Eco-friendly synthesis of ionic helical polymers and their chemical properties and reactivity

Article abstract of DOI:10.1039/c8ra05686b

Reaction of N-(2,4-dinitrophenyl)pyridinium chloride (salt(Cl^-)) with sodium dicyanamide (Na(CN)₂N) resulted in anion exchange between Cl^- and (CN)₂N^- to yield a new Zincke salt, salt((CN)₂N^-). Reactions of salt((CN)₂N^-) with piperazine, specifically (R)-(-)- or (S)-(+)-2-methylpiperazine under eco-friendly conditions, such as in aqueous solution, in the absence of a catalyst, and at room temperature, resulted in pyridinium ring opening to yield ionic high-molecular-weight polymers with 5-2,4-dienylideneammonium dicyanamide units or chiral 5-(2-methylpiperazinium)-2,4-dienylideneammonium dicyanamide units, namely, polymer(H;(CN)₂N^-), polymer(R-Me;(CN)₂N^-), and polymer(S-Me;(CN)₂N^-). UV-Vis measurements revealed that the π-conjugation system expanded along the polymer chain due to the orbital interaction between the electrons on the two nitrogen atoms of the piperazinium ring. Circular dichroism (CD) measurements revealed a helical conformation of the main chain in polymer(R-Me;(CN)₂N^-) and polymer(S-Me;(CN)₂N^-). The reaction of polymer(H;(CN)₂N^-) with p-phenylenediamine (PDA) caused recyclization of the 2,4-dienylideneammonium unit and resulted in depolymerization to yield N-(4-aminophenyl)pyridinium dicyanamide. Cyclic voltammetry analysis suggested that the polymers obtained in this study undergo electrochemical oxidation and reduction.

Full text of DOI:10.1039/c8ra05686b

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Eco-friendly synthesis of ionic helical polymers and
their chemical properties and reactivity†
Cite this: RSC Adv., 2018, 8, 29988
Isao Yamaguchi, * Yuki Tanaka and Aohan Wang
ꢀ
Reaction of N-(2,4-dinitrophenyl)pyridinium chloride (salt(Cl )) with sodium dicyanamide (Na(CN)
2
N) resulted
ꢀ
ꢀ
ꢀ
in anion exchange between Cl and (CN)
2
N
to yield a new Zincke salt, salt((CN)
2
N ). Reactions of
ꢀ
salt((CN)
2
N ) with piperazine, specifically (R)-(ꢀ)- or (S)-(+)-2-methylpiperazine under eco-friendly
conditions, such as in aqueous solution, in the absence of a catalyst, and at room temperature, resulted in
pyridinium ring opening to yield ionic high-molecular-weight polymers with 5-2,4-dienylideneammonium
dicyanamide units or chiral 5-(2-methylpiperazinium)-2,4-dienylideneammonium dicyanamide units,
ꢀ ꢀ
ꢀ
namely, polymer(H;(CN) N ), polymer(R-Me;(CN) N ), and polymer(S-Me;(CN) N ). UV-Vis measurements
2
2
2
revealed that the p-conjugation system expanded along the polymer chain due to the orbital interaction
between the electrons on the two nitrogen atoms of the piperazinium ring. Circular dichroism (CD)
ꢀ
measurements revealed a helical conformation of the main chain in polymer(R-Me;(CN)
2
N ) and
Received 3rd July 2018
Accepted 20th August 2018
ꢀ
ꢀ
polymer(S-Me;(CN) N ). The reaction of polymer(H;(CN) N ) with p-phenylenediamine (PDA) caused
2
2
recyclization of the 2,4-dienylideneammonium unit and resulted in depolymerization to yield N-(4-
aminophenyl)pyridinium dicyanamide. Cyclic voltammetry analysis suggested that the polymers obtained in
this study undergo electrochemical oxidation and reduction.
