Cationic microporous polymer networks by polymerisation of weakly coordinating cations with CO₂-storage ability

Abstract

Microporous organic polymer networks with weakly coordinating cations in their backbone have been synthesised by metal catalysed C–C bond forming reactions. A functionalised tetraphenylphosphonium ion was synthesised and successfully used as a tecton in a co-polymerisation with tetrakis(4-bromophenyl) methane using nickel catalysed Yamamoto coupling and with triethynylbenzene in a palladium catalysed Songashira–Hagihara reaction. The microporous materials showed an apparent BET surface area of 1455 m² g⁻¹ and 540 m² g⁻¹, respectively. The Yamamoto product provide a CO₂ uptake of 2.49 mmol g⁻¹ at 273 K and 1 bar. After ion exchange with chloride CO₂ uptake is further increased to 2.85 mmol g⁻¹.

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Introduction

Microporous polymer networks are of increasing interest regarding their potential applications e.g. in the areas of separation, catalysis and gas storage.^1–7 Several classes of these materials, which are built by polymerisation of rigid, two or three dimensional organic tectons have been described such as polymers of intrinsic microporosity (PIMs),⁸ covalent organic frameworks (COFs, CTFs)^9–11 and conjugated microporous polymers (CMPs).^12,13 Depending on the functional groups of the tecton different polymerisation reactions have been described to build such networks, such as oxidative polymerisations,^5,14 polycondensations¹⁵ and trimerisations^16,17 as well as Sonogashira,^12,18,19 Suzuki^20,21 or Yamamoto^22,23 metal-catalysed C–C coupling reactions.

Some microporous polymer networks show exceptionally high surface areas which allow the storage of high amounts of gases such as hydrogen or carbon dioxide.^19,24,25 For example tectons based on tetraphenylmethane subunits exhibit BET surface areas of up to 5640 m² g⁻¹.²³ Related tetrahedral tectons were reported, using silicon or germanium as central atom to connect the four phenyl rings.^26–31 Beside group 14 elements, the exchange of the central carbon with elements from group 13 or 15 should yield negatively or positively charged tectons. The first group reporting a charged microporous polymer network based on such quaternary tectons were Zhang et al.,³² who used the positively charged phosphorus analogue of tetraphenylmethane, i.e. tetraphenylphosphonium cations with halides as counter ions. By polymerising this tecton via Yamamoto C–C coupling reaction a polymer was obtained that contained a mixture of phosphonium and phosphine moieties within the polymer resulting from partial decomposition of the phosphonium tecton during polymerisation. Recently we reported the use of tetrakis(4-bromo, 2,3,5,6-tetrafluorophenyl)borate as a tecton to generate a microporous anionic borate network (ABN) by Sonogashira–Hagihara coupling with 1,3,5-triethynylbenzene as comonomer.³³ The counter cations, located within the pore channels, are highly accessible and can be easily exchanged, comparable to the extra framework cations found in zeolites. Ion exchange followed by the introduction of a ligand yielded an immobilised charged organometallic complex which can be used as a catalyst in the aerobic oxidation of alkenes.

Results and discussion

Here, we present the synthesis of two charged co-polymer networks containing a positively charged tetrahedral monomer, i.e. tetraphenylphosphonium and a neutral comonomer. The first polymer network is in principle the cationic counterpart of the previously reported anionic microporous polymer network (ABN). Instead of the negatively charged tetrakis(4-bromo, 2,3,5,6-tetrafluorophenyl)borate, the positively charged tetrakis(4-bromophenyl)phosphonium bromide was used as a monomer in a Sonogashira–Hagihara coupling with 1,3,5-triethynylbenzene. The second polymer network is generated by a Yamamoto coupling of the tetrakis(4-bromophenyl)phosphonium bromide with tetraphenylmethane. As the direct coupling of tetrakis(4-bromophenyl)phosphonium bromide yields to substantial decomposition of the monomer as reported by Zhang³² we envisioned that the use of a mixture of charged and non-charged monomers might help to stabilise the charges within the network in the same way as for zeolites. Löwenstein's rule for zeolites states that it is impossible to build in two aluminum atoms side by side in a tetrahedral composite. Even though this rule cannot simply be applied to organic networks, it inspired us to dilute the charged tectons with neutral ones to prevent the decomposition which was observed by Zhang et. al.³⁴ and also by our own experiment. Note that also for the negatively charged borate tectons, we observed partial decomposition when a direct coupling was attempted, while the addition of a neutral comonomer yielded pure borate networks.³⁴

