Synthesis of metallo-supramolecular polymers with a bis-ono-tridentate ligand by coordination programming

Article abstract of DOI:10.1246/cl.140253

A novel ONO-type bis-tridentate ligand L1 was synthesized by a diazocoupling reaction of 3,3'-dihydroxybenzidine with 6-octyl-2-naphthol in 58% yield. Azo-hydrazo tautomerization in L1 was indicated by the NMR and IR spectra. The 1:1 complexation of L1 and Mn(III) or Co(II) ions was confirmed by titration experiments at room temperature. Mn-, Co-, and Crbased metallo-supramolecular polymers were prepared via the 1:1 complexation of the ditopic ligand with the metal ions at 100 °C. The Mn- and Cr-based polymers exhibited reversible redox properties.

Full text of DOI:10.1246/cl.140253

Received: March 20, 2014 | Accepted: April 8, 2014 | Web Released: April 15, 2014
CL-140253
Synthesis of Metallo-supramolecular Polymers with a Bis-ONO-tridentate Ligand
by Coordination Programming
Miki Kanao1,2 and Masayoshi Higuchi*1,2
1National Institute for Materials and Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044
2JST-CREST, Chiyoda-ku, Tokyo 102-0076
(E-mail: HIGUCHI.Masayoshi@nims.go.jp)
A novel ONO-type bis-tridentate ligand L1 was synthesized
by a diazocoupling reaction of 3,3¤-dihydroxybenzidine with
6-octyl-2-naphthol in 58% yield. Azo-hydrazo tautomerization
in L1 was indicated by the NMR and IR spectra. The 1:1
complexation of L1 and Mn(III) or Co(II) ions was confirmed
by titration experiments at room temperature. Mn-, Co-, and Cr-
based metallo-supramolecular polymers were prepared via the
1:1 complexation of the ditopic ligand with the metal ions at
100 °C. The Mn- and Cr-based polymers exhibited reversible
redox properties.
as THF, chloroform, DMF, and DMSO. The ligand structure was
1
characterized by H-, 13C NMR (Figures S1 and S2, Supporting
Information (SI)),9 and IR spectroscopy. The peaks in the
1H NMR spectrum were assigned with the help of the 1H-1H and
13C-1H COSY spectra. A proton signal of the hydroxy group in
the biphenyl ring appeared at 10.4 ppm. The downfield shift of
the proton signal is probably due to the intramolecular hydrogen
bonding between the hydroxy proton and the nitrogen atom of
the azo group. In the 13C NMR spectrum, a characteristic signal
attributed to the carbon at the 2-position of the naphthol moiety
appeared at 168.6 ppm, which is in lower magnetic field than
that of usual naphthols and relatively close to the chemical shift
of carbonyl carbons. This shift suggests an equilibrium between
the hydroxy and quinoid structures in the naphthol moiety in
DMSO, which is caused by the azo-hydrazo tautomerization
(Figure 2).10 A signal of the tertiary carbon with a hydroxy
group in the biphenyl moiety was observed at 148.9 ppm and the
other carbon signals existed in the range from 110 to 140 ppm.
In the IR spectrum, the strong absorption of the N=N stretching
Metallo-supramolecular polymers,1 which are synthesized
by the 1:1 complexation of metal ions and ditopic ligands, have
received much attention due to their unique properties such as
electrochromism,2 nonvolatile memory,3 and stimuli-responsi-
bility.4 In addition, the concept of “coordination programming”
has further developed metallo-supramolecular polymer chem-
istry.5 Creation of a new ditopic ligand is the key to introduce
various metal ion species into the metallo-supramolecular
polymer backbone. We have already reported syntheses of
bis(terpyridine)s and bis(phenanthroline)s and succeeded in the
preparation of Fe(II)-, Ru(II)-, Co(II)-, Cu(II)-, Ni(II)-, and
Eu(III)-based metallo-supramolecular polymers using the ditopic
ligands.2,3 This time we tried to synthesize a new ditopic ligand
with aromatic azo groups as coordination sites, because azo
compounds are well known for showing photochromism,6
fluorescence,7 and a high complexation ability to metal ions
such as manganese.8 Here we report the synthesis of a novel bis-
ONO-tridentate ligand, the Mn-, Co-, and Cr-based metallo-
supramolecular polymers formation using the ditopic ligand,
and the optical and electrochemical properties of the Mn-based
polymer.
appeared at 1618 cm¹1. The C-O stretching and O-H bending
¹1
of aromatic alcohol also appeared around 1217 and 1303 cm
,
¹1
respectively. A broad absorption around 3423 cm was as-
signed to the hydroxy group of the ligand.
L1 has a strong absorption at 570 nm in DMSO. When a
base was added to the solution, the peak shifted to 577 nm and
a new peak appeared at 737 nm due to the deprotonation of
the ligand (H4L) to the anionic species (H2L2¹ or L4¹
)
(Figure 3a).8c,11 L1 also showed solvatochromism: the -max
appeared at 570 nm in DMSO and DMF, 538 nm in chloroform,
545 nm in THF, and 555 nm in triethylamine. It is considered
that the different polarity of the solvents changed the position
of the equilibrium in the tautomerization between the hydrazo-
form and the azo-form (Figure 2), giving rise to the different
absorption wavelength.8c,11
A new bis-ONO-tridentate ligand L1 was synthesized by a
diazocoupling reaction of 3,3¤-dihydroxybenzidine with 6-octyl-
2-naphthol in 58% yield (Figure 1). In this reaction, an azo
group was selectively introduced at the 1-position of the
naphthol derivative. L1 was soluble in organic solvents such
Since manganese complexes have interesting properties12
such as redox, magnetism, and catalytic activity, it is expected
that metallo-supramolecular polymers containing Mn ions are
useful as a functional material for a wide range of applications,
including electrochromic devices, photosystem II materials, EL
device, and magnetic materials. First, the coordination behavior
of L1 to Mn(OAc)3¢2H2O was investigated in the presence
of a base by titration experiments using UV-vis absorption
HCl, NaNO2
water, 0-5 °C
1
2
2
NaOH
water/MeOH
0-5 °C r.t.
L1
Figure 1. Synthesis of L1.
Figure 2. Azo-hydrazo tautomerization in L1.
Chem. Lett. 2014, 43, 1075–1077 | doi:10.1246/cl.140253
© 2014 The Chemical Society of Japan | 1075