DOI: 10.1039/c8ra05686b
rsc.li/rsc-advances
attributed to the decrease in p-conjugation lengths with the
spontaneous conversion of piperazinium rings from the boat to
Introduction
p-Conjugated polymer main chains generally consist of the chair form.
aromatic rings and unsaturated bonds that build an expanded
The development of non-catalytic syntheses of polymers with
p-conjugation system. The existence of aliphatic cycles in the expanded p-conjugated system along the polymer chain in
polymer main chain usually prevents expansion of the p- water at room temperature is attractive because it offers many
conjugation system along the polymer chain. We reported the advantages with regard to cost, safety, and environmental
ꢀ
ꢀ
synthesis of ionic polymers—polymer(H;Cl ), polymer(R- concerns. The reaction of salt(Cl ) with piperazines in water
ꢀ
ꢀ
Me;Cl ), and polymer(S-Me;Cl )—using ring-opening reactions yielded a mixture of unidentied products. We expect that the
on the pyridinium unit of salt(Cl ) with piperazine or chiral 2- introduction of a hydrophilic anion into a Zincke salt may
ꢀ
1,2
methylpiperazines in reuxing ethanol.
enable reaction with an amine in water, and incorporation of
electron-accepting groups may contribute to the formation of
an intramolecular hydrogen bond with the amine group in the
intermediate adduct, which maintains the boat form of the
piperazinium ring. Based on this hypothesis, we employed
ꢀ
These polymers comprise aliphatic piperazinium rings a Zincke salt with a dicyanamide anion, salt((CN)
2
N ), in this
ꢀ
linked by p-conjugated penta-2,4-dienylideneammonium chlo- study. The reaction of salt((CN)
2
N ) with piperazines in water
ride units. The expanded p-conjugated system along the poly- without a catalyst at room temperature yielded ionic polymers
mer chain is derived from through-space interactions between with high molecular weights. To the best of our knowledge, this
the orbitals of nitrogen electrons of the piperazinium ring with is the rst report of a non-catalytic synthesis of ionic polymers
the boat form. However, the UV-Vis absorption of the polymers with expanded p-conjugated system along the polymer chain
shied to a shorter wavelength when the solutions were allowed via the reactions under environmentally-friendly conditions.
to stand in air. We assumed that the spectral changes were The eco-friendly synthesis of such polymers will lead to new
methods for the development of functional materials.
Herein, we report the synthesis of ionic polymers with
Department of Chemistry, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, expanded p-conjugated system along the polymer chain from
Japan. E-mail: iyamaguchi@riko; shimane-u.ac.jp
Electronic supplementary information (ESI) available: H, C NMR, ESI-TOF
MS, IR, UV-Vis spectra and viscosity data. See DOI: 10.1039/c8ra05686b
a monomer with a dicyanamide anion and the chemical
1
13
†
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structures, reactivity, viscosities, optical properties, and elec- precipitate was collected by ltration, washed with ether twice
ꢀ
1
trochemical properties of the resulting polymers.
and dried in vacuo to afford model((CN) N ) (0.28 g, 74%). H
2
NMR (400 MHz, DMSO-d ): d 7.90 (d, J ¼ 12.0 Hz, 2H), 7.51 (t, J ¼
6
1
2.4 Hz, 1H), 7.24–7.28 (m, 4H), 7.00 (d, J ¼ 8.0 Hz, 4H), 6.84 (t, J
Experimental
General
¼
7.2 Hz, 2H), 6.05 (t, J ¼ 12.4 Hz, 2H), 3.73 and 3.76 (8H), 3.32
13
(
6
8H). C NMR (125 MHz, DMSO-d ): d 163.0, 160.0, 150.0, 129.0,
ꢀ
3
Salt(Cl ) was prepared according to the literature. Other 119.6, 116.0, 102.8, 53.3, 48.7, 47.6, 45.8. ESI TOF-MS: calcd for
reagents were purchased and used without further purication.
Solvents were dried, distilled, and stored under N . Reactions
were carried out using standard Schlenk techniques under solution (3 mL) of polymer(H;(CN)
nitrogen.
C
25
H
31
N
4
(cation), 387.2543. Found, 387.2560.
