Ni(II)-Based Metallosupramolecular Polymer with Carboxylic Acid Groups: A Stable Platform for Smooth Imidazole Loading and the Anhydrous Proton Channel Formation.

The Ni(II)-based metallosupramolecular polymer with carboxylic acid groups (polyNi) was synthesized via a 1:1 complexation of Ni(II) salt with (4,4'-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(pyridine-2,6-dicarboxylic acid) for the first time. The divalent state of Ni(II) in the polymer was confirmed by the X-ray absorption fine structure analysis. Smooth loading of imidazole molecules into polyNi proceeded with the help of the carboxylic acid groups to form the imidazole-loaded polyNi (polyNi-Im). Thermogravimetric analysis of polyNi-Im revealed that approximately three imidazole molecules were incorporated per repeating unit of polyNi. The Fourier transform infrared spectrum of polyNi-Im showed a new peak at 3219 cm-1, which shows an ∼73 cm-1 enhancement to -N-H of pristine imidazole. The peak suggests the formation of an imidazolium cation in the polymer. Powder X-ray diffraction indicated no degradation of the polymer structure during the imidazole loading because the diffraction pattern of polyNi-Im was almost the same as that of polyNi except for the presence of peaks corresponding to the imidazole molecules. Interestingly, the scanning electron microscopy measurement showed a large morphological change to uniform spherical particles by loading imidazole to the polymer. PolyNi-Im exhibited good proton conductivity (1.05 × 10-2 mS/cm) at a high temperature (120 °C), which is around 7 orders of magnitude higher than that of pristine polyNi because of the proton conduction pathway formation along the polymer chains by the incorporated imidazole molecules.

Introduction

Anhydrous proton conductive (PC) materials working at a high temperature (∼120 °C) are essential to the development of proton-exchange membrane fuel cells.[1−4] PC materials have been widely studied under relative humid (RH) conditions.[5−30] For example, Nafion-based polymers attain conductivities of 0.1–0.01 S/cm under 60–80 °C and 98% RH,[31] but they have temperature limitations (up to ∼80 °C). Therefore, anhydrous PC materials working at a temperature higher than 100 °C are highly required for real applications. They are prepared in two different ways. One is the incorporation of nonvolatile acids such as H3PO4 and H2SO4.[32−37] The other is the loading of organic molecules as proton carriers into the pores of the metal–organic frameworks (MOFs), porous networks, and covalent organic frameworks (COFs).[38−48]

Among the proton carrier molecules, nonvolatile heterocyclic compounds (e.g., imidazole) have received the highest attention because of the fast proton transfer (PT) and high boiling points.[38,43−45] In the hydrous PC process, it is well known that protonic substituents such as carboxylic groups (−COOH) act as proton sources as well as support hydrogen-bonding networking of water molecules.[29,49−54] However, in the anhydrous PC systems, the effect of protonic substituents on proton conductivity has not been investigated well.[55]

We reported the proton and ionic conductivity of metallosupramolecular polymers (MSPs) under high humid conditions so far.[23−30] MSPs are synthesized by the complexation of a metal ion and an organic ligand.[56−61] The consecutive metal complex constituents of an MSP devote to the proton channel formation. Recently, we revealed anhydrous PC properties of an imidazole-loaded Pt(II)-based MSP with carboxylic acid groups as the substituent.[55] However, the polymer composed of an asymmetrical ditopic ligand and a precious metal needed a complex synthetic procedure and was too expensive to be produced. Here, we designed a linear Ni(II)-based MSP with carboxylic acid groups using a new symmetrical ditopic ligand with a pyridine moiety having two carboxylic acid groups. With this polymer, we investigated the loading of imidazole molecules for anhydrous PC channel formation and compared the PC properties at high temperatures between the original polymer and the imidazole-loaded polymer.

