Covalent organic polymers are an emerging class of materials with potential applications in areas such as molecular separation,gas sorption,and catalysis ^[1,2]. The successful design and synthesis of these polymers linked by B-O ^{[3, 4, 5]},C-C ^[6,7],C-N ^[8],and C=N ^{[9, 10, 11, 12]} bonds have been reported. Recent studies have shown that conjugated organic polymers (COPs) with extended π-conjugation possess high thermal stability and structure diversity ^{[13, 14, 15]}. However,the synthesis of these COP materials usually requires expensive precursors and intricate techniques. Further modification of these COPs for practical use is difficult because of limited synthetic diversification. Although nuclear magnetic resonance techniques have been employed,the precise chemical structures of COPs are mostly unknown. For catalytic applications,it is desirable that COP materials should be stable toward heat,air,and solvents,and be designable.

In organometallic chemistry a wide range of ligands have been used to bind metals,and bis(imino)pyridines ligands are among the most common (Scheme 1). This is presumably because of their relatively facile functionalization,stability and ability to bind a large number of transition metal ions (d and f blocks) ^{[16, 17, 18, 19, 20]}. With advantages of separation,recycling,and reuse,heterogeneous catalysts are widely used in the production of chemicals. To date the use of COPs as catalytic materials is rare. We thus investigated the incorporation of bis(imino)pyridine ligands into COPs to prepare solid catalysts upon coordination with metal ions. Herein we report,for the first time,a method of preparing bis(imino)pyridine-containing conjugated polymers (COP-BIP) based on Schiff-base chemistry. The COP-BIP material was highly stable up to 440 °C and was insoluble in water and aprotic solvents. The bis(imino)pyridine ligands,as binding sites,formed complexes with Pd ion to give a heterogeneous catalyst,which demonstrated excellent catalytic activity in a classic Suzuki-Miyaura reaction. We also show that matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy is a direct and reliable method for the determination of the COP structure.

The COP-BIP material was prepared from the reaction between 2,6-pyridinedicarboxaldehyde and 1,4-diamine benzene at 80 °C in acetonitrile via an imination reaction with the elimination of water,as shown in Scheme 1. The mixture became yellow and then turbid. At 24 h,a precipitate had formed and was collected by centrifugation. A small amount of acetic acid can remarkably accelerate the reaction,but it was not used in our synthesis so that a pure material could be obtained. Solvents such as methanol,ethanol,dimethylformamide,and dimethyl sulfoxide were used and gave identical materials.

Scheme 1. Synthesis of the COP-BIP material.

A Suzuki-Miyaura cross-coupling reaction was carried out as follows. About 0.5 mmol 4-bromoanisole in 1 mL ethanol (HPLC grade),0.75 mmol phenylboronic acid in 1 mL ethanol,1.0 mg PdCl₂/COP-BIP,and 0.1 mmol K₂CO₃ were added sequentially into a 5 mL vial and stirred at 50 °C for 3 h. The conversion of 4-bromoanisole was determined by gas chromatography using a calibration curve.

Monitoring the reaction by time-dependent liquid ultraviolet-visible (UV-Vis) spectroscopy showed that the C=N absorption at 375 nm constantly increased (Fig. 1),indicating that the reaction between -CHO and -NH₂ was consistent with that reported in the literature wherein the bright yellow color was due to the C=N chromophore ^[10].

Fig. 1. Monitoring the formation of the C=N bond during the synthesis at 375 nm by liquid-phase UV-Vis spectroscopy.

The successful build-up of a conjugated polymer was confirmed by Fourier transform infrared (FT-IR) spectroscopy (Fig. 2). A strong vibration at 1623 cm^-1 clearly indicated the formation of imine bonds (C=N). The aldehyde (C=O) peak at 1710 cm^-1 and the two broad bands at 3437 and 3357 cm^-1 assigned to unreacted -NH₂ suggested the co-existence of terminal -CHO groups and -NH₂ groups.