ꢀ
2
Reaction of polymer(H;(CN)
2
N ) with PDA. Aer the DMSO
ꢀ
N ) (0.43 g, 2.0 mmol) and
2
PDA (0.22 g, 2.0 mmol) was stirred at room temperature for 72 h,
IR and NMR spectra were recorded on a JASCO FT/IR-660 the precipitate from the solution was removed by ltration, and
PLUS spectrophotometer and JEOL AL-400 and ECX-500 spec- the ltrate was concentrated under vacuum. The resulting paste
trometers, respectively. The IR measurement was conducted was extracted with methanol (200 mL), the solvent was removed
using a KBr method. UV-Vis and CD spectra were obtained by by evaporation and dried in vacuo to yield N-(4-aminophenyl)
1
a JASCO V-560 spectrometer and a JASCO J-720WS, respectively. pyridinium dicyanamide (APD) (0.34 g, 71%). H NMR (400
Multi angle light scattering (MALS) and GPC measurements MHz, DMSO-d
6
): d 9.17 (d, J ¼ 6.0 Hz, 2H), 8.62 (t, J ¼ 8.0 Hz,
were conducted with a SHOKO science DAWN HELEOS II and 1H), 8.20 (d, J ¼ 6.8 Hz, 2H), 7.49 (d, J ¼ 7.2 Hz, 2H), 6.75 (d, J ¼
a TOSO HLC-8220 with polystyrene gel columns (Shodex LF-804) 8.8 Hz, 2H), 5.90 (s, 2H). ESI TOF-MS calcd for C13H N
11 4
using DMF containing 0.06 M LiBr as an eluent, respectively. (cation): 171.0917. Found: 171.0924.
ꢀ
Cyclic voltammetry was performed on a DMSO solution con-
Reaction of model(H;(CN)
2
N ) with PDA. Aer the acetone
ꢀ
taining 0.10 M [Et N]BF with a Hokuto Denko HSV-110. 1 cm ꢁ solution (3 mL) of model(H;(CN)
2
N ) (0.045 g, 0.10 mmol) and
4
4
1
cm and 1 cm ꢁ 2 cm Pt plates and Pt wire were used as PDA (0.011 g, 0.10 mmol) was stirred at room temperature for
1
working, counter, and reference electrodes, respectively.
48 h, the solvent was removed under vacuum. The H NMR and
ꢀ
2
N ). Salt(Cl ) (2.82 g, 10.0 mmol) was ESI TOF-MS measurements suggested that the resulting solid
ꢀ
Synthesis of salt((CN)
dissolved in 10 mL of MeOH and sodium dicyanamide (1.07 g, contains APD and 1-(4-aminophenyl)piperazine (APP). H NMR
2.0 mmol) was added to the solution under N
. Aer the data of APP (400 MHz, DMSO-d ): d 6.89–6.96 (m, 5H), 3.12 (t, J
solution was stirred at room temperature for 18 h, the precipi- ¼ 4.8 Hz, 1H), 3.02 (t, J ¼ 4.8 Hz, 4H), 2.83 (t, J ¼ 5.2 Hz, 4H). ESI
tate was ltered out, and the ltrate was concentrated under TOF-MS: calcd for C13 (cation of APD), 171.0917; [APP +
vacuum. The resulting orange solid was extracted with acetone H ], 163.1235. Found, 171.0923, 163.1234.
1
1
2
6
11 4
H N
+
(
200 mL). The solvent was removed by evaporation and dried
ꢀ
1
under vacuum to afford salt((CN)
2
N ) (3.09 g, 99%). H NMR
Results and discussion
): d 9.38 (d, J ¼ 6.8 Hz, 2H), 9.13 (t, J ¼
13
(
1
400 MHz, DMSO-d
.6 Hz, 1H), 8.93–9.00 (2H), 8.41–8.46 (3H). C NMR (125 MHz,
6
Synthesis
6
DMSO-d ): d 149.2, 148.9, 146.1, 143.1, 138.8, 131.9, 130.3,
ꢀ
Reaction of N-(2,4-dinitrophenyl)pyridinium chloride (salt(Cl ))
1
28.1, 121.5, 119.1.
Synthesis of polymer(H;(CN)
mmol) and piperazine (0.086 g, 1.0 mmol) were dissolved in
mL H O under N . Aer the solution was stirred at room
with sodium dicyanamide resulted in anion exchange between
Cl and (CN) N to yield N-(2,4-dinitrophenyl)pyridinium
dicyanamide (salt((CN) N )) (Scheme 1). The reactions of
ꢀ
ꢀ
2
N ). Salt((CN) N ) (0.31 g, 1.0
2
ꢀ
ꢀ
2
ꢀ
2
3
2
2
ꢀ
salt((CN) N ) with piperazine, R-(ꢀ)-2-methylpiperazine, S-
2
temperature for 18 h, the precipitate was collected by ltration,
washed with acetone (200 mL) and dried in vacuo to afford
(
+)-2-methylpiperazine and 2-methylpiperazine in water at room
temperature led to ring-opening of the pyridinium ring to afford
ꢀ
1
polymer(H;(CN)
): d 7.92 (2H), 7.56 (1H), 6.03 (2H), 3.55, 3.81 (8H).