Results and Discussion

A Ni(II)-based MSP with carboxylic acid groups (polyNi) was synthesized via a 1:1 complexation of Ni(II) salt with a ditopic ligand (L), (4,4′-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(pyridine-2,6-dicarboxylic acid) (Scheme ). The ligand was prepared by the Suzuki coupling reaction of a bromo derivative of dicarboxyl ester group-containing pyridine with a diboronic ester of fluorene using a Pd catalyst, followed by hydrolysis. PolyNi was obtained by the complexation of an equimolar amount of L and Ni(CH3COO)2·4H2O at 60 °C in MeOH for 12 h under the nitrogen atmosphere. The chemical structure of the ligand molecule and polyNi was confirmed by various spectroscopic techniques. The complete characterizations of the ligand (Figures S1–S7) and polyNi are given in the Supporting Information.

Scheme 1

Synthesis of a Ni(II)-Based MSP (polyNi)

The electronic state of Ni in polyNi was investigated by X-ray absorption fine structure (XAFS) measurements at the synchrotron facility (BL-12C KEK). The X-ray absorption near-edge spectroscopy (XANES) patterns of Ni foil, NiO, and polyNi at the Ni K-edge are shown in Figure . The Ni K-edge spectrum of polyNi was almost not shifted from 8344 eV (red line of the standard sample of NiO). The results indicate that the oxidation state of Ni ions of polyNi is divalent. From the extended XAFS (EXAFS) region, the fitting by using the crystal structure of the reported Ni complex[62] revealed that the coordination lengths around the Ni ions, Ni–N and Ni–O, are 2.071, 2.214, and 2.204 Å (R-factor; 1.7%, Table S1, the EXAFS oscillation is shown in Figure S8). These values are similar to the distances in the crystal structure. These results confirmed that the Ni ions of polyNi have an octahedral coordination structure. Additionally, the double peaks appeared around 865 eV (Ni 2P3/2) and 883 eV (Ni 2P1/2) in the X-ray photoelectron spectrum of polyNi (Figure S9) also confirm the presence of Ni(II) ions.[63]

Figure 1

XANES patterns of Ni foil, NiO, and polyNi.

XANES patterns of Ni foil, NiO, and n class="Chemical">polyNi.

Imidazole-loaded polyNi (polyNi-Im) was obtained by the incorporation of imidazole to the polymer with the help of the carboxylic acid groups. Imidazole was loaded into a polyNi film by exposing the imidazole vapor at 120 °C for 5 h (Scheme ). The number of imidazole molecules loaded to polyNi per repeating unit was determined by thermogravimetric analysis (TGA). The TGA results of polyNi-Im and polyNi are shown in Figure . The profile of polyNi-Im shows ∼26.8% extra weight loss compared with polyNi at the 30–270 °C temperature range because of 26.8% weight imidazole loading, which is estimated to be approximately three imidazole molecules per monomer unit. The liberation of the incorporated imidazole molecules starts at 125 °C and ends at ∼270 °C.

Scheme 2

(a) Schematic Representation of Imidazole Interaction in the Polymer Chain To Form a Proton Conduction Channel and (b) Possible PT Mechanism

Figure 2

TGA results of polyNi and polyNi-Im.

The TGA curve reveals that the loss of imidazole molecules in polyNi-Im occurs in two steps: In the first step, the release of imidazole molecules starts at 125 °C (first step) and is completed around 200 °C, and in the second step, it begins at 200 °C and ends at 270 °C. It is estimated by TGA that the percentage of imidazole loss in the first step is ∼8.7% of total polyNi-Im weight, followed by ∼18.1% of the total amount in the second step. Hence, we can assume that two types of imidazole molecules exist in polyNi-Im: one is the self-hydrogen-bonded (SHB) imidazole molecules that have less interaction with the MSP, which is degraded below 200 °C (in the first step), and the other is the imidazole cation that had a strong interaction with the MSP (in the second step). The weight loss in the second step (200–270 °C) corresponds to the removal of imidazole molecules which had a strong interaction with the carboxylic acid groups of the MSP to form imidazolium ions.

