vesicle structure and prevent it
6
from dissociation. The key factor governing the “breathing” vesicle is the precise protonation
7
degree of PAD upon CO2 stimuli. Should the relationship between basicity and protonation
8
degree be known, it undoubtedly helps to choose the right base to meet the accurate protonation
9
degree. Prompted by this understanding, an attempt was made to establish the relationship based
10
on the above data (Figure 3).
11 12
Figure 3. The relationship of basicity and protonation degree for the melamine derivatives in
13
water.
14
As exhibited in Figure 3, the protonation degree is plotted as a function of pKaH to study their
15
inherent relationship. One should bear in mind that such a correlation is generally based on
CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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Yin Page 16 1
compounds 1, 2 and 3 owing to their conversions to the bicarbonate salts spread over a much
2
wider range, allowing to correlate the protonation degree and basicity.
3 4 5
From the inset in Figure 3, one can find that the protonation degree varies linearly with pKaH below 5.8 as δ = –180.3 + 39.6pKaH
(3)
6
According to eq. (3), the protonation degree can be calculated to be close to 100% when pKaH
7
hits 7.1. In other words, the organobase will be almost entirely protonated in water with bubbling
8
CO2 when its pKaH value is higher than 7.1. But actually, the basicity of 4, 5 and 6, 7 is larger
9
than 7.1, which makes a little difference in the conversion.
10
Reversibility of melamine and its derivatives
11
CO2-switchable bases normally display inconsistent switch “on” and “off” abilities, and
12
different bases usually show different reversibility in light of the reported studies.3,28 Generally,
13
the “switch-on” process upon CO2 bubbling is easier and faster than the “switch-off” one which
14
usually not only requires longer time of inert gas bubbling but also needs higher temperature in
15
some cases, and even irreversible for some strong bases. For instance, our previous study43 found
16
natural sodium erucate can form high-viscoelastic wormlike micelles in water and shift to low-
17
viscosity solutions in short time when CO2 is introduced. However, bubbling N2 while heating
18
cannot make a converse process. On the contrary, streaming CO2 into the solution of N-
19
erucamidopropyl-N,N-dimethylamine gives rise to the high viscoelastic solution, and it can be
20
converted to the initial state only through bubbling air at ambient temperature.16
21
The difference between switch “on” and “off” processes plays a crucial role in fabricating
22
CO2-switchable systems. On the one hand, easy reverse brings less stability; on the other hand,
CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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difficult conversion is hard to impart switchability. Thus the reversibility of these amines were
2
examined by conductometry in this work (Table 2).
3
As shown in Figure 4, the conductivity of compound 7 in aqueous solution increases
4
significantly upon CO2 streaming, ascending from 850 µS⋅cm–1 to 2730 µS⋅cm–1 (the
5
corresponding figures of other compounds are shown in Figure S37, SI). Displayed in Figure 5
6
are the snapshots in wet dichloromethane when sequentially bubbling CO2 and N2 inside. Since
7
the variation of ionic strength exerts more obvious effect in solubility in organic solvent
8
compared to water for compound 7, dichloromethane instead of water was used here. When CO2
9
is bubbled, the initial solution becomes cloudy, which is ascribed to the dramatic increase of the
10
ionic strength; and then the removal of CO2 by bubbling N2 makes the suspension clear again,
11
which results from the drop of the ionic strength. Importantly, the time needed to “switch-on”
12
amine aqueous solutions is shorter than that of “switch-off” process even the CO2 bubbling rate
13
is smaller than that of N2. For example, it takes only 12 min for the solution of 4 to be protonated,
14
while 79 min is needed in deprotonation process. Interestingly, as depicted in Table 2, the
15
protonation equilibrium time (the time taken to reach a stable conductivity value) by bubbling
16
CO2 for all of these bases is very close, but much different deprotonation equilibrium time by
17
displacing CO2 with N2 is evidenced, manifesting that the location and number of substitutes
18
along the melamine ring make variable reversibility.
19 20
Table 2. Conductivity (κ) of different stages and protonation, deprotonation time of 11.9 mM
21
different bases in pure water
CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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Page 18 of 30
Yin Page 18 Compound κ0*
1 2 3
tp**
tdep**
κCO2*
κN2*
(µS⋅cm-1)
(µS⋅cm-1)
(µS⋅cm-1) (min)
(min)
water
1
52
10
8
20
1
8
330
26
9
19
5.0
2
5
440
50
13
22
5.3
3
17
550
55
14
35
5.8
4
210
1070
477
12
79
8.9
5
266
1020
486
12
77
9.4
6
580
1770
683
13
105
8.2
7
846
2700
1177
12
105
8.7
pKaH
* κ0, κCO2, κN2 refer to the conductivity of the initial solutions, and those treated by CO2 or N2 to equilibrium (no longer change with time). ** tp and tdep denote the equilibrium time of protonation and deprotonation.