Other polymers were synthesized in a similar manner.
2
N ) (0.35 g, 71%). H NMR (400 MHz, DMSO-
ꢀ
ꢀ
polymer(H;(CN)
2
N ),
polymer(R-Me;(CN)
2
N ),
polymer(S-
N ), in 71%, 34%, 36%
and 21% yields, respectively (Scheme 2). However, the reaction
of salt(Cl ) with piperazine in water at room temperature
d
6
ꢀ
ꢀ
Me;(CN)
2
N ) and polymer(rac-Me;(CN)
2
ꢀ
1
Polymer(rac-Me;(CN)
DMSO-d ): d 7.92 (2H), 7.58 (1H), 6.07 (2H), 3.57–4.17 (7H), 1.31
2
N ). Yield ¼ 21%. H NMR (400 MHz,
ꢀ
6
resulted in unidentied products.
and 1.22 (3H).
ꢀ
The model compound model(H;(CN) N ) was synthesized by
ꢀ
1
2
Polymer(R-Me;(CN)
DMSO-d ): d 7.92 (2H), 7.57 (1H), 6.07 (1H), 3.57–4.16 (7H), 1.31
2
N ). Yield ¼ 34%. H NMR (400 MHz,
ꢀ
the reaction of 1-phenylpiperazine with salt((CN)
2
N ) in 74%
6
yield (Scheme 3). The results of these reactions are summarized
in Table 1.
and 1.22 (3H).
ꢀ
1
Polymer(S-Me;(CN)
2
N ). Yield ¼ 36%. H NMR (400 MHz,
DMSO-d
6
): d 7.93 (2H), 7.58 (1H), 6.10 (2H), 3.57–4.15 (7H), 1.31
and 1.22 (3H).
ꢀ
Synthesis of model compound. Salt((CN) N ) (0.31 g, 1.00
2
mmol) was dissolved in 4 mL water and 1-phenylpiperazine
(0.32 g, 2.0 mmol) was added to the solution under N
2
. The
solution was stirred for 18 h at room temperature. The resulting Scheme 1 Synthesis of monomer.
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Scheme 2 Synthesis of polymers.
Scheme 3 Synthesis of model compound.
The polymers obtained in this study were completely soluble NMR and IR spectra
in polar organic solvents such as N,N-dimethylformamide
1
ꢀ
Fig. 1 shows the H NMR spectra of polymer(H;(CN)
2
N ), poly-
(
(
DMF), dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone
ꢀ
ꢀ
mer(S-Me;(CN) N ) and model(H;(CN) N ) in DMSO-d . The
2
2
6
ꢀ
ꢀ
NMP). Polymer(rac-Me;(CN)
2
N ), polymer(R-Me;(CN)
2
N ) and
1
peak assignments are shown in the gure. The H NMR spectra
ꢀ
polymer(S-Me;(CN)
polymer(H;(CN) N ) was insoluble in the solvent.
Model(H;(CN) N ) was soluble in acetone, MeOH, DMF and
2
ꢀ
N ) were partially soluble in methanol but
ꢀ
of the DMSO-d solutions of polymer(R-Me;(CN) N ) and poly-
6
2
2
ꢀ
mer(S-Me;(CN) N ) were similar, and each spectrum showed
2
ꢀ
2
three signals, at approximately d 7.9, 7.6, and 6.1 in a 2 : 1 : 2
integral ratio, arising from the protons in the penta-2,4-
dienylideneammonium group. The H NMR spectra of the
solution of polymer(H;(CN)
signals corresponding to the penta-2,4-dienylideneammonium
group at almost the same positions corresponding those of
polymer(S-Me;(CN) N ) in a 2 : 1 : 2 integral ratio, arising from
the protons in the penta-2,4-dienylideneammonium group.
These observations suggest that the p-electrons are delocalized
along the penta-2,4-dienylideneammonium group. The integral
ratio of the peaks corresponding to the methyl group and
piperazinium protons of the polymers supports the structures
shown in Scheme 1. The presence of three signals corresponding
to hydrogen atoms H , H and H of model(H;(CN)
a 2 : 2 : 1 integral ratio suggests that the p-electrons are delo-
calized across the aminopenta-2,4-dienylidene group. The
DMSO. The weight-averaged molecular weights (M 's) of NMP
w
ꢀ
ꢀ
solutions of polymer(H;(CN)
2
N ), polymer(R-Me;(CN)
2
N ) and
1
ꢀ
polymer(rac-Me;(CN)
2
N ), as determined by light scattering
ꢀ
DMSO-d
6
2
N ) also showed three
4
4
4
method, were 53.2 ꢁ 10 , 3.95 ꢁ 10 and 13.7 ꢁ 10 , respec-
ꢀ
tively. The M
w
value of polymer(R-Me;(CN)
2
N ), as determined
4
by GPC, was 3.27 ꢁ 10 (M
w
/M
n
¼ 1.46).