The degradation of polyNi-Im above 270 °C is completely due to the polymer. The derivative TGA plot of polyNi-Im (Figure S10) also indicated a stepwise weight loss at 125–270 °C. As three imidazole molecules are loaded per monomer unit of polyNi, we can consider that two imidazole molecules strongly interacted with the carboxylic group (imidazolium cation) and the other one is less interacted (SHB) to form the imidazole channel in the polymer. These results prove that the carboxylic acid groups play a key role in the incorporation of guest molecules into polymer chains with the help of the interaction between COOH groups and imidazole in the MSP. The powder X-ray diffraction (PXRD) patterns of imidazole, polyNi, and polyNi-Im are shown in Figure a. PolyNi showed broad peaks in its PXRD spectrum, indicating the almost amorphous nature. The diffraction pattern of polyNi-Im is almost the same as that of polyNi except for the presence of peaks at 2θ = 20.42, 20.64, 26.93, and 30.92. These peaks correspond to the imidazole molecules. A new broad peak around 2θ = 12° of polyNi-Im in Figure a indicates the alignment of the polymer chains induced by the imidazole loading to polyNi. Further, we have also performed Fourier transform infrared (FT-IR) spectroscopy analysis to analyze the incorporation of imidazole molecules and their interaction with the carboxylic acid groups of polymer chains (Figures b and S7 and S11). The ligand L showed a characteristic peak at 1728 cm–1 and a broad peak at 3433 cm–1 (Figure S7). These peaks are responsible for C=O and O–H stretching frequencies of COOH in L, respectively. Similar peaks with a shift at 1732 cm–1 (blue shift) and 3393 cm–1 (red shift) have appeared in polyNi (Figure S11). The shift of these peaks is due to the interaction between C=O and Ni in polyNi. This result confirms the formation of polyNi with the free −COOH group. Additionally, we also found a peak at 1638 cm–1. This is the peak corresponding to the carboxylate ion. The FT-IR spectrum of pristine imidazole (Figures b and S11) showed the main peaks at 3019, 3045, 3125, and 3144 cm–1. The peaks at 3019 and 3045 cm–1 shift the stretching frequencies of −C–H, and the peaks at 3125 and 3144 cm–1 correspond to the −N–H and SHB −N–H stretching frequencies, respectively. After loading the imidazole molecules to polyNi, the stretching frequency of −O–H shifted from 3393 to 3420 cm–1 with the decrement of peak intensity suggesting the formation of carboxylate anions in polyNi-Im (Figure b).

Figure 3

PXRD and FT-IR patterns of imidazole, polyNi, and polyNi-Im: (a) PXRD data and (b) FT-IR spectra in the range of 2500–3850 cm–1.

PXRD and FT-IR patterns of imidazole, n class="Chemical">polyNi, and polyNi-Im: (a) PXRD data and (b) FT-IR spectra in the range of 2500–3850 cm–1.

PolyNi-Im also showed the stretching frequencies corresponding to the pristine SHB imidazole molecule at 3045 cm–1 (−C–H) and 3142 cm–1 (−N–H). Additionally, we also found one new peak at 3219 cm–1 (∼73 cm–1 enhancement to −N–H of pristine imidazole). This result confirms that two types of imidazole molecules (one is SHB imidazole and the other is imidazolium cation) exist in polyNi-Im. The formation of imidazolium cations in polyNi-Im is also confirmed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) spectroscopy. The MALDI-TOF spectrum of polyNi-Im showed an m/z peak at 856.20 [(L(−2H) + 2Ni + 2imidazole)] as it is a combination of a ligand (carboxylate form), an imidazolium cation, and a Ni(II) ion at a peak of 880.55 [(L(−2H) + 2Ni + 2imidazole + Na)] (Figures S13 and S14 in the Supporting Information). The incorporation of two types of imidazole molecules into an MSP with carboxylic acid groups was also reported in our previous paper.[55] Moreover, we performed scanning electron microscopy (SEM) analysis for polyNi and polyNi-Im. For SEM analysis, 1 mg of the compound was dispersed in 2 mL of MeOH solvent, drop-cast on a glass slide, and then allowed to dry.

