Proton Conductivity Tetrafluoroaryl Phosphonic Acid Functionalized Polyphosphazenes – Synthesis, Characterization, and Evaluation of Proton Conductivity

: A convergent approach for the incorporation of tetrafluoroaryl phosphonate moieties into cyclic triphosphazenes and linear phosphazene resins is described. Our high yield procedure is based on the treatment of chlorinated poly-and cyclotriphosphazenes with p -HO(C 6 F 4 )P(O)(OR) 2 (R = Me, Et) in the presence of potassium carbonate. Characterization of the modified cyclotriphosphazenes was accomplished by NMR and IR spectroscopy as well as by mass spectrometry. Similarly, a phosphazene resin decorated with phosphonic esters is charac-Introduction


Introduction
Polymer-based electrolyte membranes are important components in electrochemical energy conversion and storage devices, functioning as conductors and separators. In fuel cells, the use of polymeric electrolyte membranes could circumvent the frequent nuisance of leakage associated with liquid electrolytes. [1] However, there are several requirements a polymeric material must fulfil prior to use as a membrane in fuel cells. In addition to an efficient proton conductivity the materials should have low diffusion coefficients for the fuel (e.g. H 2 or MeOH) and oxidant (e.g. O 2 ), and moreover should be mechanically and chemically robust. [2] The most common materials used in fuel cells are polymeric perfluoroalkyl sulfonic acids, e.g. Nafion®. These materials provide an excellent proton conductivity in the presence of moisture at temperatures up to 80°C. [3] Despite the broad application of the sulfonic acids in polymer electrolyte fuel cells, these materials suffer from major drawbacks.
At temperatures above 80°C the proton conductivity decreases rapidly due to evaporation of water from the membrane.
terized by NMR and IR spectra and GPC. Exchange of the ethyl group by a trimethylsilyl group in the novel phosphazene derivatives was effected by the reaction with trimethylsilyl bromide. The resulting silyl phosphonates were converted into the corresponding phosphonic acids by exposure to an excess of methanol. Proton conductivities of the novel phosphonic acid derivatives of poly-and cyclotriphosphazenes were studied by electrochemical impedance spectroscopy under anhydrous conditions.
This limits the working temperature for fuel cells and requires additive humidification and cooling to provide an efficient power output of the fuel cell. [4] Furthermore, Nafion® is not suitable for the use in methanol fuel cells due to high methanol crossover rates. [5] Overall, the use of Nafion® based membranes restricts the operation of fuel cells to a narrow temperature window (T < 80°C) and prohibits the use of methanol as an inherently safer fuel than hydrogen gas. [6] Polyphosphazenes render a promising class of polymers for the design of novel proton exchange membranes that may overcome the mentioned drawbacks. These polymers are defined by an -N=P-backbone bearing two substituents (inorganic, organic) at the phosphorus atoms. [7] They provide durability at high temperatures and severe oxidative stress, due to the thermal and chemical stability of the -N=P-backbone. [7] Furthermore, the properties of polyphosphazenes can be easily tuned by the choice of sidegroups introduced to the polymer. The most common synthetic route to introduce specific sidegroups is by the use of [P(Cl) 2 N] n as polymeric precursor, which can be simply functionalized by nucleophilic substitution. [8] Several ionomers based on polyphosphazenes have been reported. [7a,9] They feature sulfonic acid groups as well as sulfonamine and phosphonic acid groups as protogenic moieties. Experimental studies on phosphonated and sulfonated phosphazenes reported that these materials are comparable to Nafion regarding their proton conductivity and superior regarding their methanol permeability. [10] However, for the design of a membrane with sufficient conductivity at 120°C, phosphonic acids seem to be one of the most promising protogenic moieties, as discussed in the literature. [11] As reported in recent papers by our group, perfluoroaryl phosphonic acid provides an even higher conductivity than its non-fluorinated counterpart at high temperatures and low humid conditions. [12] As shown by, Desmarteau et al. [13] fluorinated phosphonic acid also provide a higher conductivity than sulfonic acid derivatives which are currently used in Nafion® based membranes. Polyphosphazenes bearing perfluorinated sulfonic acid groups have already been reported and exhibit higher conductivity and stability than their aliphatic derivative. [14] In view of these observations, polyphosphazenes functionalized with fluorinated phosphonic acids should be promising candidates for membranes in high temperature fuel cells and are, to the best of our knowledge, hitherto unknown.
Here we report on the first examples of cyclotriphosphazenes and polyphosphazenes functionalized with fluorinated 4-oxy-aryl phosphonic acids.

