Prescott, Hillary A.: The Crystal Structures and Thermal Behavior of Hydrogen Monofluorophosphates and Basic Monofluorophosphates with Alkali Metal and N-containing Cations

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Kapitel 3. Synthesis

Methods of synthesis

The hydrogen sulfates studied in [11, 12] were synthesized from sulfuric acid by partial neutralization with the corresponding carbonate, oxide, or hydroxide. The absence of commercially available, pure monofluorophosphoric acid required alternative routes of synthesis for the preparation of the acid and acid salts with little or no impurities. A variety of methods have been used to prepare basic monofluorophosphates in the last 100 years [16]; however, there are fewer possibilities for the synthesis of the hydrogen monofluorophosphates. This is due to the accelerated hydrolysis of the fluorophosphate ion at pH values below 3 and above 9 [19]. Therefore, the two most promising methods of preparation found in the literature were first carried out and evaluated with 31P and 19F NMR.

The method for preparing hydrogen monofluorophosphates via cation exchange [26] started with prepared monofluorophosphates (Reaction 3 and 4). The solution of a monofluorophosphate, Na2PO3F, K2PO3F, or (NH4)2PO3F, was passed through a chromatography column of a H+ charged ion-exchange resin. In a second step, the eluate of dilute, aqueous H2PO3F was partially neutralized with an equivalent amount of monofluorophosphate (Reaction 4).


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M2PO3F rarr aq H2PO3F (cation exchange)

Reaction 3

aq H2PO3F + M2PO3F rarr MHPO3F

Reaction 4

The authors characterized the obtained products by paper chromatography, 31P and 19F NMR spectroscopy, elemental analysis, and X-ray powder diffraction. A 0.01 M aqueous solution of these hydrogen monofluorophosphates was reported to have pH 3.8 and thus the equivalence point of the acid to be pH 3.5 [26].

The other method implies direct synthesis of the monofluorophosphoric acid as patented in 1949 [76]. It involved the reaction of P2O5 with concentrated HF, in which the H2O content was le 33 mol%, and yielded variable mixtures of HPO2F2 and H2PO3F (Reaction 5). The reaction mixture was heated at 358 K for 8 hours to yield a 50:50 mixture of H2PO3F and HPO2F2, when x = 1.

P2O5 + (2+x) HF + (1-x) H2O rarr (2-x) H2PO3F + x HPO2F2

Reaction 5

The products were separated by vacuum distillation; HPO2F2 was distilled off, while H2PO3F remained in the residue. The product was characterized by elemental analysis.

A comparative evaluation showed the following. In the cation exchange synthesis, significant amounts of phosphate were found in the eluate after cation exchange and neutralization. The synthesis with P2O5 and HF resulted in side products: a mixture of monofluorophosphoric, orthophosphoric, polyphosphoric, and difluorodiphosphoric acids was formed when temperatures slightly exceeded 358 K and higher fluorinated products, such as HPF6, were obtained when HF was in slight excess. Product separation failed with H2PO3F found in both the distillate of HPO2F2 and the residue after repeated distillations at several different pressures.

Therefore, the method of cation exchange (Reaction 3 and 4) was adopted and modified for an effective synthesis of the hydrogen monofluorophosphates. The only exception was the attempted synthesis of NH4HPO3F by reacting (NH4)2PO3F with NH4HSO4 as described in [70]. Although the synthesis of NH4HPO3F from (NH4)2PO3F was successful, the relatively low solubility of NH4HPO3F made the complete separation of the monofluorophosphate from (NH4)2SO4 problematic. Repeated washing of the byproduct, (NH4)2SO4, was required for a complete product separation. Therefore, cation exchange was used in further syntheses of NH4HPO3F.