4
Apart from these, it should be noted that the conductivity of these base aqueous solutions has
5
not been entirely switched back to their starting point during the observation time. This might be
6
caused by the incomplete deprotonation and the residual protonated base contributes to the
7
higher conductivity, which implies that it is difficult to completely deplete CO2 using inert gas.
8
Nevertheless, the compounds 1, 2 and 3 show better reversibility than other bases, as supported
9
by the final conductivity values after bubbling N2.
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1 2
Figure 4. Variation of conductivity of compound 7 in water (11.9 mM) when successively
3
treated by CO2 and N2.
4 5
Figure 5. Snapshots of compound 7 in wet dichloromethane.
6
Combining with the basicity of these amines, we can easily explain why conductivity results
7
differ from amine to amine. The maximum conductivity upon bubbling CO2 dramatically
8
increases with the rise of basicity, ascending from 330 µS⋅cm–1 of melamine to 1020 µS⋅cm–1 of
9
compound 5. It should be noted that although compounds 6 and 7 are less basic than compounds
10
4 and 5, respectively, they display much higher conductivity, which originates from the treble
11
CO2-responsive tertiary amino groups they bear. Furthermore, the conductivity of the initial
12
solutions appears to increase from melamine to compound 7. This might stem from their partial
13
protonation by H2O.3 Stronger bases are more easily protonated, which leads the initial CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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conductivity to increase from compound 1 to compound 5. Similarly, 6 and 7 have treble CO2-
2
sensitive groups compared with 4 and 5, respectively, thus, their initial conductivities are almost
3
three times those of 4 and 5. In addition, although their “switch-on” time treated with CO2 does
4
not show direct relation to basicity, the “switch-off” time by N2 is closely related to it. The
5
changes in conductivity of base solutions derive from the absorption and desorption of CO2. The
6
rate limiting step for the absorption process involves CO2 rather than amine,44 so that the
7
“switch-on” time does not depend on amine choice. Essentially, the distinct performance
8
between switch “on” and “off” processes, as well as the different reversibility originate from the
9
different deprotonation capability. The good deprotonation ability implies good reversibility.
10
To gain insight into the mechanism, the deprotonation ability of these amines was investigated
11
at 25, 40 and 60 °C, respectively, with bubbling N2, and the time when the protontaion degree is
12
reduced to half of the maximum value (t1/2) was used to evaluate their deprotonation capability.
13
As shown in Figure 6a, obvious difference was observed in the deprotonation process among
14
these amines. It is found that the time of CO2 desorption increases with the rise of basicity,
15
especially when the pKaH of amine is larger than 8.2, where the t1/2 experiences a dramatic
16
increase. Furthermore, an increase in temperature evidently leads to the decrease of the t1/2
17
because the high temperature weakens the stability of formed ammonium bicarbonate salts and
18
speeds up their deprotonation. For example, the strongest base among these amines, compound 5,
19
whose t1/2 is more than 300 min at 25 oC and still longer than 200 min at 40 °C; however, it
20
declines to only 126 min when temperature is increased to 60 °C. Such a rise in temperature is
21
clearly an effective approach to improve the reversibility. The reversibility, which plays an
22
essential role in CO2-switchable system, is directly dominated by deprotonation ability, but no
23
methods can quantitatively evaluate it so far. Thus, we manage to establish the relationship
CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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between deprotonation ability and basicity to estimate the reversibility by fitting the curves
2
depicted in Figure 6a as t1/2 = Ae(–pKaH/a) + Be(–pKaH/b) + t0
3
(4)
4
where A, a, B, b and t0 are constants, the corresponding value under different conditions are
5
given in Table 3. The corresponding fitting curves can be found in Figure 38 of SI.
6
Table 3. Constants of eq.(4) under different conditions T (°C) With N2 bubbling
A
a –10
B
b
t0
–2.25
–5.74
–10
–0.35
–4.11
–10
–0.37
1.95
–6
–0.63
4.21
–6
–0.61
2.87
25
1.37×10
–0.32
0.69
40
0.33
–1.95
4.29×10
60
5.70×10
–10
–0.37
5.70×10
Without N2
40
7.98×10
–6
–0.637
7.98×10
bubbling
60
3.25×10
–6
–0.61
3.25×10
7 8
According to the fitting results, it can be calculated that 594 and 254 min are needed for
9
protonated 5 to release half of the absorbed CO2 in aqueous solution at 25 °C and 40 °C,
10
respectively, so that we did not observe the t1/2 during the test time, which lasts for a maximum
11
of 200 min.