Figure a–d (also Figures S15 and S16) shows the SEM images of polyNi and polyNi-Im. Figure a,b reveals the flake-type structure of polyNi. Interestingly, the assembled structure of polyNi changed after the imidazole vapor exposure. Figure c,d, polyNi-Im, shows several monodisperse-type spherical individual particles, probably because of the increasing close packing of the hydrophobic alkyl chains on the fluorene rings during imidazole vapor exposure forming spherical-like structures of polyNi-Im. We also performed energy-dispersive X-ray (EDX) imaging of polyNi and polyNi-Im powder samples and assembled structures; to confirm the elemental compositions of polyNi and polyNi-Im, field emission SEM (FESEM) has been performed, as shown in the Supporting Information (Figures S17–S20). The powder samples were directly cast on a carbon tape, and for assembled structures, the samples were drop-cast on a glass slide for the measurement. The EDX spectrum confirmed the presence of C, N, O, and Ni in both states. The incorporation of imidazole molecules into the metallopolymer chains may provide good PC even under anhydrous conditions.

Figure 4

SEM images of polyNi (a,b) and polyNi-im (c,d).

SEM images of polyNi (a,b) and n class="Chemical">polyNi-im (c,d).

As we anticipated to achieve PC properties at temperatures above 100 °C, we therefore were deliberate to study the PC properties of polyNi and polyNi-Im under anhydrous conditions using different temperatures. To measure the PC values, the pellet form samples of polyNi and polyNi-Im were used, which were made by compressing separately using a press. The pellets were then placed in between the two gold electrodes of a sample holder SH2Z from Toyo Industries, Japan. Proton conductivities of the samples were measured by ac impedance spectroscopy. Figure a shows the Nyquist plots for the PC measurement of polyNi-Im at temperatures ranging from 30 to 120 °C. Generally, the diameter of the semicircle gives the resistance value of the sample. The PC of polyNi-Im is increased with increasing temperature (Figure b) as the diameter of the semicircle decreases with the decreasing magnitude of Zreal (inset of Figure a).

Figure 5

(a) Nyquist diagrams for polyNi-Im at different temperatures in anhydrous conditions (the inset shows zoomed-in Nyquist diagrams for polyNi-Im); (b) bar diagram of conductivities of polyNi-Im at different temperatures; (c) Nyquist plot of polyNi-Im at 120 °C and corresponding equivalent circuit, where Ri is the impedance of the circuit, Rb is the resistance for proton conduction, and Cp is the constant phase element; and (d) Arrhenius plot for activation energy measurement for polyNi-Im.

The PC of polyNi-Im at 30 °C is calculated to be 3.18 × 10–10 S/cm, which is enhanced to 1.05 × 10–5 S/cm at 120 °C under completely anhydrous conditions (Figure c). Therefore, polyNi-Im showed a substantial increment (∼5 orders of magnitude) of conductivity as the temperature increased from 30 to 120 °C. The Nyquist plots of polyNi are shown in Figure S21. polyNi showed a PC of 1.28 × 10–9 S/cm at 30 °C, which is decreased at 120 °C (3.45 × 10–12 S/cm) (Figure S21c) under anhydrous conditions. Therefore, polyNi-Im showed ∼7 orders of magnitude higher PC than polyNi under similar experimental conditions at 120 °C. We also examined the reproducibility and accuracy of the above-mentioned PC values of polyNi-Im and polyNi by studying 3 times different batches of samples. Each experiment showed almost the same results with good consistency, marked by standard deviations in Figures b and S21b not exceeding ±8%. The PC of polyNi-Im drastically increased with the increase of temperature, but the PC of pristine polyNi was quite decreasing with the increase of temperature. In the case of polyNi, conductivity was quite decreasing with the increase of temperature because of the release of physically adsorbed or inherent water molecules from, but in the case of polyNi-Im, while the temperature increased, effective anhydrous PC channels were formed by the interaction between the loaded imidazole and the carboxylic acid groups of polyNi. This result indicates that a drastic improvement in the PC of polyNi-Im emerges directly from the incorporated imidazole molecules.