Cation exchange was carried out by using either Na2PO3F or (NH4)2PO3F as a starting


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reagent in first experiments. (NH4)2PO3F was obtained separately by melting uronium phosphate and ammonium hydrogen difluoride together at 443K [70]. Later on, only the comercially available sodium salt was used to reduce the steps of preparation. The acidic eluate was neutralized by a carbonate, hydroxide, or amine instead of a basic monofluorophosphate as described in [26] except for in the syntheses of sodium salts, because Na2PO3F was readily available. Amounts of phosphate formed were kept at a minimum by adding an aqueous solution of the base or amine dropwise into the eluate at a rate, at which the monitored pH remained between 3 and 6. In [26], product isolation involved the precipitation of products from an almost completely evaporated aqueous solution with large amounts of nonaqueous solvents, such as Et2O or EtOH. To improve yields, this working-up procedure was replaced by freeze drying of the solution after partial neutralization. This prevented the condensation of phosphorus and escape of HF during the complete evaporation of the eluate and enabled isolation of the raw products for analysis before recrystallization. Sufficient amounts of raw products were then available for the crystallization experiments described below.

Crystallization

Several methods were used for the crystallization of the freeze dried, raw products to obtain pure, crystalline hydrogen monofluorophosphates for the single crystal structure analysis and further characterization. These included

Slow evaporation in a desiccator of an aqueous solution yielded crystalline batches with approximately the same composition as the original solution-a two phase mixture of the hydrogen monofluorophosphate and hydrogen phosphate. Which compound was the major phase depended on how long the solution was left to stand. The phosphate became the major phase, when the solution was left to stand for longer periods of time.

Fractional recrystallization with slow evaporation was also unsuccessful. A slow, but steady hydrolysis of the HPO3F- anion was observed instead of product separation. The filtered and dried fraction of crystals were characterized by XRD and fluoride analysis. The first fraction, a mixture of the MHPO3F and MH2PO4 phases, was similar to that found in the raw product. The XRD patterns of the second fraction showed a very pure phase of the hydrogen phosphate. Fluoride analysis confirmed increased amounts of free fluoride in


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each further fraction reflecting the hydrolysis of the HPO3F- anion. An initial fluoride content of 0.74% for the raw product increased to 1.46% for the first fraction and to 8.85% in the second fraction.

Therefore, a method of accelerated recrystallization was used. The raw product was dissolved in H2O, MeOH, or EtOH. A second solvent was then gradually added, MeOH or EtOH for an aqueous solution or Et2O for a MeOH or EtOH solution, until a precipitate formed. The solution was refrigerated overnight. The alkali metal hydrogen monofluorophosphates and most of the salts with cations containing nitrogen were crystallized from H2O with EtOH. The compounds with NEt3, NHEt2, and N,N´-dmu were obtained crystalline from EtOH with Et2O. This method yielded adequate amounts of pure hydrogen monofluorophosphates for a further characterization and worked particularly well for the hydrogen monofluorophosphates with N-containing cations.

The fourth method was also quite successful in yielding pure monofluorophosphates. It involved the filtration of an oversaturated aqueous solution of the raw product. The trick here was to use very little H2O and lots of product (Sect. 2.3 beta-RbHPO3F). The filtered solution was then left to stand for 1-3 days until crystals of the hydrogen monofluorophosphate were formed. The crystals formed were filtered and dried.

The overall success of recrystallization was directly dependent on the purity of the freeze dried product. Crystal batches with particularly high levels of phosphate impurities were difficult to recrystallize (Sect. 2.3 KHPO3F). The influence of the cation on recrystallization was also observed and will be discussed later on.

Confirmation of the P-F bond

31P and 19F NMR spectroscopy of aqueous solution and fluoride analysis were used to ensure that fluorine was bonded to the phosphorus atom in the crystalline compounds. The doublet shown in the 31P and 19F NMR spectra verified the existence of the P-F bond in solution. Phase purity was confirmed by the absence or low intensity of the hydrogen phosphate singulet in the 31P spectrum. An integration of the signals, the phosphate singulet and the monofluorophosphate doublet, in the 31P spectrum enabled the approximation of the H2PO4-/HPO3F- ratio. A study of the rate of hydrolysis with a Na2PO3F/H2SO4 solution showed that the phosphate signal increased slightly in intensity after about ten days for the solution with pH 3.76 corresponding to the pH of the alkali metal hydrogen monofluorophosphates and to the same extent after one day for pH 2.67 corresponding to the hydrogen monofluorophosphates with N-containing cations. A phosphate singulet was not observed in the spectra of the Na5[N(CH3)4](PO3F)3·18H2O,