ꢀ
2
The reduced viscosities (h /c) of the DMSO solutions of the
sp
polymers are summarized in Table 1. The h /c values of the
sp
polymers in DMSO increased as their concentrations, c, were
ꢀ
reduced (Fig. S1†). The hsp/c values of polymer(H;(CN)
2
N ),
ꢀ
ꢀ
polymer(R-Me;(CN)
2
N )
and
polymer(rac-Me;(CN)
2
N )
ꢀ
1
ꢀ1
changed from 0.78, 0.62 and 0.56 g dL (c ¼ 0.10 g dL ) to
ꢀ
ꢀ
1
ꢀ1
0
0
.95, 0.74 and 0.71 g dL (c ¼ 0.050 g dL ) through values of
c
00
b
00
a
00
ꢀ
2
N ) in
1
ꢀ1
.85, 0.66 and 0.64 g dL (c ¼ 0.071 g dL ). These results
suggest that the polymers behaved as polymeric electrolytes in
13
C
4
ꢀ
2
dilute solutions. The h /c values of polymer(R-Me;(CN) N )
sp
NMR data also support this view, showing three signals arising
from the penta-2,4-dienylideneammonium group (Fig. S2†).
ꢀ
and polymer(rac-Me;(CN) N ) were roughly the same,
2
ꢀ
ꢀ
although the M
considerably higher than that of polymer(R-Me;(CN)
results imply that polymer(R-Me;(CN)
ture than polymer(rac-Me;(CN)
w
value of polymer(rac-Me;(CN)
2
N ) was
N ). The
N ) has a stiffer struc-
N ) in solution. It has been
2
ꢀ
Reactivity
2
ꢀ
ꢀ
2
The reaction of model(H;(CN) N ) with PDA caused recycliza-
2
reported that helical polymers show greater reduced viscosi-
tion of the 2,4-dienylideneammonium unit and resulted in the
formation of N-(4-aminophenyl)pyridinium dicyanamide (APD)
and N-phenylpiperazine (Scheme 4a). The formation of these
ties due to their stiff structures when compared to random coil
5
polymers.
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Table 1 Synthesis results and properties of polymers and model compound
a
ꢀ1
b
c
d
d
c
Product
Yield, %
h
sp/c , g dL
M
w
Absorption , nm
E
a
, V
E
, V
ꢀ
4
4
—f
ꢀ0.60
1.34
ꢀ0.61
1.35
ꢀ0.61
1.35
ꢀ0.70
Polymer(H;(CN)
2
N )
71
34
0.78
0.62
53.2 ꢁ 10
3.95 ꢁ 10
472
473
0.67
0.71
ꢀ
Polymer(R-Me;(CN)
2
N )
ꢀ
ꢀ
4
Polymer(S-Me;(CN)
2
N )
36
21
74
0.60
0.56
1.16 ꢁ 10
13.7 ꢁ 10
472
480
424
0.70
0.72
1.32
ꢀ
ꢀ
2
Polymer(rac-Me;(CN) N )
4
ꢀ
ꢀ
e
Model((CN)
2
N )
e
ꢀ1.36
a
ꢂ
b
c
d
In DMSO at 30 C. Determined by MALS. In DMSO. Measured by cyclic voltammetry. Cast lm of the polymer on a Pt plate in an acetonitrile
solution of [Et N]BF (0.1 M). Sweep rate was 50 mV s . In an acetonitrile solution of [Et N]BF (0.1 M). Not observed.