The synthesis of a phosphonium monomer containing polymerisable bromine groups was undertaken by firstly reacting 1,4-dibromobenzene and phosphorus trichloride to yield tris(4-bromophenyl)phosphine. A fourth bromophenyl group was introduced via palladium catalysed coupling reaction to obtain tetrakis(4-bromophenyl)phosphonium bromide (see ESI†). This tetrahedral monomer was then able to participate in a nickel catalysed Yamamoto reaction together with the neutral comonomer tetra(4-bromophenyl)methane to produce an insoluble yellow network in 97% yield (Scheme 1). The resulting cationic tetraphenylphosphonium-tetraphenylmethane bromide network (CPN-1-Br) is solely composed of phosphorous in the form of cationic phosphonium. Infrared spectra of the resulting polymer network clearly indicates the disappearance of the characteristic arylbromide bond at 1065 cm⁻¹ (Fig. S1).† Together with the high yield this points to the successful inclusion of both monomers into the network.

Scheme 1 Synthesis of the CPN-1-Br.

A second charged phosphonium microporous polymer network (CPN-2-Br) was formed by the palladium catalysed Sonogashira–Hagihara cross-coupling of tetrakis(4-bromophenyl)phosphonium bromide with 1,3,5-triethynylbenzene using optimised reaction conditions used previously for the synthesis of the anionic charged network³³ (Scheme 2). Again high yields of ∼100% and the disappearance of the characteristic arylbromide bond at 1065 cm⁻¹ (Fig. S1†) point to the inclusion of both monomers.

Scheme 2 Synthesis of CPN-2-Br.

The chemical identities of both materials were further confirmed by ¹³C- and ³¹P solid state NMR. For the Yamamoto linked network CPN-1-Br, signals at δ = 143, 138, 129, 125 and 63 ppm in the ¹³C {¹H} MAS NMR indicate the presence of five different carbon environments (Fig. 1a). The signal at 63 ppm can be assigned to the central quaternary C-atom connected to four phenyl groups. The intense signal at 125 ppm can be assigned to the C_Ar–H of the tetraphenylphosphonium unit and the signal at 129 ppm can be assigned to the C_Ar–H of the tetraphenylmethane unit. The signal at 137 ppm corresponds to C_Ar–C_Ar–atoms as well as the carbons connected to phosphonium, while the aromatic carbons adjacent to the central carbons in the tetraphenylmethane unit exhibits a chemical shift of 143 ppm.

Fig. 1 a) ¹³C-NMR and (b) ³¹P-NMR spectra of CPN-1-Br.

Fig. 2a shows the ¹³C {¹H} MAS NMR of CPN-2-Br. The resonance at 89 ppm can be assigned to the acetylene groups and the signal at 122 ppm to the aromatic carbons of both monomers next to the triple bond. The intense signals at 126 ppm and 129 ppm can be assigned to the C_Ar–H of the tetraphenylphosphonium unit as well as of the 1,3,5-triethynylbenzene. The signal of the C_Ar–P–atoms is observed as a shoulder of the broad signal in the area between 133 ppm to 143 ppm. Notably, the ³¹P-NMR spectra of both materials (Fig. 1b and 2b) show just one intense signal at 24 and 25 ppm with a comparable shift to that seen for the monomer, tetrakis(4-bromophenyl)phosphonium bromide. No resonance due the cleaved corresponding phosphine can be observed in contrast to the network previously reported.³² We conclude therefore that the additional non-charged monomers were able to stabilise the charged phosphonium units in the polymer network.

Fig. 2 a) ¹³C-ssNMR and (b) ³¹P-ssNMR of CPN-Br.

The networks were also characterised by nitrogen gas adsorption/desorption at 77 K. The Yamamoto network CPN-1-Br showed a Type I isotherm with a significant amount of gas adsorbed at low pressures followed by a large adsorption at higher pressures attributed to gas condensing into the voids between the polymer particles (Fig. 3). As often observed for the relatively soft polymer networks, the desorption branch does not close on the adsorption at low pressures.³⁴ An apparent BET surface area of 1455 m² g⁻¹ was calculated for the network over a relative pressure range of 0.01–0.15 P/P₀ (see Fig. S2†). The total pore volume was calculated to be 1.0 cm³ g⁻¹. The Sonogashira network CPN-2-Br shows a lower BET surface area of 540 m² g⁻¹ compared to CPN-1-Br. A pore volume of 0.33 cm³ g⁻¹ was calculated for CPN-Br.