Synthesis and Characterization of the Precursor p-HO(C 6 F 4 )P(O)(OR) 2 ; R = Me, Et
As previously described by our group, bis(diethylamino)pentafluorophenylphosphane (1) is readily functionalized with LiOMe to furnish the corresponding phosphane 2 in a 66 % yield (Scheme 1, I). [15] An alternative synthesis makes use of the coupling of lithium tetrafluoroanisole and ClP(NEt 2 ) 2 (3) (Scheme 1, II). The employment of this new synthetic route allows the preparation of phosphane 2 in higher yields (85 %) and quantities up to 20 g. Hydrolysis of aminophosphane 2 with aqueous HCl and subsequent mild oxidation of the obtained phosphinic acid 4 with DMSO/I 2 led to the formation of phosphonic acid 5 (Scheme 2). [15] The analytic data of the described derivatives 1-5 are extensively elaborated in our previous report. [15] Treatment of 5 with oxalyl chloride in the presence of catalytical amounts of DMF in dichloromethane quantitatively furnished phosphonic acid chloride 6 as a yellow solid (Scheme 3). Dealkylation of the methoxy functionality of 6 is achieved by BBr 3 in dichloromethane (Scheme 4). [17] After the dealkylation reached completion, the addition of alcohols (MeOH, EtOH) to the reaction mixture leads to the cleavage of the boron-oxygen bond and to the esterification of the phosphonic acid. Removal of all volatile compounds and recrystallization from tetrahydrofuran yields the pure methyl ester 7 (78 %). The ethyl ester 8 is isolated in an 85 % yield after column chromatographic workup. Colorless single crystals of 8, suitable for an X-ray diffraction analysis, are grown from tetrahydrofuran. The compound crystallizes in the triclinic space group P1. Hydrogen bridging of O4-H1 to O1# in the crystal leads to the formation of infinite chains ( Figure 1). The O4-O1# distance amounts to 255.5(1) pm.