4 4 4 4
ꢀ1
e
f
pentadienylideniminium group by reaction with an amine has
not been previously reported. The assumption that the above
reactivity is due to the presence of dicyanamide anions in the
polymer and model compound is conrmed by the result that
ꢀ
ꢀ
polymer(H;Cl ) and model(H;Cl ) did not react with PDA. Pyr-
idinium salts are an important class of compounds that are
8
used as initiators of cationic polymerization, cationic surfac-
9
10
tants, non-linear optical materials, and phase transfer cata-
11
lysts. The reactive pyridinium salts could be used as starting
species for functional materials such as dyes, nonlinear optical
polymers, and polymer catalysts. However, reports on such
reactive pyridinium salts are limited because of difficulty in
12
synthesis of them. In fact, APD could not be obtained by the
reaction of salt(Cl ) with PDA. In contrast, the ring-closure
ꢀ
reaction yielded APD. Scheme 5 shows a possible reaction
mechanism for the recyclization of the 2,4-dien-
ꢀ
ylideneammonium unit of model(H;(CN) N ). Nucleophilic
2
addition of one of the amine groups of PDA to the carbon atom
a
(
C ) bonded to the piperazinium ring occurs rst. The fact that
a
the electron density of C is lowest among the 2,4-dien-
1
ꢀ
ylideneammonium carbons is conrmed by the appearance of
the H NMR signal at the lowest magnetic eld position among
Fig.
Me;(CN)
1
H
NMR spectra of polymer(H;(CN)
2
N ), polymer(S-
ꢀ
ꢀ
1
2
N ) and model(H;(CN)
2
N ) in DMSO-d
6
.
the 2,4-dienylideneammonium protons.
1
The newly formed C–N bond exchanges between the amino-
phenyl and piperazinium rings, followed by the ring-closure of
the 5-piperizine-N-(4-aminophenyl)-2,4-pentadienylideniminium
unit to yield N-(4-aminophenyl)pyridinium dicyanamide and N-
phenylpiperazine. The reaction of N-(2,4-dinitrophenyl)penta-2,4-
dienylidene-1-N-arylinium chloride with piperizine to provide N-
compounds was conrmed by their ESI TOF-MS and H NMR
ꢀ
spectra (Fig. S3 and S4†). The reaction of polymer(H;(CN)
2
N )
with PDA also caused recyclization of the 2,4-dien-
ylideneammonium unit of the polymer and resulted in depo-
lymerization to yield APD (Scheme 4b).
It has been reported that 5-anilino-N-phenyl-2,4-
pentadienylideniminium chloride is converted spontaneously
and quantitatively to N-phenylpyridinium chloride and aniline
(
5-piperidino-2,4-pentadienylidene)piperizinium chloride has
6,7
also been previously reported.
3
in solution. However, the ring-closure of the 2,4-
Scheme 4 Recylization reactions.
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Scheme 5 Possible reaction mechanism.
It is known that piperazine derivatives usually assume the and methanol were 473 and 475 nm, respectively. These wave-
13
energetically stable chair form rather than the boat form.
lengths were essentially the same as that in DMSO. The change
However, the piperazinium and 2-methylpiperazinium rings of in the UV-Vis spectra over time of freshly prepared MeOH
ꢀ
ꢀ
the polymers obtained in this study primarily assume the boat solutions of polymer(R-Me;Cl ), polymer(S-Me;Cl ) and poly-
ꢀ
ꢂ
form, which contributes the expansion of p-conjugation mer(H;Cl ) at 30 C in air corresponds to the two-step conver-
system, as revealed by the following UV-Vis measurements. The sion of 2-methylpiperazinium and piperazinium rings from the
1,2
intramolecular hydrogen bond between the nitro group and the boat to the chair form via a half-chair intermediate. When the
NH group in the intermediate adduct seems to play an impor- 2-methylpiperazinium rings of the polymers adopt the chair
tant role in the predominant formation of the piperazinium conformation, the distance between its two nitrogen atoms is
ring with the boat form in the polymers, as illustrated in too large to allow through-space interaction; therefore, there is
Scheme S1.† The piperazinium and 2-methylpiperazinium rings no expanded p-conjugation in these polymers. We proposed
ꢀ
ꢀ
ꢀ
of polymer(H;Cl ), polymer(S-Me;Cl ) and polymer(H;Cl ) a possible reaction mechanism for the predominant formation
converted from the boat to chair form when solutions of each of the thermodynamically unstable boat form of piperazinium
1
,2
1
were allowed to stand in air. The possibility that the UV-Vis ring in the polymers at the initial stage. However, the UV-Vis
ꢀ
spectral changes were attributed to decomposition of the poly- spectra of solutions of polymer(R-Me;(CN) N ), polymer(S-
mers could be ruled out from the result that the viscosities of Me;(CN) N ) and polymer(H;(CN) N ) in DMSO were almost
2
ꢀ
ꢀ
2
2
the polymer solutions did not change with time. Whereas such unchanged when the solutions were allowed to stand in air. The
interconversion of the piperazinium and 2-methylpiperazinium UV-Vis spectra of DMF and methanol solutions of
ꢀ
rings of the polymers with dicyanamide anions was not polymer(H;(CN)
2
N ) were also unchanged when the solutions
observed. In other words, the polymers can remain their were allowed to stand in air. This result suggests that the 2-
extended p-conjugation system in solution. We speculated that methylpiperazinium and piperazinium rings did not change
this result may be the intramolecular interaction between the their conformations in solutions and maintained the p-conju-
nitrogen atom of the piperazinium ring in the boat form and gation lengths of the polymers.