12
Since high temperature deteriorates the stability of protonation products and some CO2-
13
switchable materials are used above room temperature, the stability of the protonated products at
14
different temperature is of importance and deserves to be examined. Thus, we investigated the
15
thermal stability of these protonated amines using the same method at 40 °C and 60 °C,
16
respectively, but without bubbling inert gas. In contrast to the reversibility, the poor
17
deprotonation ability of ammonium bicarbonate salt demonstrates good thermal stability. As
18
shown in Figure 6b, basicity of amines is also found to play a key role in stabilizing their CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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Page 22 of 30
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protonated species. Stronger basicity imparts the ammonium bicarbonate salt with better stability,
2
and the dramatic increase of t1/2 also occurs around the pKaH value of 8.2. Besides, protonated
3
species from compounds 1, 2 and 3 are very unstable as they are much less basic than other
4
amines. As expected, a temperature increase obviously lowers the stability. For instance, the t1/2
5
was not observed even the test time was more than 40 h at 40 °C; however, it decreases to 32 h
6
when heated at 60 °C. Thus, the effect of temperature must be carefully considered when CO2-
7
switchable compounds are used in practical applications. For example, Crespy and co-workers22
8
employed the protonated N,N-dimethyldodecylamine (pKaH 9.97) as the CO2-switchable
9
surfactant to prepare poly(methyl methacrylate) and polystyrene latexes at 40 °C, 60 °C and 80
10
°C. Since the polymerizations were carried out at higher temperatures, CO2-switchable surfactant
11
with larger basicity was employed. If there is a method to evaluate the thermal stability of
12
protonated bases, practical uses will be easier. With this objective in mind, the curves shown in
13
Figure 6b were fitted, and it is found that the fitting results also satisfy eq. (4). The
14
corresponding parameters are shown in Table 3 and the fitting curves can be found in Figure S39
15
of SI. Hence, the thermal stability of other organobases with known basicity can be
16
quantitatively forecasted according to eq. (4).
17
It is worth noting that the inert gas bubbling evidently accelerates the deprotonation of
18
ammonium bicarbonate salts when Figure 6a is compared with Figure 6b. This might be caused
19
by two reasons. First, the rapidly moved inert gas molecules transfer energy to ammonium
20
bicarbonate salts in solution to promote their decomposition. Moreover, the introduction of inert
21
gas can instantly drive the released CO2 out and avoid its re-dissolution.45
CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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1 2
Figure 6. The relationship of basicity and deprotonation capability for the melamine and its
3
derivatives in water. (a) With N2 bubbling; (b) Without N2 bubbling.
4
In addition, the number of CO2-sensitive group in an amine molecule also plays an important
5
role on its switchability. As shown in Table 2, although the basicity of 4 and 5 is larger than that
6
of 6 and 7, the conductivity of compounds 6 and 7 is respectively much higher than that of 4 and
7
5, and the conductivity change between the initial value and final one after treatment of CO2 is
8
also larger than that of 4 and 5. The large difference between the properties before and after CO2
9
bubbling means effective CO2-switchability, thus organobases with more CO2-sensitive groups
10
show more effective CO2-switchability than the organobases with fewer corresponding groups.
11
But more CO2-sensitive groups take longer time to achieve the final equilibrium value in the
12
deprotonation process in comparison 6, 7 with 4, 5.
13
Conclusion
14
A series of amino-substituted melamine derivatives with different basicities was synthesized
15
and characterized. Then CO2-switchability of melamine and these derivatives were tested by
16
examining the protonation forms, protonation degree, and reversibility. Results showed all these CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment
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Page 24 of 30
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bases have good CO2-responsiveness. However, they display different switch “on” and “off”
2
capabilities, which were found directly related to their basicity. Based on these results, we
3
established the quantitative relationships of basicity–protonation degree and basicity–
4
deprotonation ability of organobases in water. In addition, organobases containing more
5
responsive groups are found to show higher effective CO2-switchability, but they need more time
6
to achieve the equilibrium in deprotonation process. These findings may be useful to choose
7
appropriate basicity and specific numbers of sensitive groups to design CO2-switchable materials
8
to meet the requirements in practical applications.
9
Acknowledgements
10
This work was financially supported by the National Natural Science Foundation of China
11
(21273223, 21173207) and Distinguished Youth Fund from Science and Technology Department
12
of Sichuan Province (2010JQ0029).
13
Supporting Information Available
14
FTIR, NMR and HRMS spectrum of compounds and additional results. This material is
15
available free of charge via the Internet at http://pubs.acs.org.
16
Author Information
17
*Corresponding Author. E-mail:
[email protected] 18
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Table of Contents
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Quantitative relationship between basicity of melamine and its amino-substituted derivatives and
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their CO2-switchability was established. The larger the basicity is, the higher the conversation is,
5
the more difficult the deprotonation process is, and the more stable the protonated species is.
CO2-switchability of melamine and its derivatives ACS Paragon Plus Environment