Fig. 3 Nitrogen adsorption (closed)/desorption (open) isotherms at 77 K for CPN-1-Br (black squares), CPN-1-Cl (red circles) and CPN-2-Br (blue triangles).

As previously shown for borate networks³³ the counter ions can be exchanged after synthesis. Here, the as synthesised bromine ions can be simply and completely exchanged by chloride ions (CPN-1-Cl), by stirring the CPN-1-Br in saturated sodium chloride solution and methanol as shown by EDX analysis (see Fig. S11 and S12†). After ion exchange the BET surface area slightly increased to 1504 m² g⁻¹ with a pore volume of 1.0 cm³ g⁻¹. Pore size distributions were calculated for the networks and showed the largest pores for CPN-1-Br and CPN-1-Cl to be centred around 16 Å (see Fig. S5†) similar to that for PPN-6.³⁵ CPN-2-Br had a smaller pore size (7 Å).

The CO₂ uptakes for the networks were measured at 273 (Fig. 4) and 195 K (Fig. S8 and S9) up to a pressure of 1 bar. It was assumed that for CO₂ with its quadrupole moment, the introduction of network charges might have a considerable influence on the adsorption capacity. Furthermore, it was interesting to investigate if ion exchange within such networks influences the sorption characteristics, e.g. due to different size or Lewis acidity of the respective counter anions. CPN-1-Br showed an uptake at 273 K of 2.49 mmol g⁻¹ while the network after anion exchange, CPN-1-Cl, showed an increased CO₂ uptake of 2.85 mmol g⁻¹ at 273 K and 1 bar. In comparison to the non-charged network (PAF-1/PPN-6) the CO₂ uptake is increased by around 40% despite a much lower surface area (1504 m² g⁻¹vs. 5600 m² g⁻¹).²⁵ This increased uptake can thus be attributed to the charges within the network. The Sonogashira network (CPN-2-Br) showed a lower uptake than the Yamamoto networks (1.55 mmol g⁻¹) in line with its lower surface area. Using the two adsorption isotherms at 195 and 273 K the isosteric heats of adsorption were calculated using Clausius–Clapeyron equation (Fig. S10†). The heat of adsorption of CPN-1-Br at zero coverage was calculated to be 31.0 kJ mol⁻¹ placing it amongst the highest reported microporous polymer networks and almost double the non-charged PAF-1/PPN-6 analogue (15.6–17 kJ mol⁻¹).^23,35 This value is similar to the acid containing PPN-6-SO₃H (ref. 35) and CMP-1-COOH (ref. 36) networks, although lower than the amine functionalised PPN-6 series (such as PPN-6-DETA).³⁷ The chloride exchanged network, showed a similar heat of adsorption while the Sonogashira network (CPN-2-Br) was slightly lower at 28.2 kJ mol⁻¹ at zero coverage rising to 31.8 kJ mol⁻¹ at a coverage of 7 cm³ g⁻¹.

Fig. 4 Carbon dioxide adsorption (closed)/desorption (open) isotherms for CPN-1-Br (black squares), CPN-1-Cl (red circles) and CPN-2-Br (blue triangles) measured at 273 K.

Conclusion

In conclusion, we have successfully synthesised two different charged microporous organic polymer networks CPN-1-Br and CPN-2-Br using tetrakis(4-bromophenyl)phosphonium bromide as a tecton. The tecton was stabilised by an analogous methane monomer which coupled trough a Yamamoto reaction with the tecton to form the CPMN-Br. The polymer network CPN-Br was obtained through a Sonogashira–Hagihara coupling with 1,3,5-triethynylbenzene. In this case 1,3,5-triethynylbenzene is the neutral, uncharged stabilising factor, which ensures the intactness of the tetrahedral phosphorus building unit during the reaction. Nitrogen sorption revealed a high surface area that, together with the charged nature of the polymer, ensured very good CO₂ uptakes for the CPN-1-Br and CPN-1-Cl in comparison to non-charged network (PAF-1/PPN-6).

Acknowledgements

We acknowledge the European Research Council for financial support within the project 278593_ORGZEO. The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007–2013) under REA grant agreement no. PIEF-GA-2011-300534.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ta02022g

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