ꢀ
a CN group of the dicyanamide anion, as illustrated in
Scheme 6.
The absorption of the DMSO solution of polymer(H;(CN) N ) at
472 nm in the presence of an equimolar amount of PDA decreased
2
UV-Vis spectra
Fig. 2 shows the UV-Vis spectra of the DMSO solutions of pol-
ꢀ
ꢀ
ymer(H-Me;(CN)
2
N ), polymer(S-Me;(CN)
2
N ), polymer(rac-
ꢀ
ꢀ
Me;(CN)
2
N ) and model(H;(CN)
2
N ). The UV-Vis data are
summarized in Table 1. The absorption maxima (lmax) of the
DMSO solutions of the polymers appeared at longer wave-
ꢀ
lengths than did that of model(H;(CN) N ), suggesting exten-
2
sion of p-conjugation across the aminopenta-2,4-dienylidene
groups and through the 2-methylpiperazinium and piper-
ꢀ
azinium rings. The lmax's of polymer(H-Me;(CN)
2
N ) in DMF
ꢀ
Fig. 2 UV-Vis spectra of polymer(H-Me;(CN)
2
N ) (purple curve),
ꢀ
ꢀ
polymer(S-Me;(CN)
2
N ) (blue curve), polymer(H;(CN)
2
N ) (green
ꢀ
ꢀ5
Scheme 6 Intramolecular hydrogen bond between the cyano group curve) and model(H;(CN)
2
N ) (black curve) in DMSO (c ¼ 1 ꢁ 10
ꢀ1
and the nitrogen atom.
M ).
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ꢀ
Fig. 3 CD spectra of polymer(R-Me;(CN)
2
N ) (blue curve) and poly-
ꢀ
ꢀ5
ꢀ1
M ).
mer(S-Me;(CN)
2
N ) (red curve) in DMSO (c ¼ 1.0 ꢁ 10
with time when the solution was le open to air (Fig. S6†). The
decrease in this absorbance corresponds to the formation of APD,
accompanied by depolymerization of the polymer.
ꢀ
Fig. 4 CV curves of the cast film of polymer(R-Me;(CN)
plate in the DMSO solution of [Et N]BF
N]BF
2
N ) on a Pt
ꢀ
4
4
(0.1 M) and model(H;(CN)
2
N )
CD spectra
in the DMSO solution (1 mM) of [Et
4
4
.