Synthesis and Characterization of Cyclic Triphosphazenes and Polyphosphazenes
Functionalization of hexachlorotriphosphazene with phosphonic esters 7 and 8 was performed in tetrahydrofuran in the presence of K 2 CO 3 (Scheme 5). [18] Substituted aryloxy phosphazenes 9 and 10 were isolated in good yields (9: 90 %; 10: 66 %). Complete substitution of the chlorine atoms was proved by 31 P NMR spectroscopy. Resonances for the symmetric phosphazene 9 are observed at 8. In addition to the spectroscopic data, suitable crystals of phosphazene 9 were grown by layering a tetrahydrofuran solution with n-pentane. The crystals were subjected to an X-ray diffraction analysis. The analysis confirms the full substitution of the chlorine atoms of the cyclic phosphazene. The distances of the phosphorus-nitrogen atoms in the phosphazene ring are ranging from 157.3(2) pm to 157.8(2) pm and agree with multiple bonding ( Figure 2). [19] Figure 2. Molecular structure of phosphazene 9. (Thermal ellipsoids are set to 50 %, organic substituents are featured in wire and stick for clarity, hydrogen atoms, solvent THF and minor occupied disordered atoms are omitted for clarity.) In addition to the functionalization of the cyclotriphosphazene we studied the functionalization of polydichlorophosphazene with phosphonic ester 8 (Scheme 5). The here employed polydichlorophosphazene was synthesized from Cl 3 P=NSiMe 3 as reported by Allcock et al. [20] The substitution by the hydroxyl fluorophenyl phosphonic ester 8 was performed analogously to the trimeric derivatives. The resulting off-white solid 11 is well soluble in DMF and DCM. GPC analysis of the obtained material was performed in DMF (+0.05 M LiBr). The elugram is depicted in Figure 3. Evaluation of the molar mass distribution relative to PMMA standards results in M n = 1860 g/ mol and M w = 1910 g/mol. The obtained dispersity D: 1.03 is as narrow as expected for polyphosphazenes derived from a controlled polymerization of Cl 3 P=NSiMe 3 with PCl 5 . [20] NMR spectroscopic analysis reveals resonances assigned to the phosphorus atom of the phosphonic ester at 3.45-5.73 ppm. Signals assigned to the phosphorus atoms of the phosphazene backbone are observed at -15.7 to -22.0 ppm. The intensities of the signals match the expected ratio of 2:1 ( Figure 4).
Dealkylation of the phosphonic ester functionality of phosphazenes 10 and 11 is achieved with Me 3 SiBr in dichloromethane at room temperature. When the silylation of the ester  P NMR spectrum of the product displays two resonances due to the phosphorus atoms of the phosphazene ring at 8.8 ppm and to the phosphonic acid at -2.6 ppm. Volumetric titration of the acid 12 in water with 0.1 M NaOH reveals the expected titration curve with two equivalence points. The pK S1 and pK S2 values amount to 2.1 and 6.7. The pK S1 value is only tentative due to the inapplicability of the Nernst equation at pK S values near 2. [21] Dealkylation of the polymeric ester 11 results in a brittle material after cleavage of the silyl ester moieties by methanol. The ion exchange capacity (IEC) of the polymer 13 was determined by volumetric titration with 0.1 M NaOH containing 0.1 mol/L NaCl and amounts to 6.8 mmol g -1 . An IR spectroscopic analysis reveals broad signals in the range of 2460-1990 cm -1 which are indicating hydrogen-bridging of P=O and P-OH functionalities. Besides the signal for O-H stretching vibrations of water (3700-3120 cm -1 ) a broad band in the range of 3120-2480 cm -1 results from P-OH stretching vibrations. At 1293 cm -1 and 1273 cm -1 the characteristic P=N stretching vibrations of polyphosphazenes are observed. The 31 P NMR spectrum of the polymeric acid 13 reveals two characteristic groups of signals. Signals assigned to the phosphorus atom of the acid functionality are observed from -1.5 to -3.5 ppm, those of the phosphorus atom in the polymeric backbones resonate from -12.0 to -23.0 ppm.

Proton Conductivity Measurements
The conductivity of the phosphonic acid functionalized triphosphazene 12 and polyphosphazene 13 is ascertained by electrochemical impedance spectroscopy under anhydrous conditions. All samples were dried in vacuo for 24 hours to remove traces of water from the solid materials, as water has a drastic influence on the conductivity of the presented materials. In addition to Nafion® powder, C 6 H 5 P(O)(OH) 2 and C 6 F 5 P(O)(OH) 2 were analysed as references ( Figure 5). As expected, all fluorinated derivatives exhibit a higher conductivity than C 6 H 5 P(O)(OH) 2 . At 120°C the conductivity of all fluorinated derivatives is two orders of magnitude higher than the one of phenylphosphonic acid (1.98 × 10 -7 S cm -1 ). Phosphazene 12 exhibits a conductivity of 1.60 × 10 -5 S cm -1 at 120°C. For the polyphosphazene 13 a conductivity of 6.58 × 10 -5 S cm -1 is determined. Noticeable is the rampant increase in conductivity of polyphosphazene 13 in comparison to the other samples, starting at 3.67 × 10 -9 at 20°C up to 6.58 × 10 -5 S cm -1 (120°C) covering four orders of magnitude. Whilst 13 exhibits the lowest conductivity of the fluorinated samples from 20°C up to 70°C (1.64 × 10 -6 S cm -1 ) the strong temperature dependent increase in conductivity leads to a significantly higher value at 120°C than that of triphosphazene 12. The values at 120°C for 13 and 12 are well comparable with the measured data for Nafion® (7.66 × 10 -5 S cm -1 at 120°C) and C 6 F 5 P(O)(OH) 2 (7.38 × 10 -5 S cm -1 at 120°C). As the conductivity of Nafion® ranges in the same order of magnitude as that of phosphazenes 12 and 13, it is obvious that they are also a powerful candidate for the use in proton conduction and other ionomer applications. As the proton conduction studies were performed under strictly water free conditions, additional studies on the influence of water on the conductivity should be considered for further investigations on these materials. Figure 5. Conductivity of cyclic phosphazene 12 and polyphosphazene 13 as a function of temperature at water free conditions. Conductivities of Nafion®, C 6 F 5 P(O)(OH) 2 and C 6 H 5 P(O)(OH) 2 are given as references.