To know the proton transport mechanism, the measurement of activation energy for PC phenomena is necessary. In general, proton transportation proceeds via two different kinds of mechanisms, one is Grotthuss mechanism [activation energy (Ea) < 0.45 eV] and the other is vehicle mechanism (Ea > 0.45 eV).[3,64] We have measured the PC of polyNi-Im for each 10 °C interval from 30 to 120 °C to calculate the activation energy. Figure d shows the fitted PC data of polyNi-Im in the temperature range of 30–120 °C to the Arrhenius equation. Interestingly, we observed two different slopes (shown in Figure d), which indicate two activation energies: 1.85 eV between 30 and 80 °C and 0.45 eV between 80 and 120 °C. Hence, at low temperatures (30–80 °C), the vehicle mechanism may be employed where the mobilized SHB imidazole molecules (which are less interacted with −COOH of a polymer chain) can transport the proton individually. However, at high temperatures, the Grotthuss mechanism is mainly payable as the activation energy is low at high temperatures. In this mechanism, all the loaded imidazole molecules interacted with −COOH or less interacted to create proton channels along with the polymer chain as shown in Scheme b to provide a Grotthuss-type mechanism by continuous breaking and formation of an alternate bond. This Grotthuss-type mechanism of PT in the system which contains imidazole molecules is well studied theoretically and experimentally.[43,65−69] However, we also observed two types of imidazole molecules, SHB imidazole and imidazolium cations, that were incorporated in polyNi-Im. For the PT process in polyNi-Im, the Grotthuss-type mechanism may be employed, as shown in Scheme b. The activation energy was changed when the temperature increases from 30 to 120 °C. This is probably due to the rotation of the incorporated imidazole molecules, and the vibration of their N–H bonds helps to the transfer of protons from one imidazole to others through the SHB pathway. To check the stability of polyNi-Im during the impendence measurement, we also performed the TGA and PXRD studies of polyNi-Im after the impendence measurement and results were compared with those obtained before the impendence measurement. We did not observe any significant difference in the TGA and PXRD spectra and those after the ac impedance measurement (Figures S22 and S23). These results indicate that polyNi-Im is quite stable during the impedance measurement. The comparative PC data of polyNi and polyNi-Im in the temperature range of 30–120 °C are shown in Table . Additionally, we also plotted log10 conductivity versus log10 frequency (Bode plot) of polyNi-Im at 120 °C to study the conductivity response with the frequency, and the Bode plot is presented in the Supporting Information (Figure S24). The comparison of proton conductivity of polyNi-Im at 120 °C with different kinds of imidazole-loaded materials is shown in the Supporting Information (Table S2). The obtained anhydrous PC value (σ ≈ 1.05 × 10–5 S/cm) of polyNi-Im at 120 °C is almost similar when compared with our previous report on anhydrous PC of an imidazole-loaded linear Pt(II)-based MSP (polyPtC-Im σ ≈ 1.5 × 10–5 S/cm).[55] This is probably due to the similar kind of mechanism that took place in both the cases (the number of free carboxylic acid groups and loaded imidazole molecules are also similar in the polymer chains) for the formation of anhydrous PC channels in polymer chains. Interestingly, the assembled structure of polyNi-Im was different because polyNi has flexible hydrophobic alkyl chains in its ligand structure. The main advantages of polyNi-Im compared with polyPtC-Im are that polyNi-Im showed more amorphous nature (easy to form a homogeneous polymer film which is required for processability) than polyPtC-Im and that the synthesis of polyNi-Im is cost-effective because nickel salts are much cheaper than platinum salts.