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[NH2Et2]HPO3F, and beta-NH4HPO3F crystals. An observed phosphate singulet of very low intensity (2%) was shown in the spectrum of [N(Me)4]HPO3F·H2O. For other compounds, the neutralized solution or freeze dried powder was characterized by NMR in the absence of a crystalline product. In this case, the overall amount of the phosphate impurity was estimated prior to recrystallization and nothing could be concluded about the crystalline product.

Fluoride analysis was particularly useful in evaluating phase purity by a double determination (Sect. 2.1 Fluoride analysis). Deviations were found between the experimental (Seel) and calculated values of even very pure crystals. Experimental values of the total fluoride content were generally lower than the calculated values by less than 20%, based on small amounts of H2O and hydrogen phosphate in the sample. Two examples were the fluoride analyses of [N(CH3)4]HPO3F·H2O and Na5[N(CH3)4](PO3F)3·18H2O with experimental (calculated) values of 9.5 (9.94) and 6.5 (7.06)%, and a free fluoride content of 0.2 and 0%, respectively. Crystals with free fluoride contents of less than 2% were of high purity. Incomplete fluoride analyses reflected difficulties with hydrolysis and product isolation.

Alkali metal and ammonium hydrogen monofluorophosphates

The alkali metal hydrogen monofluorophosphates were obtained with the compositions, MHPO3F and M3H(PO3F)2. The synthesis of compounds with a M/H ratio of 1:1, MHPO3F, was rather straight-forward for M = NH4 [77], Rb, and Cs [78]. The cesium salt, CsHPO3F, crystallized easily and could be identified by XRD.

Dimorphism of the NH4 and Rb compounds made crystallization and product identification by XRD complicated. Therefore, the modifications were confirmed by measuring the cell on the single crystal diffractometer.

After repeated syntheses, the formation of the alpha and beta-modifications of NH4HPO3F could be controlled by the H2PO3F/(NH4)2PO3F ratio. One experiment also showed that beta-NH4HPO3F could be obtained by recrystallization at 333K (Sect. 2.3 beta-NH4HPO3F); the alpha-modification was isolated after recrystallization of the same raw product at room temperature. An extensive study of this dimorphism could not be carried out within the scope of this thesis, but further investigation thereof are planned. Structure determinations of beta-NH4HPO3F measured at 310 and 180 K were almost identical with only slight deviations in the a lattice constant and P-F bond lengths between the two [77]. A comparison of the simulated powder data of alpha- and beta-NH4HPO3F and the powder data published for NH4HPO3F [26] showed that the data for NH4HPO3F [79] could be assigned


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to a mixture of both modifications except for three peaks at d-values of 5.30, 4.30, and 3.44 Å. The strongest peaks at 3.71 and 3.49 Å probably belong to the â-modification and suggest that this was the major phase. This is not surprising because the NH4HPO3F salt in [26] was obtained at pH 3.8, which would more likely be reached by a 3:2 molar ratio of NH4/PO3F as is the case for beta-NH4HPO3F.

In comparison with the NH4 compound, the conditions for the formation of a and beta-RbHPO3F were not fully understood, but also seemed to be dependent on the H2PO3F/Rb2PO3F ratio. The modifications could be obtained separately by repeated syntheses. Crystals isolated for both modifications were measured twice at ca. 180 K; the second measurement yielded improved R1-factors for the structure refinements of both modifications. Crystals of beta-RbHPO3F seemed to crystallize more easily than those of alpha-RbHPO3F. The structure of beta-RbHPO3F has been more difficult to interpret. Further investigation could acquire information on a possible phase transition and the thermal stability of each modification, which could not be obtained within the framework of this thesis.