(a)
(b)
1
0.8
0.6
0.4
0.2
0
0.4
0.3
0.2
nL
(ML)n
0
1
2
300
500
700
Molar ratio of [Mn]/[L]
Mn(OAc)3 2H2O
Wavelength / nm
L1
KOH
Figure 3. (a) UV-vis spectra of L1 in DMSO during the
addition of Mn(OAc)3¢2H2O (broken line: L1 (20 ¯M), solid
line: L1 (20 ¯M) + triethylamine (80 ¯M) + [Mn(III)] (0-40
¯M)). (b) Absorbance change at 380 nm as a function of the
molar ratio of [Mn]/[L1].
EtOH/DMF
100 °C, 24 h
spectroscopy (Figure 3). The experiments were performed under
air conditions to promote auto-oxidation of Mn(III) to Mn(IV).
When a DMSO solution of Mn(OAc)3¢2H2O (16 mM) was
gradually added to a DMSO solution including L1 (20 ¯M)
and triethylamine (80 ¯M), the absorptions at 577 and 732 nm
decreased and those at 380 and 880 nm increased due to the
complexation (Figure 3a). The color of the solution changed
from violet to gray. The absorbance change at 380 nm was
plotted as a function of the molar ratio of [Metal]/[L]
(Figure 3b). The absorbance at 380 nm enhanced linearly on
increasing the molar ratio up to 1.0, and the spectral change
became almost silent for further addition of Mn ions. This
absorbance change suggests that the complexation of L1 and Mn
ions proceeds at a 1:1 ratio. A similar complexation behavior
was also conformed in the absence of the base, too: the final
spectrum was almost similar to that shown in Figure 3, though
the initial spectrum was different. It indicates that the ability of
Mn ions to complex with L1 is much higher than that of protons,
probably because of the thermodynamically higher stability of
Mn complex moieties by the multidentate coordination of L1.
The complexation of L1 with Co(II) or Cr(III) ions was also
investigated. During the addition of Co(OAc)2 to a DMSO
solution of L1, the absorbance at 365 and 615 nm increased and
that at 525 nm decreased until the molar ratio of L1 and Co(II)
ions became 1:1. It is expected that Cr(III) ions also form a
1:1 complex with L1 according to a previous study,13 but the
complexation with Cr(OAc)3 was not observed at room temper-
ature. As for the complexation of L1 with Cu(II), Ni(II), and
Zn(II) ions, their titration results revealed that the molar ratio in
the complex was not 1:1 but 1:2.
A Mn-based metallo-supramolecular polymer polyMnL1
was synthesized by the 1:1 complexation of L1 and Mn(OAc)3¢
2H2O (Figure 4). KOH (0.800 mmol) was added to an ethanol/
DMF (1/1, v/v) solution (500 mL) of L1 (0.200 mmol).
After the solution was heated up to 50 °C, Mn(OAc)3¢2H2O
(0.200 mmol) was added to the solution. Then, the solution was
heated at 100 °C for 24 h under air. After the reaction, the
solvent was evaporated and the residue was washed using water
and methanol. PolyMnL1 was obtained as dark violet powder in
89% yield (IR (KBr) ¯ (cm¹1): 3043, 2953, 2941, 2852, 1585,
1560, 1546, 1492, 1456, 1438, 1388, 1344, 1313, 1284, 1203,
1163, 1112, 999, 908, 837, 827, 798, 563; UV-vis spectrum (in
NMP): -: 334, 590, 890 nm). While L1 is soluble in common
Figure 4. Synthesis of polyMnL1.
organic solvents, polyMnL1 is soluble only in highly polar
solvents such as THF, DMF, and 1-methyl-2-pyrrolidone
(NMP). An NMR spectrum of polyMnL1 was not obtained
due to the paramagneticity.12 In the IR spectrum, the N=N
¹1
stretching bond was shifted from 1618 (L1) to 1585 cm
(polymer) because of the complexation. The Ar-O stretching