Table 1

Comparative Proton Conductivity Data of polyNi and polyNi-Im

	conductivity (σ) S/cm
temperature (°C)	polyNi	polyNi-Im
30	1.28 × 10–9	3.18 × 10–10
40	1.34 × 10–9	4.25 × 10–9
50	2.09 × 10–9	6.88 × 10–8
60	2.38 × 10–9	2.72 × 10–7
70	2.47 × 10–10	1.23 × 10–6
80	1.60 × 10–11	1.84 × 10–6
90	1.06 × 10–11	4.76 × 10–6
100	6.62 × 10–12	7.26 × 10–6
110	5.13 × 10–12	8.162 × 10–6
120	3.45 × 10–12	1.05 × 10–5

Conclusions

We have successfully synthesized a linear Ni(II)-based MSP with carboxylic acid groups (polyNi) using a new symmetrical ditopic ligand with a pyridine having two carboxylic acid groups. The coordination structure of polyNi was confirmed by XANFS studies along with various spectroscopic and microscopic characterizations. The carboxylic acid groups effectively assisted as hosts for imidazole loading to generate anhydrous proton conduction channels through the MSP chains. The imidazole-loaded polymer (polyNi-Im) was analyzed in detail by FT-IR spectroscopy, TGA, PXRD spectroscopy, and SEM techniques. PolyNi-Im exhibited a proton conductivity of 1.05 × 10–2 mS/cm at 120 °C under anhydrous conditions, which is around 7 orders of magnitude higher than that of pristine polyNi. This is an efficient and easy way to develop anhydrous proton conduction materials for real applications. Also, this polymer design will also be applicable to the other low-cost transition-metal ions such as Fe and Cu. The high durability, processability, and amorphous nature are the advantages of MSPs beyond MOFs and COFs to the practical applications.

Experimental Section

Materials

All the solvents and reagents were of spectroscopic grade and were used in this work as obtained. Dimethyl sulfoxide (DMSO), methanol (MeOH), tetrahydrofuran (THF), and potassium hydroxide (KOH) were purchased from Wako Chemical Co. Inc. Potassium carbonate (K2CO3) was obtained from Kanto Chemical Co. Inc. Phosphorus pentabromide (PBr5), tetrakis(triphenylphosphine)palladium(0), 9,9-dihexylfluorene-2,7-diboronic acid bis (1,3-propanediol) ester, and nickel(II)acetate tetrahydrate (Ni(CH3COO)2 4H2O) were purchased from Sigma-Aldrich Co., Ltd. Imidazole was obtained from nacalai tesque, INC. Chelidamic acid monohydrate was purchased from TCI Co., Ltd. Dimethyl 4-bromopyridine-2,6-dicarboxylate (1) was synthesized from chelidamic acid monohydrate using the reported procedure.[55]