Fig. 2. FT-IR spectra of the COP-BIP material. (1) Freshly synthesized material dried at 110 °C in air for 2 h; (2) Treated at 250 °C in H2 for 2 h.

The conjugated polymer exhibited high stability as evidenced by thermal gravimetric analysis (TGA). As shown in Fig. 3,an apparent decomposition (weight loss of approx. 3%) occurred at 440 °C in N₂ and at 420 °C in air. These results indicate that COP-BIP is more stable than other covalent organic polymers such as triazine-based frameworks (about 410 °C in air) ^[20],melamine-based polymer (about 400 °C in N₂) ^[8],COF-LZU1 (about 310 °C in N₂) ^[2],and POFs (about 400 °C in N₂) ^[10]. By comparison,a polymer with a N,O,N unit ^[21],which is different to the N,N,N unit of COP-BIP,is stable at 300 °C in N₂. The imine can be reduced to the amine by hydrogenation. A test conducted in H₂ at 250 °C found no apparent change in structure as observed by IR (Fig. 2). Therefore,COP-BIP was very stable under both oxidative and reductive conditions. We conclude that the high stability of COP-BIP arises from the bis(imino)pyridine sites and the fully conjugated structure.

Fig. 3. TGA curves of COP-BIP in N2 (a) and artificial air (N2:O2 = 4:1) (b). Experiment conditions: ramp rate 10 °C/min,N2 flow speed 30 mL/min.

COP-BIP is insoluble in conventional solvents such as hydrocarbons,alcohols,ketones,nitriles,amides,and sulfoxides. This inherent insolubility responsible for the desirable properties of the COP materials makes a detailed structural analysis difficult. MALDI-TOF mass spectrometry is used to characterize oligomers and can provide direct and unambiguous structural information ^[22]. We have reported the use of this technique in peptides analysis ^[23]. In this study,the slow mixing of the two monomer solutions generated oligomers during the initial reaction,and this could be analyzed by MALDI-TOF mass spectrometry. It is noteworthy that the precise structures of COP materials are seldom reported. In this study,the MALDI-TOF analysis confirmed the mass and thus helped to establish the COP-BIP structure. Because of the tendency of MALDI-TOF mass spectrometry to favor the ionization of lower molecular weight species,the spectra of these oligomers should not be considered to represent the true molecular weight distribution ^[24].

The mass spectra of the COP-BIP oligomers indicated that at a m/z ratio less than 3000 it had distinctive peaks (Fig. 4(a)). Three series of single polymers were present with the same repeat unit of 207 m/z,which is the molar mass of the repeat unit (see square brackets in Scheme 2). Figure 4(b) shows an enlarged view of the m/z ratio from 600 to 1000. Two sets of peaks (n = 2 and 3,where n is the number of the repeat unit) were differentiated by an interval of 207 m/z. These results agree well with the expected masses from the three different end-groups (Scheme 2 and Table 1). For n increased to 1 above,all the calculated mass spectra were found in the experimental MALDI-TOF (Table 1 and Fig. 4(a)). Oligomers of N-n-N and O-n-O can be viewed as exclusive N-n-O (225 + 207n) reactions with 1,4-diamine benzene to form N-n-N (315 + 207n),and with 2,6-pyridinedicarboxaldehyde to form O-n-O (342 + 207n),respectively. The isotopic distributions obtained experimentally by this technique are in good agreement with the predicted values (obtained by using the free software IsoPro 3.1 (a free 30 day evalutation version) written by M. Senko),as demonstrated for N-2-O,N-2-N,and O-2-O in Fig. 4(c).

Table 1 Calculated mass signals of the oligomers with varying repeat units n*. n 1 2 3 4 5 6 7 >7 N-n-O — 639 846 1053 1260 1467 1674 ... N-n-N 522 729 936 1143 1350 1557 1764 ... O-n-O 549 756 963 1170 1377 1584 1791 ... * The table data show the maximum isotopic mass. The calculation was based on the formula shown in Scheme 2.

Table 1 Calculated mass signals of the oligomers with varying repeat units n*.