vibration also moved to 1217 from 1205 cm¹1. Similarly, Co-
and Cr-based polymers polyCoL1 and polyCrL1 were also
prepared at 100 °C by the 1:1 complexation of using L1 and
Co(OAc)2 or Cr(OAc)3, respectively (polyCoL1: yield: 92%; IR
(KBr) ¯ (cm¹1): 3043, 2953, 2941, 2852, 1589, 1546, 1491,
1446, 1350, 1305, 1211, 1161, 997, 825, 798, 500; UV-vis
spectrum (in NMP): -: 357, 591, 620 nm; polyCrL1: yield:
>99%. IR (KBr) ¯ (cm¹1): 3043, 2953, 2941, 2852, 1589, 1545,
1491, 1473, 1298, 1161, 995, 831, 804, 500; UV-vis spectrum
(in NMP): -: 377, 619, 672 nm).
UV-vis spectra of L1 and the obtained polymers were
measured in NMP (Figure S3 (SI)). L1 showed characteristic
peaks at 321 and 542 nm, which are assigned to the π-π* and
n-π* transitions. PolyMnL1 showed a broad band from 300
to 1000 nm and has a characteristic small band at 900 nm. The
absorption at 642 and 900 nm were assigned as metal to ligand
charge transfer (MLCT) and the d-d transition of the metal,
respectively. The absorption of polyCoL1 appeared at 620 nm
and has a shoulder peak at 579 nm. In case of polyCrL1, three
characteristic absorption peaks were observed at 614, 673, and
919 nm.
Cyclic voltammogram of polyMnL1 was measured in both
the solution and film state (Figures 5 and S4). A polyMnL1 film
showed a reversible redox wave attributed to the Mn(III)/(IV)
couple (E1/2 = ¹0.46 V vs. Ag/AgCl).12 As shown in Figure 5,
the current in the redox wave gradually increased for the first
several cycles, following which, it became constant. This
behavior indicates the introduction of counter cations in the
electrolyte solution into the polymer film during the redox of
Mn ions, because the original polyMnL1 is a neutral compound.
The redox wave appeared at a similar voltage in the solution,
too. In polyCoL1, the reduction peak appeared at ¹0.025 V,
but an oxidation peak was not observed. In polyCrL1, a redox
wave appeared at 0.495 V vs. Ag/AgCl in the film state, based
on the redox of Cr(III)/(IV).
1076 | Chem. Lett. 2014, 43, 1075–1077 | doi:10.1246/cl.140253
© 2014 The Chemical Society of Japan

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Figure 5. Cyclic voltammogram of a polyMnL1 film in
CH3CN (working electrode: glassy carbon; counter electrode:
Pt wire; scan rate: 50 mV s¹1, electrolyte: Et4N¢ClO4).
In summary, a novel ONO-type bis-tridentate ligand L1 was
synthesized by a diazocoupling reaction of 3,3¤-dihydroxyben-
zidine with 6-octyl-2-naphthol in 58% yield. Azo-hydrazo
tautomerization in L1 was indicated by the NMR and IR spectra.
The 1:1 complexation of L1 and Mn(III) or Co(II) ions was
confirmed by titration experiments at room temperature. Mn-,
Co-, and Cr-based metallo-supramolecular polymers were
prepared via the 1:1 complexation of the ditopic ligand with
metal ions at 100 °C. The Mn- and Cr-based polymers exhibited
reversible redox properties. This study provided a new ditopic
ligand for metallo-supramolecular polymer synthesis. Using the
ligand, various functional polymers will be prepared in the near
future.
6
7
8
9
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas. “Coordination Programming”
Area 2107, No. 24108742 from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
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Chem. Lett. 2014, 43, 1075–1077 | doi:10.1246/cl.140253
© 2014 The Chemical Society of Japan | 1077