Instrumentation

The 1H NMR spectrum was recorded at 300 MHz using a JEOL AL 300/BZ instrument. Chemical shifts were specified here to tetramethylsilane. The FT-IR spectra of compounds were recorded on a Shimadzu FT-IR-8400S instrument. The weight-average molecular weight of polyNi was measured using a size exclusion chromatography–viscometry–right-angle laser light scattering (SEC–viscometry–RALLS) system having components such as a liquid chromatograph, a pump, a solvent degasser refractive index detector, a column oven, and a Viscotek 270 dual detector (eluent: DMSO; low speed: 1 mL min–1; column operating temperature: 28 °C; polymer concentration used for the experiment: 1.0 mg mL–1; total injection volume: 20 mL; and standard: polystyrene-99K). An Elementar vario MiCRO cube was used for performing elemental analyses. PXRD was performed using a Rigaku Smart Lab 3. The EDX spectroscopy analyses were performed using a Hitachi field emission scanning electron microscope SU8000. X-ray photoelectron spectroscopy (XPS) experiment was performed on a KRATOS Axis Nova (Al Kα = 1.4866 keV). For proton conductivity measurements, pellet forms of polyNi and polyNi-Im were used. The thicknesses of all the polymer pellets were 0.4 mm with a 4 mm diameter. The pellet was then put in between the two gold electrodes of a sample holder. The sample holder used in the conductivity experiments was equipped with two electrodes with an attached micrometer. A Solartron 1260 impedance gain/phase analyzer coupled with a Solartron 1296 dielectric interface was used for ac impedance measurements. A frequency range of 1 Hz to 30 MHz was used to determine the resistance of the polymer pellets. Z-View software was used to analyze the impedance results employing an equivalent circuit simulation to complete the Nyquist plot and obtain accurate resistance values. To calculate the proton conductivity, we have utilized the equation σ = L/AR, where L is the width of the sample, A is the sample area, and R is the sample resistance which was obtained from the Nyquist plot.

Synthesis of Ligand (L)

Synthesis of Tetramethyl-4,4′-(9,9-dihexyl-9H-fluorene-2,7-diyl)bis(pyridine-2,6-dicarboxylate) (3)

2,2′-(9,9-Dihexyl-9H-fluorene-2,7-diyl)bis(1,3,2-dioxaborinane) (280 mg, 0.557 mmol), dimethyl-4-bromopyridine-2,6-dicarboxylate (305 mg, 1.11 mmol), potassium carbonate (240 mg, 1.73 mmol), and Pd(PPh3)4 (64 mg, 0.055 mmol) and DMSO (30 mL) were added to a 50 mL, three-neck, round-bottom flask, and the solution was charged with nitrogen for 20 min. Later, the reaction mixture was degassed by several sequences of freeze–pump–thaw cycles about 40 min. The final resultant solution was stirred at 100 °C for 12 h under a nitrogen atmosphere. After completion of the reaction, the solvent was removed under reduced pressure at 100 °C. The reaction mixture was cooled to room temperature, and CHCl3 (50 mL) was added. The catalyst was removed by filtration and washed thoroughly with CHCl3. The filtrate was then washed with H2O. The organic layer was separated, dried over Na2SO4, filtered, concentrated, and purified by column chromatography on silica gel (CHCl3/hexane = 50:50), affording the desired product 3 (0.250 g, 62.2%). 1H NMR (300 MHz, CDCl3, 298 K): 8.63 (s, 4H), 7.90 (d, 2H), 7.81 (d, 2H), 7.79 (s, 2H), 4.08 (s, 12H), 2.09 (t, 4H), 1.05 (m, 16H), 0.71 (t, 6H). MALDI-TOF: found m/z, 721.34 [M + H]; C43H48N2O8 requires (m/z), 720.34. FT-IR (KBr) cm–1: 2953, 2926, 1742, 1718, 1605, 1583, 1541, 1521, 1454, 1417, 1390, 1369, 1308, 1292, 1262, 1225, 1198, 1171, 1138, 1082, 1023, 991, 933, 900, 864, 819, 776, 744, 723, 695, 677, 646, 627, 596, 543.