Fig. 3 shows the CD spectra of the DMSO solutions of poly-
ꢀ
ꢀ
mer(R-Me;(CN)
2
N ) and polymer(S-Me;(CN)
2
N ). The CD
ꢀ
spectrum of polymer(R-Me;(CN)
2
N ) displays a relatively strong
ꢀ
ꢀ
Polymer(R-Me;(CN) N ) and model(H;(CN) N ) underwent
2
2
positive Cotton effect, with zero-crossings centered at the p–p*
transition of the polymer chain (at approximately 470 nm in the
electrochemical oxidation of the piperazinium ring and
reduction of the CN groups of the dicyanamide anion. The
UV-Vis absorption spectra, see Fig. 2). The positive Cotton effect
ꢀ
oxidation peak potential of polymer(R-Me;(CN)
2
N ) was
ꢀ
in the CD spectrum of polymer(R-Me;(CN)
2
N ) is consistent
+
ꢀ
lower (Epa ¼ 0.71 V vs. Ag /Ag) than that of model(H;(CN)
2
N )
ꢀ
2
with a similar trend for the polymer(R-Me;Cl ). Similarly,
+
(
E
pa ¼ 1.32 V vs. Ag /Ag) because of the longer p-conjugation
ꢀ
polymer(S-Me;(CN)
2
N ) showed a relatively strong negative
length of the polymer. The oxidation peak potential of poly-
Cotton effect. These Cotton curves suggest that the main chains
ꢀ
mer(S-Me;(CN) N ) was the essentially same as that of poly-
mer(R-Me;(CN) N ). The dicyanamide anions of the
polymers, except for polymer(H;(CN) N ), and the model
2
ꢀ
ꢀ
of polymer(R-Me;(CN)
2
N ) and polymer(S-Me;(CN)
2
N ) have
ꢀ
2
oppositely oriented, highly ordered structures, such as right-
ꢀ
2
and le-handed helical conformations. The ellipticities for the
compound participated in two-step electrochemical reduc-
ꢀ
ꢀ
Cotton curves of polymer(R-Me;Cl ) and polymer(S-Me;Cl )
gradually decreased with time when the methanol solutions
were allowed to stand in air. These decreases are presumably
due to time-dependent dynamic conformational changes in the
polymers; the helical conformation of the polymers dis-
appeared as the 2-methylpiperazinium ring converted from the
boat to the chair form via a half-chair intermediate form.
+
tions at approximately ꢀ0.6 V and ꢀ1.3 V (vs. Ag /Ag). The
reason for the disappearance of the reduction peaks in
ꢀ
polymer(H;(CN)
2
N ) is unclear.
Conclusions
Whereas such decrease in ellipticities for the Cotton curves was
In conclusion, ionic helical polymers were obtained from the
reaction of N-(2,4-dinitrophenyl)pyridinium dicyanamide with
chiral 2-methylpiperazines in water at room temperature. UV-
Vis measurements revealed that the 2-methylpiperazinium
rings of the polymers with dicyanamide anions did not undergo
conformational changes in solution, in contrast to those of the
polymers with chloride anions. This increased stability
ꢀ
not observed in the CD spectra of polymer(R-Me;(CN) N ) and
2
ꢀ
polymer(S-Me;(CN)
2
N ). The results are consistent with the fact
that the UV-Vis spectra of the DMSO solutions of the polymers
were almost unchanged with time.
Cyclic voltammograms
Cyclic voltammetry (CV) measurements suggest that the contributed to the preservation of the p-conjugated length and
polymers and model compound obtained in this study helical conformations of the polymers in solution. The poly-
underwent electrochemical oxidation of the 2-methylpiper- mers obtained in this study reacted with PDA, resulting in
azinium and piperazinium rings and reduction of the depolymerization. The results obtained in this study indicate
dicyanamide anions. The oxidation and reduction potentials that Zincke salts with dicyanamide anions can be used to
of the polymers and model compound in DMSO containing synthesize stable ionic helical polymers under environment-
0
.10 M [Et
4 4
N]BF are summarized in Table 1. Fig. 4 shows the friendly reaction conditions. Other polymers were synthesized
ꢀ
ꢀ
CV curves of polymer(R-Me;(CN)
2
2
N ) and model(H;(CN) N ). in a similar manner.
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6
S. Kunugi, T. Okubo and N. Ise, J. Am. Chem. Soc., 1970, 92,
5
Conflicts of interest
641–5645.
7 E. M. Marvell and I. Shahidi, J. Am. Chem. Soc., 1970, 92,
646–5649.
There are no conicts to declare.
5
8
E. Takahashi, F. Sanda and T. Endo, J. Polym. Sci., Part A:
Polym. Chem., 2002, 40, 1037–1046.
Acknowledgements
The authors are very grateful to the Japan Society for the
Promotion of Science – Grant-in-Aid for Scientic Research(C)
9 N. O. Mchedlov-Petrossyan, N. A. Vodolazkaya,
A. A. Kornienko, E. L. Karyakina and C. Reichardt,
Langmuir, 2005, 21, 7090–7096.
(No. 15K05609).
1
0 B. J. Coe, J. A. Harris, I. Asselberghs, K. Wostyn, K. Clays,
A. Persoons, B. S. Brunschwig, S. J. Coles, T. Gelbrich,
M. E. Light, M. B. Hursthouse and K. Nakatani, Adv. Funct.
Mater., 2002, 12, 110–116.
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29994 | RSC Adv., 2018, 8, 29988–29994
This journal is © The Royal Society of Chemistry 2018