Conclusion
A convergent approach for the incorporation of the tetrafluoroaryl phosphonate building block into cyclic and polymeric phosphazenes is described. The synthesis is based on the reaction of the precursors p-HO(C 6 F 4 )P(O)(OR) 2 (R = Me: 7, Et: 8) with polymeric or trimeric chlorinated phosphazenes in the presence of K 2 CO 3 . The reaction produces the corresponding organophosphazenes in good yields. The cyclotriphosphazenes 9-10 are fully characterized by spectroscopy (NMR, IR) and mass spectrometry. The crystal structure of the cyclotriphosphazene 9 proves the complete substitution of the chlorine atoms of hexachlorotriphosphazene. Addition of Me 3 SiBr to phosphazene 10 and subsequent addition of methanol led to the corresponding cyclophosphazene decorated by phosphonic acid functions (12). The pK S values were measured by volumetric titration (pK S1 = 2.1; pK S2 = 6.7). The corresponding polyphosphazenes are syn- thesized in accordance to the cyclic trimers and are characterized by spectroscopy (NMR, IR) and GPC. The ion exchange capacity of polymeric phosphonic acid 13 was measured by volumetric titration and amounts to 6.8 mmol g -1 . The proton conductivities of the phosphonic acid analogues of the cyclic trimer 12 and the homologous polyphosphazene 13 were investigated by electrochemical impedance spectroscopy under water free conditions. Both phosphazenes (12: 1.60 × 10 -5 S cm -1 : 13: 6.58 × 10 -5 S cm -1 ) exhibit a high proton conductivity which is comparable to Na-fion® (7.66 × 10 -5 S cm -1 ) at 120°C which renders these materials promising candidates for the design of novel ionomers for high temperature applications.

Experimental Section
The starting material (Et 2 N) 2 PCl [22] was synthesized as described in the literature. For the synthesis of Cl 3 P=NSiMe 3 , [23] polydichlorophosphazene [20] and compounds 2, 4 and 5 please refer to the supporting information. All other chemicals were obtained from commercial sources and used without further purification. Standard high-vacuum techniques were employed throughout all experiments. Nonvolatile compounds were handled in a dry N 2 atmosphere using Schlenk techniques. IR spectra were recorded with a Bruker Alpha FT-IR spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with an ATR unit with a diamond crystal for liquids and solids. The NMR spectra were recorded on a Bruker Model Avance III 300 spectrometer ( 31 P 121.5 MHz; 19

Synthesis of p-HO(C 6 F 4 )P(O)(OMe) 2 (7):
A solution of acid chloride 6 (3.54 g, 13.8 mmol) in DCM (30 mL) was cooled to 0°C and borontribromide (6.90 g, 27.6 mmol) was added. The reaction mixture was stirred at room temperature for 72 hours. A sample of methanol (20 mL) was added at 0°C to the solution. Evaporation of the solvent and recrystallization from THF yielded 2.96 g of the pure product (10.8 mmol, 78 %) as colorless crystals. 1

Syntheses of Polyphosphazene 13:
A sample of polyphosphazene 11 (1.23 g; 1.9 mmol)) was dissolved in dichloromethane (40 mL). At 0°C Me 3 SiBr (1.23 g; 1.9 mmol) was added dropwise. The reaction mixture was stirred for 36 hours. The solvent was evaporated and methanol (10 mL) and water (10 mL) were added to the crude product. After stirring the mixture for one hour the solvents were evaporated to yield 910 mg of the polymeric phosphazene 13 (1.