Synthesis of 4,4′-(9,9-Dihexyl-9H-fluorene-2,7-diyl)bis(pyridine-2,6-dicarboxylic Acid) (L)

Compound 3 (150 mg, 0.193 mmol) was dissolved in THF solvent and taken in a 100 mL, two-neck, round-bottom flask. To this, an aqueous solution of sodium hydroxide (30 mg, 0.772 mmol) was added, and the final solution was refluxed for 12 h. The resulting mixture was cooled at room temperature, and then 2 M hydrochloric acid was added. The precipitate was filtered and washed with water to remove the salts. Moreover, the compound was purified by washing with chloroform and hexane solvents and dried in vacuo to give a yellow solid (122 mg, 95%). 1H NMR (300 MHz, DMSO, 298 K): 8.48 (s, 4H), 8.06 (m, 4H), 7.91 (d, 2H) 2.05 (broad, 4H), 0.98 (broad, 16H), 0.66 (t, 6H); Elemental analysis Calcd for [L + THF + 2H2O] = C43H52N2O11: C, 66.82; H, 6.78; N, 3.62. Found: C, 66.12; H, 5.83; N, 3.88 MS: MALTI-TOF: found m/z, 665.20 [M + H]; 687.43 [M + Na]; 703.62 [M + K]; C39H40N2O8 [M] requires 664.28. FT-IR (KBr) cm–1: 3433 (broad peak, O–H in COOH), 3258, 3185, 3090, 2955, 2930, 2857, 2531, 1924, 1728 (C=O in COOH), 1600, 1549, 1456, 1384, 1339, 1285, 1212, 1185, 1066, 1007, 918, 907, 889, 826, 789, 759, 747, 717, 682, 633, 612.

Synthesis of Polymer (polyNi)

PolyNi: Ligand L (50 mg, 0.075 mmol) was dissolved in 15 mL of methanol and taken in a 100 mL, two-neck, round-bottom flask. To this, 5 mL of methanolic solution of Ni(CH3COO)2 4H2O (18.71 mg, 0.075 mmol) was added and heated at 60 °C for 24 h under the nitrogen atmosphere. The precipitate was separated by filtration and purified by washing with solvents, such as chloroform, methanol, and hexane, which removed the ligand and soluble metal ions. Additionally, the compound was dried under reduced pressure overnight to obtain polyNi as a green solid (30 mg). Elemental analysis Calcd for [polyNi + 5CH3COOH + 2MeOH] = C51H66N2O20Ni: C, 56.42; H, 6.13; N, 2.58. Found: C, 55.69; H, 5.46; N, 3.40 FT-IR (KBr) cm–1: 3393 (broad peak, O–H stretching in COOH), 2953, 2928, 2857, 1732 (C=O stretching in COOH), 1636, 1599, 1456, 1437, 1395, 1387, 1350, 1319, 1283, 1263, 1173, 1138, 1076, 1036, 1007, 953, 930, 908, 889, 806, 743. The molecular weight (Mw) of the polymer was measured using the SEC–viscometry–RALLS method. Mw = 2.5 × 104 Da.

Synthesis of Imidazole-Loaded Polymer (polyNi-Im)

PolyNi-Im: To synthesize the imidazole-loaded Ni(II)-based MSP (polyNi-Im), first, we have prepared a polyNi film using 15 mg of polyNi dispersion in 0.5 mL of MeOH by spreading over a glass slide (area = 75 mm × 25 mm). Later, the film was degassed by heating to 120 °C under vacuum overnight, and imidazole vapor at 120 °C was exposed over polyNi for 5 h. The imidazole-incorporated polyNi was scratched from the glass slide to give polyNi-Im. FT-IR (KBr) cm–1: 3420 (broad peak, O–H stretching in COOH), 3219 (−N–H stretching frequency), 3142 (−N–H stretching frequency), 3045 (−C–H stretching frequency in imidazole), 2953, 2928, 2857, 1740 (C=O stretching in COOH), 1640, 1599, 1428, 1389, 1327, 1286, 1263, 1172, 1138, 1099, 1069, 1038, 932, 889, 835, 812, 743, 664, 629. MALDI-TOF m/z: [(L(−2H) + Ni2 + 2imidazole)] calcd for C45H46N6O8Ni 856.27; found, 856.20 and [(L(−2H) + Ni2 + 2imidazole + Na)] calcd 880.27; found, 880.55.