Scheme 2. Three types of conjugated COP-BIP chain and their calculated molecular weights. N indicates the NH2-terminated groups (MW = 92 or 223),and O is the CHO-terminated groups (MW = 133 or 209). n indicates the number of repeat units in square brackets (MW = 207).

Fig. 4. (a) MALDI-TOF mass spectra of COP-BIP in the range of m/z = 500-3000; (b) Enlarged view of the m/z ratio from 600 to 1000; (c) Calculated (dashed line) and observed (solid curve) isotopic distributions for N-2-O,N-2-N,and O-2-O.

The unreacted end-groups of -CHO and -NH₂ can further react with aniline and benzaldehyde,respectively,to form a benzene-terminated polymer under the same conditions,the so called “end-cap” ^[10]. This reaction can be used to gauge the amount of terminal amine and aldehyde groups in COP-BIP. End-capping of the -CHO groups can be clearly seen in the partially capped oligomers shown in Fig. 5(a). For example,the O-3-O oligomer (963 m/z) has two -CHO groups at the ends. After reacting with aniline,it should increase its mass by 75 m/z,which was present as (O-3-O + 75,1083 m/z) in Fig. 5a. The oligomer (O-3-O + 75) reacted with aniline,and its mass was increased to (O-3-O + 75 + 75,1158 m/z). The same calculation rule was applied to the N-n-O oligomers. No increase in mass caused by the N-n-N oligomers was found in the MALDI-TOF analysis. The mass signal intensities increased as follows: O-3-O < (O-3-O + 75) < (O-3-O + 75 + 75),indicating the end-capping approached completion although the MALDI-TOF analysis was semi-quantitative. Similar results were found when end-capping with benzaldehyde (Fig. 5(b)). The N-n-O and N-n-N oligomers were capped,and their mass increased by 88 m/z.

Fig. 5. (a) End-capping reaction with aniline. (b) End-capping with benzaldehyde. Note: N-2-O + 88 (727 m/z) had a similar mass to N-2-N (729 m/z),and thus their isotopic partly overlapped.

Bis(imino)pyridines,as binding sites,have many potential applications such as ligands for metal ions to produce catalysts. Herein,we prepared a Pd catalyst with bis(imino)pyridine as the ligand (PdCl₂/COP-BIP). It was prepared by soaking COP-BIP in a boiling PdCl₂-acetonitrile solution for 6 h. The catalyst was then washed three times with hot acetonitrile to remove weakly adsorbed PdCl₂. Elemental analysis (Table 2) and TGA gave a comparable Pd weight content (8.5% and 8.1%,respectively),suggesting that about 20% of the bis(imino)pyridine coordinated with the Pd ion. The PdCl₂/COP-BIP catalyst showed similar thermal stability to COP-BIP (Fig. 6). The catalyst was evaluated in a Suzuki- Miyaura C-C coupling reaction,which is used in homogeneous media for the facile formation of C-C bonds (Table 3) ^[25]. Under reaction conditions less demanding than that reported in the literature for MOF and COP materials ^[2],the conversion of p-bromoanisole reached (confirmed by MS) > 99% with a selectivity for p-phenyl anisole of > 99% over the 0.16% Pd catalyst. The turnover number (TON) was calculated as 625,indicating that PdCl₂/COP-BIP possessed high catalytic activity during the catalysis of the Suzuki-Miyaura coupling reaction. The catalyst was filtered out and reused in the same reaction another three times without an apparent loss in catalytic activity (Table 4). No Pd leaching was detected in the reaction solution,which was confirmed by ICP analysis. The superior catalytic activity and catalyst stability of PdCl₂/COP-BIP are attributed to the bis(imino)pyridine ligands incorporated in the conjugated organic polymer.

We present an approach for the synthesis of a fully conjugated organic polymer (COP-BIP) and demonstrate a reliable method to determine its structure. The COP-BIP material benefits from a cheap and straightforward strategy,in contrast to the expensive and elaborate protocols previously reported. The COP-BIP polymer containing bis(imino)pyridine ligands may find wide application in catalysis,adsorption,and photochemistry.