In the case of KHPO3F, repeated syntheses obtained charges with particularly high amounts of phosphate. Purity could not be improved, thus, making the crystallization and characterization of this compound difficult. Crystallization seemed to be chemically hindered, as if the crystal structure was rather unstable. This was confirmed by the pseudo-orthorhombic twinning of very small crystals (0.1 x 0.1 x 0.1 mm) measured. The simulated powder data agreed with that of the KHPO3F phase [26, 80].

Crystalline NaHPO3F·2.5H2O [81] was obtained by the recrystallization of NaHPO3F [26, 71] from H2O. The moderate pH (5.5) of Na2PO3F enabled the addition of the complete amount of Na2PO3F required for the synthesis prior to the collection of the eluate. Thus, neutralization was carried out at a starting pH not higher than 5.5 and yielded a product with very low degree of hydrolysis that recystallized easily.

Attempts to synthesize acid salts with other M/H ratios resulted in the formation of second modifications of the hydrogen monofluorophosphate, e.g. beta-NH4HPO3F, or the basic monofluorophosphates except for K3[H(PO3F)2]. This phase was rather impure with a free fluoride content of 11.9% (calculated value of 6.05 and 13.76% for KHPO3F) probably based on hydrolysis.

Hydrogen monofluorophosphates with N-containing cations

The next system studied was that of the hydrogen monofluorophosphates with organic cations containing nitrogen. In this case, higher yields of crystalline products were


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obtained for a complete characterization: elemental analysis, NMR, and XRD. The XRD patterns of these compounds were easier to interpret than those of the alkali metal and NH4 compounds.

In the first synthesis with tetramethylammonium hydroxide, crystals of [N(CH3)4]H2PO4·H2O [74] were found at first. However, six months later, cubic crystals of the hydrogen monofluorophosphate hydrate (Sect. 4.6.2) could be isolated and characterized completely. The crystals were stable in solution. Both the diethyl and triethylammonium compounds were obtained crystalline in higher yields and could then be characterized completely. The crystallization of the guanidinium acid salt was more difficult; crystals for measurement were, therefore, first formed by the evaporation of a drop of the aqueous solution on the slide. Crystals of a pure product formed later were then characterized.

The isolation of [PipzH2]HPO3F was not straight-forward. After a first synthesis, which obtained the hydrogen phosphate, a repeated synthesis yielded single crystals of the hydrogen monofluorophosphate despite significant amounts of phosphate. Further characterization was not possible. Other nitrogen heterocycles, N,N´-dimethylpiperazine and 1,4-diazabicyclo[2.2.2]octan, were also used as a counterion, but in both cases, single crystals with a high enough quality for measurement could not be isolated.

The [N,N´-dmuH]HPO3F compound was obtained in an amazingly pure and crystalline form despite its very low pH of 1.5 in solution. This was confirmed by fluoride analysis, whereas crystals of uronium hydrogen monofluorophosphate could not be isolated.

The mixed salts, Cs3(NH4)2(HPO3F)3PO3F [78] and Na5[N(CH3)4](PO3F)3·18H2O, were both synthesized using partial cation exchange of (NH4)2PO3F and Na2PO3F, respectively. While the Na/[N(CH3)4] was recrystallized with ease forming needles, the Cs/NH4 salt could not be isolated in high yields. An incomplete fluoride analysis of the Cs/NH4 compound measured 8.2% free fluoride in the sample, which is lower than the calculated value of 9.16% probably based on the hygroscopicity of the sample and consequent hydrolysis. In comparison to the NH4/Cs structure, 0% free fluoride was found in needles of the Na/[N(CH3)4] compound and a phosphate singulet in the NMR spectrum was not observed for this product.

A final synthesis of the guanidinium compound, [C(NH2)3]2PO3F, yielded a rather impure product with a experimental free fluoride content of 3.5%; the experimental C, H, N contents also deviated significantly from the theoretical values.

Despite experimental difficulties described here, single crystals were obtained and


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measured by single crystal X-ray diffraction. The results of these investigations are treated in the following chapter.

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